Question

A three phase induction motor can develop maximum torque at startin if it uses a)star-delta starer b)rotor resistance starter

Answer 1

Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges.

Star-delta motor starter panel

To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed.

Star-delta starter panel

Two methods used for reduction of starting voltage are: Star delta starting and Auto transformer starting.


Working Principle of Star-Delta Starter

This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings as illustrated in the figure below. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three.

Scheme – Working Principle of Star-Delta Starter

After a period of time the winding are reconfigured as delta and the motor runs normally. Star/Delta starters are probably the most common reduced voltage starters. They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply.

Traditionally in many supply regions, there has been a requirement to fit a reduced voltage starter on all motors greater than 5HP (4KW). The Star/Delta (or Wye/Delta) starter is one of the lowest cost electromechanical reduced voltage starters that can be applied.

The Star/Delta starter is manufactured from three contactors, a timer and a thermal overload. The contactors are smaller than the single contactor used in a Direct on Line starter as they are controlling winding currents only. The currents through the winding are 1/root 3 (58%) of the current in the line.

There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star current while the motor is connected in star.

The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.


Star-delta Starter Consists following units

Contactors (Main, star and delta contactors) 3 No’s (For Open State Starter) or 4 No’s (Close Transient Starter).

Time relay (pull-in delayed) 1 No.

Three-pole thermal overcurrent release 1 No.

Fuse elements or automatic cut-outs for the main circuit 3 Nos.

Fuse element or automatic cut-out for the control circuit 1No.