Speed Control of Three Phase Induction Motor

• The three phase induction motor or synchronous motor is used as AC drive. 
• The three phase induction motor has following advantages : 

              1. Low cost

              2. Low maintenance

              3. Simple construction

              4. Reliability and high efficiency

• There are following methods of speed control of three phase induction motor.

               A. Stator voltage control

               B. Variable voltage variable frequency control

               C. Slip power recovery method

               D. Stator resistance control

• The speed control of three phase induction motor is done by first and second method whereas the speed control of slip ring induction motor is done by third and fourth method.

Stator voltage control

• The speed control of three phase induction motor is done by stator voltage control. 
• The output torque of the three phase induction motor is directly proportional to square of supply voltage for a given slip. 
• The torque – slip characteristics of three phase induction motor is shown in figure A. 
• As the supply voltage increases for a given constant torque, the slip decreases.

        T α sV² / R

           Where

              T = Torque

              R = Rotor resistance and

               s = Slip

• There is one resistor connected between stator terminal and supply voltage. 
• The speed control of the motor is achieved by adjusting the resistor but there are higher heat losses in this method and eventually the efficiency decreases.
• If the speed control of the induction motor is done by thyristor control, it will result in low losses, high efficiency, less weight and smooth control.
• If the torque of the induction motor is given in the synchronous watt

                T α I₂² R₂ / s

• The stator current of the induction motor is sum of magnetizing current and reflected rotor current. If the magnetizing current is neglected at very low voltage

               T α I₁² R₂ / s

• The stator and rotor current is inversely proportional to √ R₂ for a given constant torque.

               I₁  α     I₂   α √ ( s / R₂ )

• Therefore we conclude that rotor resistance play important role to determine rotor current. 
• The squirrel cage induction motor draws high current particularly at starting and low speed.

• The stator voltage control method is applicable for small and medium HP induction motor. 
• The stator voltage is adjusted by controlling the firing angle of two anti parallel SCRs. 
• The speed control is achieved without change of frequency in this way. 
• The voltage and current waveform gets distorted particularly at low speed. 
• This method is rarely used in the industries because the induction motor draws heavy current from the supply.

Variable voltage – Variable frequency control

• The synchronous speed of the three phase induction motor is given by

           Ns = 120f / P

           Where

                f = Supply frequency

               P = Number of poles and

             Ns = Synchronous speed

• The speed of the three phase induction motor can be changed from above or below normal speed by changing in the frequency. 
• The stator flux should be constant for constant torque motor. 
• The ratio of voltage to frequency is kept constant in order to keep flux constant in the three phase induction motor.

• The air gap flux of the induction motor increases if the frequency decreases and voltage is kept constant. 
• This will result in saturation of the core and magnetizing current and stator losses increases. Therefore the motor efficiency decreases.
• If the supply frequency increases (or flux decreases) in the three phase induction motor, it affects the torque. 
• The supply voltage should not increase beyond certain limit in order to consider insulation of the stator winding. 
• If the speed control of the induction motor is done by frequency, the ratio of voltage to frequency must be constant. 
• There are several methods to control speed of induction motor by variable voltage and variable frequency ( Figure A ).

• The basic circuit for speed control of three phase induction motor is shown in the figure C. 
• The three phase bridge converter controlled rectifier converts alternating voltage into direct voltage. 
• The combination of L and C is called as DC Link. 
• The function of inductor L is to smooth direct voltage and capacitor C provides constant voltage to voltage source inverter. 
• The three phase controlled rectifier is line commutated whereas the voltage source inverter is forced commutated. 
• The voltage source inverter provides variable voltage, variable frequency supply to the three phase induction motor. 
• The variable voltage, variable frequency supply is achieved by adjusting firing angle of controlled rectifier and voltage source inverter. 
• The speed control of three phase induction motor is achieved by current source inverter and cycloconverter. 

Slip power recovery method

• This method is only applicable to speed control of Slip ring induction motor. 
• The rotor input power of the three phase induction motor is equal to sum of rotor mechanical output and rotor copper loss.

              Rotor input power = Power across air gap = T

              Rotor copper loss = Rotor input

              Rotor mechanical output = ( 1 - s ) Rotor input

              Where

                 T = Induction motor torque

               ω_s = Synchronous angular speed and

                 s = Slip

• The rotor copper loss and rotor mechanical power output is becomes equal at half the synchronous speed of the induction motor. 
• This will result in efficiency of three phase induction motor becomes 50%. If the stator losses are also considered, the air gap power dissipates in the rotor circuit at low speed. 
• The speed of the three phase induction motor at low speed is not economical.
• The slip power dissipates in the external resistor in the slip ring induction motor.
•  If the slip power is utilized, the speed of the induction motor increases. 
• The slip power is given back to supply network in the slip power recovery method or supply is given to auxiliary motor which is connected to same induction motor.

Sub - synchronous speed control ( < Ns ) 

• The power circuit diagram for speed control of Slip ring induction motor is shown in the figure D. This is called as Modified surbius system. 

• The three phase rectifier convert alternating voltage into direct voltage and phase controlled inverter converts direct voltage into alternating voltage. 
• The three phase bridge rectifier operates at slip frequency whereas the inverter operates at supply frequency. 
• The inductor smoothen the DC output. 
• The three phase rectifier and inverter operates at naturally commutated. The output voltage of the three phase rectifier ( if the commutation is neglected )

           V_dc = 1.35sV_r........................(1)

           Where V_r = Line to line rotor voltage ( When rotor is stationary )

                        s = Slip

• The back emf of the three phase inverter is

           V_b = 1.35 V_L Cos α................(2)

           Where V_L = AC line voltage and

                        α = Firing angle

• If the motor torque at no load is neglected, when comparing equation (1) and (2)

                V_dc + V_L = 0

                1.35sV_r + 1.35 V_L Cos α = 0

                 s = – ( V_L / V_r ) Cos α  

                   = K Cos α

                Where K = Turns ratio of stator and rotor winding

                              = Stator winding turns / Rotor winding turns

• Therefore we can say that the speed control of induction motor is done by firing angle of inverter. 
• If the value of K is unity, the motor speed control is achieved from zero to maximum. 
• Actually the value of K always greater than unity therefore the voltage available at the slip ring is very low. 
• The step down transformer is connected between supply and inverter side which step down the supply voltage at slip ring voltage.
• The system power factor reduces when the motor speed decreases. 
• The power factor of the system is increased by using forced commutated inverter so the reactive power does not flow through supply lines.

Super synchronous speed control ( > Ns ) 

• The speed of the three phase induction should be lower than the synchronous speed when the power flow from rotor of the slip ring induction motor to the supply side. 
• When the induction motor speed requires greater than the synchronous speed, regenerative braking operation is necessary. 
• If the power flows from supply to rotor side or rotor to supply side, the motoring and braking operation is possible at sub synchronous speed and super synchronous speed. 
• If the motor speed requires greater than the synchronous speed, the rotor injected voltage should be reverse than the phase sequence of supply voltage. 

• There are two phase controlled converters used for achieving speed of the motor above or below synchronous speed. One bridge circuit works as rectifier or inverter at slip frequency whereas the second bridge circuit works as inverter or rectifier at supply frequency. The cost of converter increases due to six thyristor in place of six diodes. The gate signal is controlled at slip frequency. When the induction motor speed slightly lower than the synchronous speed, the slip frequency emf ( E₂’= sE₂ ) reduces and this will result in natural commutation of thyristors. Due to above reason, forced commutation circuit is necessary.

Induction motor speed control by chopper

• This method is used to control speed of slip ring induction motor by varying external rotor resistance. 
• The chopper is used to control rotor resistance smoothly and in step. 
• The chopper is turned on and turned off at regular interval in order to change rotor resistance. 
• The rotor resistance becomes R₁ when chopper is turned on. Similarly the rotor resistance is equal to R₁ + R₂ when the Chopper is in off condition. 
• The effective rotor resistance R = R₁ ( T_ON ) + R₂ ( T_OFF ) / ( T_ON + T_OFF )

• The ripple is reduced by by filter circuit. The harmonics increases as the ripple increases. This will result in higher copper losses. 
• The transistor is used for low power application whereas the GTO is used as high power application. 
• The three phase controlled rectifier may be used in place of three phase rectifier.

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