Primarily, electric drives can be divided into two groups, DC drives and AC drives , DC drives have already been discussed in this chapter. Now AC drives are described at there introductory level only.advantages and disadvantages of AC drives with respect to DC drives are as under:
Advantages of AC drives
- For the same rating, AC motors are lighter in weight as compared to DC motors.
- AC motors are less expensive as compared to equivalent DC motors.
- AC motors require low maintenance as compared to DC motors.
Ac motors can work in hazardous areas like chemical, petrochemical etc. Whereas DC motors are unsuitable for such environments because of commutator sparking.
Disadvantages of AC drives
- power converters for the control of AC motors are more complex.
- Power converters for AC drives are more expensive.
- power converters for AC drives generate harmonics in the supply systems and load circuit. As a result, AC motor get derated.
The advantages of AC drives outweigh there disadvantages. as such, AC drives are used for several industrial applications. In general, there are two types of AC drives:
- Induction motor drives
- Synchronous motor drives
Induction -motor drives
Three phase induction motors are more commonly employed in adjustable speed drives than three phase synchronous motors. Three phase induction motors are of two types, squirrel -cage induction motors (SCIMs) and slip ring induction motors (SRIMs). Stator windings are both types carry three phase windings. Rotor of squirrel cage induction motors is made up of copper or aluminium bars short circuited by two end rings. Rotor of slip ring induction motor carriage three phase winding connected to three slip rings on the rotor shaft. when three phase supply is connected to 3 phase stator winding, rotating magnetic field is produced. The speed of rotating field is called synchronous speed.
Speed control of 3 phase induction motors
3 phase induction motor are admirably suited to fulfill the demand of loads requiring substanially constant speed . Several industrial applications, however, need adjustable speeds for their efficient operation. the object of the present section is to describe the basics principle of speed control techniques employed to three-phase induction motors through the use of power electronics converters. The various methods of speed control through semiconductor devices are as under:
- Stator voltage control
- Status frequency control
- Stator current control
- Static rotor resistance control
- Slip energy recovery control
Method 1 and IV are to both squirrel cage induction motor and slip ring induction motors whereas method v and vi can be used for slip ring induction motors only. these methods are now described in what follows:
Stator voltage control: it seem from that motor torque Te is proportional to the square of the stator supply voltage. A reduction in the supply voltage will reduce the motor torque and therefore the speed of the drive. If the motor terminal voltage is reduced to for the purposes of wearing the voltage applied to a three phase induction motor so as to achieve a speed control, three phase AC voltage controller is usually implied as shown in figure. Three phase AC voltage controller feeding a three phase induction motor. By controlling the firing angle of the thyristor connected in antiparallel in each phase. The rms value of the stator voltage can be regulated. As a consequence, motor torque and the speed of the drive is controlled. As shown in figure for load torque TL, is the operating point at a rated voltage and OA which is the motor speed. For reduced stator voltage is the operating point and OB is the reduced motor speed for a load torque TL. This method is suitable for motors having large value of Sm. For low slips motors , the range of speed control is very narrow.
Stator voltage control method offers limited speed range. It is usual to use 3 phase voltage controller. Their use, however,introduces pronounced harmonic contents and input supply power factor for the voltage controller is quite low. These are, therefore, used for low power drives like fans, blowers and centrifugal pumps requiring low starting torque. for these types of loads the load torque is proportional to speed squared and input current is maximum when slip is equal to 1/3 , this is proved in example.
Stator voltage and frequency control
Figure shows that air flux gap is kept constant, the maximum torque remains an altered. Ashwin equation indicates that a starting torque increases even if their flux is kept constant. At low values of frequencies,the effect of resistance cannot be neglected as compared to the reactances. this has the effect of reducing the magnitude of maximum torque at lower frequency as shown in figure. In practice, at low frequencies,the supply voltage is increased to maintain the level of maximum torque. this method of speed control is also called volts control.
If stator is tense is neglected, then from figure b , load torque TL for a certain load is also shown. It is seen from this figure that both voltage and frequency are varied (usually below their rated values), speed of the drive can be controlled. The control of both voltages and frequency can be carried out (so as to keep V/f constant) thoda use of three phase inverter for cycloconverters. inverters are used in low and medium power drives where is cycloconverter are suitable for high power drives like cement Mills, locomotives etc.
variable voltage and variable frequency can be obtained from the voltage source inverter. For such circuit configuration as shown in figure,three phase AC is converted to constant DC by diode rectifier. Voltage and frequency are both very good bye pulse width modulation inverter. The circuit between the rectifier and the inverter consists of an inductor L and capacitor c, called filter circuit. The function of filter circuit is to smooth DC input voltage to the inverter. The circuit in between rectifier and inverter is called DC link. As shown in figure,regeneration is not possible because of diode rectifier. Also,inverter would inject harmonics into the three phase AC supply.
As shown in figure, three phase AC is converted to DC by diode rectifier. chopper where is the DC input voltage to the inverter and frequency is controlled by the inverter. Use of chopper reduces the harmonic injection into the AC supply. Regeneration is not feasible in the scheme.
As shown in figure (c) use of 3 phase controlled rectifier, DC link consisting of L and C and force commutated voltage source inverter. Voltage regulator by controlled rectifier and frequency is varied within the inverter. Hair regeneration is possible if three phase full converter is used. Regeneration is also visible in the scheme as shown in figure. It uses a three phase double converter, L-C filter and inverter.level of DC input voltage to the inverter is regulated in double converter where is frequency is varied within the voltage source inverter. It may be observed from above that volts/hertzs. Control offer speed control from stand steel upto rated speed of induction motor. This method is similar to the armature voltage control method used for the speed control of DC motor.
Summary of characteristics of adjustable frequency induction motor drives
speed torque characteristics of a three phase induction motor drives depend upon the methods of control techniques employed. For a different stator frequency, a family of speed torque characteristics as shown in figure can be obtained.