What Does A Start
And Run Capacitor Do?

I received this question the other day and thought it was a great question and worth a blog post. 

Question: Can someone explain me what is the function of the start and run capacitor used for motors?

Answer: I'm not an expert, but after doing some reading here is what I found. I hope it helps. 

The start capacitor provides that electrical "push" to get the motor rotation started. It does this by creating a current to voltage lag in the separate start windings of the motor. 

It is a component that stores an electrical charge and then releases it. Capacitors are most often used to start motors, and compressors and can be found in central air conditioners and furnace, as well as other heating and cooling devices. Capacitors are also used in refrigerators and washing machines. 

Since this current builds up slower, the armature has time to react to the rotating field as it builds up, and to begin rotating with the field.

Once the motor is very close to it's rated speed, a centrifugal switch disconnects the start cap and start windings from the circuit. 

Most smaller, single phase motors usually have a permanent magnet armature that is pushed / pulled around by the rotating inductive field produced by the stator (outside) windings. 

According to  capacitorformotor.com/run_capacitor.html the inductive field rotates as a result of the positive / negative alternations of the 60HZ AC current flowing through the windings. 

McCombs Supply has several start capacitors, including part number SPP5.

Interesting note: Without a start capacitor, like when one burns up, when the voltage is applied, the motor will just sit and hum. But if you were to grab the shaft and give it a spin, the motor would (usually) start and run normally.

What is a run capacitor?

A motor  run capacitor is used to continuously adjust current or phase shift to a motor’s windings in an effort to optimize the motors torque and efficiency performance.

McCombs Supply has several run capacitors to choose, including  part number POCD403.

How to Select a Motor Run Capacitor

Voltage: Select a capacitor with a voltage rating at or above the original capacitor. If you're using a 370 volt capacitor, a 370 or 440 volt capacitor will work, according to the website for TEMCo industrial Power Supply.

Round or Oval? Round capacitors are by far the most common, but many motors still use oval designs. Electrically speaking, there is no difference. Fit is the only question here. If space in the mounting box is not limited, the case style does not matter.

Here is a capacitor training video.

Capacitors are used to store electric charge. Most capacitors sold here at McCombs Supply are manufactured under the Packard, Titan PRO™ and Supco brands.

Helpful tips

• When device receives too much electricity, the capacitor will burst. When a capacitor fails, you will usually notice a bubble in the housing. When it fails you will often see a bulge or leak. 
• Capacitors hold an electrical charge. To avoid being electrocuted, it is important to discharge the capacitor before handling it. To discharge a capacitor, with the appliance unplugged, use a metal screw driver with an insulated handle to short across the terminals of the capacitor.
• It is important to discharge a capacitor. If this is not done there can be strong jolt of electricity. Here is how you discharge a capacitor. This link by wikihow.com shows you 5 steps for discharging a capacitor.
  

Here is an video illustrating how to discharge a capacitor.

McCombs Supply has a variety of capacitors to choose from like the  17.5-440R . When ordering a capacitor, know the microfarad and volts for your capacitor. 

You will find on the  McCombs Supply website run capacitors ranging from 2mfd through 100mfd in 370 and 440 volts. Both round and oval in shape. Start capacitors range from 21mfd through 1000mfd in 125, 165, 250 and 330 volts. All start capacitors are round in shape and have a plastic housing.

Capacitor Tester

We also have the following capacitor testers for sale. Both are battery operated.

Supco  MFD10 Digital Capacitor Tester, Ranges from .01 - 10000 mfd's. It sells for $61.20. 

                       

The second tester McCombs offers is the Packard  CT1 Digital Capacitor Tester, Ranges from 20 MF to 200PF. 

It sells for $40.85.

Here is a video on testing a capacitor.

Is it time to replace your Motor Run Capacitor?

As a general rule, a motor run capacitor will out last the same motor's start capacitor.

It will also fail or wear differently, making it slightly more involving when trying to determine if it's time to replace it. Start capacitors will commonly fail completely, making the decision on when to replace obvious, limiting time spent troubleshooting.

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About Us

McCombs Supply Company, Inc.

In business since 1953, McCombs Supply Co., Inc. is a third-generation family-owned business that specializes in fast, reliable shipping.

Located in Lancaster, Pennsylvania, McCombs Supply Co., Inc. offers replacement repair parts for dishwashers, dryers, microwaves, range-ovens, refrigerators, washing machines, HVAC, commercial refrigeration, commercial cooking, pellet stove parts and more. Some of our manufacturers we distribute for include – Backer-Chromolox, Broan-Nutone, Exact Replacement Parts (ERP), EZ Flo, Fasco, Mastercool, ICM, JW Harris, Packard, Robertshaw, Sealed Unit Parts Company (SUPCO), White Rodgers, Englander, Breckwell, Harman, Lennox, Napoleon, St. Croix and Quadra-Fire.

We supply contractors, service technicians, housing developments, hospitals, retirement communities, institutions, and do-it-yourselfers.

Today, we have nearly one million SKUs in our system. We are constantly upgrading our cutting-edge technology to serve our customers better.

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