Induction motors are a common type of ac motor, used in both industry and on board ships, with a number of induction motor starters.
The type of induction motor starters that are chosen, depend on a number of factors.
Direct On Line Starters (D.O.L)
Direct on line, or D.O.L for short, are a simple way to switch on smaller ac Induction Motors.
DOL is used to start smaller induction motors, which have a current rating of up to 10 amps.
In a DOL starter system, as Contactor is used to switch on the induction motor.
The Contactor is similar to a large electrical relay, and its function is to switch on and off, the large currents drawn by the induction motor.
When the operator presses the start button on the control panel, a voltage is supplied to an insulated coil inside the Contactor.
The coil works as an electro-magnet, and exerts a magnetic pull on the switch contacts also inside the Contactor casing.
The magnetic field causes the switch contacts to close, therefore allowing current to flow into the induction motor, and starts it.
The switch on the control panel that is used to start the motor, only works when it is pushed in. As soon as the operator releases their finger, the power ceases.
This is obviously not convenient to have to hold the button in, therefore an extra ‘auxiliary contact’ is included in the Contactor, which is wired to ‘lock’ the supply current on, even once the button is released.
The Contactor will remain locked on, allowing the induction motor to run, until a separate off button is pressed.
The off button breaks the link to the auxiliary contact, which releases the Contactor, and cuts the electrical supply to the motor.
In a three-phase DOL starter system, a single phase supply is taken from one of the input phases, and fed into the primary side of a single phase step down transformer.
The output from the transformer is used to supply the coil inside the Contactor, which closes the contacts, and makes the motor start.
Star-Delta Starters, which the Americans call Wye-Delta Starters, are used for starting larger induction motors.
What I mean by larger induction motors, are motors that are draw over 10 Amps of current at full load.
Induction motors have a metal plate on them which specifies the maximum current drawn by the motor.
This will be described as FLC, which is short for Full Load Current.
The reason we don’t use DOL starters for larger ac induction motors, is something called ‘inrush current’.
When you first start an induction motor, the current drawn by the motor is a number of times higher than the motors steady operating current.
Its similar to when you accelerate a car from standstill, in that more energy is used to get it going, than when its cruising at desired operating speed.
The problem with having a high initial current on motor startup, is that you need bigger capacity cables & contactor to cope with the large current.
Needing larger cables increases costs, as a motor with a FLC of 20 Amps, might have an ‘Inrush’ current of five or more times the FLC.
20 Amps x 5 = 100 Amps!
Star-Delta starters reduce the initial starting (inrush) current by starting the motor in a ‘Star’ wiring configuration.
A three phase induction motor has three sets of coils in its Stator windings. This results in six connections coming out of the Stator (two ends of each of the three coils).
To be continued…..