Causes of Hard-Starting Electric Motors

Fix Hard Starting Air Conditioning / Heat Pump Compressor Motor & Other Electric Motors
Motor hums, only starts with a push, or has other startup trouble

Most-Common Causes of Electric Motor Start Trouble

• Bad or failed starting capacitor:
  
   the air conditioner compressor (and some other electric motors) may already have a starting capacitor installed, but the starting capacitor might have failed, causing the air conditioner compressor to start with difficulty or not at all.
  
  A bad starter capacitor can also disable the fan in the outdoor compressor/condenser, or the blower fan in the indoor air handler unit.
  
  Capacitors are electric devices that get an electric motor running at start-up or that help keep a motor running once it has started. If the capacitor has failed the symptom is that the motor won't start. You may hear it humming or observe that it's getting hot. If you observe this we suggest that to avoid damage you turn off the system while waiting for repairs.
  
  We give more detail about failed electric motor starting capacitors, and we explain possible visual diagnosis of a failed starter capacitor (bulged ends) without having to perform electrical testing,
  
  at HARD STARTING COMPRESSOR MOTORS.
  
  Thanks to reader George Fazio for this air conditioner start-up diagnosis suggestion and for the photo (above left).

• Bad centrifugal switch in the electric motor:
  
  Thanks to a reader, Paul J. Ste. Marie we can add that a bad centrifugal switch can also keep an electric motor from starting or running as it should.
  
  See details
  
  at ELECTRIC MOTOR CENTRIFUGAL SWITCH or PTC PRD

• Low line voltage 
  
  supplied to the equipment or excessive power drop on a long circuit wire
• Un-balanced cooling or refrigeration systems -
  
  such as a compressor motor that is having trouble starting when the refrigerant pressures are high on one side and low on the other side of the cooling system. This hard starting condition happens when a compressor is turned off in the middle of an "on" run cycle.
  
  You can guess that you have this condition on an air conditioner if the system starts just fine when it's been shut down for a half hour or more (refrigerant pressures are equalized on both sides of the compressor) but starts with difficulty (the compressor motor "hums" and does not start quickly or may even trip the circuit breaker) when trying to restart a few minutes after having been shut off.
• Old, aging compressor motors 
  
  or other electrical motors that are at or near the end of their life may have trouble starting and may be able to function for some additional time given the "help" provided by a starting capacitor.
  
  The underlying trouble might be failing motor bearings.
  
  See HARD STARTING COMPRESSOR MOTORS and also
  
  TIGHT or SEIZED AC COMPRESSORS for more details about old or failed compressor motors.
  
  

   

• Troubles with an electric motor itself  such as used on an air conditioner air handler, condensing coil fan, or a well water pump:
  
  A loose or bad connector or a motor winding that's damaged, perhaps opening or shorting, can cause motor start trouble as can a defective motor control or switch.
  
  see ELECTRIC MOTOR OVERLOAD RESET SWITCH
  
  

Sort out the Types of Motor Capcitors

Here, to help sort out what sort of replacement capacitor you may need for a hard starting motor we review the basic capacitor types.

Separately at CAPACITOR TYPES, for MOTORS we explain electric motor capacitors, what they are and how they work, in detail.

Why would they call the capacitor a dual run if there seems to be a problem in the starter winding circut.?

A reader asked us:

My question is a simple one but i just can't find an answer anywhere. A lot of online videos show a compressor motor or fan motor not starting.

In every single video I see them changing what they say is the dual run capacitor. One part for compressor motor, one part is for the fan motor.

But why would they call the capacitor a dual run if there seems to be a problem in the starter winding circuit ?

I would think that the capacitor is a starting, not run capacitor. - On 2017-11-24 by matthew

Explanation by (mod) -

Thanks for asking, I agree that these capacitor types and names can be confusing.

Electric motors and how they run and use capacitors vary:

1. Some electric motors use no capacitors at all. We'll skip those for now.

2. Other electric motors require a capacitor to get the motor spinning (the start capacitor) and another capacitor keep the motor spinning after it's up to speed (the run capacitor).

Two Basic Types of Electric Motor Capacitors

So first off we have

Start capacitor - start the motor spinning - we need more of a kick to start up a motor that's not moving. Typical start capacitors on equipment you'll typically see are in the 70 to 120 uF or microfarads. It's giving the motor a big kick in the patuchka to get it started up.

Run capacitor - keep the motor spinning - A typical run capacitor doesn't need so much oomph as the motor is already spinning, so it'll be in the 7-9 uF range. It's giving the motor just a little extra torque force to keep the motor spinning.

Two Types of "Dual Capacitors"

The term "dual capacitor" can be confusing because there are two kinds of "dual capacitors", one dual type is distinguished by two speed ranges it controls, the other dual capacitor type is distinguished by its intended use in running two different actual motors themselves.

Dual Capacitors or Dual Function Capacitors - 2 speed ranges on one motor

Three- lead capacitors to combine "start" function and "run" function in a single device. So a dual capacitor for a fan motor will have three leads,

Common - incoming power

Start - power used just while the motor is starting up

Run - power used while the motor is running

I like to call thse "dual function" capacitors as they combine two functions: start, and run, in one physical device.

They would not work any differently than if two physically separate capacitors were installed, one for starting and one for running, but in a single device less space is needed.

Typically a speed-sensitive centrifugal switch (ELECTRIC MOTOR CENTRIFUGAL SWITCH or PTC PRD) drops the Start Capacitor out of the circuit when the motor gets close to its intended full running speed.

Dual Run Capacitors or "Dual Motor" Capacitors - 2 separate motors

Three lead capacitors to combine the "run" function for two physically separate motors in a single device.

So a dual capacitor for an air conditioning condenser unit will have three leads

Common - incoming electrical power

F - for Fan - to run the fan motor

H - for HERMetically sealed compressor motor - to run that big compressor motor

I like to call these dual motor capacitors since the same cap runs two different motors. - DF

In a companion page CAUSES of HARD STARTING ELECTRIC MOTORS FAQs you'll find a number of examples of types of electric motors having start-up problems along with our suggestions about both probable cause and best course of repair for each case.