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The only way to reverse the direction of rotation of such a motor is to swap the power supply connections to one pair of field windings.
A two phase supply can be obtained from a three phase a.c. supply, by using one phase voltage and the opposite line voltage.
8.1.3 THREE PHASE
To produce a rotating field from a three phase a.c. supply requires the use of a six pole stator and three pairs of field windings. The stator of a three phase a.c.
motor is the same as that of a rotating field a.c. generator.
The direction of rotation of the field depends on the order in which the windings are energised. To reverse the direction of rotation, it is only necessary to swap the connection to any two of the field windings.
8.2 TYPES OF AC MOTOR
The two main types of a.c. motor used on aircraft systems are the induction motor and the synchronous motor. Hysteresis and shaded pole motors are however often found in instruments, and as they are both a.c. motors, they will also be examined at this time
8.2.1 INDUCTION MOTOR
The rotor of an induction motor consists of a number of copper or aluminium bars connected by two end rings to form a cage. The cage is enclosed in a laminated
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When the rotor is placed in a rotating magnetic field, the bars are cut by the rotating flux, causing emf's to be induced in them, because the bars are shorted by the end rings, currents then flow in the bars. Current flow in the bars produces a magnetic field around them, which reacts with the main field of the machine, causing the rotor to turn.
At switch-on, the emf's induced in the rotor bars are at the same frequency as the supply voltage and because the circuit is highly inductive the current lags the voltage by almost 90 degrees. This means, that by the time the rotor field has been produced, the main field has moved on by almost 90 degrees and the rotor field can only react with the trailing edge of the main field, resulting in a small starting torque.
As the rotor speed increases, the frequency of the emf's in the rotor decrease, reducing the inductive reactance. The brings the current more in-phase with the induced emf's, producing a good running torque.
It is not possible for the rotor to rotate at synchronous speed (the speed of the
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When running, the field around the rotor bars induces an emf into the stator windings, this ‘back-emf’ is almost 180 degrees out of phase with the applied voltage and therefore opposes it, resulting in a small effective voltage across the field and a low current drawn from the power supply. If the load on the motor is increased, it slows down, this causes the phase angle of the back-emf to change, increasing the effective voltage, the current from the supply and the motor torque.
The increase in motor torque accelerates the motor back to its original running speed.
When first started, the back-emf is almost at 90 degrees to the applied voltage and therefore not opposing the supply voltage. The effective voltage is therefore almost equal to the supply voltage and the current demand is high. In order to reduce the starting current, some motors are designed to be started with the field windings connected in star and run with them connected in delta. This increases the impedance during starting and reduces the current drawn from the supply, but it does not improve the poor starting torque.
If it is required that an induction motor be started ‘on-load’, then the poor starting torque must be improved. To achieve this, the rotor current must be made to appear more in phase with the voltage. This can be achieved by increasing the resistance of the rotor windings, however, if the resistance is left in the rotor circuit once the motor is running, there will be:
an increase in the slip speed
a greater speed variation with load changes an increase the current taken from the supply
A compromise often used on aircraft induction motors is to fit a second, high resistance, cage into the rotor. This gives an improved starting torque, with minimal running problems.
8.2.2 SYNCHRONOUS MOTOR
The synchronous motor gets its name from the fact that the rotor runs at
synchronous speed (the speed of the field), for it to do this, the rotor must be a permanent magnet or an electro-magnet.
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In order for the magnet to lock-on to the field, it must be brought up to about 75%
of synchronous speed, to achieve this the majority of synchronous motors have the cage of an induction motor built into them. The motor starts as an induction motor and when sufficient speed has been attained, the electromagnet is
energised, allowing the rotor to lock onto the field. Once running, no emf's are induced in the rotor bars, however, they are useful in holding the rotor and rotor windings in place and also assist in smooth running during load changes.
The rotor, although running at synchronous speed, will lag behind the field, the angle of lag is proportional to the load placed on the motor.
If whilst running the load is increased, the angle of lag increases, changing the angle of the back-emf and increasing the effective voltage. This increase in effective voltage increases the current taken from the supply, producing an increase in torque to cope with the load increase. Should the angle become too great, the magnetic link will snap, the motor will run down, stop, and possibly burn out due to the high current taken from the supply as a result of the loss of back emf.
8.2.3 SHADED POLE MOTOR The shaded pole motor uses only a single set of poles to create the appearance of a rotating
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When the field winding is energised, an alternating flux appears across the main poles. The alternating main field induces emf's in the shaded ring or shorted winding and causes a current flow within it that produces a second alternating magnetic field. The field in the shorted ring lags the main field by approximately 90 degrees. The overall effect is to produce a field that appears to move through an angle determined by the relative positions of the two sections of each main pole. The field is not fully rotating, only moving through a small angle, therefore the starting torque is low and the motor can only be used for small, fixed loads.
The operation of the rotor is as for an induction motor.
8.2.4 HYSTERESIS MOTOR
The construction of hysteresis motors vary. The motor is so named because the material used for the rotor has a large hysteresis loop. This type of motor
requires a two phase a.c. supply and is often used as a servo motor, one phase being supplied from a reference source, the other from a control circuit. The current in the control phase is made to either lead or lag the reference phase by 90 degrees, depending on the direction of rotation required.
The motor shown employs a cobalt steel ring rotor. When the field is energised, a North pole appears at A and a South pole at A1. Poles B and B1 are not magnetised. The field across A-A1 induces a South pole in the rotor at X and a North pole in the rotor at Y.
As the supply changes, A and A1 die away as B becomes a North pole and B1
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