Starting Methods for Induction Motors.

Starting Methods for Induction Motors.

The desire at all Times would be to start on full voltage, as this would give simplicity of starting equipment, a good starting torque and hence acceleration. However, other factors must be considered and the starting method chosen will depend
a. The starting torque required, which is determined by the load
being driven.
b. The starting current, which is a function of the driven load and the design characteristics of the motor.
c. The requirement or otherwise by the load, of very gradual and smooth acceleration.
d. The mains capacity to supply the load.

The starting methods available form two, distinct groups.

[a] Starting on full voltage. i.e. Direct on-Line and Rotor resistance.


[b] Starting on reduced voltage; je. Star-Delta. Auto Transformer, Stator Resistance.

Direct on Line Starter:-This is a single stage starting method applied to standard
or special three phase squirrel cage induction motors. which 'connects the motor to the supply without a change in voltage. It gives a good starting torque, having alterative torque characteristics for different rotor designs. By the choice of ratio of rotor resistance to reactance, the maximum. torque position in the torque slip characteristic of an induction, motor can be determined. For the non specialist rotor design the starting torque is typically 1. 5 Tn. However this method of starting is not suited to low capacity mains, as the motor so started, draws five to eight times its normal running current, when starting. Because of this high starting current demand, the' supply authority places a restriction on the kW. rating, which may be started without notification and permission. This is usually three kW
The Direct on Line starter uses the minimum of components ensuring low capital and maintenance costs with high reliability. Typical applications are: shaping machines, milling machines, drills, hoists, low power pumps etc.
Rotor Resistance Starter:.
The torque of an induction motor is a maximum when the rotor resistance is equal to the rotor reactance. At the instance of start the rotor reactance is at its maximum value. Because the frequency of the rotor induced current is at its maximum 50 Hz . As the motor accelerates the frequency of the rotor current decelerates progressively until at full speed it will be approximately 1.5 Hz. Therefore the reactance has gone from a maximum value to a minimum value over the period of start. While the resistance has remained relatively constant. Hence maximum or pull out torque, only occurred at one speed and only for an instant, as the rotor accelerated through that speed.
where optimum torque is required through the starting period it is therefore necessary to vary the resistance of the rotor circuit in step with the reducing reactance. This can. only be done by constructing a special rotor-a wound rotor-and connecting external resistances in its circuit. These resistances, together with their s\4itching, and ancillary equipment form the rotor resistance starter. The function of this starter is to insert the appropriate external resistance in the rotor circuit at the instant of start and then progressively reduce that resistance Value as the rotor accelerates. with the motor operating at full speed, all the external resistance is removed- and the slip rings short circuited, it is effectively operating as a squirrel cage machine.

The reduction in resistance can be manual or automatic, time or current controlled, whole current or current transformer activated. The external resistance can be wire wound: liquid or liquid converted to vapour. The wire wound is the most widely used and can be air or oil cooled. The oil cooling is the most efficient and where starting frequency is high the tank enclosure itself can be water cooled.
having regard to cost and complexity, it is the most complex and expensive form of starting, especially as a wound rotor is utilized. However the torque requirement will dictate its choice or otherwise.

In general its application would be to any high torque- load which cannot be satisfied using a squirrel cage rotor, of normal or special design. e.g. Elevators, hoisting machinery, cranes, piston type compressors, piston pumps.

Star-delta Starter: -
This is a two stage starter used in conjunction with a three phase, squirrel cage, six terminal induction motor. Its purpose is to-reduce the starting current to the motor, and it does this by reducing the starting ¼oThage to phase voltage value. This is accomplished by initially connecting the stator winding in star formation and subsequently changing to delta, i.e. full voltage, when the rotor has attained it maximum speed at the reduced voltage. A typical value for this speed would be eighty percent of full speed and the change over point is generally a function of time, though it could be current, or speed controlled.

As the torque produced by the stator is proportional to the square of the applied voltage, when the torque available using this method of starting is reduced to one third of that available , under direct on line starting, clearly then it is only suitable for application which have minimum starting torque. Requirements. In the standard form of star delta starter the supply is disconnected from the motor during the change over period. This results in loss of momentum, and the duration of the disconnection is critical in minimising switching peaks.

Compared to the direct on line starter it requires additional components, eg. Contactors, timer, six terminal motor, larger enclosure. In general it is suited to lathes, drilling -machines, saws, metal cutters, grinders, Applications which, can be started off load, and for reasons of mains capacity cannot be started direct-on-line.

Auto-Transformer Starter: -
A single or multi stage starter used in conjunction with a three phase: squirrel cage, three terminal, induction motor. Its function is to supply a reduced voltage to the stator during the starting period, in order to reduce the starting current. This it does, in one or more steps, depending on the requirements of the load. The number of steps in the starting sequence as well as the voltage available at each step can be specified to match the load and supply requirements. eg. Three steps of values, 50%, 60%, and 75% of line voltage.
As the steps are graded it gives smooth acceleration without,­ disconnecting the motor from the supply, which avoids any danger of switching transients or momentary loss of speed. However, because it is a reduced voltage method of starting there is a significant torque reduction which limits its application to low inertia loads. As it is only in circuit for the starting period its windings are short time rated, which reduces cost but makes starting times and transformer temperature protection critical. Compared to direct-on-line starting it requires additional components e.g. contactors timers, auto-transformer. Transformer protection, larger enclosure, etc.

Stator resistance starter;­
A single or multi stage starter applicable to a three phase squirrel cage three terminal induction motor which gives a reduced starting voltage by inserting resistances in the stator circuit during the starting sequence. These resistances are taken out of circuit in one or more steps as the motor accelerates. While this -is usually timed controlled, it can also be current controlled or indeed respond to a signal generated by shaft speed. The single Stage starter usually permits approximately three times full load current as an inrush-current, while the multi stage would restrict this about twice full load value.

As this is a reduced voltage method of starting the torque is severely restricted, therefore its application must be carefully chosen. It does give an extremely smooth acceleration for the following reason:
On the initial application of voltage the inrush current is a maximum, thereby giving maximum voltage at the motor terminals. As the motor accelerates its current demand decreases, thus giving a reduction in voltage drop across the resistance and an increase in voltage at the motor terminals.

While there is a power loss in the resistances during starting there is an improvement in the power factor, which is desirable. Compared to the direct on line starter it requires additional components. E.g. stator resistance contactors, timers, larger enclosure.