1 Electrical Machines Lab Experiment-No. ROTOR RESISTANCE SPEED CONTROL OF WOUND ROTOR INDUCTION MOTOR AIM: To vary the speed of the wound rotor induction motor using rotor rheostat control. Theory The main advantage of wound rotor induction motor over the squirrel cage motor is that it offers one more degree of freedom to the Engineers for starting & speed control. Wound rotor Induction Motor is used where the load requirements are high starting toque & variable speed, or where the motor is to be started under heavy load. Typical applications of these motors are crane & hoist control. Resister controllers in the rotor circuit are used to achieve smooth start & speed control. Resent investigations have shown that certain desired torque speed characteristics can be achieved by insertion of relatively simple passive frequency sensitive networks. Careful selection of network parameters leads to highly reduced starting current & & improved torque/ current ratio. To understand the industrial systems incorporating the wound rotor Induction Motors it is therefore necessary to study the effect of rotor impedance on the performance of Induction Motors. In this method of speed control of three phase induction motor external resistance are added on rotor side. The equation of torque for three phase induction motor is The three phase induction motor operates in low slip region. In low slip region term (sx)2 becomes very small as compared to R2. So, it can be neglected. And also E2 is constant. So the equation of torque after simplification becomes,
2 Now if we increase rotor resistance, R2 torque decreases but to supply the same load torque must remain constant. So, we increase slip, which will further results in decrease in rotor speed. Thus by adding additional resistance in rotor circuit we can decrease the speed of three phase induction motor. The main advantage of this method is that with addition of external resistance starting torque increases but this method of speed control of three phase induction motor also suffers from some disadvantages: 1. The speed above the normal value is not possible. 2. Large speed change requires large value of resistance and if such large value of resistance is added in the circuit it will cause large copper loss and hence reduction in efficiency. 3. Presence of resistance causes more losses. 4. This method cannot be used for squirrel cage induction motor. From the figure 1 it is clear how does the speed vary with the change of rotor resistance.
3 The model used to simulate this experiment Figure 2 simulation the wound rotor induction motor Parameters of components 1- A synchronous machine block: a- Configuration: Preset model: No Mechanical input: Torque Tm Rotor type: wound rotor Reference frame: Rotor Mass units: SI b- Parameters:
4 Pn (VA): 2850 Vn (Vrms L-L): 380 f(hz): 50 Another data are remain constant c- Advanced: Sample time: -1 3 phase source: This block consists of three A.C. Voltage sources, as shown below: 1- Phase A: 2- V= K/ (sqrt (3)*sqrt(2)) 3- f= 50Hz 4- Phase= 0 degree 5- Sample time= -1 6- Phase B: 7- V= K/ (sqrt (3)*sqrt(2)) 8- f= 50Hz 9- Phase= -120 degree 10- Sample time= -1 11- Phase C: 12- V= K/ (sqrt (3)*sqrt(2)) 13- f= 50Hz 14- Phase= -240 degree 15- Sample time= -1
5 PROCEDURE: 1 - Open File ---> New---> Model. 2 - Open Simulink Library and browse the components. 3 - Connect the components as shown in the above circuit, figure 2. 4 - Simulate the circuit to start motor with rotor resistance starter. 5 observe the waveforms of speed, torque, stator current and rotor current. 6 - The rotor resistance is varied and corresponding values of speed, voltage, currents and torque are noted down. Rotor Resistance 50 40 30 20 10 Voltage (V) Stator current (A) Rotor current(a) Speed (rpm) Torque (N.m) Report 1- Plot the relation between the rotor resistances with speed. 2- As you added resistance externally to the rotor circuit, what happened to rotor speed? Why? 3- As rotor speed decreased, what happened to the torque output of the motor? Why? 4- As rotor speed decreased, what happened to the current being induced into the rotor circuit?