Rotor Resistance Control and Slip Power Control using Chopper

Rotor Resistance Control and Slip Power Control using Chopper
Closed Loop Control of Induction Motor

1. ROTOR RESISTANCE CONTROL PREET PATEL (151310109032) POWER ELECTRONICS - II

2. CONTENTS Rotor resistance control of an Induction motor Closed loop Speed Control

3. ROTOR RESISTANCE CONTROL of an INDUCTION MOTOR Rotor Resistance Control  is also one of the methods by which we can control the speed of the  Induction motor . The speed of the wound induction motor can be controlled by connecting an external resistance in the rotor circuit through slip rings. This method is not applicable to cage rotor induction motor.

4. As we know that, So now if we consider that motor torque remains constant, then resistance is directly proportional to slip, which in turn will vary the speed. This method creates more losses, so this is only adopted for a narrow range speed and used in lifting up of hoist, cranes and excavators.  

5. Depending on the rotor resistance, the speed of the rotor falls or increases. The variation of speed torque characteristics with respect to change in rotor resistance is shown in the figures below.

6. Slip Power Control using Rotor Resistance along with Chopper The conventional method for variation of rotor resistance was done by inserting a variable rheostat in series with the 3 phase winding of motor. This method of speed control faced problems and this was not economically viable. So in solution to this problem, thyristor is used to which is connected across enables external resistance and effective resistance is varied simultaneously.

7. Effective value of resistance across terminal A and B i.e., R AB is varied by varying duty cycle of the transistor T r , which in turn varies the resistance of the rotor circuit. Inductance is added to the circuit to reduces the ripple and discontinuity in the dc link current I d . Rotor current waveform is shown in the when the ripple is neglected.

8. Thus the rms value of the current will be: Resistance between A and B terminal will be zero when the transistor is on and it will be R when it is off. Therefore average value of resistance between the terminals is given by Where, δ is the duty ratio of the transistor. Therefore the total rotor resistance is varied from Rr to 0.5(1-δ)R as δ is varied from 1 to 0. Electrical power consumed by the R AB resistor per phase is     Power consumed per phase  

9. Closed loop Speed Control CURRENT CONTROLLER BASE DRIVE CURRENT LIMITER SPEED CONTROLLER δ ω m ω m I d B A T r R I d L d CHOPPER IM SPEED SENSOR ω m*

10. Closed loop speed control scheme with inner current control loop is shown in figure, which is employed for successful performance achieved by using transistor chopper, which is of low cost and I d has a constant value at the maximum torque point.

11. If the current limiter is made to saturate at this current, the drive will accelerate and decelerate at the maximum torque giving very fast transient response. For plugging to occur, arrangement will have to be made of reversal of phase sequence. Compared to conventional rotor resistance control, static rotor resistance control has several advantages such as smooth and stepless control, fast response, less maintenance compact size, simple closed-loop control and rotor resistance remains balanced between the three phases for all operating points.

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