Can the wrong capacitor burn out a motor?

[JacobP]

Good afternoon,

Earlier today, I repaired our outside A/C condenser by replacing the fan and capacitor. I noticed two things in the process: 1) the fan called for a 5uF run capacitor and the one on the unit was a 7.5uF and 2) the fan casing looked badly discolored. From this, I assume that the fan was running hot and that is why it failed early (it was only a few years old).

What I don't understand is *why* this mismatch would cause the motor to run hot. My understanding of capacitors is relatively limited, but I understand that they help store charge. But I don't understand why having more stored charge that expected would cause the fan to run hotter.

I would appreciate any help you can offer in understating this.

[MagicSmoker]

Well, I was going to tell you to read the Wikipedia entry on motor run capacitors, but it's terrible.

Put simply, capacitors not only store charge - which is static behavior - they also shift the phase of alternating current relative to the voltage (more specifically, they cause the current to lead the voltage) and exhibit a lossless form of resistance that is inversely proportional to frequency called reactance. So, too much run capacitance results in too much current through that winding of the motor - causing it to overheat - and does not provide the correct amount of phase shift which results in lower torque for a given amount of current, also causing the motor to overheat. So, doubly bad.

Too little capacitance also results in reduced torque, and can also cause the motor to overheat from forcing to work at too high of a slip (roughly speaking, AC induction motor efficiency is reduced from 100% by the slip percentage) and, of course, if the capacitance is really too low then the motor won't even start, but it will still draw current from the mains and then burn up anyway.

So best to use the right value of capacitor. And while 7.5uF might not seem too far off from 5uF on an absolute basis, it is 50% higher on a relative basis...

[Seekonk]

Yea, those people at the factory have no idea what they are doing.  So, what makes you think that capacitor had anything to do with it?  One of the capacitors may have been bad, but that difference in capacitance is not your problem.  7.5uF is perfectly fine, I see these differences quite often in AC units. Have a friend that lets me scavenge parts off these units.  A lot of caps are way under tolerance from what I've seen.  Better a little high than low.

[SeanB]

Measure the 7.5uF capacitor, it likely will be well under 3uF. The capacitor fails slowly, going lower and lower in value, simply from the self healing that occurs in the inside foils, and this gradual erosion eventually stops the unit totally.  The motor looking like it has been sitting in an oven at high temperature is simply the result of the fan being cost cutted down, the manufacturer runs a thinner wire, and this has a higher resistance, and the high temperature over long periods slowly kills the motor. They rely on the insulation being able to withstand class H temperatures ( 130C winding temperature) for at least the warranty period, and as well use fewer turns of copper ( or copper coated aluminium in the most likely case) by winding the motor for the low part of the voltage range, winding the typical compressor for EU mains for 200 V instead of the harmonised 230VAC, and as well using just enough core to handle the magnetising field with only minor heating and running it well into saturation.

Yes they fail, but most from simply being poor designs, the capacitor value going low is the most common killer, but a high capacitor will also kill the motor as well, but they run for a long time, with much higher voltages across the capacitor that self heals it faster.

[ebastler]

Great -- we have three anwers, and they are

• "The capacitor value is probably too high."
• "The capacitor value has nothing to do with the problem."
• "The capacitor value is probbly too low."

Glad you asked, Jacob, and I'm sure you can take it from there...
Sorry about the mess, and I don't pretend that I know the right answer.

[Zero999]

Great -- we have three anwers, and they are

• "The capacitor value is probably too high."
• "The capacitor value has nothing to do with the problem."
• "The capacitor value is probbly too low."

Glad you asked, Jacob, and I'm sure you can take it from there...
Sorry about the mess, and I don't pretend that I know the right answer.

All the sorts of answers you'll get when you don't provide enough information.

No make, model number, datasheet, photographs were given.

[RobertHolcombe]

Run cap value is critical, use 5-10% tolerance of the rated value. MagicSmoker explained both failure circumstances perfectly. In both cases the motor will suffer a loss of maximum torque so may run fine until heavily loaded, but will burn out prematurely regardless.

[noidea]

Run cap value is critical, use 5-10% tolerance of the rated value. MagicSmoker explained both failure circumstances perfectly. In both cases the motor will suffer a loss of maximum torque so may run fine until heavily loaded, but will burn out prematurely regardless.

+1
I used to work on some AC units where the manufacturer of the A/C unit used to increase the capacitor size (20uF) from what the motor OEM specified (15uF) to try and max the performance from an indoor blower motor on one of thee units they manufactured.
No guesses for which ones suffered the highest failure rates.....

[albert22]

Is there a way to determine the right capacitor for an unknown motor ? Perhaps measuring current at different loading or phase angle ?

[Seekonk]

I posed this question on an engineering forum with a number of motor experts.  This is the general opinion.

1. The motor nameplate is the minimum capacitance that should be used.
2. The manufacturer will use a larger value if the motor will see a higher than normal load for increased performance.
3. This will make the motor heat up a little more but it is unlikely to cause it to burn out. An open cap or bad bearings made it fail

So, reducing capacitor size to nameplate may result in increased slip and lower airflow and more heat.  When the manufacturer uses a larger cap they are pressing the design limits and motor may have shorter life statistically. Choose your poison.  If the motor is in the high airflow, that is about an ideal situation for a motor to get rid of heat.

If this has never been repaired before, that higher capacitor value means they are pushing the motor at its extreme limits. Like the slogan says,

Nothing can stop a Trane...... But an engineer.

[HighVoltage]

From my experience, if possible, you want to stay withing +/-5% of the stated capacitor value from the motor plate.
That is also why companies that repair these motors have drawers and drawers full of so many different capacitors.

For calculating the capacitor, start with around 40 ?F / kW and measure the motor performance and adjust accordingly.