How capacitors work on a motor

Written by g.k. bayne
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How capacitors work on a motor
Capacitors can be roughly compared to batteries, and have a similar appearance too. (Getty Images)

Electrolytic capacitors store an electrical charge. Similar to a battery, the storage of the charge is released 60 times a second. The purpose for this fast charge and release of a capacitor is because all AC (alternating current) power comes in the form of 60 Hertz. This is the electrical power going from maximum (120 volts) to minimum (0 volts) 60 times a second. All electrical power that comes from the home wall outlet is cycling at 60 times per second or 60 Hertz. The capacitor takes advantage of this cycling to supply motors with an extra "burst" of power. Motor capacitors can even resemble the shape and size of a large round D-cell type battery. The storage capacity of a motor capacitor can be in the hundreds of volts for each charge and discharge cycle.

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Like a battery

Electrolytic capacitors store an electrical charge. Similar to a battery, the storage of the charge is released 50 times a second. The purpose for this fast charge and release of a capacitor is because all AC (alternating current) power comes in the form of 50 Hertz. This is the electrical power going from maximum (230 volts) to minimum (0 volts) 50 times a second. All electrical power that comes from home plug sockets in the UK is cycling at 50 times per second or 50 Hertz. The capacitor takes advantage of this cycling to supply motors with an extra “burst” of power. Motor capacitors can even resemble the shape and size of a large round D-cell type battery. The storage capacity of a motor capacitor can be in the hundreds of volts for each charge and discharge cycle.

Starting capacitors

Some electrical motors may need an extra push to get them rotating, especially if these motors are driving a heavy load such as a furnace fan or air compressor. The capacitor is placed within the motors electrical circuit by a switch on the motor itself. As the motor starts, the capacitor feeds the motor extra power to "jump" start the rotation. When the motor reaches a predetermined speed, the switch shuts off power to the starting capacitor while it is under full speed operation. If the motor drops below this speed, while electrical power is applied, the capacitor will switch back into the circuit to give the motor a boost to resume normal speed operation.

Running capacitors

Electric motors that drive a heavy load or must ensure a proper rotation critical to the equipment it drives may require the use of a run type capacitor. The run capacitor creates a "lead" or "lag" to the electrical power supplied to the motor. The run capacitors can also be called a "power factor" correction capacitor. Regardless of the nomenclature, lead, lag or power factor correction, the run capacitor increases the efficiency of the individual motors operation.

When electric motors operate, especially very large motors, they can create a derogative effect on the electrical distribution system. This can create an unbalance on the overall loading of the motor and overheat the windings. This unbalance can also raise the power consumption of the motor thus increasing the overall utility bill. A run type capacitor will act as a balance by "injecting" or "boosting" the electrical power just as a start capacitor does for certain applications.

When the electric motor is running, the load may change. During this load change, the motor can take a few seconds to "catch up." As described above that all power comes at 60 Hertz, a few seconds can mean many cycles of the motor to "catch up" to the load change of the motor. This "lag" in power consumption from the motor to the load can cost more electricity. Since the capacitor stores a charge and can release that charge as fast as 60 cycles per second, a large capacitor "bank" can boost the electrical system for a fast response to load changes. This storage of charge allows the motor to quickly adjust and not place a burden on the electrical system by being slow or overheating the windings of the motor.

The power factor capacitors or run capacitors can be as small as single round unit, and up to large multiple banks the size of a small refrigerator. Regardless of size, they all perform the same operation in storing an electrical charge and releasing that charge when needed by the motor circuit during operation.

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