03-03-2012, 04:11 PM
Electromagnetic clutch
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Friction-plate clutch
Main article: Friction-plate electromagnetic clutch
A friction-plate clutch uses a single plate friction surface to engage the input and output members of the clutch.
[edit] How it works
Engagement
When the clutch is required to actuate, current flows through the electromagnet, which produces a magnetic field. The rotor portion of the clutch becomes magnetized and sets up a magnetic loop that attracts the armature. The armature is pulled against the rotor and a frictional force is generated at contact. Within a relatively short time, the load is accelerated to match the speed of the rotor, thereby engaging the armature and the output hub of the clutch. In most instances, the rotor is constantly rotating with the input all the time.
Disengagement
When current is removed from the clutch, the armature is free to turn with the shaft. In most designs, springs hold the armature away from the rotor surface when power is released, creating a small air gap.
Cycling
Cycling is achieved by interrupting the current through the electromagnet. Slippage normally occurs only during acceleration. When the clutch is fully engaged, there is no relative slip, assuming the clutch is sized properly, and thus torque transfer is 100% efficient.
Applications
Machinery
This type of clutch is used in some lawnmowers, copy machines, and conveyor drives. Other applications include packaging machinery, printing machinery, food processing machinery, and factory automation.
Automobiles
When the electromagnetic clutch is used in automobiles, there may be a clutch release switch inside the gear lever. The driver operates the switch by holding the gear lever to change the gear, thus cutting off current to the electromagnet and disengaging the clutch. With this mechanism, there is no need to depress the clutch pedal. Alternatively, the switch may be replaced by a touch sensor or proximity sensor which senses the presence of the hand near the lever and cuts off the current. The advantages of using this type of clutch for automobiles are that complicated linkages are not required to actuate the clutch, and the driver needs to apply a considerably reduced force to operate the clutch. It is a type of semi-automatic transmission.
Electromagnetic clutches are also often found in AWD systems, and are used to vary the amount of power sent to individual wheels or axles.
A smaller electromagnetic clutch connects the air conditioning compressor to a pulley driven by the crankshaft, allowing the compressor to cycle on only when needed.
Locomotives
Electromagnetic clutches have been used on diesel locomotives, e.g. by Hohenzollern Locomotive Works.
