(a) Explain with the aid of simple sketches, the DOL starter control on 3 phase induction motor.

(b) Explain starting method of reducing voltage and describe the star-delta method of starting for 3 phase squirrel cage induction motor with the aid of sketch.

(c) Explain with the aid of simple sketch, soft starter.

Kind of Motor starter (3phase, AC motor)

1. Direct on line starter (DOL) (Full voltage starter)

2. Star delta starter (Reduced voltage starter)

3. Auto transformer (Reduced voltage starter)

4. Soft starter (using thyristor)

(a) DOL starter

  • Very simple and cheap starting arrangement
  • Use for majority of induction motor drives.
  • Very large starting current equivalent up to 6-8 times of full load current.
  • When large motor start this way, large starting surge current causes disturbance of voltage on supply lines.
  • Malfunction of other electrical equipment connect on that same line.

DOL starter operation

Power circuit operationControl circuit operation
Manual closing of breakerControl circuit voltage available
Closing of line contractorPress start button S2
 Motor runContractor energized
Contactor drops out, motor stopAux contact latches contactor
 Main contactor  close and motor run
 Running lamp ON
 Press stop button S1, contactor de-energized, motor run
 When over-load occurs, OCR trip, motor stop
 OCR reset manually to restart

(b) Star delta starter

  • Starting is star connect, running is delta connect.
  • Star-delta starter reduced starting current.
  • Starting current of delta connected, motor reduce 1/3 if motor is star connected for starting.
  • Torque is reduced to 1/3, reduce shaft acceleration and increase run-up time of drive.
  • Circuit construction more complex, required 3 contractor and timer.


  • Star-delta starting reduces the current surge, more complex, requiring three contactors and timers.
  • The malfunction may occur if the control timer sequence is not completed or the star contactor remains stuck, when a mechanical interlock prevents the delta contactor from closing.
  • This will cause overheat and eventual burn out the motor unless tripped by the overcurrent relay.

(c) Soft starter

  • Use back-to-back connected Thyristors or Triacs in supply lines which are gated to delay “turn-on” within each AC half-cycle.
  • This delayed switching applies a reduced average AC voltage to the motor.
  • The applied motor voltage is gradually increased by starter software program until full voltage level is reached.
  • To achieve maximum efficiency, electronic switching circuit is bypassed for normal running.
  • A “soft” starter use voltage controller over the motor operating load range called “energy manager” application.
  • The controller monitors the motor P.F which is a measure of the motor loading.
  • On light load and full voltage, the P.F is low so the controller reduces the motor voltage which reduces current while improving P.F and efficiency.
  • One of the benefits of this soft starter is possibility to adjust motor torque to exact need whether the application is loaded or not.
  • Another feather of soft starter is the soft stop function, which eliminates water hammering in the pipe system.

Describe the star-delta method of starting a cage-type, 3 phase induction motor with the aid of sketch and disadvantage of this method of starting.

  • Three sets of stator windings have six end connections, brought out to a starter box.
  • Contactors in starter box, six ends to be Star-connected for starting, Delta-connected after rotor runs up to about 80% of its rated speed.
  • Star starting effect of reducing phase voltage to 57.7% of the line voltage.
  • Starting current and torque only a third of with direct on-line starting.
  • Low current start is obtained at the expense of torque and star delta motors only used with light starting loads.
  • Automatic switching to delta running condition preferred to manual changeover, too soon or too slowly switch-over, result in a current surge.
  • In delta running condition, phase voltage is equal to line voltage and the motor as a straight-forward squirrel-cage type.
  • Built-in interlocks or double-throw switches prevent star and delta contacts from closed together.
  • The starter is also designed, star contacts made before possible to change to the run position.


  • Star-delta starting reduces the current surge, more complex, requiring three contactors and timers.
  • The malfunction occurs if the control timer sequence is not completed or the star contactor remains stuck, when a mechanical interlock prevents the delta contactor from closing.
  • This will cause overheat and eventual burn out the motor unless tripped by the overcurrent relay.

(a) Explain the principle and operation of induction motor.

(b) Describe the construction of induction motor.

(c) Define the term synchronous speed and slip.

(d) Define power factor correction.

(a) Principle

Induction motors depend on interaction of a revolving magnetic field created in stator by AC current, with an opposing magnetic field induced on rotor, resulting interaction produces torque, coupled to desired loads


  • When 3-phase A.C supply voltages connected to phase stator windings, resulting currents produce a multi-pole magnetic flux (Φ) and rotated around stator core
  • Stator rotating magnetic flux cut through rotor conductors to induce alternating emf into them
  • Rotor conductor connected together at ends, induced emf set up rotor currents
  • Rotor current produce magnetic flux which interact with stator rotating flux which produce torque on rotor conductor bars
  • In AC induction motor magnetic field induced in rotor opposite in polarity of magnetic field in stator
  • Magnetic field rotate in stator, rotor rotate to maintain its alignment with stator magnetic field.

(b) Construction

  • The induction motor has two main components, the stator and the rotor.
  • The stator have 3 phase windings, space 120’ apart, lying in slots into a laminated steel magnetic core.
  • Ends of the stator windings are terminated in the stator terminal box connected to the incoming 3-phase supply cables.
  • Stator windings are wound for specific numbers of pole-pairs connected in either star or delta connection.
  • Rotor consist of copper or aluminum conductor bar which connect together at their end by short circuiting ring to form cage winding
  • Conductor bar set in laminated steel magnetic core

(c) Synchronous speed

The flux rotated around the stator core by the switched sequence of the RY-B currents at a speed called synchronous speed (ns).

The value of synchronous speed depends on

(1) Number of magnetic pole-pairs (p) fixed in the stator winding.

(2) Frequency (f) of the supply voltage connected to the stator winding.

Synchronous speed Ns = F/P revs/sec (or) Ns = f x 60/P rev/min


Slip speed is difference between the synchronous speed (ns) of the rotating magnetic flux and actual rotor speed. (nR).

Slip is usually expressed as a percentage of the synchronous speed.

Slip = synchronous speed (Ns) – actual rotor speed (NR)

Slip (%) = (Ns – NR/Ns ) x 100%

(d) Power factor correction

  • For constant load, PF of synchronous motor varied from lagging value to leading value by adjusting DC field excitation.
  • Keeping same load, when field excitation increase on synchronous motor, motor operates at leading PF.
  • If reduce field excitation motor operate at lagging PF.
  • In large industrial complexes, synchronous motor used to correct (improve) PF obtained by DC over excitation current

(a) What are the possible causes of the motor failure?

(b) State the routine maintenance for a.c. motor.

(c) Explain the meaning of “single phasing “.

(d) State the possible causes of “single phasing “.

(a) The possible causes of the motor failure based on field experience are

1. Failures due to mechanical problems.

2. Insulation failure

3. Rotor-bar failure

4. Over-current failure

(b) Routine maintenance for AC motors

1. Keep motor clean and free from dirt and oil.

2. Check for dampness around motor or inside motor

  • Reduce insulation strength of motor winding.
  • Keep motor dry internally as well as externally.
  • If not in use for long time, run the motor for few hours to dry out moisture.

3. Periodic inspection of motor for accurate shaft alignment

  • For directly coupled motor – check alignment between motor shaft and load shaft
  • For belt-type system – check belt condition and belt tension.

4. Check bearing condition on a regular basis.

  • Bearing lubricated with correct lubricant in proper quantity.
  • Excess as well as lesser quantity can do harm.

5. Check for any abnormal noise or excess vibration from the motor or coupling. Do vibration analysis if necessary.

6. Check motor heating – If motor heats up quickly, check and clean air filters to get adequate air flow.

(c) Single phasing

1. In a 3-phase supply, loss of current through one phase call single phasing.

2. Effect of single phasing, current flow within two remaining lines is increased and the motor becomes very noisy due to uneven torque developed in the rotor.

(d) Possible cause of Single phasing

Possible causes of single phasing are

1. Fuse blow

2. Wire broken

3. Contact loose

4. Contact faulty