(a) To determine the equivalent circuit parameters of a 3-phase squirrel-cage induction
motor from laboratory test data.
(b) To determine the performance characteristics of an induction motor under load conditions. (c) To control the no-load speed of an induction motor.
2.1 Three-phase squirrel-cage induction motors
An induction motor is a transformer with a rotating secondary winding. It has two essential components: an outer stationary stator and an inner rotating rotor. (Figure 1) [pic]
Figure 1: Induction motor
The three-phase induction motors could be categorized into two main types according to the different rotors: the squirrel-cage induction motors and the wound induction motors. The squirrel-cage induction motors are commonly used when the load requires little starting torque, such as lifts and fans, while the wound induction motors are necessary used when the load requires high starting torque. The induction motor used in this experiment is a there-phase squirrel-cage induction motor.
2.2 Power converter
Since it is necessary to adjust the voltage magnitude according to maintain a constant air-gap flux, when the motor supply frequency is varies, a power-converter is used in the experiment A power converter is power-electronics based device which can supple power with varied frequency. It first converts the normally available fixed frequency ac supply to a dc supply by using a rectifier. The dc supply is then converted to a variable-frequency ac supply, which can maintain a constant air-gap flux in the motor by an inverter.
2.3 Synchronous speed
When the balanced three-phase currents flow through three-phase stator windings of the motor, a rotating magnetic field or magnetic motive force (mmf) is produced. The speed of the rotating mmf is called the synchronous speed and it depends on the supply frequency and the number of the poles in the motor, as shown in equation 1.
[pic] (Equation 1)
[pic] rad/sed (Equation 2)
The rotor of the motor tends to follow the rotating mmf but operates at a speed slightly lower than the synchronous speed, because of the absence of the induced emf in the rotor coils. The rotor speed is denoted as Nr and the phenomenon that the rotor is continuously falling behind the stator rotating mmf is called slip. The mathematical expression is shown as below
Three-phase induction motor1 unit
DC generator1 unit
Power Converter1 unit
Power Analyzer1 unit
Three-phase variable resistive load bank1 unit
Three-phase load switch1 unit
4.1 Nameplate data and preliminaries
4.1.1 Record the nameplate (or rated) data of the motor.
4.1.2 Measure the resistance per phase (R1) of the stator winding using power analyzer. 4.2 No-Load Test
4.2.1 Make sure that the shaft of the induction motor is decoupled from the shaft of the DC generator. 4.2.2 Connect the three-phase stator winding of the induction motor in delta configuration. Connect the three-phase variable AC supply to the stator winding of the motor. Also connect the power analyzer to the circuit to measure the input voltage, current and power. The whole configuration is shown as figure 2. 4.2.3 Set the dial of the three-phase variable AC supply to zero position. Switch on the power supply and gradually increase the voltage to the rated value for delta connection. Record the line current, line voltage and two power reading W1 and W2. Also record the speed of the motor using a tachometer. 4.2.4 Reduce the variable voltage to zero and switch off the power supply. [pic]
Figure 2: No-Load Test
4.3 Blocked-Rotor Test
4.3.1 Loosen the screws on the base plate of the induction motor and move it as close as possible to the DC generator. Lock the shaft of the induction motor with the mechanism so that it cannot rotate. Tighten...