DC Motor Drives

33.5.1 DC Motor Drives

or linear) machines. These would decelerate naturally to rest. Braking can speed up the process cycle for the sake Until recently, the DC motor drive was the most commonly of productivity.

used type of electric variable speed drive, with only very few

896 Y. Shakweh

Electric Variable Speed Drives

by phase-controlled rectification and this technique has now almost entirely replaced the Ward-Leonard systems previously

Special drives

AC Drives

DC Drives

used.

The AC/DC converter offers a variable DC voltage, which

BDCM Induction Motor

Synchronous

is capable of four-quadrant operation (positive and negative DC voltage and DC current output). Permanent Magnet-

Motor

SRM

excited brushed motors have been used in numerous appli-

Wound rotor

Squirrel cage

cations for sometime, particularly in non-regenerative drive

LIM/LSM

applications.

Stepper Kramer

Motor output torque is approximately proportional to

Motor

AC/AC

DC LINK

armature current and motor speed is approximately propor-

tional to converter output voltage. Speed control by sensing

armature voltage is therefore feasible giving an accuracy of

Integral motor

around 5%.

FCI(IM)

LCI(SM)

Provided the motor excitation is kept constant, the DC drive power factor is proportional to motor speed. Since most

FIGURE 33.3 Classification of electric VSD. pumps, compressors, and fans demand a torque proportional to the square of speed, constant excitation systems are used and so the above relationship applies.

A typical power factor, at maximum rated speed for a DC exceptions is the least expensive. The mechanical commutator drive is 0.85. This relationship applies to many other types of is an electromechanical DC to AC bidirectional power flow electric drives. power converter, as the currents in the rotor armature coils

If a slow dynamic response is satisfactory, regeneration to are AC while the brush-current is DC. The DC drive is well- the mains supply is achieved either by reversing the motor known, well-proven and widely applied; yet its popularity is field or armature connection. Alternatively regeneration with in relative decline due to the emergence of the more robust, faster response is achieved by connecting another Thyristor lower cost squirrel cage induction motor drive.

Bridge in anti-parallel with the main bridge. In this case fast Unfortunately the mechanical commutator though not bad response is possible with changeover time of <15 ms between in terms of losses and power density has serious commutation full torque motoring to full torque regenerating. The 6-pulse current and speed limits and thus limits the power per unit drive configuration is acceptable for powers up to 1 MW. This to 1–2 MW at 1000 rpm and may not be accepted at all in limitation arises not from any semiconductor device limita- chemically aggressive or explosion-prone environments. The tion, but is due to AC line current harmonics the converter application of the DC drive has been restricted to hazardous generates. areas due to the very limited availability of flameproof DC

A force-commutated or “chopper” converter for DC motors machines. Commutator and brush maintenance is difficult in uses the principle of variable mark-space control using a such environments. Furthermore, continuous sparking at the Thyristor or transistor solid-state switch. With a diode front- brushes is virtually inevitable at full load output.

end converter a fixed, smoother DC supply is derived from Due to the inherent ease of speed control of the sepa- the mains by uncontrolled rectification and rapidly applied, rately excited DC machine, DC drives found popularity in removed, and reapplied to the machine for adjustable intervals, early electric drive applications, by varying the applied arma- thus applying a variable mean DC voltage to the DC motor, ture voltage. This variable armature voltage is simply generated refer to Fig. 33.4b.

FIGURE 33.4 DC drive: (a) with fully controlled anti-parallel supply bridge and (b) diode rectifier with DC chopper.

33 Drives Types and Specifications 897 This type of DC drive has the advantage of high (near unity)

Within this range, the motor frequency is increased power factor at all motor speeds and much reduced harmonic

until, at maximum speed.

spectrum. (c) Machine Limit (Pullout Torque): Once the machine limit has been reached the torque falls off in propor- tion to the square of motor frequency. Operation at

33.5.2 Induction Motor Drive

the higher end of this speed range may not be feasi-

33.5.2.1 Squirrel Cage Induction Motor

ble as the motor power factor worsens. This in turn results in a higher stator current than the rated value.

Squirrel cage induction motors are simpler in structure than The motor heating may be excessive unless the duty DC motors and are most commonly used in the VSD industry.

factor is low.

They are robust and reliable. They require little maintenance and are available at very competitive prices. They can be

Induction motors are used in applications requiring fast and designed with totally enclosed motors to operate in dirty and precise control of torque, speed, and shaft position. explosive environments. Their initial cost is substantially less

The control method widely used in this type of applica- than that of commutator motors and their efficiency is com- tion is known as Vector control, a transient response at least parable. All these features make them attractive for use in equivalent to that of a commutator motor can be achieved. industrial drives.

The voltage, current, and flux linkage variables in this cir- The three-stator windings develop a rotating magnetic flux cuit are space vectors from which the instantaneous values rotating at synchronous speed. This speed depends on the of the phase quantities can be obtained by projecting the motor pole number and supply frequency: The rotating flux space vector on three radial axes displaced 120 ◦ from each intersects the rotor windings and induces an EMF in the rotor other. The real and imaginary components of the space vec- winding, which in turn results in circulating current. The rotor tors are separated, resulting in separate direct and quadrature currents produce a second magnetic flux, which interacts with axis equivalent circuits but with equal parameters in the the stator flux to produce torque to accelerate the machine. As two axes. the rotor accelerates, the induced rotor voltage falls in magni-

Changes in the rotor flux linkage can be made to occur only tude and frequency until an equilibrium speed is reached. At relatively slowly because of the large value of the magnetizing this point the induced rotor current is sufficient to produce the inductance of the induction motor. Vector control is based torque demanded by the load. The rotor speed is slightly lower on keeping the magnitude of the instantaneous magnetizing than the synchronous speed by the slip frequency, typically 3%. current space vector constant so that the rotor flux linkage

In order to ensure constant excitation of the machine, and remains constant. The motor is supplied from an inverter that to maximize torque production up to the base speed, the ratio provides an instantaneously controlled set of phase currents of stator voltage to frequency needs to be kept approximately that combine to form the space vector, which is controlled constant.

to have constant magnitude to maintain constant rotor flux Induction motor drive has three distinct operating regions: linkage. The second component is a space vector, which is in space quadrature with the instantaneous magnetizing current

(a) Constant Torque: The inverter voltage is controlled space vector. This component is instantaneously controlled to up to a maximum value limited by the supply volt-

be proportional to the demand torque. age. As the motor speed and the voltage are increased To the extent that the inverter can supply instantaneous sta- in proportion, constant V /F, the rated flux linkage is tor currents meeting these two requirements, the motor is maintained up to the base speed. Values of torque up capable of responding without time delay to a demand for to the maximum value can be produced at speeds torque. This feature, combined with the relatively low inertia up to about this base value. The maximum avail- of the induction motor rotor, makes this drive attractive for able torque is proportional to the square of the flux

high-performance control systems.

linkage. Typically, the induction motor is designed to Vector control requires a means of measuring or estimating provide a continuous torque rating of about 40–50% the instantaneous magnitude and angle of the space vector of its maximum torque. of the rotor flux linkage. Direct measurement is generally not (b) Constant Power: For higher speed, the frequency of the feasible. Rapid advances are being made in devising control inverter can be increased, but the supply voltage has configurations that use measured electrical terminal values for to be kept constant at the maximum value available

estimation.

in the supply. This causes the stator flux linkage to decrease in inverse proportion to the frequency. Con- stant power can be achieved up to the speed at which

33.5.2.2 Slip-ring (Wound-rotor) Induction Motor

the peak torque available from the motor is just suf-

Drive

ficient to reach the constant power curve. A constant Wound rotor induction motors with three rotor slip rings power speed range of 2–2.5 can usually be achieved. have been used in adjustable speed drives for many years.

898 Y. Shakweh In an induction motor, torque is equal to the power crossing line-commutated converters and inverters, current harmonics

the air gap divided by the synchronous mechanical speed. In are produced and these can be reduced to acceptable values. early slip-ring induction motor drives, power was transferred However, as the slip-energy recovery network is only power- through the motor to be dissipated in external resistances, con- rated in direct proportion to the speed reduction required nected to the slip ring terminals of the rotor. This resulted in (assuming constant load torque), the magnitudes of the har- an inefficient drive over most of the speed ranges. More mod- monic currents generated are proportionally less than with ern slip ring drives use an inverter to recover the power from drives where the solid-state converters have to handle the whole the rotor circuit, feeding it back to the supply system.

drive power. Harmonics of the rotor rectifiers are transmitted The speed of slip-ring induction motor can be controlled through the rotor and appear as non-integer harmonics in the by:

main supply.

The main disadvantages of the slip-ring induction motor • Stator frequency control as with a cage rotor machine drive are: (a) the increased cost of the motor in comparison • Rotor frequency control with a squirrel cage, (b) the need for slip ring maintenance, • Rotor resistance control (c) difficulty in operating in hazardous environments, (d) the • Slip energy recovery (Kramer system). For capital cost need for switchable start-up resistors, and (e) the poor power reasons the last two are commonly used factor compared with other types of drive.

Addition of rotor resistance especially for starting large induction motors is well-known. The basic effect produced by

33.5.3 Synchronous Motor Drives

adding rotor resistance is to alter the speed at which maximum motor torque is developed. Unfortunately, power dissipation as To understand the way the synchronous machine operates, let heat in the rotor resistance bank takes place, earlier means to us assume that the induction motor were to rotate at the syn- overcome this shortcoming were to convert the rotor power to chronous speed by an external means. Under this condition the DC, and feed a DC motor on the same shaft. The rotor slip frequency and magnitude of the rotor currents would become energy, when running at reduced speed, is therefore recon- zero. If an external DC power supply were connected to the verted to mechanical power. This is the “Kramer” system. The rotor winding, then the rotor would become polarized in a disadvantages of this approach were the extra maintenance and similar way to a permanent magnet. The rotor would pull into capital costs.

step with the air-gap-rotating magnetic field, generated by the The static Kramer system overcomes these shortcomings by stator but lagging it by a small constant angle, referred to as the replacing the DC machine with a line commutated inverter load angle. The load angle is proportional to the torque applied which returns the slip-energy directly to the AC line, either to the shaft, and the rotor keeps rotating at synchronous speed, directly (on lower power systems) or via a transformer. A key provided that the DC supply is maintained to the rotor field advantage of the Kramer drive system is that the slip energy winding. The magnetic flux produced by the rotor winding recovery equipment (DC machines or static inverter) needs intersects the stator windings and generates a back EMF, which only be rated for a fraction of the maximum motor rating. makes the synchronous motor significantly different from the This is true when a small speed range is required and provided induction motor. that a separate means is provided of starting the motor. This is

As with the induction motor drive, the requirement is to because the motor rotor current is proportional to torque and keep the ratio V /F constant (i.e. varies both the stator fre- the rotor voltage inversely proportional to speed.

quency and applied voltages in proportion to the desired motor Naturally if the slip energy recovery network can be rated speed). to withstand full rotor voltage (developed at standstill) a con-

The supply bridge converter is phase controlled generating trolled speed range of zero to maximum could be achieved. an adjustable DC current in the DC link choke. To generate However this is generally only feasible on smaller motors maximum torque from the synchronous motor this current is (below 2000 kW) where the rotor voltage is sufficiently low for switched into the motor stator windings at the correct phase an economic inverter package. Secondly if a full speed range is position with respect to rotor angular position as detected by needed the slip energy recovery network has to be rated at full position sensor by the Inverter Bridge. When running above motor power, so static Kramer drives become uneconomic for about 10% speed, the back EMF generated by the synchronous wide speed ranges. The overall system power factor would be motor is sufficient to commutate the current into the next very low for a wide speed range system.

arm of Inverter Bridge. So, as this type of inverter is machine For the above reasons Kramer drives are very suitable for (motor) commutated, the inverter configuration is merely that high power drives (>200 kW) where a small speed range is of a conventional DC drive. The complexity, expense, and required. Pump and fan drives present therefore good eco- limited power capability of the force-commutated circuitry is nomic applications. Kramer drives have also been used for therefore avoided. low speed range endurance dynos using the recovery sys-

The motor back EMF is insufficient for Thyristor commuta- tem to control torque of induction generator. As with all tion at low speeds. The technique here, therefore, is to rapidly

33 Drives Types and Specifications 899 phase back the supply converter bridge to reduce the DC link power and reactive power control and for efficient wide speed

current to zero and after a short delay (to ensure that all thyris- range control in high power applications. tors in machine bridge are turned off) reapply DC current

For high power applications, synchronous motors are when the correct Thyristor trigger pattern has been reestab- preferred because of the ability to control reactive power lished. As the motor speed and thus back EMF, increase to flow through appropriate control of excitation. Synchronous

a value sufficient for machine commutation, changeover to motors tend to have wider speed range and higher efficiency. continuous DC link current operation is effected.

However, synchronous motors are generally more expensive During the starting mode the correct Inverter Bridge fir- than induction motors. ing instant is determined by rotor position sensor, which

With modern high power PWM-VSI drives, synchronous is mounted on the motor shaft whose angular position is motor can be driven for same inverter with vector control detected by opto or magnetic probes. When in the machine- methods. commutated mode, sensing of stator voltage is used. To develop maximum torque in the low speed or pulsed mode, angular rotor position sensing is necessary. However if less

33.5.4 Special Motors

than full load torque availability at low speed can be tolerated, Motors under this category employ power electronics convert- the inverter system can be set to produce a low fixed frequency ers for normal operation. Generally, this type of motor has a in the pulsed mode. This frequency is then increased, as motor large number of phases in order to limit torque pulsation and rotation is detected (either in steps or on a pre-set ramp rate) self-start from any rotor initial position. This is a new breed of until sufficient back EMF is generated to facilitate changeover motors, which can be fed through a unipolar or bipolar cur- to the voltage-sensing mode.

rent. Also they have singly salient or doubly salient magnetic

As previously stated the key advantage of this type of drive structures with or without permanent magnets on the rotor. is that all Thyristor devices are line or machine commu- tated. Expensive and complex forced commutation circuitry is avoided and fast turn-off thyristors are unnecessary. Inverter

33.5.4.1 Brushless DC Motor (BDCM) Drive

systems of this type can therefore be built at very high powers, This type of machine has a similar construction to a standard up to 100 MW. Also, as a result of avoiding force commutation, synchronous machine, but the rotor magnetic field is produced converter efficiency is high.

by permanent magnet material. A position sensor is used to The thyristors in the machine Inverter Bridge must be trig- ensure synchronism between the rotor position and the stator gered at such an angle to give sufficient time for commutation magneto motive force (MMF) via drive signals to the inverter. from one device to the next. This results in the synchronous The use of new magnet materials characterized by high coer- motor operating at a high leading power factor of around 0.85. cive force levels has reduced magnet sizes, and largely overcome However, as far as the mains supply is concerned, the total the demagnetization problem. The absence of the field copper drive has the characteristics of a DC drive where power factor losses improves the machine efficiency. is proportional to speed.

As the permanent magnet is the source for excitation, the Another important characteristic of this type of drive is that BDCM can be viewed as a constant flux motor. A limited it is inherently reversible and regenerative. For regenerative amount of flux weakening can be achieved by increasing the operation the Inverter Bridge is triggered in the fully advanced load angle of the stator current. Achieving a useful constant position so, in effect, it becomes a plain diode bridge. A DC power range is not usually practical with this type of motor. output voltage, approximately proportional to motor speed, is

A large demagnetizing component of stator current would be therefore generated at the DC side of the supply Converter required to produce a significant reduction in magnet flux, and Bridge. This converter bridge is now triggered in the regenera- this would increase the stator loss substantially. tive mode thus returning power to the supply system. Reversing

The required base torque determines the motor size and operation is achieved by altering the sequence in which the the losses are essentially independent of the number of stator thyristors in Inverter Bridge are triggered.

turns. At speeds up to the base speed of the constant power This type of drive is widely applied over a wide power range range, the efficiency of the motor is essentially the same as as it embodies an efficient brushless motor and relatively sim- for one designed for rated voltage at base speed. For oper- ple and efficient converter. At lower powers, say below 30 kW, ation above base speed, the stator current from the inverter permanent magnet synchronous motors is more common.

is reduced in inverse proportion to the speed. This mode of Unlike the induction motor, the synchronous type requires operation in the high-speed range reduces the dominant sta- two types of converter. The first for main power conversion tor winding losses relative to a machine in which the flux while the second is low power for field excitation. The field is reduced and the current kept constant. The losses in the converter feeds the rotor exciter winding through slip rings inverter are, however, increased due to its higher current rat- and brushes or alternatively a brushless exciter can be used. A ing. For an electric road vehicle that must carry its energy store, coordinated control of the two converters provide for active the net energy saving may be sufficiently valuable to overcome

900 Y. Shakweh the additional cost of the larger inverter. A further advantage stator winding loss will be the same. Also, the stator iron losses

of this approach is that, if the DC supply to the inverter is lost, will be similar for the two motors. the open circuit voltage applied to the inverter switches will be

The reluctance motor is capable of operation in the constant within their normal ratings.

power mode of operation. As for all AC drives, when the supply The BDCM has higher volumetric power density compared voltage limit is reached above the base speed, the flux linkage to other types of motors (induction or synchronous). They are is reduced in inverse proportion to the shaft speed, and the particularly suited for the high values of acceleration required torque is inversely proportional to speed squared. in drives (e.g. machine tools). They are often operated with high acceleration for a short time followed by a longer period of low torque. At such low values of load factor, the cooling

33.5.4.3 Linear Motors

capability is frequently not a limitation. The major interest is There are applications in which linear motion, as opposed to in obtaining the maximum acceleration from the motor. The rotational, is required. A linear machine has the same operat- short-term stator current of a BDCM is limited to the value ing principles as those applied to all other rotating machines. required for magnet protection. These values of acceleration The PWM-VSI converters and motor control principles dis- are significantly higher than that can be achieved with either cussed in this chapter are also applicable to this type of induction or DC motors of similar maximum torque rating.

motor.

There are two types of linear motors:

• LIM – Linear Induction Motor

33.5.4.2 Switched Reluctance Motor (SRM) Drive

• LSM – Linear Synchronous Motor with permanent

magnetic excitation

This motor can be regarded as a special case of a salient syn- chronous machine in which the field MMF is zero and the

The LSM type has the following advantages over the LIM: torque is produced by reluctance or saliency action only. The

Better power factor

rotor has no winding. The SRM drive needs an inverter whose

• More responsive control • Higher efficiency

frequency is locked to the shaft speed, but since the torque

is linearly proportional to the square of the stator current, the use of unidirectional current involves little sacrifice in

The disadvantages of LSM are:

performance. • Very accurate position feedback is required Generally, the use of position sensors in the SRM and BDCM • The use of PM – expensive and heavy is something of a disadvantage in both cases. The SRM does

not require permanent magnets, which can cost and may Transport, material handling, and extrusion processes are a involve demagnetization risks and limit top speeds due to cen- few examples in which linear motors have successfully been trifugal forces. The SRM hence has a simpler construction and employed. is more robust. However the need to magnetize the motor from the AC side adds to inverter costs and may increase peak current levels significantly, hence raising stator copper losses.

33.5.4.4 Stepper Motors

Switched reluctance synchronous motors have a cylindrical Stepper motors are either built in a similar manner to BDCM, stator with three AC windings and a solid rotor (without any with permanent magnets embedded in or bonded to the rotor winding) with a moderate orthogonal axis magnetic saliency or a rotor with no magnets. The latter type is made of a fer- up to 4 (6) to 1. High magnetic saliency is obtained with rite magnetic material and its circumference is cut to form a multiple flux barriers. The conventional SRMs are to some number of slots, forming teeth lengthwise to the rotor axis. extent (up to 100 kW) used in low dynamics variable speed

Torque production can be based on (a) magnetic reluctance drives with open loop speed control, as the speed does not (as in SRM), (b) magnetic attraction (as in BDCM), or (c) decrease with load. Consequently the control is simpler than both magnetic reluctance and attraction. with induction motors.

Stepper drives do not offer dynamic speed control, and the The main drawback of the conventional SRD is the low main action is to accelerate at full torque to full speed, maintain motor power factor and the relatively poor torque density, the speed and decelerate at full torque. In comparison to the which leads to a higher kVA rating of the power converter reluctance type stepper motor, the permanent magnet type (approximately 20%). The main advantage of the synchronous offers greater torque for a given speed, particularly at start and reluctance motor over the induction motor of similar rating low speeds. is the higher efficiency. Compared to the squirrel cage induc-

Most drives incorporate controllers with connections for a tion motor, the rotor loss is small or negligible in synchronous communications link for supervisory control by PLC, hard- reluctance machines. If the saliency ratio is sufficient to pro- wiring connectors for analog/digital inputs and outputs, and duce a power factor equal to that of the induction motor, the some are equipped with software for communications with

33 Drives Types and Specifications 901 TABLE 33.12 Control features for servo and stepper motor drives, Reference [6]

Control features

Servo drive

Stepper drive

Acceleration/deceleration time

Accelerate at maximum torque, time is dependent on maximum torque and inertia Maximum speed

Adjustable

Part of the motor specification Speed control

Part of the motor specification

Permit a range of speed settings

Not necessarily available

Torque control

Always operate at maximum torque Auto tuning

Many offer speed & torque control

A feature of some servo drive

Not applicable

Commonly available by digital control signal Zero speed clamp

Reversing

Commonly available by digital control signal

Applies full torque to hold the position constant Dynamic braking

Applies full torque to hold the position constant

Controlled deceleration, may require dissipative brake

Usually standard

resistor

Regenerative braking

Dedicated circuit for controlled braking

Not applicable

Travel limits

Definition of travel limits in the forward and reverse

Standard

directions

Jog or inch

Digital command to “jog” one step (with defined distance)

Optional feature

Closed loop configuration

Most drives accept external signals for closed loop control Programming functions

Most drives accept external signals for closed loop control

Many drives incorporate programming functions as in PLCs, reset all functions to default states, return to a home position, enable or disable repetition or a pre-set sequence, select a particular set of control inputs, increased or decreased speed, change the torque boost, etc.

a computer or handheld keypad. Table 33.12 lists typical

33.5.4.6 Integrated Motors

options. The Integral Motor consists of a standard AC motor with an Unlike above motor drives, the stepper motor can achieve integrated frequency inverter and EMC filter. It is robust and

precise position control without the need for any external specified for reliable operation, and often designed to han- feedback.

dle rough working conditions, including ambient temperature – 25–40 ◦

C and dusty, corrosive as well as humid environments (Enclosure IP55). This type of drive uses a standard induc-

tion motor with the AC/AC converter integrated in the motor Actuators are widely used in industry, primarily for posi- frame often as a separate converter box mounted directly above

33.5.4.5 Actuators

tioning tasks. Their designs are based on all sorts of force the motor frame in place of the terminal box. The power producing principles. Reference [7] describes several types of popularity of this type of drive is limited to 0.5–7.5 kW. direct drive electric actuators, including (a) the DC actuator

This type of motor offers the following advantages: (Moving coil type), (b) induction actuators, (c) synchronous

• Save space by eliminating the need for a separate con- actuators (moving magnet DC type), (d) reluctance actuators,

troller

and (e) inductor actuators (polarized reluctance type). • Reduce installed costs because cabling between motor and Electric actuators are used increasingly in control systems

converter is eliminated

and automated electromechanical equipment. Typical specifi- • Eliminate motor problems caused by high voltage tran- cation factors include: range of motion, type of motion (linear

sient due to output cable capacitance or rotary, stepwise or continuous), resolution needs, speed of

• Minimize EMC due to high dV /dt response, environmental conditions, supply conditions, allow-

able electromagnetic noise emission level, need for integrated The integrated drive includes most features including start, position and velocity sensors, maintenance needs, eligibility, stop, forward, reverse, speed and torque controls, controlled cost, peak, and continuos torque, etc.

acceleration and deceleration, etc.

The main demands of industry for high performance systems are:

(a) A convenient supply and low power consumption

33.6 PWM-VSI DRIVE

(b) Reliability and robustness (c) Low initial cost and maintenance

In recent years, the popularity of PWM VSI has increased (d) Fast response

beyond recognition. Its dynamic performance and controlla- (e) Linear “torque-excitation” characteristics

bility is better than the DC drive. Its power range has extended

902 Y. Shakweh TABLE 33.13 Drive comparison

Control features Cyclo

Speed Limited

Dynamic response Excellent

Torque pulsation Low

High

Very low

Very good

Power factor at Poor

Poor

Very good

Very good

low speed Stability

Good

Moderate

Very good

Very good

Motor Custom

Regeneration Inherent

Inherent

Needs extra

Inherent

(c) (d) Volumetric power

FIGURE 33.5 MV stack topologies. density

Moderate

Good

Very good

Excellent

∗ The AC output voltage of the matrix-converter is always less than the input voltage – derating is expected or a larger frame size is required.

improved thermal performance of power components can be achieved by operating at medium voltages instead of low volt- ages. Many variable-speed drive applications will benefit from

to areas dominated for years by traditional solutions such as the availability of economic MV alternatives. the cyclo-converter and LCI drives.

When adequately rated high blocking voltage devices are available, a simple 2-level inverter or alternatively 3-level

33.6.1 Drive Comparison

Neutral Point Clamped (NPC) has always been the choice to meet required output voltages. These topologies offer a simple

Table 33.13 shows a direct comparison between the cyclo- and cost-effective solution. converter, LCI, and PWM-VSI drives. The DC drive and the

Series connection of power devices is the traditional solu- Slip Power Recovery converter type are not listed because AC tion for high power high voltage Thyristor-based drives. This drives have already replaced DC drives in most applications approach is perceived to be complex with fast switching IGBTs due to low maintenance and better reliability of AC motors. because of simultaneous switching and correct static and Slip recovery is only suitable for applications with a limited dynamic voltage sharing of series devices. speed range and requires a slip ring wound rotor.

The “Multi-level” inverter drive is seen to offer a better In comparison with the cyclo-converter and LCI cur- solution for high power, high voltage inverter drive. The out- rent source converter drives, the PWM-VSI drive offers the put waveform is high quality, even at very high modulation following advantages:

frequencies, which inherently results in lower harmonic con- tent in the output voltage waveform (less losses, less torque

• Excellent dynamic response pulsation, and lower insulation voltage stresses). • Smooth torque/speed control over full speed range Reference [8] and Fig. 33.5 categorizes MV converter (0–200 Hz)

topologies as follows:

• High volumetric power density • Ride through of dips in supply voltage

(a) Series Connected 2-Level (SC2L) • Use of standard motors (squirrel cage induction motor

(b) 3 Level Neutral Point Clamped (3LNPC) or synchronous motor)

(c) Multi-level: Diode Clamped Multi-Level (DCML), • Improved AC supply power factor over full speed range

Capacitor Clamped Multi-Level (CCML) • Reduced cabling and transformer size and cost in com-

(d) Isolated Series H-Bridge (ISHB) parison with cyclo-converters • No significant torque pulsation • Lower noise level

33.6.3 Control Strategies

• Low maintenance Several control techniques can be found in the VSD industry,

refer to Fig. 33.6. These are:

33.6.2 Medium Voltage PWM-VSI

• Open loop inverter with fixed V/Hz control • Open loop inverter with flux vector control

The maximum power rating of LV VSD is limited by prac- • Closed loop inverter with flux vector control (induction tical current ratings of power components such as motor,

motor)

cable and transformer (typically 1500 A), giving a limit of about 2 MVA at 600 V. At this rating, motor manufacturers

Table 33.15 summarizes the main features, advantages, and always prefer a MV machine design – significant saving and disadvantages of each technique.

33 Drives Types and Specifications 903 TABLE 33.14 Comparison between different MV converter stack topologies

Topologies Advantages

Disadvantages

2-level with series devices • Simple & proven technology • Static and dynamic voltage sharing of series devices (SC2L)

• Same converter design over supply voltage range • High dV/dt due to synchronous commutation of series • Standard fully developed PWM control

devices

• Provision for series redundancy of power switches per • High switching frequency harmonic content in inverter

inverter phase arm (n+1)

output voltage

3-level NPC (3LNPC) • Well-proven • Series redundancy is difficult to achieve • Reduced harmonic content

• More complex PWM control is needed, than 2 level • Better utilization of switches

• Requires extra clamping diodes • Reduced dV/dt (half the SC2L equivalent)

• Requires split DC link • Requires mid-point voltage balance control • Even number of power devices per arm is always needed • Switches requires snubbers

Diode clamped multi-level • Reduced harmonic contents • Series redundancy is very difficult to achieve (DCML)

• Reduced dV/dt • Very complex PWM control is needed • Requires many steering diodes • Requires split DC link • Requires voltage balance control of split DC link capaci-

tors • Uneven current stresses on power devices • Requires snubbers

Capacitor clamped • This configuration has all the advantages of a multi-level • Possible parasitic resonance between decoupling capaci- multi-level (CCML)

converter plus Simpler arrangement, modular building

tors

• Complex to provide series redundancy • Less components

block

• More complex PWM control strategy than for 2-level • Snubberless operation is possible

• Voltage redistribution of capacitors during supply voltage • Easier capacitor voltage balance than 3LNPC

surges • Too many capacitors (bulky stack design & poor capacitor

utilization at high ratings) • Complex converter arrangement (for low stray induc-

tance) • Inverter rating is limited by the load current flowing

through the capacitors

Series connected isolated • Modular design of the converter power modules • Employs a special (bulky and expensive) transformer h-bridges (ISHB)

• The basic building block is based on a DC supply bridge, • Complex to achieve series redundancy

• Different supply transformer designs are required for • In the AC supply the combined diode bridge rectifiers

decoupling capacitor and a H-bridge arrangement

applications operating at different AC line voltages

act like a multi-pulse bridge (18p for 4-level and 24p for

• Power pulsation for poor power factor loading

5-level), reducing harmonic injection into the AC supply.

• Poor utilization of capacitors

• Its output has very low harmonic contents in spite of the • Not suitable for common DC bus applications

low switching frequency

• Dynamic Braking difficult

33.6.4 Communication in VSDs

and drives, using a single communication link. The major ben- efits seen are (a) reduced installation and cabling cost, and

The use of a high speed advanced digital communication better overall immunity of the system. Both factors result in (Fieldbus) to build industrial automation system for real-time more reliable operation and reduced maintenance costs. control or simply for data logging has become well-established There exist two main types of network: in modern industries. Digital communication resulted in

replacing wiring looms with a digital serial network, this (a) Centralized network – requires a network master con- resulted in a lower cost installation and a more reliable

troller, typically a PLC. The master device is entirely solution. Over the last few years many industrial Fieldbuses

responsible for controlling communications over the emerged and endusers, system integrators and original equip-

network, while the slave devices tend to be “dumb” ment manufacturers (OEMs) chose the optimum system for

devices with no local intelligence. their applications.

(b) Decentralized network – which require some local

A Fieldbus is a digital communication system that allows a intelligence at each node, but no overall mas- control system to exchange data with remote sensors, actuators

ter device. This is ideal for real time application

904 Y. Shakweh

DC environment, as all nodes are effectively running in

parallel. Most modern VSDs are equipped with hardware and soft-

ware, which enable local and remote communication with plant automated system via a Fieldbus system. The most pop-

(a)

Frequency V/f

V PWM

AC ular Fieldbuses are Profibus, Interbus, Ctnet, Sercos, Worldfib,

Refernce Ratio

f Modulator

Motor

and Devicenet.

(b)

AC

Speed Torque

Vector

f Modulator 33.6.5 PWM Techniques Motor

V PWM

Control Control

Control

Different PWM techniques have been employed in PWM-VSD

converters. Figure 33.7 identifies the most commonly used

Speed Torque &

AC 33.6.6 Impact of PWM Waveform

Optimal

Control Flux Control

Pattern

Motor

33.6.6.1 PWM Voltage Waveform

Fast switching of IGBTs (typically <1 µs) results in high dV/dt,

Speed

Feedback

typically 3–5 kV/µs, and possible voltage overshoot at turn off which can last for a few microseconds. The fast rate of

rise/fall of voltage combined with high peak voltage at the turn FIGURE 33.6 Electrical drive control techniques. (a) DC drive; (b)

(d)

off, result in a premature failure of motors as well as EMC. frequency control (PWM scalar control); (c) flux vector control (field-

References [9–11] deal with the effect of PWM waveforms of oriented control); and (d) direct torque control.

VSD.

The following is a brief summary of the effect of the unfiltered waveforms.

TABLE 33.15 Comparison between various control methods used in VSD Drive type

DC drive

AC drive

Control method • Field-oriented

• Direct torque control Features

• Frequency control

• Flux vector control

• Field orientation via

• Use advance control mechanical commutator

• Voltage and frequency

• Field-oriented control –

theory • Controlling variables are

control

similar to DC drive

• Controlled variables are armature current and field

• Simulation of variable

• Motor electrical

magnetizing flux and current

speed drive using

characteristics are

motor control • Torque control is direct

modulator

modelled (observer)

• Typical torque dynamic • Typical response 10–20 ms

• Flux provided with a

• Closed loop drive

constant V /F ratio

• Torque controlled

response is <5 ms

• Open loop drive

indirectly

• Load dictate torque level

• Typical torque dynamic

• Typical torque dynamic

response 10–20 ms

response 100 ms

Advantages • Accurate and fast torque

• Simple control

• Low cost

• Good torque response

• No feedback requirements • High dynamic speed

• No feedback devices are

• Accurate speed control

• No need for an observe response

required

• Full torque at zero speed

• Simple

• Performance approaching

• Simple to control

DC drive

Disadvantages • Reduced motor reliability

• Field orientation not used

• Feedback is needed

• Regular maintenance

• Motor status ignored

• Costly

• Motor costly to purchase

• Torque is not controllable

• Modulator is needed

• Needs encoder for

• Delaying modulator used

feedback

33 Drives Types and Specifications 905

PWM Switching Strategies

Current-Controlled PWM

Voltage-Controlled PWM

Random PWM Current Control Current Control

Fixed Switching

Current Control

Frequency PWM

Random Elimination

Random

Optimal & Harmonic

Switching

Randomized

Space Vector PWM

Conventional PWM

Frequency

Pulse Position Switching

Sinusoidal Modulation Wave

Modified Modulation Wave (Sinusoidal with addition of third harmonic,trapezoidal, and other

waveforms)

Natural Sampled

Regular Sampled

Asymmetric Modulation

Symmetric Modulation

FIGURE 33.7 Classifications of PWM techniques.

33.6.6.2 Effect on Motor

33.6.6.4 Effect on EMC/Insulation/Earthing

• Premature insulation failure due to partial discharge as a • Inductive and capacitive couplings between live compo- result of peak voltage, high dV/dt and high frequency.

nents and earth result in common-mode and differential- • Motor shaft voltage, which forces current into the shaft

mode noise. This could lead to malfunctioning of nearby bearing, leading to early bearing failure.

sensitive equipment.

• Motor stray capacitance (between windings and earthed • The voltage to earth applied on drive components pul- frame) leads to earth-current flow caused by high dV/dt.

sates at the switching frequency, adding voltage stresses • High dV/dt creates nonuniform distribution of voltage

(worst at low speeds, low modulation index). This poses across the winding, with high voltage-drop across the first

additional insulation requirements on main power com- few turns and consequential failures.

ponents (motor, cable, output filter, and transformer). • In a large motor, voltage differential on the frame is likely

A 4th wire may be required between the motor frame and to develop in spite of protective earthing of the motor.

the converter virtual earth so that a low impedance path More than one earthing point is needed.

is provided for the motor earth current. • Strict rules must be observed when cabling and earthing.

33.6.6.3 Effect on Cables

• Voltage doubling effect at the rising/falling edges of

33.6.6.5 Motor Insulation

voltage waveform due to wave propagation in long High peak voltages can be experienced at the motor terminals cables [9].

especially when long cable is employed (10–100m depending • Earth-current flows in cable stray capacitance due to on the size of motor). This is usually caused by voltage dou- dV/dt.

bling phenomena of a transmission line with unequal line and • Restriction on cable type used and earthing methods load impedance. Motor line voltage can reach twice the DC employed.

link voltage with long cables.

• Cable type (armored, screen, multi-core). Fast voltage rise times of 5000 V/µs can be measured at the • Likelihood of cross talk with other surrounding cables motor terminals. Under this condition the motor insulation running in parallel.

becomes stressed and can lead to a premature breakdown of a • For PWM drives, the cost of cabling is likely to be standard motor insulation. When motor fails due to insulation significant due to special requirements of cables, and stress caused by high peak voltage and fast voltage rise times, termination methods employed.

failure occurs in the first turn as phase-to-phase short or phase

906 Y. Shakweh to stator short. The highest voltage is normally seen by the first

• The fundamental cause of the shaft voltage is magnetic turn of the winding.

asymmetry between the stator and the rotor or possi- Standard motor capabilities, established by the National

bly a phase shift of the motor voltage waveform. System Electric Manufacturers Association (NEMA) and expressed in

ground may also contribute to this condition through the MG-I standard (part 30), indicate that standard NEMA

unbalance system voltage.

type B motors can withstand 1000 V peak at a minimum rise • NEMA-500 recommends the consideration of insulated time of 2 µs (500 V/µs). Reference [10] describes the effect of

bearing for motor frame of certain sizes. PWM inverter waveform on motor insulation in more detail.

Partial Discharge

33.6.6.7 EMC

The phenomena which starts deterioration of the motor insu- The main sources of electromagnetic emission of PWM-VSI lation is called Partial Discharge (PD). When electric stresses drives are described in [12] as follows: in insulation voids exceed the breakdown voltage of the air, a

AC/DC Converter: Supply harmonics caused by supply partial discharge occurs. Successive PDs destroy the insulation bridge rectifier (100 Hz–2.5 kHz): As already explained the slowly.

input bridge circuit with a SCR or diode bridge is a source of supply harmonics in the input current.

DC/AC Inverter: Harmonics caused by the switching of the

Voltage Strength between Phase to Phase and Phase to Frame

inverter bridge (3 kHz–20 MHz): the inverter bridge uses fast switching devices to create PWM voltage output. The inverter

Both NEMA and IEC are proposing: (a) maximum 1000 V at is a source of a wide band of frequencies, typically extending rise time less than 2 µs and (b) a maximum rate of rise of from the basic switching frequency (usually several kilohertz)

500 V/µs. It is believed that low voltage standard motors can to the radio high-frequency bands at 20 MHz. The radio fre- withstand a lot larger voltage stresses than specified by NEMA quency current spreads out into both the supply and motor and IEC, possibly up to 1300 V, almost regardless of the rise connections. An EMC filter is often used to limit spread of time.

high frequency harmonics into the supply. Control Electronics: The control circuit employs a micro-

Voltage Strength between Turn to Turn

processor with clock frequency of several megahertz, typically In low voltage AC motors, the conductor insulation is designed

20 MHz. The clock wave produces frequencies, which are for 245 V RMS (350 V peak). The insulation strength is multiple of 20 MHz up to 300 MHz. however higher depending on the impregnation method.