GROUP 1 METHODS

14.2 GROUP 1 METHODS

In this group there are three alternative possibilities that are practical. The first possibility uses an intermediate device between the sinusoidal supply and the motor to vary the magnitude of the voltage applied to the motor. Secondly, simple switching methods are available when two or three discrete speeds are required. These are usually obtained by special stator winding arrangements for squirrel-cage induction motors.

Examples in this sub-group are:- • Star-delta stator winding.

• Pole-changing motors, e.g. PAM and NS motors. • Special motors that have connections made to their rotor windings.

References 4 and 5 give descriptions of the PAM and other switched winding methods. These methods find little application in the oil industry. The third possibility includes systems that allow the speed to be continuously varied over part

or all of the torque-speed characteristic of the motor. This is achieved by making special connections to the rotor or secondary circuit of the motor. Examples in this sub-group are:

• Wound rotor induction motors. • AC commutator motors, e.g.

– Schrage motor – Double-fed motor – Three-phase series motor

• Special combinations of machines that use the slip frequency energy of the rotor circuit e.g. – Kramer combination

– Scherbius machine All of these possibilities have become obsolete due to the availability of highly reliable elec-

tronic controllers.

14.2.1 Simple Variable Voltage Supplies

These methods provide continuously variable control of the speed over part or all of the torque-speed characteristic of the motor. One of the simplest ways of causing an induction motor speed to change is by altering the magnitude of the applied voltage to its stator. This will cause the motor torque to

change in proportion to the square of the voltage, i.e. T αV 2 .

Thus, at the new voltage a new torque will be produced and this will match the load require- ments at some new value of speed. The shape of the torque-slip (speed) curve of the motor will be

VARIABLE SPEED ELECTRICAL DRIVERS

Figure 14.1 Variable applied voltage with a normal design of an induction motor.

the same as that for full voltage operation, but scaled up or down by the ratio (V /V r ) 2 where V is the applied voltage for the new speed and V r is the rated voltage, see Figure 14.1.

It may be seen that if this method is used to control the speed of a standard, almost constant- speed type of induction motor, then the actual range of speed control obtained will, in fact, be small before stalling occurs. The situation could be improved by using a motor with a high rotor resistance as shown in Figure 14.2. The rotor resistance at full-load has been increased by a factor of 10 in order to demonstrate the effect on the torque-speed characteristic.

However, this method is not used for medium and large industrial drives, because of the practical difficulty in designing a high resistance rotor. If a slip-ring wound rotor design is used then an external high resistance can be added, but this method is seldom acceptable in the oil industry because of restrictions imposed by hazardous area classification.

The voltage applied to the stator can be varied in two ways: • In steps using a transformer that has various taps on its secondary winding. This gives a course

control and is used for ‘open loop’ control, i.e. no feedback regulation is used. • Continuously by using some form of thyristor controller which will allow feedback action in the

form of ‘closed loop’ control to be used to accurately regulate the speed. However, if such a scheme is used then it is the customary practice to adjust the applied frequency so as to maintain

a constant air-gap flux, see 14.3.2 and 14.6.

14.2.2 Pole-changing of the Stator Winding

If an induction motor has more than two poles, e.g. four or eight, then it can be arranged to operate at two different synchronous speeds, one being half of the other. This technique is one of several which

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Figure 14.2 Variable applied voltage with a high resistance rotor design for an induction motor.

come under the general heading of ‘pole-changing’ motors. The method just outlined is applied to squirrel-cage motors but not to wound rotor motors.

In a multi-pole motor for eight-pole operation, the adjacent poles change in polarity from North to South around the air gap.

If half of the pole windings have their connections reversed so that the current flows in the opposite direction around the windings, then those windings will produce poles of opposite polarity. Hence each pair of adjacent poles will have the same polarity.

Therefore the resulting number of effective North and South poles will be halved and so the synchronous speed will be doubled.

Care should be taken when specifying the duty of multi-speed motors to ensure that the windings are appropriately rated for continuous or short-term duty since this may affect the amount of heat and temperature rise produced in the windings and also the effectiveness of any shaft-mounted cooling fans that may be employed.

14.2.3 Pole Amplitude Modulated Motors

A variation on the theme of pole changing is a particular type of squirrel-cage induction motor, called the Pole Amplitude Modulated (PAM) motor. PAM motors should be used for low speed applications thereby requiring many poles e.g. 10, 12, 16. In addition, the various speeds required should not

be widely different. This means that the number of effective poles will not be too dissimilar, e.g. 8-pole and 10-pole operation. Commonly used two-speed pole ratios are 4/6, 6/8, 6/10, 6/12, 8/10, 10/12, 12/14, 12/16, 16/20 and 16/40. Low speed motors have many poles e.g. 16 and 24, and so

VARIABLE SPEED ELECTRICAL DRIVERS

complicated winding reconnections can be devised to produce more than just two speeds from the motor, as described in Reference 4. However, this is mainly of academic interest since the demand in the oil industry for such motors is rare. Three-speed ratios are 4/6/8, 6/8/10 and 8/10/12. Fractional ratios of speeds can be obtained by reversing and reconnecting only a small number of the poles or leaving some poles unexcited. Hence, an irregular distribution of poles around the stator is produced and this tends to produce harmonic torques throughout torque-slip characteristics.

Occasionally in refineries there is a need for large gas compressors to operate at two different speeds for long periods of time. If these two speeds can be matched to the pole arrangements of a multi-pole motor, then pole changing can be used satisfactorily.

These motors have been used successfully on large multi-speed air fans for power plant steam boilers. Most of the research on PAM motors took place between about 1958 and 1975 and is well documented in the proceedings of the IEE of the UK during this period.

14.2.4 Wound Rotor Induction Motors

A more versatile and satisfactory method of speed control of an induction motor is to make use of the rotor impedance. There are two basic approaches, firstly by simply adding resistance into the circuit by means of rotor slip-rings and an external static resistance bank or, secondly, by injecting a slip frequency AC voltage into the rotor circuit in such a way that the rotor current can be changed in magnitude or phase angle for any particular speed. The second method can be achieved by using rotor slip-rings or a rotor commutator, which looks and functions rather like those used on DC machines. In both approaches the essential effect is that the time phase of the flux produced by the rotor current, relative to the main flux produced by the applied voltage to the stator, is reduced to a minimum. Maximum torque is produced when this effect is achieved. If the rotor circuit is made predominantly resistive at any particular slip then the desired effect is achieved.

The simplest method of achieving the effect is to insert extra resistance into the rotor circuit. The rotor of the induction motor has to be specially wound so the winding can be split into three sections. Each section is connected to shaft-mounted slip-rings. The conductors of the rotor winding are carefully insulated from the iron core and from each other. The extra resistance is an external static unit mounted near to the motor.

If, for example, a water pump needs to be run at reduced flow rate for much of its operating time then a reasonably accurate method is to use a wound rotor motor with an external resistance. The resistance can be in the form of wire elements with various fixed tappings (for coarse control and starting) or an electrolytic tank using a water and caustic soda solution (for fine control and starting). In practice, the tendency is for this electrolytic tank to be preferred for large motors. A wide range of speed control with good torque performance is obtained by this method.

Until the introduction of thyristor and power transistor controllers a wound rotor motor with added resistance was one of the most common and simplest methods of speed control and is used for motors up to 10 MW. The main disadvantage is that the resistance bank is wasteful of energy, and the removal of the heat produced can prove difficult. The stability of the resistance of the electrolyte is also a problem since the resistance varies considerably with temperature and chemical composition of the electrolyte. Reasonably good speed regulation can be obtained by closed loop control, even though the stability of the electrolyte can introduce complications. Precise regulation is obtainable by other, more sophisticated, methods as will be described in following pages. However, now that

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power electronic controllers are available for even the largest motors, the use of wound rotor motors has been largely superseded and no longer used in the oil industry.