- Centrifugal Switch Definition: A centrifugal switch is an electrical component that activates based on the centrifugal force generated by a motor’s rotating shaft to control motor start-up.
- Operational Mechanism: The switch engages a boost circuit to start the motor and disconnects it once the motor reaches sufficient speed, facilitating smooth and efficient motor operation.
- Symbol and Schematic: The symbol of a centrifugal switch in electrical schematics represents its function and connection within an electrical or electronic circuit.
- Testing Protocols: Testing a centrifugal switch involves checking its operational consistency and ease of access for maintenance without disassembling the motor.
- Impact of Malfunction: If the centrifugal switch fails to disconnect after the motor starts, it can cause the starting winding to burn out, highlighting the importance of proper switch function for motor longevity.
What is a Centrifugal Switch?
A centrifugal switch is a speed-operated electrical switch. Weights on a rotating shaft fly outward and open or close contacts at a set speed. On a single-phase electric motor that job is usually dropping the start winding after the rotor is up. The switch does not set shaft speed.
How do Centrifugal Switches work?
You will see this switch on many signal phase induction motors, including resistor split-phase induction motors and capacitor-start motors.
At rest the start-circuit contacts are closed. After the rotor reaches the cut-out speed, the contacts open and take the start winding (and start capacitor, if fitted) out of circuit.
The mechanism is a speed switch, not a speed governor. Split-phase is one family of single-phase induction motors. Shaded-pole and many permanent-split-capacitor motors have no such switch.
A vehicle centrifugal clutch is a different part. It transmits torque with shoes or a drum. This motor switch only makes and breaks an electrical circuit. Do not call the switch a clutch.
On a typical capacitor-start or split-phase motor the switch sits on the shaft, often behind the end bell. With the rotor stopped the start contacts are closed.
At start, those closed contacts let electricity reach the start winding and, on capacitor-start motors, the start capacitor. That displaced field raises starting torque. A resistor split-phase motor has no start capacitor. Once speed is high enough the contacts open.
A single-phase main winding alone does not produce a true rotating field at standstill, so starting torque is too small. A second winding, displaced in space and time, is needed to start.
The start circuit is already closed at rest. After cut-out speed (often about 70% to 80% of rated speed) the switch opens that circuit. The motor then runs on the main winding. The original wording “until” was backwards.
Centrifugal Switch Symbol
On a schematic the device is drawn with an electronic symbol, the same class of pictogram used for wires, batteries, resistors and transistors. The stored figure below is one drawing of that speed switch.
Like any switch, it either completes or breaks a path so electrical current can flow or stop. The actuator here is shaft speed, not a hand or a coil.
A motor start switch opens on rising speed (cut-out) and usually recloses on falling speed (cut-in), at a lower set point. Direction-sensitive versions exist, but the common start switch is a speed switch.
How to Test a Centrifugal Switch?
The list below is a design wish-list, not a test method. To test a real motor: isolate power, check start-circuit continuity at rest (closed), then confirm the start circuit opens once the rotor is up. Many switches sit inside the end bell, so access is not always from outside.
- Throughout its life cycle, the process should be uniform.
- For simplicity of design and low production cost, the number of components of the equipment should be minimal.
- It should have marginal elements of friction.
- A maker may offer a changeable cut-out/cut-in ratio without a new frame. That is a factory option, not a field test.
- Some designs put the stationary contacts on the end bell so you can inspect them from outside. Many others sit inside the motor. Do not assume you can test, clean or replace the switch without opening the assembly.
What happens if the centrifugal switch does not open?
If the start switch stays closed, the start winding (and start capacitor, if fitted) stay energised and overheat. The winding can open and the next start will fail. If the switch stays open at rest, the motor usually hums and does not start. Left on, the main winding can overheat too.
What is the effect if the centrifugal switch is not disconnected after the motor starts?
Cut-out is commonly about 70% to 80% of rated speed. If the start circuit stays in, heavy current keeps flowing in a winding rated for a short start. That winding fails. The motor can still approach running speed; the stored claim that speed and current cannot reach a maximum is not reliable.
What is the purpose of a centrifugal switch in the end of an open motor?
On an open-drip-proof or similar single-phase motor the shaft-mounted switch drops the start winding as the rotor nears running speed. Gasoline-engine speed switches exist for other jobs (for example ignition cut-out). They are not the same part as a motor start switch.
Do all single phase motors have a centrifugal switch?
No. Shaded-pole motors and permanent-split-capacitor (PSC) motors keep the auxiliary winding in circuit and have no centrifugal start switch. Split-phase and capacitor-start motors usually do. A PSC motor can look like a two-phase machine while running, and skipping the switch removes one failure point, but it is not a rule for every single-phase motor.
Centrifugal Switch in Induction Motors
A single-phase induction motor has a main stator winding plus an auxiliary (start) winding. Single-phase AC current feeds the main winding.
The main winding alone does not make a rotating field at standstill, so starting torque is near zero. The auxiliary winding is displaced in space and, with resistance or a capacitor, in time.
Those two fields together produce starting torque. After the rotor is turning, rotor currents help sustain a rotating field, so the start winding can be dropped. The original “pulsating field that does not include the stator filed” sentence is garbled and was replaced.
At a set fraction of synchronous speed the start circuit must open so the auxiliary winding is no longer fed.
That opening is the job of the shaft-mounted centrifugal switch (or, on some motors, a potential or current relay).
Why is the Centrifugal Switch Used in Most Single-Phase Induction Motors?
Many workshop and appliance motors (drill presses, furnaces, table saws, pumps, grinders, washers, dryers) are capacitor-start or split-phase and do use this switch with a start winding. PSC and shaded-pole units in the same products may not.
Single-phase motors need some form of start or auxiliary circuit. On very small fans the auxiliary circuit can stay connected.
Leaving a start winding or start capacitor in circuit wastes energy and adds heat. Above a few tenths of a horsepower, makers often drop that circuit after start. 1/10 hp is only a rough teaching threshold, not a code limit.
Without a start method the rotor may sit and hum. If left energised, winding insulation can fail. “About 30 seconds” is informal; trip time depends on protection and size.
With the start winding in, the motor starts. That winding is rated for a short duty. Drop it at cut-out speed, before it stays in at full speed and overheats.
On the mechanism, spring force holds the weights in until centrifugal force (rising with speed, and with radius as the weights move out) overcomes the spring and opens the contacts.
In the stored figure, weights on the shaft (marked P) operate switch S to drop the start winding. A start capacitor, when fitted, increases starting torque. It is not required on a resistor split-phase motor.
What Type of Split Phase Motor Does Not Generally Contain a Centrifugal Switch?
The usual capacitor-start capacitor-run split-phase motor (two-value capacitor motor) does use a centrifugal switch or a potential relay. That device drops the start capacitor. The run capacitor stays in. The stored figure below shows a switch.
That motor has a cage rotor and two stator windings, main and auxiliary, displaced about 90 electrical degrees in space.
Two capacitors are used: a larger start capacitor in only during start, and a smaller run capacitor that stays in while the motor runs.
Names in use are capacitor-start capacitor-run and two-value capacitor motor. The stored figure shows both capacitors.
A Permanent Split Capacitor (PSC) Motor is a single-phase motor with one capacitor left in series with the auxiliary winding at all times. It has no centrifugal start switch.
It also uses a cage rotor and main plus auxiliary windings. One capacitor sits in series with the auxiliary winding.
That single capacitor stays in for start and run (single-value capacitor motor), so no start switch is fitted.
Applications of a Centrifugal Switch
The motor start switch above is one use. Separate speed-sensitive switches are also built for overspeed or underspeed trips. Those are not the same as a computer safety interlock.
Typical uses, keeping motor-start and speed-trip devices distinct:
- Overspeed trip switches on some motors and generators (a speed switch, not the start-winding switch).
- Speed or slack-belt switches on conveyors, escalators and lifts. Ordinary DC motors do not use a single-phase start-winding centrifugal switch.
- Underspeed or zero-speed switches appear on blowers, fans and conveyors.
- An underspeed trip can stop a machine before loss of speed damages the process or the drive.
