Tuesday, March 18, 2008

Assignment 2

AC MOTOR DESIGNS AND USES

Introduction

Motors are electromagnetic devices that are used to convert electrical energy into mechanical work. There are three classes of AC motors

1. synchronous motors,

2. induction motors

3. Series wound motors.

The most common motor classes are synchronous and induction motors.

NEMA MG 1-2003 has the following definitions:

An induction machine is an asynchronous machine that has a magnetic circuit interlinked with two electric circuits, or sets of circuits, rotating with respect to each other. Power is transferred from one circuit to another by electromagnetic induction.

A synchronous machine is an alternating-current machine in which the average speed of normal operation is exactly proportional to the frequency of the system to which it is connected

A common differentiator is the phase of the motor. Single-phase motors use a single discrete waveform while two and three phase motors use two and three discrete polyphase waveforms that are spaced 180 and 120 degrees apart.

  • Single-phase AC induction motors: In this type of motor, only one discrete waveform is used. It has a rotating magnetic field to create the starting torque. These are used in devices like fans, washing machines, clothes dryer and other small household appliances. Important types are shaded pole motors and split phase induction motors.
  • Three-phase AC induction motors: These are used in high power applications. The phase difference between the three discrete waveforms of the input polyphase creates a rotating magnetic field. They are the workhorses of the industry and are used in heavy-duty electrical networks, locomotives and other applications. Using the principles of electromagnetic induction, current is induced in the conductors of the rotor by a rotating magnetic field. This creates a counterbalance field that makes the rotor turn in the direction of the magnetic field. The rotor rotates at a slower rate than the magnetic field. These motors will work even if one phase is disconnected.
  • Single-phase AC synchronous motors: These motors rotate in a synchronous manner with the main current frequency. They have magnetized rotors and do not need an induced current. This prevents backward slippage against the main frequency which makes them very accurate. They are used in audio turntables, mechanical clocks, tape drives, telescope drive systems, strip chart recorders and other applications.
  • Three-phase AC synchronous motors: These motors provide high and accurate performance and are used in traction motor applications and in TGV locomotives. Connections to the rotor coils are given on slip rings and a separate field current is given. This produces a continuous magnetic field that causes the rotor to rotate synchronously with the rotating magnetic field. These motors can also be used as alternators. To reduce starting problems, the motors are driven by transistorized variable frequency drives or with squirrel cage winding with a common rotor.
  • Stepper motors: The design is similar to three-phase AC synchronous motors and is a hybrid of a DC motor with solenoid. They have an internal rotor with permanent magnets that is controlled by external magnets which are operated electronically. The motor does not rotate continuously but steps from one position to another when the windings are activated and deactivated in a sequence. This allows them to turn forwards or backwards. They are used in sophisticated positioning drives and in servo controlled systems.

Conclusion

As seen above the motors design depends upon the function in with the machine is intended to be used for. This therefore makes the functions of machines dependant on the nature of the work the machine is going to be doing.

Reference

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