Load Profile Types

33.4.1 Load Profile Types

power electronics converter are definite constraints in a drive In the literature, four different load profiles have been specification. described, e.g. Reference [3] (Table 33.10). These are:

Table 33.11 categorizes operating duties into eight major types, Reference [4].

1. Torque proportional to the square of the shaft speed (Variable torque)

2. Torque linearly proportional to speed (Linear torque)

33.4.2.2 Mean Output

3. Torque independent of speed (Constant torque) Variation of the required motor output during the periods

4. Torque inversely proportional to speed (Inverse torque)

of loaded operation is among the most frequent deviations from the duty types defined in Table 33.11. In such cases the

33.4.2 Motor Drive Duty

load (defined as current or torque) is represented by the mean load. This represents the root mean square(RMS) value, calcu-

lated from the load versus time characteristics. The maximum The size of the driven motors is generally chosen for continu- torque must not exceed 80% of the breakdown torque of an ous operation at rated output, yet a considerable proportion of induction motor.

33.4.2.1 Duty Cycle

TABLE 33.10 Load characteristics Type I

Type IV • T = f(n 2 )

Type II

Type III

• T = f(1/n) • P = f(n 3 )

• T = f(n)

• T = Constant

• P = Constant • Low start-up torque

• P = f(n 2 )

• P = f(n)

Mostly dominated by DC drives, but • Best suited for energy saving

Information about process is

At start-up the torque may be higher than

modern PWM VSI is taking over. • Torque-speed curve is

needed (e.g. density,

nominal. Examples static friction with

Certain loads such as winding and required when specifying a

consistency, viscosity,

conveyor belts. Vertical and horizontal

reeling machinery require closed drive

temperature)

forces need to be taken into consideration

loop controls • Axial and centrifugal pumps

for inclined conveyors.

• Lift-machines • Axial and centrifugal

• Mixers

• Extrusions, draw-benches

• Reciprocating rolling mill ventilators

• Stirrers

• Paper and printing continuous

• Winding machines • Screw and centrifugal

machines

• Lathes compressors

• Volumetric gear pumps / pistons

• Winders • Centrifugal mixers

pumps etc.

• Reelers • Agitators

• Piston compressors

• Conveyor machines

• Wire drawers

• Web-feed printing machines Power Torque

• Lift-machine

894 Y. Shakweh TABLE 33.11 Definition of load cyclic duties – VDE0530, in accordance with IEC 34-1

Duty type

Representation

Description

S1: Continuous running duty Operation at constant load of sufficient duration for the thermal equilibrium to be reached. Specify by indicating “S1” and required output.

S2: Short-time duty Operation at constant load during a given time, less than required to reach thermal equilibrium, followed by a rest and de-energized period of sufficient duration to re-establish machine temperatures within 2 ◦

C of the coolant. S3: Intermittent periodic duty with a

A sequence of identical duty cycles, each including a period of high start-up torque

operation at constant load and a rest and de-energized period. In this duty type the cycle is such that the starting current does not significantly affect the temperature rise.

S4: Intermittent periodic with a high A sequence of identical duty cycles, each cycle including a start-up torque

significant period of starting, a period of operation at constant load and a rest and de-energized period.

S5: Intermittent periodic duty with A sequence of identical cycles, each cycle consisting of a period high start-up torque and electric

of starting, a period of operation at constant load, a period braking

of rapid electric braking and a rest and de-energized period.

S6: Continuous operation periodic A sequence of identical duty cycles, each cycle consisting of a duty

period of operation at constant load and a period of operation at no-load. There is no rest and de-energized period.

S7: Continuous operation periodic A sequence of identical duty cycles, each cycle consisting of a duty with high start-up torque

period of starting, a period of operation at constant load and electric braking

and a period of electric braking. There is no rest and de-energized period.

S8: Continuous operation periodic A sequence of identical duty cycles, each cycle consisting of a duty with related load/speed

period of operation at constant load corresponding to a changes

predetermined speed of rotation, followed by one or more periods of operation at other constant loads corresponding to different speeds of rotation. There is no rest and de-energized period.

If the ratio of the peak torque to the minimum power Careful assessment of duty types S2 to S8 reveal that there requirements is greater than 2:1, the error associated with using exist two distinct groups:

the root mean square (RMS) output becomes excessive and the mean current has to be used instead. No such mean value

1. Duties S2, S3, and S6 permit up rating of motors rel- approximation is possible with duty type S2, which therefore

ative to the output permissible in continuous running necessitates special enquiry.

duty (S1).

33 Drives Types and Specifications 895

2. Duties S4, S5, S7, and S8 requiring derating relative

Torque

to the output permissible in continuous running duty (S1).

Motoring Speed

33.4.2.3 Thermal Cycling

QIII

QII

The drive duty cycle also affects the reliability and the life

Reverse

Forward

expectancy of power devices. Repetitive load cyclic duty results

Motoring

Braking

in additional thermal stresses on power devices. Frequent acceleration and deceleration of drives results in repetitive junction temperature rise and falls at the cyclic duty. The life

FIGURE 33.2 Operating regions of electric VSD. expectancy of devices is often determined by the maximum

allowed number of cycles for a given power device junction temperature rise.

• Mass or potential energy loads: Typically hoists or lifts – Although this is true for all types of power devices, it is more

which would run on or even accelerate. Braking must critical for IGBTs where wire bonds and solder layers are used.

apply full power to maintain constant speed while the In modern IGBT-based converter design, the maximum

load is lowered.

junction temperature rise of the IGBTs is limited to a level, which ensures a conservative number of thermal cycles over

The drive losses, mechanical resistance, and the transmission the lifetime of the drive. Typical junction temperature rise is efficiency, work in favor of deceleration, reducing the braking

C for power demand. The energy regenerated by potential energy non-repetitive cyclic duty (e.g. fan pumps).

C for a repetitive cyclic duty (e.g. steel mill) and 40 ◦

loads depend on maximum power and both the overrun time and the decelerating time.

The braking time and the duty cycle time are decided by

the requirements of the process system, but note particularly Fully regenerative electric VSDs offer a rapid regenerative the effect of varying the braking duty cycle time and the dynamic braking in both forward and reverse directions. Oper- deceleration time. ation in motoring implies that torque and speed are in the same

33.4.2.4 Multi-quadrant Operation

For DC injection braking the kinetic energy of the motor - direction (QI, III). In regenerative braking the torque is oppo- load system is converted to heat in the motor rotor. For fast and site to the speed direction (QII, IV) and the electric power flow frequent generator braking the power electronics converter has in the motor is reversed. (See Fig. 33.2.)

to handle the generated power either by a controlled dynamic Positive power flow of electric energy means that electric brake chopper (with braking resistor) or through bidirectional power is drawn from the power supply via the power electron- power flow. The power losses in the converter can assist in ics converter by the motor while negative power flow refers to dynamic braking. electric power delivered by the motor in the generator mode

For a fast speed response, modern variable speed drives may to the power electronics converter. This could be regenerated develop a maximum transient torque up to base speed and back to the supply or dissipated, as a heat in the dynamic brake maximum transient power up to maximum speed, provided dissipative mechanism.

that both the motor and the power electronics converter can For regenerative drive, the power electronics converter has handle these powers. For a 200 kW dynamometer drive appli- to be designed to be able to handle bidirectional power flow. cation, a rapid change of torque from full positive torque to In low and medium power converter (say <500 kW) with slow full negative torque is required in less than 10 ms. dynamic braking demands, the generated power during the braking period is interchanged with the strong filter capacitor

of power electronics converter, or DC (dynamic) braking is

33.5 Variable Speed Drive Topologies

used. In this section drive topologies are classified according to the

33.4.2.5 Dynamic Braking Energy

motor they employ. Various publications dealt with this subject

e.g. References [3, 5]. The most common motors are illustrated There exist two types of energy stored in VSD, which need to in Fig. 33.3.

be dealt with during dynamic braking: • Inertia or kinetic energy loads: Typically moving (rotating