Experiment No.:25
Aim of the Experiment:
Speed control of Three-phase Induction Motor using three-phase AC voltage controller.
Objective:
Measurement of different parameters experimentally such as speed, torque, current of Induction motor drive.
Apparatus Required:
| Sl. No. | Name | Specification | Quantity |
| 01 | IGBT power Module | 01 nos. | |
| 02 | PWM Controller | 01 nos. | |
| 03 | Three-phase Squirrel case Induction Motor with spring balance load | Stator Voltage:400V | 01 nos. |
| 04 | Tachometer | Digital type | 01 nos. |
| 05 | CRO | 01 nos | |
| 06 | Patch Chord | PVC insulated copper wire | As per required |
Circuit Diagram:
Theory:
Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā An Induction motor is practically a constant speed motor, that means for the entire loading range, the change in speed of the motor is quite small.
Different speed control methods of induction motors in stator sides are-
- By changing the applied voltage.
- By changing the applied frequency.
- Constant V/f control of induction motor.
- Changing the number of stator poles.
Here we are discussing changing the applied voltage.Ā
From the torque equation of induction motor
Rotor resistance R2 is constant and slip s is small then (sX2)2 is so small that it can be neglected.
Therefore, T ā sE22 where E2 is rotor induced emf and E2 ā V. Thus, T ā sV2, which means, if supplied voltage is decreased, the developed torque decreases.
Hence, for providing the same load torque, the slip increases with decrease in voltage, and consequently, the speed decreases. This method is the easiest and cheapest, still rarely used, because
A large change in supply voltage is required for a relatively small change in speed.
2. A large change in supply voltage will result in a large change in flux density, hence, this will disturb the magnetic conditions of the motor.
- AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value of the alternating voltage applied to a load circuit by introducing IGBT drives between the load and a constant voltage ac source.Ā
- The RMS value of alternating voltage applied to a load circuit is controlled by controlling the triggering angle of the IGBTs in the ac voltage controller circuits.
- In phase control the IGBTs are used as switches to connect the load circuit to the input ac supply, for a part of every input cycle. That is the ac supply voltage is chopped using IGBTs during a part of each input cycle.
- The IGBTs switch is turned on for a part of every half cycle, so that input supply voltage appears across the load and then turned off during the remaining part of input half cycle to disconnect the ac supply from the load.
- By controlling the phase angle or the trigger angle āĪ±ā (delay angle), the output RMS voltage across the load can be controlled. The trigger delay angle āĪ±ā is defined as the phase angle (the value of Ļt) at which the IGBTs turns on and the load current begins to flow.
Procedures:
1. Connect three-phase input supply to R, Y, B input terminal of IGBT power module through isolation transformer.
2. Connect three-phase motor to R, Y, B output terminals of IGBT power module.
3. Connect the PWM controller to the module.
4. Now by changing the Duty cycle of IGBT we can change the output voltage and frequency.
5. Now take the readings as per observation table.
Observation Table: With No-Load
| Sl. No | Output Voltage(V) in volts | Output Current(I) in Amp. | Frequency (f) | V/f ratio | Speed (N) | W1 in kg | W2 in kg | W=(W1-W2) in kg | Torque (T) |
| 1 | |||||||||
| 2 | |||||||||
| 3 | |||||||||
| 4 | |||||||||
| 5 | |||||||||
| 6 |
Observation Table: With Load
| Sl. No | Output Voltage(V) in volts | Output Current(I) in Amp. | Frequency (f) | V-f ratio | Speed (N) | W1 in kg | W2 in kg | W=(W1-W2) in kg | Torque (T) |
| 1 | |||||||||
| 2 | |||||||||
| 3 | |||||||||
| 4 | |||||||||
| 5 | |||||||||
| 6 |
Plot the graph of Torque vs Speed.
Conclusion:
Written by student.