Induction Motor Drives Solid State Drives - Drive Characteristics (UNIT I) Induction Motor Drives Department of Electrical & Electronics Engineering, Chettinad College of Engineering & Technology, Karur

Control of induction motors Control of induction motors can be classified into two ways Scalar Control Vector Control Scalar control includes the control techniques listed below Stator voltage control (with constant frequency) Supply frequency control (with constant voltage) Constant V/f control Pole changing Rotor resistance control Slip power recovery scheme

Stator Voltage Control A simple method of controlling speed is to vary the stator voltage at constant supply frequency. This can be achieved by using an AC voltage controller. By varying the instant of firing, the stator voltage of the IM can be controlled. This method is being used as a solid state soft-start for constant speed induction motors, where the stator voltage is applied gradually to limit the stator current.

Stator Voltage Control ωms sm ωm, s V 0.75V 0.5V 0.25V T O Tm TL Range of speed control

Stator Voltage Control Torque is proportional to voltage squared and current is proportional to voltage. Starting torque, Running torque and Maximum torque are reduced as voltage decreases. But the slip at which the maximum torque occurs remains the same for whatever voltage. This method is suitable for applications where torque demand reduces with speed. Some applications are fan, pump, propellers etc. This method enables speed control below rated speed because the supply voltage cannot be increased beyond rated value.

Stator Voltage Control As the voltage is decreased to decrease speed, the slip increases. Hence, the efficiency of the motor is very low at slow speeds.

Stator Voltage Control When the voltage is reduced to decrease speed, slip increases. As slip increases, rotor current increases to maintain rotor input power constant (rotor input power is constant if stator copper and core losses are omitted). As rotor current increases, rotor copper losses increases, decreasing the motor output power, efficiency and torque. Increase in rotor copper loss causes over heating of the rotor. So for this kind of application, Class D induction motors with a normal full load slip (s=0.1 to 0.2) or wound rotor induction motors with external rotor resistance is used so that power dissipation takes place externally. Thus motor size decreases, but maintenance and cost increases due to ext resistance.

Constant Voltage Variable Frequency Control ωm,s 3 2 1 T O Teωm2 = const Tm Rated Curve

Constant Voltage Variable Frequency Control Frequency is varied only above rated frequency and not below. If the frequency is decreased below rated value with supply voltage held constant, flux increases and saturation of core occurs. Decreasing flux below rated value is also undesirable because it decreases motor’s torque capability. Induction motors are designed to operate at the knee point of magnetization characteristics to make full use of magnetic material. Therefore increase in flux will saturate the motor. This will increase the magnetizing current, distort the line current and voltage, increase the core loss and stator copper loss, and produce a high pitch acoustic noise. φ Im saturation Knee Point O

Constant Voltage Variable Frequency Control When frequency is increased above rated value, the synchronous speed Ns increases. So, starting, running and maximum torques of the motor decreases. The slip at which the maximum torque also changes. The loci of the maximum torque value for various increased frequency gives a curve with Teωm2=constant. In the region above rated speed, induction motor behaves like a DC series motor. This method is generally not adopted for the reason that this method changes the rated flux.

Constant V/f Control Induced EMF E1 in stator winding is given by Let ‘a’ represent the factor by which the supply frequency is increased or decreased, with respect to rated supply frequency. In constant V/f method, it is desired to maintain the flux constant for any frequency or Im is maintained constant.   V1 R1 X1 Xm Xr E Rr/s Im I1 Ir’

Constant V/f Control Im for rated frequency is Im for reduced frequency is

Constant V/f Control With supply voltage constant, If frequency is increased beyond rated value, saturation occurs and if frequency is decreased below rated value, torque capability decreases. So it is necessary to maintain the flux of a motor constant. To maintain the flux in motor at rated value, V is also varied whenever f is varied such that their ratio is constant. If frequency is doubled, voltage is also doubled. If frequency is halved, voltage is also halved. For slow speeds (or low frequencies), maximum torque decreases slightly in motoring region and increases in braking region. This is due to decrease in flux in low frequency region because of decrease in voltage due to stator impedance drop. So in low frequency regions, stator drop must be compensated by an additional boost voltage to restore maximum to its actual value.

Constant V/f Control Variation of supply voltage with frequency is only possible for frequencies below rated frequency. For higher frequencies, voltage cannot be increased beyond its rated value. So beyond rated frequency, only the frequency is increased holding the supply voltage constant. This causes decrease in flux. In this region the motor is said to operate in field weakening mode. In this mode since voltage is constant and frequency is increases torque decreases.

Constant power region Even for a>1 the motor is operated in the low slip region. Since the slip is small Ir’ flows through a resistive circuit. So Φr becomes zero. If the drop due to rotor resistance is neglected, then 𝑅 2 ′ /𝑠 represents only the rotor output power. So, If the stator resistance is neglected, If Ir’ and hence Is is maintained constant with the help of a current control loop, then the variable frequency control with constant voltage gives constant power operation. When operating at maximum permissible current, the motor develops maximum constant power as shown in figure. Also from maximum torque equation for a>1, it is observed that the maximum torque reduces inversely with speed.

Constant power region For a>1, since the slip is small, Therefore for a given Ir’ and hence for a given Is, slip speed increases linearly with ‘a’. In other words, change in speed from no-load to full load increases if frequency is increased above rated frequency. Frequency is continuously increased to increase the motor speed. For a particular increased frequency and speed, motor torque equals the breakdown (or maximum) torque. This speed is called critical speed. Beyond this critical speed running the motor at maximum constant current will stall the motor. So, this speed marks the end of constant power region.

DC series motor characteristics Speed control beyond the critical speed (or beyond constant power region), makes the motor behave like a DC series motor. Speed control beyond critical speed is required in traction applications, where the torque demand in the high speed range is low. In this region, torque is not allowed reach breakdown torque by maintaining slip speed constant. Since slip speed is constant, for any increase in ‘a’, rotor current Ir’ should decrease linearly. As a result mechanical power developed also decreases. Torque in this region is inversely proportional to speed squared.

ωm T -Tmax Tmax O ωms Increasing frequency Constant Power Locus Constant torque Locus

Drive operating Regions

Drive operating Regions The different operating regions of a VFVS drive is shown in figure. The Inverter maximum torque capability, but short- time or transient torque capability, is limited by the peak inverter current and is lower than the machine torque capability. The steady state torque capability is further limited by the power semiconductor junction temperature Tj. The steady state torque envelope can be increased by improving the cooling system design. The operating point can be anywhere within inverter torque envelope for transient operation and anywhere within steady state torque envelope for steady state operation. Inverter transient torque limit is 50% higher than steady state torque limit for 60 seconds.

Constant torque and field weakening regions

Constant torque and field weakening regions Below rated speed both voltage and frequency are varied in such a way that flux is constant. Since V Above rated speed, frequency is decreased holding voltage constant. So flux decreases. As a result induced EMF E2 decreases. Current is maintained constant by increasing slip. This is equivalent to field weakening mode of operation in a DC separately excited motor.

National Electrical Manufacturers Association (NEMA –US) SCIM are classified into 5 types to meet industrial needs Class A SCIMs have low starting torque, high starting current and low operating slip. They have low rotor resistance, and therefore operating efficiency is high at low slips. Class B SCIMs are used for constant speed industrial drives. Starting torque, starting current and maximum torque are lesser than Class A motors. Class B motors have higher slip characteristics Class C and D have high rotor resistance and hence have low starting current and high starting torque. Class E has high operating efficiency