Resistance Start Split Phase Induction Motor

SANDEEP KUMAR08104051

Construction Viewpoints

This motor has two stator windings: main (running) winding and an auxiliary (starting) winding.Connected in parallel but their magnetic axis are space displaced by 90° electrical.Main winding is designed to have lower resistance i.e. thick wire and higher reactance.Auxiliary winding is designed to have higher resistance i.e. thin wire and lower reactance.Since Xm α N² thus Auxiliary winding has less no. of turns as compared to main winding.

use of thin wire for auxiliary winding is permissible because it is required in the starting only & use of thick wire for main is essential because it has to remain permanently in the ckt. That’s why main winding has low R/X ratio than auxiliary winding.

In case of split phase motors that are sealed in refrigeration units, uses electromagnetic relay instead of centrifugal switch. the relay coil is connected in series with main winding & pair of relay contacts is connected to auxiliary winding which is generally kept open.

Principle of Operation

Since main winding has higher leakage reactance as compared to auxiliary winding . . Main winding current I ͘� m lags aux. winding current Ia as shown in phasor diagramSo when 1- Ø supply is given, current Ia reaches its maximum value and after angle α (30° -40°) from Ia, current Im reaches its maximum value As a result of this stator field 1stly reaches its maximum value along main winding and then after α along aux. winding.This means the stator flux rotates from main winding to aux. winding

Now the rotating (alternating) flux induces the emf in the rotor windings since these windings are short circuited so currents start flowing in these windings and hence produces rotor flux which in turn tries to oppose the stator flux acc to Lenz’s law so the rotor starts rotating in the direction of stator fluxThe starting winding is disconnected automatically by means of centrifugal switch at about 70% to 80% of synchronous speed.

Once the rotor is up to full speed, it will continue to run with the second winding disconnected. This is because the rotor circuit is both resistive and inductive. If we consider the magnetic field rotating in the same direction as the rotor, the frequency of the current will be low, so the rotor current will be primarily limited by the rotor resistance. In the case of the counter rotating field, the frequency of the induced current will be almost twice line frequency and so the inductance of the rotor will play a much greater role in limiting the rotor current. In other words, once the motor is up to speed, it will lock on to one field only and the second winding can be disconnected.

V

Ia

Im

α

I

Ø

Generation of Rotating Field

α

Role of Centrifugal Switch

A centrifugal switch is an electric switch that operates using the centrifugal force created from a rotating shaftdesigned to activate or de-activate as a function of the rotational speed of the shaft.consists of weights mounted to the shaft of the motor and held near the shaft by spring forces At rest, levers attached to the weights press a low-friction, non-conductive plate against a set of electrical contacts closing the contacts and connecting the starting winding and auxiliary winding to the power sourceWhen the motor approaches 70 t0 75% speed, centrifugal force overcomes the spring force and the weights swing out, raising the plate away from the electrical contacts.

The direction of rotor can be reversed by interchanging the terminals of either main winding or of the aux. winding.

Torque Speed characteristics

At starting the torque is about 150 percent of its full load value. As the motor speed picks up, the torque is increased (except for a slight decrease at low speed) and may reach higher than 250 percent of full-load value. The switch is opened and the motor runs on its main winding alone and the motor reaches its equilibrium speed when the torque developed is matched by the load.

TORQUE-SPEED CURVES OF DIFFERENT TYPES OF SINGLE-PHASE INDUCTION MOTORS

Applications

The split-phase design is one of the oldest single-phase motors and ismost widely used in the ratings of 0.05 to 0.33 hp. A split-phase motor is usedin machine tools, washing machines, oil burners, and blowers etc.

Advantage Disadvantages

►Operate at constant speed

►Reversible at low speed

►Rapid acceleration

►Relatively low cost

►Repeated start/stop cycles heat the windings (high start resistance)

►Less useful for large inertial loads

►Requires large wiring to handle starting currents

Questions

What would happen if centrifugal switch does not disconnect the auxiliary winding??

If we disconnect the one the phase of a 3- Ø IM, will it

continue to run ?

Can a 3- Ø IM run on 1- Ø supply??