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Posted By: zed Capacitor in motors - 06/28/05 12:49 AM
Hello all, this is my first post in this type of communication.
Can anyone explain how a capacitor provides a "second phase" in an electric motor.
I have also had a small electric motor that was low on power until we replaced the cap that was wired to it. How did the cap provide this extra power?
Thanks for any answers.
Cheers Zed from NZ
Posted By: Trumpy Re: Capacitor in motors - 06/28/05 06:36 AM
Gidday there Zed,
Welcome to ECN, mate!. [Linked Image]
A capacitor-start motor, doesn't actually create a "second phase".
What happens, is the capacitor causes the usually lagging current of the start winding, to be shifted by 30 degrees, to bring it more into line with unity power factor.
Upshot of this, is that the start winding provides more useful power (having a higher power factor), through a lower current draw, during the starting phase.
Basically the capacitor takes the inductive component out of the windings of the motor.
Hope this helps.
If not, feel free to ask any other questions you may have.
Posted By: Scott35 Re: Capacitor in motors - 06/28/05 06:38 AM

Welcome to ECN!!! (the Electrical-Contractor Network's forums).
Thanks for posting your questions, and I will try to answer them for you.


Can anyone explain how a capacitor provides a "second phase" in an electric motor.

Actually, the Capacitor "Assists" the "Auxiliary Winding" (AKA "Start Winding"), to achieve better starting characteristics on Single Phase "Split Phase" Induction Motors.

A typical 1Ø "Squirrel Cage Rotor" type Induction Motor will have a "Main" or "Run" Winding - and an "Auxiliary" or "Start" Winding.
The Start Winding is connected to the Motor's Circuitry via a Normally Closed Centrifugal Switch. This is known as the "Start Switch".
The Run Winding (or Windings), however, are not connected to any centrifugal switch, and remain connected throughout the entire operation of the Motor.

When the Motor's speed reaches aprox. 80% of the rated full speed, the Start Switch opens, and the Aux. Winding is no longer used. The Motor is now using only the Main / Run Winding for normal operation.
If the speed drops down below that 80%, the Start Winding will become active once again - until the speed reaches the 80% mark, when at this point, the Start Switch disengages - opening the Circuit to the Start Winding once again.

The reason for a Single Phase Induction Motor to include a Starting type Winding, is due to the "Non-Rotating" Magnetic Field of a 1Ø Circuit.
The 1Ø Field is stationary in space, but alternating in time.
Another way to view this is the sum of two equally and oppositely revolving Field Phasors.

Actually, the best description would be a "Semi-Visual/Mostly Imagination" scenario:
The Starting of the Motor would be similar to placing a 10 foot board on a fulcrum point at exactly 5 feet from either end, and placing exactly 10 Pounds of weight on each end.
The board just sits there stationary in space, because it is balanced at both ends.

To get the board to move, one side's weight must be reduced somehow.

If we remove 5 Pounds of weight from one end, the board will "fall" towards the end with the 10 Pounds of weight.
Now we have movement.

Take this analogy a bit farther, and imagine the fulcrum point is an Axle, which is 6 feet above the ground, and the ends of the board are 5 feet from the axle point (plus the weights are attached to the board's ends).
With 10 Pounds at each end, the board is stationary (doesn't move), because it is in a balanced state.

To get the board moving, we create an imbalance of weight by removing 1 or 2 Pounds from one end.
Now the board will begin to rotate towards the heavier side (or opposite from the reduced side).
The weight represents the magnetic field's intensity.

That is how the 1Ø Induction Motor is Started. The Start Winding "Reduces" the Magnetic Field of "One Side" of the Run Winding, which allows the Rotor to begin spinning.

Typically, the Motor is known as "Split Phase" - which refers to the Start Winding as an "Offset Phase" - split from the Main Winding.
The "Resistance-to-Reactance Percentage" between the Start and Run Windings is quite different.
The Start Winding is designed to have a much lower value, as opposed to the Run Winding - which creates a Phase Lead/Lag between the two Windings, and ultimately results in a reduction in the Field on one end of the Run Winding.

Motor Types - 1Ø Induction - Descriptions

A Motor with just the Start Winding used, is known as a "Split Phase Resistance Start" type Motor, and is adequate for large Fan type applications.

When more Starting Torque is needed, the addition of a Capacitor - in series with the Start Winding - is done. This gives a greater Phase offset, plus improved Starting Torque.
This is known as a "Split Phase Capacitor Start" type Motor, which typically gets used for Pump applications (like Air Compressors).

There are two variations of the Capacitor Motor described above.

One is the "Split Phase Capacitor Start-Run" type Motor, which uses a large value "Start" Capacitor to start the motor (up to the 80% speed point), and once that point is reached, a low value "Run" Capacitor is switched in to the Circuit - while the large value "Start" Capacitor is switched out of the Circuit.
The Aux. Winding is used throughout the Motor's operation - via one of the Capacitors.

The other type is known as the "Permanent Split-Capacitor" type Motor ("PSC" for short).
This Motor has the Aux. Winding connected 100% of the Motor's operation, via a very low value Capacitor. There is no Start Switch on these Motors.
Since these Motors have a low starting torque, they are commonly used on Ceiling Fans and Oscillating Fans.

Another type of Split Phase Induction Motor is a "Shaded Pole" type.
This Motor has no physically connected Aux. Winding - nor does it use a Capacitor or any switches.
The "Shaded Pole" is a short-circuited winding, which is wound "at an angle" to the main winding.
It performs the tasks of Starting by reducing the Field at a fixed end of the run winding.

BTW, Polyphase Motors - such as 2 Phase and 3 Phase types, are "Self Starting", and therefore do not require an auxiliary starting means.



I have also had a small electric motor that was low on power until we replaced the cap that was wired to it. How did the cap provide this extra power?

I can only assume this assisted with Starting Torque, or the Motor was either a PSC or "Capacitor Start/Run" type.

Here's a few Schematics of 1Ø Motors.
More may be found at the Technical Reference area under these topics (click on the underlined blue text to go to that page):

Schematics for 1Ø Motors - Series 1

Schematics for 1Ø Motors - Series 2

Schematics for 1Ø Motors - Series 3

Schematics for 1Ø Motors - Series 4

[Linked Image]

Image 1: Split Phase Resistance Start Motor.

[Linked Image]

Image 2: Split Phase Capacitor Start Motor.

[Linked Image]

Image 3: Permanent Split Phase Motor.

Good luck on the Motor studies!

Let us know if you have more questions!


1: Found several spelling errors, and hope all were fixed!
2: Tried to clear up a few statements, and add more details to a few others.
3: Per the Schematics above, the numbers refer to notes found with the original posted items. See the Technical Reference area for details.


Mike (Trumpy):
I didn't know you had replied to this topic, until I submitted this one! (2 minutes apart!)
Talk about being on the same Hz!!!

[This message has been edited by Scott35 (edited 07-02-2005).]
Posted By: Trumpy Re: Capacitor in motors - 06/28/05 06:40 AM
Didn't see that post by Scott35!. [Linked Image]
Posted By: zed Re: Capacitor in motors - 06/29/05 07:34 PM
Thanks guys, I am looking into the information that you have provided.
Scott is you current email address ?
Posted By: Larry Fine Re: Capacitor in motors - 07/02/05 03:35 AM
If I may, a series cap actually produces a "phase shift" because the current leads the main current, rather than "lagging" it. Here's why:

If you picture a voltage sine wave across a load, the current is in phase with the voltage, because the current is at its peak during the voltage peak (during little change).

However, a capacitor in series with a load charges (and discharges) at the highest rate during the time the voltage sine wave is changineg (alternating from - to + and + to -).

Therefore, the current in a capacitive circuit peaks ahead of the voltage (sometimes by a full 90 deg.), and a two-phase magnetic field is "synthesized" by positioning the start-winding poles between the run-winding poles.
Posted By: Scott35 Re: Capacitor in motors - 07/09/05 08:19 PM

Thanks for posting the additional information, regarding this subject! [Linked Image] [Linked Image]

As to Zed's message regarding the E-mail address of "S.E.T. Electric", please refer to the message dated 07-09-2005 @ 3:52 PM at:

Reply Message # 4

Reply Message 4

Let's see if this works!

Posted By: Deang Re: Capacitor in motors - 09/13/05 12:45 PM
Hi guys,

I stumbled onto this site while researching the use of motor run capacitors in loudspeaker filters (crossovers). Yeah, I'm a long way from home.

I guess my question would be this: Is the harmonic information found in audio signals, and passing through a filter comprised of motor run capacitors -- harmful to these capacitors?
Posted By: Larry Fine Re: Capacitor in motors - 09/13/05 03:39 PM
There should be no such issue using motor caps for audio (which I know quite a bit about also). You might also check with Parts Express for audio components, including caps and inductors.
Posted By: Scott35 Re: Capacitor in motors - 09/21/05 10:16 PM
As Larry said, there should not be any trouble using a Motor Start / Run Capacitor for High Pass Filter components, in Audio System Passive Crossover Networks... at least Harmonic Damage,...


These Capacitors might not sound very good - especially if they are the ones in Series with Tweeters; in which case using something in the Mylar or Polypropylene (sp???) flavors is better sounding.

The Standard Electrolytics, such as a Motor type Capacitor, might not have an effect on the sound, when placed in Parallel to the Loudspeaker ("drains" out a certain upper frequency ones), which is likely where you plan to use these types anyway - since they come in larger sizes per individual device, the need to Parallel 2 or more devices is eliminated.
(last part added to cover reasoning for "oddball" Capacitor usage).


p.s. we have some Schematics of Passive Crossover Networks in the Technical Reference area.
You will find it "between" this forum section and the Computers/Internet forum section. Just click the link for the "Menu", and search near the end of the list.
Posted By: SteveFehr Re: Capacitor in motors - 12/04/05 03:15 PM
Small DC motors use caps to decrease EMI and AC harmonics; the capacitor is placed across the DC terminals so that any AC current caused from arcing at the commutators is shorted through the caps. It's especially common in R/C, as the EMI can seriously mess with the radio control. I'm sure anything trying to pass the FCC would have to have these caps, too.

[This message has been edited by SteveFehr (edited 12-04-2005).]
Posted By: JoeTestingEngr Re: Capacitor in motors - 12/04/05 04:12 PM

I stumbled onto this site while researching the use of motor run capacitors in loudspeaker filters (crossovers). Yeah, I'm a long way from home.

I guess my question would be this: Is the harmonic information found in audio signals, and passing through a filter comprised of motor run capacitors -- harmful to these capacitors?

I would just like to add that the cap is just a cap when you think about it starting a motor. It has alot more going for it (or against it) that we need to think about if we use it in another application. That cap has equiv. series resistance and inductance that lump in with its more dominant capacitance. They might not come into play within the design tolerances of your network. There will be a frequency though, where that cap will be a tuned cicuit, with its own L & C, and Q factor determined by its own series resistance. This point is likely above the audio band.
Temperature stability might even be an issue with automotive applications. You wouldn't want your midrange center f or 3dB badwidth shifting on you from winter to summer would you?
I'm not saying that these things would definately cause problems, just that I wouldn't rule out the possibility.

[This message has been edited by JoeTestingEngr (edited 12-04-2005).]
Posted By: WFO Re: Capacitor in motors - 12/05/05 12:17 AM
"Can anyone explain how a capacitor provides a "second phase" in an electric motor."

Everyone knows that motors run on smoke. When the smoke escapes from the motor, it will no longer run.
Capacitors are also called condensors because they condense the smoke into the motor, giving it more torque (or oomph to use more technical terms).

Scott35's explanation is good too. [Linked Image]
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