AC Drives Dr. Adel A. El-Samahy Department of Electrical Engineering University of Helwan

Torque Speed C/C Example 3: A three-phase asynchronous machine, wound-rotor, 500 volts, 50 HZ, has the following data: N f.l =l480 r.p.m, I 2f.l =310 A. T max /T f.l = 3.1 Calculate: -the slip for maximum torque. -Starting torque ratio T st. / T f.l -Starting current ratio I 2st / I2 f.l

solution Example 3: =1500 rpm Since Then

solution Example 3: s cr =s fl [T max /T fl ±[(T max /T fl ) 2 -1] ½ ]=0.08 I 2st =I 2fl [(T st /T fl ). 1/ S fl ] ½ = 1885 A

Speed Control of 3phase I. M.  Speed control means change the drive speed as desired by the process to maintain different process parameter at different load  Energy Saving.  Speed control is a different concept from speed regulation where there is natural change in speed due change in load on the shaft.  Speed control is either done manually by the operator or by means of some automatic control device.  Low speed starting requirement

Speed Control of 3phase I. M. The speed and the torque of I.M can be varied by one of the following means: Stator Voltage Control Rotor Voltage Control Frequency Control Stator Voltage and Frequency Control Stator Current Control Voltage, Current, and Frequency Control

Stator Voltage Control T proportional to V 2 If V is reduced to bV such that b≤1, The torque equation will be modified to

Stator Voltage Control

As the stator voltage is reduced, the air gap flux and the developed torque and hence the speed is also reduced. The stator voltage can be varied by: 1.AC voltage controller 2.Voltage fed variable DC link inverters 3.PWM inverter Due to limited speed range, AC voltage controller are normally used to provide the voltage control AC voltage controllers are very simple, but the harmonic contents are high and the input power factor to the controllers is low. They are used mainly in low power applications, such as fans, blowers, and centrifugal pumps, where the starting torque is low They are also used for starting high power induction motor to limit the in-rush current

Stator Voltage Control AC voltage Controller

Stator Voltage Control

AC voltage Controller

Stator Voltage Control Disadvantages: limited speed range  when applied to Class B (low-slip) motors Excessive stator currents at low speeds  high copper losses Distorted phase current in machine and line Poor line power factor Hence, only used on low-power, appliance-type motors where efficiency is not important e.g. small fan or pumps drives

Stator Voltage Speed Control Excercise: A three-phase 460 V, 60 Hz, 4poles, Y connected I.M has the following parameters: R1 =1.01 , R2 = 0.69 , X1 = 1.3 , X2= 1.94 , and Xm = 43.5 , no load losses is negligible. The load torque which is proportional to the speed squared is 41N.m at 1740 rpm. If the rotor speed is 1550 rpm, determine: 1.Load torque 2.Rotor current 3.Stator voltage 4.Stator current 5.Motor input power 6.Slip for maximum rotor current 7.Maximum rotor current 8.Torque at maximum rotor current

Rotor Voltage Control

This method increases the starting torque while limiting the starting current. However, this is an inefficient method and there would be unbalances in voltages and currents if the resistances in the rotor circuit are not equal. You may note that the magnitude of maximum torque does not affected by changing the added resistance to the rotor circuit, only s cr is affected

Rotor Voltage Control The three-phase resistor may be replaced by a three-phase diode rectifier and a chopper as shown in Fig, where the GTO operates as a chopper switch, The inductor, L d acts as a current source, I d and the chopper varies the effective resistance, which can be found from Eq.: R e = R(l- k)

Rotor Voltage Control

Note: that the dc current in the inductor is speed independent

Rotor Voltage Control

Equivalent Circuit of static Kramer

Rotor Voltage Control Again, by replacing the bridge rectifiers by three three-phase dual converters (or cycloconverters) as shown in Fig., the slip power flow in either direction is possible and this arrangement is called a static Scherbius drive. static Kramer and Scherbius drives are used in large power pump and blower applications where limited range of speed control is required. Since the motor connected directly to the source, the power factor of these drives is generally high.

Rotor Voltage Control Excercise: A three-phase 460 V, 60 Hz, 6poles, Y connected wound rotor I.M whose speed is controlled by slip power recovery, the motor has the following parameters: R1 =0.041 , R2 = , X1 = 0.29 , X2= 0.44 , and Xm = 6.1 , Ns/Nr =0.9. L d is high enough such that I d has negligible ripple, no load losses, rectifier losses, and GTO chopper losses are negligible. The load torque is proportional to the speed squared is 750N.m at 1175 rpm. 1.If the motor has to operate with minimum speed of 800 rpm, determine the resistance R: 2.With the value of the resistance R, if the desired speed is 1050 rpm calculate the Id, duty cycle of the chopper, dc voltage, efficiency, and input power factor

Frequency Control The torque and speed of induction motors can be controlled by changing the supply frequency. When motor is supplied by the rated voltage and rated frequency, the flux is the rated value. If the voltage is maintained fixed at its rated value while the frequency is reduced below its rated value, the flux increases. This would cause saturation of the air-gap flux, and the motor parameters would not be valid in determining the torque-speed characteristics. At low frequency, the reactance's decrease and the motor current may be too high. This type of frequency control is not normally used. If the frequency is increased above its rated value, the flux and torque would decrease.

Frequency Control

If the synchronous speed corresponding to the rated frequency is called the base speed  b, the synchronous speed at any other frequency becomes  s =  b And

Frequency Control The typical torque-speed characteristics are shown in the following figure for various values of 

Frequency Control The typical torque-speed characteristics are shown in the following figure for various values of 

Frequency Control If R 1 is negligible, the maximum torque at the base speed as The maximum torque at any frequency The slip at maximum torque at any frequency

Frequency Control

Frequncy Control Excercise: A three-phase, 11.2-kW, 1750-rpm, 460·V, 60-Hz, four- pole Y-connected induction motor has the following parameters: R 1 = 0, R 2 = 0.38 ., X 1 = 1.14 ., X 2 = 1.71 ., and X m = 33.2 . The motor is controlled by varying the supply frequency. If the maximum torque requirement is 35 N· m, calculate (a)the supply frequency, (b)the speed  m.

Frequency Control

HOW SPEED IS CONTROLLED USING VFD Rectifier:  The rectifier in a VFD is used to convert incoming ac power into direct current (dc) power. DC bus:  DC output of rectifier flows through the dc link to inverter input. Inverter:  The “Insulated Gate Bipolar Transistor” (IGBT) is a common choice in modern VFDs.  The IGBT can switch on and off several thousand times per second and precisely control the power delivered to the motor.  The IGBT uses a method named “pulse width modulation” (PWM) to simulate a current sine wave at the desired frequency to the motor.

Voltage and Frequency Control

The developed torque equation at any frequency The slip at maximum torque at any frequency

Voltage and Frequency Control The torque speed characteristics are shown in the following figure

Voltage and Frequency Control The torque speed characteristics are shown in the following figure

Voltage and Frequency Control Voltage and frequency may be controlled by one of the following circuits

40

Voltage and Frequency Control

Voltage and Frequncy Control Excercise: A three-phase, 11.2-kW, 1750-rpm, 460-V, 60-Hz, four- pole, Y-connected induction motor has the following parameters: R 1 = 0.66 , R 2 = 0.38 , X, = 1.14 , X; = 1.71 . and X m = 33.2  The motor is controlled by varying both the voltage and frequency. The volts/hertz ratio which corresponds to the rated voltage and rated freql1ency, is maintained constant. (a) Calculate the maximum torque Tm and the corresponding speed  m for 60 and 30 Hz. (b) Repeat (a) if R 1 is negligible.