Difference between a motor capacitor and a regular capacitor?

[SmallWalrus]

Hi all,

I've been looking at fixing a little desk fan that needs a little push before its blades can start spinning, and something I have been pondering about is if a motor start capacitor is the same as a regular non polar capacitor.

What will happen if I simply put in a regular capacitor that's rated to the same voltage and capacitance? (Not going to try this myself without hearing from someone more knowledgeable than me first!)

[electroguy]

i've wondered the same thing myself! they are a lot bigger that's for sure! lets see what the experts say.

[Mint.]

Im interested, bumping.

[Zero999]

It should be fine as long as the new capacitor has sufficient current rating.

[SmallWalrus]

It should be fine as long as the new capacitor has sufficient current rating.

So any old el cheapo ceramic capacitor will do?

[Randall W. Lott]

It may be polypropylene.  This specific type can handle pulses much better than other types of capacitors.

Certain ratings must be held before a capacitors can be placed reliably across mains.  These types can extinguish themselves, rather than explode when they get overloaded.

It's very important for safety.

Courtesy of Wikipedia:

• Live to neutral capacitors are flame retardant, and in Europe are required to use use class X dielectrics
• Live or neutral to earth: The requirements for L to N use apply, plus the dielectric must be self healing and fusible. In Europe these are class Y capacitors.

[T4P]

MKP is suitable . For Class X2 , its MKP
For Class Y2 , it can be MKP or Ceramic ( Blue ones )

[ciccio]

Motor starting capacitors are not the same as the ones usually wired live to neutral or  live to ground  - neutral to ground (these are of lower capacitance and their function is the reduction of conducted noise).

Motor starting capacitors (or motor run capacitors)  are used to create a second phase, rotated of some angle in respect to live, to start the motor in the correct direction. The motor has two windings, one between live and neutral and the other between the capacitor and neutral, with the other terminal of the capacitor wired to live.
A failure of the capacitor will not result in a short circuit between live and neutral (the winding is still in series), so (to my knowledge) a standard film capacitor, with suitable voltage rating, may be used, and more expensive  self-healing type (X or Y) can be avoided.
Best regards

[G7PSK]

Many fans and other small motors do not have a run capacitor they have a shunt winding around the laminated field which often consist of a piece of copper wire around 2.5mm dia welded into a loop the induced current in this forms a field that is at 90 deg to the main field that is also all the capacitor does but that requires a separate winding. The capacitor will also be low esr and of substantially heavier build as it has to withstand the forces imposed on the plates, mechanical failure is the most common thing on these you can often hear them vibrating shortly before failing. I have tried ordinary capacitors in the past but they quickly fail on bigger motors as soon as they are turned on.   

[Randall W. Lott]

If my statement wasn't completely accurate, I apologize as I stand corrected.  The bottom line is to make sure you find a safe replacement.

[ron]

I don't recall ever seeing a desk fan motor with a start capacitor. and the original poster didn't actually say that his motor had a cap that he wanted to replace.  The way I read it he is thinking about adding a cap.

If that is the case, cleaning and lubing the bearings (or bushings) might be the solution. 

[vk6zgo]

If I remember correctly,fans use an induction motor,without any capacitor.It looks the same as the one on extractor fans,& old style Gramophone/phonograph turntable motors.
The usual reason for these sticking is binding bearings---you may be able to lubricate them,or not,depending on the construction.
Some very large,& old, floor fans may use Universal motors.

[IanB]

If I remember correctly,fans use an induction motor,without any capacitor.It looks the same as the one on extractor fans,& old style Gramophone/phonograph turntable motors.

The motors without capacitors are "shaded pole" induction motors. These are usually practical only in small sizes, so probably found only in smaller fans.  I remember finding them in fans I have dismantled in the past. Big fans would use a more powerful kind of motor.

[vk6zgo]

If I remember correctly,fans use an induction motor,without any capacitor.It looks the same as the one on extractor fans,& old style Gramophone/phonograph turntable motors.

The motors without capacitors are "shaded pole" induction motors. These are usually practical only in small sizes, so probably found only in smaller fans.  I remember finding them in fans I have dismantled in the past. Big fans would use a more powerful kind of motor.

The OP was talking about a "small" fan!
I have seen larger ones with a commutator ("universal")motor,
Now that I think of it,though,I seem to remember still larger fans with capacitor start.

[IanB]

The OP was talking about a "small" fan!
I have seen larger ones with a commutator ("universal")motor,
Now that I think of it,though,I seem to remember still larger fans with capacitor start.

Maybe age comes into it too. I think you would likely find quite different motors in fans and other devices from decades ago compared to what you might find today. I'm thinking of a desk fan from the 60's with a solid cast iron base and a proper durable motor with metal fan blades. Not like the cheap plastic crap you find today...

[vk6zgo]

I think you probably have to go back further than that!I remember seeing fans from the 1930s still in everyday use in the late 1960s.

They already knew how to make crap in the 1960s!

[T4P]

Maybe age comes into it too. I think you would likely find quite different motors in fans and other devices from decades ago compared to what you might find today. I'm thinking of a desk fan from the 60's with a solid cast iron base and a proper durable motor with metal fan blades. Not like the cheap plastic crap you find today...

Plastic ... You don't say ? Mine's made of plastic but to be fair it is not the cheap ones , it's the 100$ push switch types.
Expensive i know, but well worth the money since it's still made in Japan .

[SeanB]

There will be a black cased rectangular capacitor in there. It will have either 2 or 3 leads, and will be rated from 1.2 to 2uF, if 3 leads there will be 2 sections, one as before and another of 0.5uF. Voltage rating is 440VAC. You can get one out of an old ceiling fan ( one scrapped for broken blades, not because it will not turn) or buy a new one from appliance repair suppliers. I pay under $1 for them, and they do fail often. To test the old one disconnect the leads and check capacitance. If lower than 10% from rated then replace, and lubricate the bushes as well, with a drop of engine oil on each bush.

I have a lovely ceiling fan going to recycling soon, shorted windings to frame. Only noted when it stopped turning, and was touched ( no earth to light rose).

[Hex173t]

I pay under $1 for them, and they do fail often.

Ugh! I paid $28 for mine!  My shopping skills stink.

[SeanB]

Small motor caps are cheap. I pay around $10  for a 50uF 440VAC motor capacitor. Sometimes I have to buy smaller motor capacitors that cost almost the same. Most expensive are motor start units, I often will put 2 motor run units in place of them, as they will definitely last longer. Space permitting though, the start caps are very small, around 1/4 the volume of the same motor run unit.