Low power motors for power tools, domestic equipment, refrigerators, vacuum cleaners etc are typically supplied at 220V a.c. 50/60Hz.
Common types are:
Split-phase induction motor
Capacitor start / run induction motor
Shaded pole induction motor
a.c. commutator motor
Split-phase induction motor on ship
A single phase induction motor has a cage rotor similar to that used in a three phase type. A single stator winding produces a pulsating magnetic field when energised with single-phase a.c. current.
This field cannot exert a rotating force on the cage rotor.
One method used to produce a rotational force is to employ two stator windings fitted 90 degree to each other with both connected across the same supply.
This is the split-phase motor.
To get the effect of a shifting magnetic field (and hence induce a rotating force into the rotor), one winding is electrically phase-shifted by adding capacitance in series with one of the windings.
Capacitor start/run induction motor on ship
When the motor has started to run, the additional phase winding circuit may be disconnected and the rotor will continue to be pulsed around by the magnetic flux.
This is called a capacitor start motor which is only useful for driving a very light load.
For starting and running, two capacitors are used in circuit, during the starting period the two paralleled capacitors create a large phase angle to the “S” winding current. As the rotor runs up to speed a switch cuts out one of the capacitors. The switch may be a centrifugal type on the rotor shaft or a current-operated time-delay relay in the motor terminal box.
This type of motor gives good starting and running torque with reasonable power factor. Most split phase motors are arranged for a 4-pole stator winding so at 50 Hz its synchronous (flux) speed will be 25 rev/s or 1500 rpm.
As with all induction motors, the rotor will slip causing the shaft speed to be about 24rev/s or 1440 rpm on no-load.
On-load, a single-phase induction motor will run with greater slip and operate with less efficiency, than a three phase version.
Shaded pole induction motor on ship
This is a low torque machine useful for low power drives such as small cooling fans in ovens and electronic equipment.
Face of each salient stator pole is partially split with one side carrying a thick copper wire called a shading ring. The pulsating a.c. flux divides into each half of the pole but is time delayed in the part with the shading ring.
This is due to an induced current in the ring which opposes flux change in the shaded part.
To the rotor, this delay appears as a flux shift across the overall pole face which drags the rotor with it by the normal induction motor action.
Obviously, the developed torque is small and the machine is not very efficient but is an inexpensive drive for very low power applications. As with all induction motors the shaft base speed is fixed by the supply frequency, so at 50 Hz the maximum speed is 3000 rpm and shaft loading will cause the rotor to slip below this value.
a.c. commutator motor on ship
This is basically a d.c. series motor construction designed to operate very effectively on an a.c. voltage supply.
At 220V a.c. the shaft speed on light load is typically 12,000 to 18,000 rpm and is easily controlled by an additional series resistance or an electronic voltage regulator.
The speed falls rapidly with increased load torque.
This type of motor is mainly used intermittently in equipment’s rated up to a few hundreds of watts. Typical examples would include power drills, sanders, jig-saws, food-mixers and vacuum cleaners.
The commutator and brush contacts will cause some sparking in normal operation which can cause radio/television interference so a high frequency voltage suppressor is usually fitted to this type of motor.
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