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Thursday, April 26, 2012

2.1 AC Motors

1. Synchronous motor
A synchronous motor is distinguished by its rotor spinning at the same rate as the oscillating field which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor, which must slip in order to produce torque.  Sometimes a synchronous motor is used, not to drive a load, but to improve the power factor on the local grid it's connected to. It does this by providing reactive power to, or consuming reactive power from the grid. In this case the synchronous motor is called a Synchronous condenser.

A synchronous motor may have either a revolving armature or a revolving field, although most synchronous motors are of the revolving-field type. The stationary armature is attached to the stator frame, while the field magnets are attached to a frame that revolves with the shaft.  The field coils are excited by direct currents, either from a small DC generator (usually mounted on the same shaft as the motor and called an exciter), or from some other source. Fig. 1 shows a directly connected exciter.

Electrical power plants almost always use synchronous generators because it's very important to keep the frequency constant at which the generator is connected. Low power applications include positioning machines, where high precision is required, and robot actuators.

Synchronous motors have the following advantages over non-synchronous motors:
     i.      Speed is independent of the load, provided an adequate field current is applied.
   ii.      Accurate control in speed and position using open loop controls, eg. stepper motors.
 iii.      They will hold their position when a DC current is applied to both the stator and the rotor windings.
 iv.      Their power factor can be adjusted to unity by using a proper field current relative to the load.
   v.      Their construction allows for increased electrical efficiency when a low speed is required (as in ball mills and similar apparatus).

2. Squirrel-Cage Motor
            The most common form of induction motor is the squirrel-cage type. This motor has derived its name from the fact that the rotor, or secondary, resembles the wheel of a squirrel cage. Its universal use lies in its mechanical simplicity, its ruggedness, and the fact that it can be manufactured with characteristics to suit most indus­trial requirements.  Squirrel-cage motor consists essentially of two units, namely stator and rotor.  The stator (or primary) consists of a laminated sheet-steel core with slots in which the insulated coils are placed. The coils are so grouped and connected as to form a definite polar area and to produce a rotating magnetic field when connected to a polyphase alternating- current circuit.

            The rotor (or secondary) is also constructed of steel laminations, but the windings consist of conductor bars placed approximately parallel to the shaft and close to the rotor surface. These windings are short-circuited, or connected at each end of the rotor, by a solid ring. The rotors of large motors have bars and rings of copper connected at each end by a conducting end ring made of copper or brass. The joints between the bars and end rings are usually elec­trically welded into one unit, with blowers mounted on each end of the rotor. In small squirrel-cage rotors, the bars, end rings, and blowers are of aluminium cast in one piece instead of welded together.

            The air gap between the rotor and stator must be very small in order for the best power factor to be obtained. The shaft must, therefore, be very rigid and be furnished with the highest grade of bearings, usually of the sleeve or ball-bearing type. A cutaway view of a typical squirrel-cage induction motor is shown in Figure 2.

     i.      Because of its simplicity of construction and because it can be built with electrical characteristics to suit almost any industrial requirement, has made it one of the most widely used machines.
   ii.      The speed of a squirrel-cage motor is nearly constant under normal load and voltage conditions.
 iii.      Suitable for medium or low-starting-torque requirements.

Squirrel-cage motors as a rule are not suitable where a high starting torque is required.

3.  Wound – Rotor Motor
A wound rotor motor is a variation of the in­duction motor but does not use a squirrel cage winding. The stator of the wound rotor motor is the same as the standard three-phase induction motor, in that it produces a three-phase rotating magnetic field. The rotor is not a squirrel cage winding.  Squirrel cage windings have cast conducting bars shorted together end rings and installed in the laminated The rotor of the wound rotor motor act consists of conductors (magnet wire) wound into coils on the rotor. (See Figure 3.)
The stator in the wound-rotor motor is identical to the stator in the squirrel-cage motor. The basic difference in the two motors lies in the rotor winding.  In the squirrel-cage motor, the rotor winding is nearly always self-contained; it is not connected either mechanically or electrically to the outside power-supply or control circuit. However, in wound-rotor motors, the rotor winding consists of insulated coils of wire that are not permanently short-circuited, but are connected in regular succession to form a definite polar area having the same number of poles as the stator. The ends of these rotor windings are brought out to collector rings, or slip rings.

The wound-rotor motor is often used in cranes, hoists, and elevators. These devices are operated intermittently and for short periods of time, where exact speed regulation and loss in efficiency are of little consequence.  Wound-rotor motors can be used to start extremely heavy loads. Hence, they are suitable for: (1) driving various types of machinery that require development of considerable starting torque to overcome friction; (2) accelerating extremely heavy loads that have a flywheel effect or inertia; and !3) overcoming back pressures set up by fluids and gases, as in reciprocating pumps and compressors.

     i.      The wound rotor motor has higher starting torque per line amps than do most AC squirrel cage motors.
   ii.      The wound-rotor motor can operate at any speed from its maximum full-load speed down to almost standstill.
 iii.      Wound-rotor motors have the ability to start extremely heavy loads. Hence they are suitable for:
Ø  Driving various types of machinery that require development of considerable starting torque to overcome friction.
Ø  Accelerating extremely heavy loads that have a flywheel effect or inertia.
Ø  Overcoming back pressures set up by fluids and gases in the case of reciprocating pumps and compressors.

     i.      Most motor starters used with wound rotor motors have a provision that will not allow you to start the motor if all secondary resistance is shorted out of the secondary.
   ii.      The wound rotor motor is a higher maintenance motor because of the windings that are fitted into the rotor.
 iii.      If lowered speed is required over longer periods, poor speed regulation and loss in efficiency may become prohibitive.

4.  Split-phase induction motor

            Another common single-phase AC motor is the split-phase induction motor, commonly used in major appliances such as washing machines and clothes dryers. Compared to the shaded pole motor, these motors can generally provide much greater starting torque by using a special startup winding in conjunction with a centrifugal switch.

            In the split-phase motor, the startup winding is designed with a higher resistance than the running winding. This creates an LR circuit which slightly shifts the phase of the current in the startup winding. When the motor is starting, the startup winding is connected to the power source via a set of spring-loaded contacts pressed upon by the not-yet-rotating centrifugal switch. The starting winding is wound with fewer turns of smaller wire than the main winding, so it has a lower inductance (L) and higher resistance (R). The lower L/R ratio creates a small phase shift, not more than about 30 degrees, between the flux due to the main winding and the flux of the starting winding. The starting direction of rotation may be reversed simply by exchanging the connections of the startup winding relative to the running winding.

            The phase of the magnetic field in this startup winding is shifted from the phase of the mains power, allowing the creation of a moving magnetic field which starts the motor. Once the motor reaches near design operating speed, the centrifugal switch activates, opening the contacts and disconnecting the startup winding from the power source. The motor then operates solely on the running winding. The starting winding must be disconnected since it would increase the losses in the motor.

5.  Repulsion motor

            Repulsion motors are wound-rotor single-phase AC motors that are similar to universal motors. In a repulsion motor, the armature brushes are shorted together rather than connected in series with the field. Several types of repulsion motors have been manufactured, but the repulsion-start induction-run (RS-IR) motor has been used most frequently. The RS-IR motor has a centrifugal switch that shorts all segments of the commutator so that the motor operates as an induction motor once it has been accelerated to full speed. RS-IR motors have been used to provide high starting torque per ampere under conditions of cold operating temperatures and poor source voltage regulation. Few repulsion motors of any type are sold as of 2005.

6.  Shaded-pole motor

            A common single-phase motor is the shaded-pole motor, which is used in devices requiring low starting torque, such as electric fans or other small household appliances. In this motor, small single-turn copper "shading coils" create the moving magnetic field. Part of each pole is encircled by a copper coil or strap; the induced current in the strap opposes the change of flux through the coil (Lenz's Law), so that the maximum field intensity moves across the pole face on each cycle, thus producing a low level rotating magnetic field which is large enough to turn both the rotor and its attached load. As the rotor accelerates the torque builds up to its full level as the principal (rotationally stationary) magnetic field is rotating relative to the rotating rotor. Such motors are difficult to reverse without significant internal alterations.

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