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The motor start components
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Capacitors are electrical devices that can discharge and electrocute. Any electrical device should be powered down before removing and replacing parts.
The start capacitor
The start capacitor is a component used on split phase motors to assist the motor start. It is used when there may be a considerable resistance to initial rotation. This could heavy components that are attached to the motor or a compressor that must start with a difference in pressures. This difference, in refrigeration systems, would be a higher pressure in the discharge of the compressor compared to a lower pressure in the suction side.
In refrigeration systems, many compressors use a PSC motor that can be converted to a start capacitor motor.
The start capacitor is much higher strength than a run capacitor.
The physical size of the start cap is not usually larger than a run cap but, unlike a run capacitor, it is not designed to to stay in the circuit. If it is left in the circuit, it will overheat and fail with a minute or two.
Below is a video describing the start capacitor.
In refrigeration systems, many compressors use a PSC motor that can be converted to a start capacitor motor.
The start capacitor is much higher strength than a run capacitor.
The physical size of the start cap is not usually larger than a run cap but, unlike a run capacitor, it is not designed to to stay in the circuit. If it is left in the circuit, it will overheat and fail with a minute or two.
Below is a video describing the start capacitor.
The start capacitor cannot be kept in the circuit longer than a few seconds. One of the most common types of devices to remove the start capacitor from the circuit after the motor has started is the potential relay.
This relay uses an interesting electrical potential.
When a motor is started and comes up to speed, the motor windings induce power in those same windings. This power travels in the opposite direction and stacks on top of the incoming power. As the motor speed increases, this power increases. This is called back EMF.
Back EMF exists in both windings, but the potential relay is energized by the back EMF between start and common.
As the speed increases to 75% of the rated motor speed, the back EMF rises high enough to energize the potential relay coil and opens the normally closed contacts, that deenergizes the circuit containing the start capacitor.
The videos below explain the operation of this relay.
This relay uses an interesting electrical potential.
When a motor is started and comes up to speed, the motor windings induce power in those same windings. This power travels in the opposite direction and stacks on top of the incoming power. As the motor speed increases, this power increases. This is called back EMF.
Back EMF exists in both windings, but the potential relay is energized by the back EMF between start and common.
As the speed increases to 75% of the rated motor speed, the back EMF rises high enough to energize the potential relay coil and opens the normally closed contacts, that deenergizes the circuit containing the start capacitor.
The videos below explain the operation of this relay.
The video below shows schematically, how the potential relay works.
The PTC device
Start capacitors are powerful devices and tend put pressure on the windings of the motor. So in some cases, where a hard start is not necessary to start the compressor and the run capacitor may not always be able to start it, sometimes a PTC device will be installed.
This device is installed between the start and run winding. The device allows power to pass thru to give a large current to the start winding. As the power passes thru the device, it is warmed by the current which increases the resistance of the device and reduces the power passing thru to almost 0. The result is that the larger power to the start winding only lasts about 1 second.
The video below shows a cutaway of this device.
This device is installed between the start and run winding. The device allows power to pass thru to give a large current to the start winding. As the power passes thru the device, it is warmed by the current which increases the resistance of the device and reduces the power passing thru to almost 0. The result is that the larger power to the start winding only lasts about 1 second.
The video below shows a cutaway of this device.
The video below takes a PTC device apart.
More coming