A repulsion motor is a type of

electric motor An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate for ...

which runs on

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...

(AC). It was formerly used as a

traction motor A traction motor is an electric motor used for propulsion of a vehicle, such as locomotives, electric or hydrogen vehicles, elevators or electric multiple unit. Traction motors are used in electrically powered rail vehicles ( electric multiple ...

for electric trains (e.g. SR Class CP and SR Class SL electric multiple units) but has been superseded by other types of motors. Repulsion motors are classified as

single phase In electrical engineering, single-phase electric power (abbreviated 1φ) is the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads ...

motors. In repulsion motors the stator windings are connected directly to the AC

power supply A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a ...

and the

rotor Rotor may refer to: Science and technology Engineering * Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator *Helicopter rotor, the rotary wing(s) of a rotorcraft ...

is connected to a commutator and brush assembly, similar to that of a

direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...

(DC) motor.''The Electrical Year Book 1937,'' published by Emmott and Company Limited, Manchester, England, pp 79–82

Construction

The motor has a stator and a

but there is no electrical connection between the two and the rotor current is generated by induction. The rotor winding is connected to a commutator which is in contact with a short-circuited pair of

brushes A brush is a common tool with bristles, wire or other filaments. It generally consists of a handle or block to which filaments are affixed in either a parallel or perpendicular orientation, depending on the way the brush is to be gripped durin ...

which can be moved to change their angular position relative to an imaginary line drawn through the axis of the stator. The motor can be started, stopped and reversed, and the speed can be varied, simply by changing the angular position of the brushes.

Voltage

Most commutator motors are limited to about 1,500

volt The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defin ...

s because higher voltages give rise to a risk of

arcing An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. An ...

across the commutator. Repulsion motors can be used at higher voltages because the rotor circuit is not electrically connected to the supply.

Principle

Repulsion motors are based on the principle of repulsion between two magnetic fields. Consider a 2-pole

salient pole A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows. In a rotating ...

motor with a vertical magnetic axis. The armature is connected to a commutator and brushes. The brushes are short circuited using a low-resistance jumper. When alternating current is supplied to the field (stator) winding, it induces an electromotive force (emf) in the armature. The direction of alternating current is such that it creates a

north pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...

at the top and a

south pole The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on Earth and lies antipod ...

at the bottom. The direction of induced emf is given by

Lenz's law Lenz's law states that the direction of the electric current induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes changes in the initial magnetic field. It is named after p ...

, according to which the direction of induced emf opposes the cause producing it. The induced emf induces current in the armature conductors and the direction of the induced current depends on the position of the brushes.

Stop positions

;Brush axis parallel to field If the brush axis is along the direction of the magnetic field, the armature behaves like an electromagnet and a N-pole is formed directly below the N-pole of the stator and a S-pole is formed directly above the S-pole of the stator. The net torque in this condition is zero. Both the N-poles repel each other and both the S-poles repel each other. The two repulsion forces are in direct opposition to each other and hence no torque will be developed. This is very fast process in this repulsion motor. ;Brush axis at right angles to field If the brushes are shifted through 90 degrees, so that the magnetic axis is perpendicular to the brush axis, the coils undergoing short circuit change. Apart from the coils undergoing short circuit, the voltage induced in the other coils between the brush terminals is neutralized and the net voltage is zero. Since there is no induced emf, there is no current in the circuit and the net torque developed is, again, zero.

Run positions

If the brush axis is displaced at an angle to the magnetic axis, a net voltage is induced at the brush terminals which will produce current in the armature. The current in the armature circuit will produce its own magnetic field, with North and South poles, but in this condition, the North Pole is not directly under the North pole of the magnetic axis and the South Pole is not directly above the South Pole of the magnetic axis. The poles of the armature are slightly displaced from those of the stator. In this condition, the N-pole of the stator field will repel the N-pole of the rotor field and the S-pole of stator field will repel the S-pole of the rotor field, so the rotor starts rotating. ;Direction of rotation The direction of rotation is determined by the position of the brushes with respect to the magnetic field of the stator. If the brushes are shifted clockwise from the main magnetic axis, the motor will rotate in a clockwise direction. If the brushes are shifted counter-clockwise from the main magnetic axis, the motor will rotate in a counter-clockwise direction. ;Torque and speed control The starting torque of a repulsion motor is determined by the angle of brush shift from the main magnetic axis. The maximum torque is obtained from a brush shift of 45 degrees. Brush shift can also be used to control the speed of a repulsion motor.

Types of repulsion motor

Types of repulsion motor are listed below. It is likely that the different types were developed to match the

torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...

/speed characteristics of the motor as closely as possible to the service it was required to provide.

Elihu Thomson

The

Elihu Thomson Elihu Thomson (March 29, 1853 – March 13, 1937) was an English-born American engineer and inventor who was instrumental in the founding of major electrical companies in the United States, the United Kingdom and France. Early life He was bor ...

motor is the original repulsion motor and is described in "Construction" above.

Deri

The

Deri Deri may refer to : People * Aryeh Deri (born 1959), an Israeli politician * Frances Deri (1880–1971), an Austrian psychoanalyst * Miksa Déri (1854–1938), a Hungarian electrical engineer * Shlomo Deri (fl. 2000s), an Israeli politician * ...

motor is similar to the Elihu Thomson type but has two pairs of short-circuited brushes—one fixed and one moveable. This allows very fine control of speed.

Latour-Winter-Eichberg

This is the "compensated" repulsion motor devised independently by Latour and by Winter-Eichberg. There are, again, two pairs of brushes but they are fixed at right angles to each other. One pair is short-circuited while the other pair is fed with variable-voltage

from tappings on the secondary winding of a small

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

. The primary winding of the transformer is in

series Series may refer to: People with the name * Caroline Series (born 1951), English mathematician, daughter of George Series * George Series (1920–1995), English physicist Arts, entertainment, and media Music * Series, the ordered sets used in ...

with the stator winding of the motor. This motor has the same torque/speed characteristics as an ordinary series-wound motor.

Atkinson

The Atkinson motor has two stator coils at right angles to each other. Speed control (by brush-shifting) is possible from 75% below

synchronous Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...

speed to 10% above. Starting torque is about 2.5 times full-load torque with twice full-load current.

Repulsion-start induction-run

These were used where high starting torque was required. They started as repulsion motors, but once they were running at a sizable fraction of full speed, the brushes were lifted mechanically and all commutator bars were short-circuited together to create the equivalent of a squirrel-cage induction motor.

Applications

Repulsion motor applications included: * High speed lifts * Electrical locomotives * Fans and pumps * Printing presses * Textile machines * Film winding machines (smooth manual speed and direction adjustment could be achieved without complicated circuitry)

References

{{Electric motor AC motors Electric motors