Shaded-pole synchronous motor
Encyclopedia
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
.
Like a shaded pole induction motor
Shaded-pole motor
A shaded-pole motor is a type of AC single-phase induction motor. It is basically a small squirrel cage motor in which the auxiliary winding is composed of a copper ring surrounding a portion of each pole. This auxiliary winding is called a shading coil. Currents in this coil delay the phase of ...
, they use field coils with additional copper shading coils (see the illustration) to produce a weakly rotating magnetic field
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
. But unlike a shaded pole induction motor (which uses a squirrel cage rotor
Squirrel cage rotor
A squirrel cage rotor is the rotating part used in the most common form of AC induction motor. An electric motor with a squirrel cage rotor is termed a squirrel cage motor.- Structure :In overall shape, it is a cylinder mounted on a shaft...
), the synchronous version of this motor uses a magnetized rotor. This rotor rotates synchronously with the rotating magnetic field: if the rotor begins to lag behind the rotating field, driving torque increases and the rotor speeds up slightly until the rotor's position within the rotating field is a point where torque = drag; similarly, if the rotation of the field slows down, the rotor will advance relative to the field, torque will decline, or even become negative, slowing the speed of the rotor until it again reaches a position relative to the field where torque = drag.
Because of this, these motors are often used to drive electric clocks and, occasionally, phonograph
Phonograph
The phonograph record player, or gramophone is a device introduced in 1877 that has had continued common use for reproducing sound recordings, although when first developed, the phonograph was used to both record and reproduce sounds...
turntables. In these applications, the speed of the motor is as accurate as the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of the mains
Mains
Mains may refer to:* Mains electricity * Mains power around the world* Electricity transmission* Public utility, "mains services", including electricity, natural gas, water, and sewage disposal...
power applied to the motor.These motors are also used in shavers.
Frequently, the rotor and its associated reduction geartrain
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
is encased in an aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, or plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
enclosure; the enclosed rotor is driven magnetically through the enclosure. Such geared motors are commonly available with the final output shaft or gear rotating from 600 RPM down to as low as 1/168 revolutions per hour (1 revolution per week!).
A further development dispenses with the shading rings altogether. The application of power gives the magnetised rotor enough of a 'flick' to move it fast enough to establish synchronism. A mechanical means prevents the rotor from starting in the wrong direction. This design will only work satisfactorily if the standstill load is near to zero and has very little inertia.
Starting issues and torque limitations
Even by the standards of shaded pole motors, the power output of these motors is usually very low. Because there is often no explicit starting mechanism, the rotor of a motor operating from a constant frequency mains supply must be very light so that it is capable of reaching running speed within one cycle of the mains frequency. Alternatively, the rotor may be provided with a squirrel cageSquirrel cage rotor
A squirrel cage rotor is the rotating part used in the most common form of AC induction motor. An electric motor with a squirrel cage rotor is termed a squirrel cage motor.- Structure :In overall shape, it is a cylinder mounted on a shaft...
, so that the motor starts like an induction motor, once the rotor is pulled into synchronism with its magnet, the squirrel cage has no current induced in it and so plays no further part in the operation.
In more recent times, the use of variable frequency controls
Variable frequency drive
A variable-frequency drive is a system for controlling the rotational speed of an alternating current electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive...
permits synchronous motors to start slowly and deliver more torque.
Further reading
- Institute of Electrical and Electronics EngineersInstitute of Electrical and Electronics EngineersThe Institute of Electrical and Electronics Engineers is a non-profit professional association headquartered in New York City that is dedicated to advancing technological innovation and excellence...
, "Analysis and Control of subsynchronous resonance : presented at the IEEE Power Engineering Society 1976 winter meeting and Tesla symposium". Piscataway, N.J., IEEE, c1976. 76CH1066-O-PWR. LCCN 77153757 //r842