Experiment No.: 19
Aim of the Experiment:
Speed control of Three-phase Induction Motor by using VVVF control method.
Objective:
Measurement of different parameters experimentally such as speed torque current of Induction motor drive.
Apparatus Required:
| Sl. No | Name | Specification | Remarks |
| 01 | Three Phase Induction Motor Speed Control Trainer | 01 nos. | |
| 02 | Three Phase Squirrel cage Induction Motor with spring load | 1440 rpm, 430V, 50Hz, 0.5 HP | 01 nos. |
| 03 | Patch Cord (connecting wire) | PVC Insulated copper | As per requirements. |
| 04 | Tachometer | Digital Type | 01 nos. |
Block Diagram:
- EMT16A: Single Phase AC Input Panel.
- EMT-33: IGBT Controlled VVVF Drive Panel.
- EMT2: AC Voltmeter Panel.
- EMT3: Dual Range AC Ammeter Panel.
Theory:
Induction Machines is the most widely used motor in industry. Initially induction motors are used for fixed applications however in the last two decades, due to evolution of power semiconductor devices and electronics converter induction motors are well established in the industrial area. A three-phase induction motor is basically a constant speed motor so it’s somewhat difficult to control its speed. The speed control of the induction motor is done at the cost of decrease in efficiency and low electrical power factor.
The Speed of Induction Motor is changed from Both Stator and Rotor Side. The speed control of three phase induction motor from stator side are further classified as:
• V / f control or frequency control.
• Changing the number of stator poles.
• Controlling supply voltage.
• Adding rheostat in the stator circuit.
V/f control or frequency control:
Whenever three phase supply is given to three phase induction motor rotating magnetic field is produced which rotates at synchronous speed given by
In three phase induction motor emf is induced by induction similar to that of transformer which is given by
Where, K is the winding constant, T is the number of turns per phase and f is frequency. Now if we change frequency synchronous speed changes but with decrease in frequency flux will increase and this change in value of flux causes saturation of rotor and stator cores which will further cause increase in no load current of the motor.
Procedures:
- At EMT16A connect input supply 230V mains IN from grid.
- From EMT16A panel output terminals 4/7 and 6/8, connect to EMT33 panel input terminals L (1) and N (2) and connect output terminals from EMT33 panel terminals 3,4,5 to EMT2 panel input R (1), Y (2), B (3) terminal.
- From EMT2 panel output terminals R (5), Y (6), B (7) terminals connect to EMT3 panel R (1), Y (2), B (3) terminals points respectively.
- From EMT3 panel output terminals R (5), Y (6), B (7), connect three phase Squirrel cage Induction Motor U(½), V(¾), W (⅚) terminals respectively.
- Before connecting to the motor terminal check the three-phase output voltage.
- Now switch on the power supply from grid to experimental setup i.e., EMT16A panel, then switch ON the MCB.
- Now Click FWD/REV button then press Start button in EMT33 panel.
- Now slowly increase the frequency by rotating the knob in clockwise direction at that time output voltage also increases from the inverter simultaneously.
- Now take the meter readings as per observation table with No-Load and Load condition.
- Now apply the load at motor shaft and calculate Torque applied to the motor by the formulae
- a. Torque in N-m = W kg*9.81* r in meter
- Where, W kg = (W1-W2) kg & r = radius of pulley in meter
- Torque in N-m= W kg*9.81*0.035
- a. Torque in N-m = W kg*9.81* r in meter
- By varying the frequency of inverter and load in the motor you can plot torque speed characteristics of IM.
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 students.