A centrifugal switch is an
electric switch that operates using the
centrifugal force
In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is paralle ...
created from a rotating
shaft
Shaft may refer to:
Rotating machine elements
* Shaft (mechanical engineering), a rotating machine element used to transmit power
* Line shaft, a power transmission system
* Drive shaft, a shaft for transferring torque
* Axle, a shaft around whi ...
, most commonly that of an
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 f ...
or
gasoline engine
A petrol engine (gasoline engine in American English) is an internal combustion engine designed to run on petrol (gasoline). Petrol engines can often be adapted to also run on fuels such as liquefied petroleum gas and ethanol blends (such as ''E ...
. The switch is designed to activate or de-activate as a function of the
rotational speed
Rotational frequency (also known as rotational speed or rate of rotation) of an object rotating around an axis is the frequency of rotation of the object. Its unit is revolution per minute (rpm), cycle per second (cps), etc.
The symbol fo ...
of the shaft.
History
A 1927 patent for a centrifugal switch (US Patent #1,630,394
) was granted to Royal Lee on May 31, 1927. It was the basis for the formation of the Lee Engineering Company.
Applications
Electric motors
Perhaps the most common use of centrifugal switches is within
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 loa ...
,
split-phase induction motor
An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor ...
s. Here, the switch is used to disconnect the starting winding of the motor once the motor approaches its normal operating speed. In this case, the centrifugal switch consists of weights mounted to the shaft of the motor and held near the shaft by spring force. At rest, levers attached to the weights press a
low-friction,
non-conductive plate against a set of electrical contacts mounted to the motor housing, closing the contacts and connecting the starting winding to the power source. When the motor approaches its normal operating speed, centrifugal force overcomes the spring force and the weights swing out, raising the plate away from the electrical contacts. This allows the contacts to open and disconnects the starting winding from the power source; the motor then continues operating solely using its ''running winding''. Motors using such a centrifugal switch make a distinct clicking noise when starting and stopping as the centrifugal switch opens and closes.
[ (Includes illustrations)]
A variation of the centrifugal switch used a changeover contact to ensure that the auxiliary windings remain in the circuit through a running capacitor. These motors are called two-value or
capacitor start capacitor run motors.
Centrifugal switches are found in many
clothes dryer
A clothes dryer, also known as tumble dryer or simply dryer, is a powered household appliance that is used to remove moisture from a load of clothing, bedding and other textiles, usually shortly after they are washed in a washing machine.
Many ...
s, supplying power to the motor in parallel with the start switch. As long as the motor continues turning, the centrifugal switch supplies power, but if something jams in the dryer and slows it down too much, the switch will cut power.
Centrifugal switches were also used in early
electric fans, particularly those made in the 1900s and early 1910s.
In
aircraft
An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or by using the Lift (force), dynamic lift of an airfoil, or in ...
, a centrifugal switch is used to control the starting and
ignition circuits, the governed speed indication circuit and the
overspeed
Overspeed is a condition in which an engine is allowed or forced to turn beyond its design limit. The consequences of running an engine too fast vary by engine type and model and depend upon several factors, the most important of which are the du ...
protection circuit of the
auxiliary power unit
An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft and naval ships as well as some large land vehicles. Aircraft APUs generally produce 115& ...
. This switch is mounted inside a rotating body. This can be set to activate or deactivate a circuit as the rpm of device increases or decreases.
Centrifugal switches are designed to minimize
frictional contact between the rotating speed sensor and the non-rotating switch body. In most cases, this is done by having contact at low speed, which is pulled away at operating speed, so there is friction only during brief periods of start-up and slow-down. However, overspeed switches work the other way, making contact at high speed, but avoiding contact at normal operating speed.
See also
*
Centrifugal governor
A centrifugal governor is a specific type of governor with a feedback system that controls the speed of an engine by regulating the flow of fuel or working fluid, so as to maintain a near-constant speed. It uses the principle of proportional con ...
*
Transfer switch
A transfer switch is an electrical switch that switches a load between two sources. Some transfer switches are manual, in that an operator effects the transfer by throwing a switch, while others are automatic and trigger when they sense one of t ...
*
References
{{DEFAULTSORT:Centrifugal Switch
Switches