How the Common Capacitor Start Pool Motor Works

WRITTEN BY:  Inyo Pools

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The Capacitor Start Pool Motor is one of the most common motors found on in-ground pool pumps. This style of motor has a supplemental circuit consisting of a start windings and capacitor to increase the starting torque by 150-175% while decreasing the starting current. When the motor reaches 2/3 to 3/4 of full speed, a start switch will take the start windings and capacitor out of the running circuit. This guide identifies the major parts of this start circuit and explains how it operates. For more information on which motors use capacitors and how a capacitor fails , see our guide on "How To Select the Right Capacitor For Your Pool Pump Motor".

Step by Step


Step 1

IDENTIFYING COMPONENTS - After you remove the motor cover on the electrical end of the motor, you will see the motor's electrical circuits. This picture shows the location of most of the components of the supplemental start circuit on a common Capacitor Start Pool Motor. Note: The Windings are not visible on this picture. They are incorporated in the actual motor. The function of each component follows.

Step 2

SUPPLY POWER - Power from the main circuit board comes into the pump motor at this point. Usually it is conveyed on three lines: to power and one ground. If the supply power is 230V, the two power lines will both be hot - 240V across them. If the supply [power is 120V, the power line will be hot and the other neutral. The power lines are connected to the power terminal at L1 and L2 terminals. The green ground wire goes to a grounding screw attached to the frame of the motor.

Step 3

CAPACITOR - This Capacitor, referred to as a Start Capacitor, is tied directly to the Start Winding. This Capacitor together with the Start Windings provide an increase of 150 -175 % of torque to the motor shaft during its initial startup. When the motor reaches 2/3 to 3/4 full speed, this circuit will be shut off. The start capacitor will generally be rated at 50-400 MFD and 125 or 250 VAC.

Step 4

SWITCH - The Start Switch is commonly shaped as a V with contacts at the point of the V. When the contacts are pushed together by the Governor, the start circuit is activated. This picture shows the contact points of the switch.

Step 5

GOVERNOR (1) - The governor controls the opening and closing of the start switch. When the motor is off, springs on the arm of the governor cause the arm to come in contact with the bottom of the start switch causing the two contact points on the switch to come together and close the start winding circuit. When the motor is turned on, the rotational force off the shaft causes the weights on the ends of the governor to spin out. When that rotational force is greater than the force of the springs, the arm on the governor drops down and the contacts on the switch separate and opens the start winding circuit. This picture shows the arm of the Governor engaged with the push rod of the Switch.

Step 6

GOVERNOR (2) - This picture shows the down position of the Governor's arm as it would appear in rotation. Note: a screwdriver is holding the arm down to simulate rotation. Also the Switch has been removed to better illustrate the Governor.

Step 7

PROTECTOR - The Thermal Overload Protector helps keep the motor from overheating. When the motor amperage exceeds max amps, the motor will overheat, and the Overload will shut the motor off until it cools. If the motor keeps cycling on and off, you either have a defective Overload or some other problem with the motor.

Step 8

WINDINGS - Windings are the wire wrapping inside the motor. In a Capacitor Start Pool Motor there are three set of windings. The main windings are broken into two circuits. They are active whenever the motor is running. The start winding is part of a third circuit that is only active at the start of the motor. This picture shows a schematic of the three winding circuits. The windings outlined in green are the main windings; the winding outlines in red is the start capacitor winding. The switch that turns the start capacitor circuit on and off is in the middle of the red box.


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Inyopools  Posted: 01/12/2015 8:57 AM  Inyo Product Specialist

roundbob - Not all motors have capacitors. Some pool motors like many above ground and spa motors are designed without a capacitor.

roundbob  Posted: 01/10/2015 23:05 PM 

I purchased a new motor from inyopools. It did not come with a capacitor. is that something I need to purchase separately?

Inyopools  Posted: 08/19/2014 11:19 AM  Inyo Product Specialist

Snapperhead - If your pump's motor has a capacitor under the back cover, that's a good place to start. If the capacitor is bad, the motor won't have enough torque to turn over. Also check the governor to make sure it doesn’t stick open when the pump is off.

Snapperhead  Posted: 08/19/2014 9:40 AM 

I have a 1.5 hp pump for an above ground pool. It doesn't start on it's own, I have to take the back cover off and spin the slot to get it started but then it is fine. Does that sound like a start capacitor? Someone else said a good cleaning inside could fix it.

Inyopools  Posted: 08/05/2013 11:44 AM  Inyo Product Specialist

Mr.C - Sounds like you have a bad capacitor. At least that's a good place to start. Here is a link to our capacitors.

Mr.C  Posted: 08/04/2013 14:34 PM 

Your explanations are great! Thank you!
Q. My pump just hums on startup. If I give it a spin manually it starts right up and runs great. By your explanations above can I assume its my capacitor? Do you sell those?