Work explanation

STARTUP OF THREE-PHASE INDUCTION MOTORS

Purpose of the work – to research and evaluate the induction motors with the phase and short-circuited rotor startup ways.

Equipment and devices

Problems of the start of the induction motors with the phase and short-circuited rotor are investigated in this laboratory work.

Voltage and current measurements are performed with the help of electromagnetic ammeter and voltmeter of electromagnetic system or with help of measuring kit “K-50”.

Starting three-phase rheostat is used for startup of induction motor having phase rotor. It is connected to the rotor circuit.

Autotransformer of type “РНТ-220-6” is used for the startup with autotransformer.

Three-pole knife-switch is used at the startup by the switching on the star-delta stator winding of the induction motor.

Terminals of devices and equipment are placed on the terminal panel of the testing bench.

Work contents

1. To familiarize with motors passports data.

2. To measure value of starting current of induction motor having phase rotor with starting rheostat and without it.

3. To analyze experimentally questions of the start of induction motors with the short-circuited rotor.

Work explanation

During the investigation of possible ways of induction motors start it is necessary to take into account the next basic facts:

- motor must have sufficiently great torque (bigger than static strength torque) at moment of start for the beginning of rotating, and reaching rated rotating velocity;

- starting current value must be limited by such value, that motor damage and normal mode of the operation of the motor disturbance are not occurred;

- the startup scheme must be simple, and number and cost of auxiliary equipment must be small.

Next problems have essential meaning: 1)time of the start; 2) the energy losses in windings and winding heating; 3)transients during the start.

Depending on start up conditions of this or that device different types of induction motors such as motor having phase rotor or short-circuited rotor. Startup favorable conditions (big starting torque at small starting current) can be reached with the help of starting rheostat (resistance putted in rotor winding circuit) in the motors with phase rotor.

If current and electromagnetic torque (fig. 5.2) of induction motor at start (S=1) without starting resistance are equal, equivalent circuit (fig. 5.1), is

rotor current and starting torque at the resistance, connected in the circuit of the rotor are calculated according to the formula:

Resistance value, which corresponds to minimal value of starting current, can be calculated by the formula:

Induction motor with contacts rings startup is performed by the connection of the stator to the network. In this case starting resistance of the rotor is maximal and then after switching-on the stator it’s is decreased to the short-circuited of the winding. Motor receives the natural characteristics. In this case rotor will be rotate with the velocity, corresponding to the electromagnetic torque equality to the torque of resistance condition. (fig. 5.3).

Induction motors with phase rotor are more expensive then motors with short-circuited rotor, and the application of the starting rheostat complicates the equipment and its service.

That’s why motors with short-circuited rotor are widely used. starting current decreasing is reached by decreasing of voltage on the terminals during start. But starting torque is decreased proportionally to square of the voltage at the same time. That’s why this method is used only for easy starting conditions, when load on axle doesn’t exceed 30% of rated one.

Normal induction motor with short-circuited rotor can not increase sufficient starting torque even at the direct switching-on to the rated voltage. In these cases special induction motors are used (two-cage or deep-slot motors).

Start of induction motor with short-circuited rotor can be performed by the next ways: with the reactor, with the autotransformer, delta to star switching-on at start.

Direct start is mostly used because networks have enough power for its start. This method is very simple, but leaps of current accompany it (5.5÷6.5)I_rated, and these leaps are harmful for the network. If direct start is impossible because of the conditions of the voltage drop, different ways of decreased voltage are used. The start necessity at the decreased voltage is very often received in cases of powerful high-voltage motors, in agriculture, and sawmills.

Date: 2015-02-16; view: 1656