Motors - Operation and Issues - 1

Methods of speed control in single phase motors

 Speed control is important in any application. Fans driven by single phase motors need to operate a different speeds. Industrial drives may need to run at different speeds too.

Speed control in single phase motors can be done using

Resistances
In this method, resistances are used to reduce the voltage available at the motor terminals. This does not provide fine speed control as the resistances will be cut out in steps. It is however a simple method. Another disadvantage is the loss of energy in the resistances as heat. This reduces the overall efficiency of the motor.

TRIAC
A Triac is a three terminal semiconductor device. The current flowing through a thyristor can be controlled by controlling the gate voltage. Triac controls are used in control of speed of fans.

Gears
Gears are also a method of speed controls. A gear assembly is coupled to the output of the single phase motor. The gears are engaged and disengaged as per the speed required.

VFDs
VFDs or Variable Frequency Drives control the speed of the motor by controlling the frequency of the power supply on the input terminals. VFDs are thus able to control the synchronous speed of the stator. This, in turn, controls the speed of the rotor.


Why is the single phase motor not self starting ?

For an induction motor to be self starting, the stator needs to have a rotating magnetic field. In a three phase induction motor, the rotating magnetic field is created by the fluxes of the three phase windings which are displaced by 120 degrees.

This rotating field induces an emf in the rotor. This results in the flow of current which results in the magnetic field in the rotor. This results in the magnetic field of the rotor trying to catch up with the stator and the rotor starts to rotate.

In a single phase motor, the magnetic field in the stator is the result of only one phase. Thus the magnetic field produced is pulsating. The field is not rotating.

Hence, the rotor is pulled in opposite directions by the stator magnetic field. This results in the rotor staying the staying position. Hence, the single phase motor needs an auxilliary winding which produces two magnetic fields displaced in time which provides a magnetic field.


Running Three Phase motors with single Phase supply

A three phase device can be run with a single phase converter by means of a static capacitor phase converter. The phase converter converts the single phase voltage into three voltages which can be connected to the three phases of the motor.

The single phase supply is connected to two of the motor phase terminals. The other terminal is connected to one of the single phase terminals through a capacitor. The capacitor introduces a phase shift which causes the third phase to be out of phase by 120 degrees. The produces the rotating magnetic field required for starting and running the three phase motor. 


In motors, the starting current is usually about six times the rated current. Hence, a bigger value capacitor is usually used as a starting capacitor. The starting capacitor is kept in line by means of a switch which is opened as the motor picks up speed


Phase Sequence Relay for motors

Phase sequence protection is an important safety for motors. Reversing the phase sequence causes the motor to reverse its direction of rotation. This can cause serious damage and injury to personnel if for instance, the motor is coupled to cutting equipment, or conveyor belts.

The phase sequence indicator works by monitoring the phase sequence continually and preventing the motor from starting if the phase sequence has been reversed in the supply.


Jogging and Inching in Motors

Jogging refers to repeated starting and stopping of a motor in short bursts to perform a particular movement such as moving a crane to a particular location. The motor is operated on full power. Special Jogging circuits are available.


Inching is similar to Jogging and the two are often confused with each other. Inching involves short thrusts of motion at reduced power. The motor is made to run at a reduced voltage.

Jogging should not be done more than 5 times in a minute. This is because jogging can cause the motor to overheat. If Jogging is to be done frequently, the starter of the motor should be derated.

Special Jogging Blocks are also available which prevent the motor from being jogged more than 5 times an hour by logging the number of starts per hour.


Plugging in Motors 

Plugging is a method of braking used in induction motors. Plugging involves interchanging the supply to two of the stator phase windings. This method is used in applications which require immediate stop applications. When the phase supply is reversed, torque is produced in the opposite direction. This leads to braking of the electric motor.

Motors which are operated this way have a plugging switch. This switch operates when the stop command is given to the motor circuit. The operation of this switch applies reverses the supply to two of the windings. When the motor comes to a complete halt, this reversed supply is disconnected.


However, this method of braking generates a large amount of heat in the rotor, even greater than that produced by a locked rotor. This is due to the kinetic energy of the rotor and the coupled load.


Plugging should not be used frequently as the heat produced in this method of braking can cause the rotor bars to overheat and, sometimes, melt.