Selection Of Electrical Motor Drives


Selection Of Electrical Motor Drives

Selection Of Electrical Drives:

Selection Of Electrical Motor Drives
selection criteria of electrical drives

Some of the went Akron on which the selection of electrical drives depend are :

  1. Steady state operation requirements :
  2. Transient operation requirements:
  3. Requirement related to the source :
  4. Capital and running cost, maintenance needs, life.
  5. Reliability
  6. Environment and location
  7. Space and weight restrictions, if any.
All of the above points are described below in detail description. So let's start the learning with detailed description.

(1) Steady State Operation Requirements :

  • Nature of speed-torque characteristics.
  • Speed regulation.
  • Speed range.
  • Efficiency.
  • Duty cycle.
  • Quadrants of operation.
  • Speed fluctuations if any, ratings.

(2)  Transient Operation Requirements:

  • Values of acceleration and deceleration.
  • Starting, braking and reversing performance.

(3)  Requirement Related To The Source :

  • Type of source, and its capacity.
  • Magnitude of voltage, voltage fluctuations, power factor, harmonics and their effect on other loads.
  • Ability to accept regenerated power.

(4) Capital And Running Cost, Maintenance Needs, Life.

(5) Reliability

(6) Environment And Location

(7) Space And Weight Restrictions, If Any.

1. Advantages Of AC Drives Compared To DC Drives :

1) For the same kW rating, ac motors are 20% to 40% light weight as compared  to DC motors.

2) AC motors are less expensive as compared to same kW rating dc motors.

3) AC motors have low maintenance as compared with DC motors.

4) AC motors can work in hazard areas like chemical and petrochemical etc. but DC
motors are not suitable for hazard areas due to commutator sparking.

2. Disadvantages Of AC Drives Compared To DC Drives :

1) The control of AC drives require complex control algorithm such as model reference, adaptive control, sliding mode control and field-oriented control which can be performed by microprocessors and microcontrollers.

2) Power electronics converters such as converters, inverters and ac voltage controllers are used to control the ac motors and these converters are expensive.

Power Electronics converters for ac drives generate harmonics in the supply system load and circuit. Therefore, ac motors will be derated.

3. Review Of Induction Motor Fundamentals


DC drives have been extensively used in the industry for variable speed applications for a long time, but gradually AC drive have been introduced into this field. This is due to the fact that cage type induction motors have several advantages over D.C. machines and the speed of induction motor can be controlled by controlling either its rotor frequency or by stator frequency.

The variation of rotor frequency can be accomplished by stator voltage variation or by variation of rotor resistance or by both. In an AC machine a rotating magnetic field is produced in the air gap by 3-Φ AC supply.

This rotating magnetic field reacts with the rotor mmf waves to develop torque. The torque in an induction machine is created by stator induction effect, whereas in a synchronous it is extracted by a separate field winding carrying D.C. Current.

Thus the speed of an AC machine is related to the stator supply frequency. Therefore supply frequency of the drives has to be changed if the speed of the machine is to be altered. This can be achieved cither by a DC link converter system or by a cyclo converter.

In order to get good torque capabilities of the motor and to avoid saturation at low frequency operation, the voltage is also varied to operate the motor at constant flux.

It is therefore natural that the cost of converter and the complexity of its control system are much higher compared to those of a DC drive of the same capacity. Due to these reasons, widespread application of AC drives are widely used.

4. Induction Motor Drives :

Basically 3-Φ induction motors are classification into two types :

  1. Slip Ring Induction Motor
  2. Squirrel Cage Induction Motor

Squirrel cage type induction motor are widely used in industry for various applications, because it is economical, rugged and reliable. Also it is available in the range of fractional horsepower to multi-megawatt ratting capacity.

Both these motors carry three phase balance distributed winding in stator. The squirrel cage induction motor has a rotor consisting of copper or aluminium bars which are short circuited at both end-rings.

For slip ring motors its rotor has a balanced three phase distributed winding and three slip rings are connected on the rotor shaft.

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