Can washing machine start capacitor test good - and still be bad?

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About a year ago the motor in my kenmore washing machine was acting up - buzzing but not turning. I suspected the start capacitor and picked up a used one for $10 at a local repair shop. These are black, about the size of a D-cell battery.
The original was 270 - 324 mfd, 110 vac. Model 3348058.
The one I got as a replacement was 189 - 227 mfd, 357021.
Even though I had a capacitance meter at work, I just went out and picked up the used cap, installed it and bingo - the washing machine works. That was a year ago.
So more recently, over the past week or two, the motor was acting up again. It would buzz, I'd bump the machine, and it would start. But yesterday even bumping / rocking the machine didn't work.
So I took the capacitor out and measured the original and this replacement. The original measured 298 mdf (micro-farad) and the replacement was 211. Both numbers pretty much exactly what they should be according to their labels.
So maybe the problem wasn't / isin't the capacitor - but a centrifugal contact in the motor? Do these motors have such contacts (start winding) ?
Or can hand-held digital capacitance meters perform correct measurements on these big caps?
I'm cross-posting this to alt.hvac, because my question about this washing machine motor capacitor and how well or accurately a capacitance meter can measure them if they're bad might also apply to compressor or fan motors.
As far as I can tell, there is just this one capacitor connected to this motor, so I don't think there are separate start and run capacitors.
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Home Guy wrote:

Rule of thumb, you don't replace cap. with lesser value. If same one is not available, use one with larger value. Even tho meter shows it OK, under voltage in circuit it can go bad.
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Tony Hwang wrote:

But why could I get the motor to start by bumping the machine?
Are there centrifugal contacts in the motor?
Or was the bump enough to get the spindle turning?
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Home Guy wrote:

You're doing the job of the cap.(shifting(leading) current phase) Motor winding is inductive, with cap. you counter the inductance. That cap. is called PSC(Phase shift capacitor). Most new appliance, even garage door opener, vacuum cleaner, furnace blower, nowadays use smaller more powerful DC motors. DC motor's efficiency is higher than AC motor using PSC. Also Diesel electric locomotive propulsion is by DC motor. Diesel engine drives generator, which in turn it drives DC motor. That is why it is called Diesel electric locomotive.
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On 3/3/2014 5:42 PM, Home Guy wrote:

Probably that. Glad you got it running, though.
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I would suspect the starting motor contacts equally likely with the startin g capacitor. The mfd value of the replacement capacitor you bought a year ago is apppreciably less than the mfd of the original capacitor, and that w ould make the starting torque less than the original design. Then, if thin gs get marginal/sticky, the motor might not be able to have enough torques to start. Shaking the machine could provide just enough oompf to get thing s moving. I would buy the correct value capacitor and try that. At $15 bu cks or so, it is much easier than to take things apart to get to the starti ng contacts on the motor.
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snipped-for-privacy@sbcglobal.net wrote:

I was surprised to hear OP bought used cap. I'd never do that.
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Tony Hwang wrote:

Locally, the price of a new cap is around $60. There's no internet source that after customs/brokerage charges I'm going to get this new for under $30.
So here's what I've done so far:
Connect both caps in parallel and the meter reads 500 mfd - which is expected. The original was 300 and the replacement was 200.
Connect both up to the motor - and the machine works (drum turns) when machine is set to spin cycle. Motor hums (drum doesn't turn) when machine is set to agitate.
Remove the 200 mfd cap and just connect the original 300 mfd - and get same result. Spin works, agitate hums.
Swap the caps so that only the 200 mfd is connected, and motor hums with spin and agitate.
This is with an empty drum. The problem arose while the drum was full of water, about to start a spin cycle (with the 200 mfd cap).
So are these caps so weak that even combined they don't have the energy to turn an empty drum set to agitate?
Or is agitate using different windings of the motor, and something else is going on?
micky wrote:

There's no belt in this washing machine. It's direct-drive.
Maybe you're thinking of a dryer.
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On 03/03/2014 07:10 PM, Home Guy wrote:

That price is way out of line ...don't buy it.
Test your capacitor by wiring it in series with a light bulb (100 watt incandescent)
If it lights, the capacitor is ok
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philo wrote:

Ok, I didn't do that (but would have if the following didn't work).
Turns out that you have to have water in the drum for the agitate to work.
To make a long story short, I had to hook up both capacitors in parallel to get the washer to function for all settings and cycles and get no motor buzzing or stalling.
These are complicated motors with lots of wires, and I thought I was going to have to dick around with it as in this thread:
http://www.applianceblog.com/mainforums/threads/ 35555-Top-Load-Kenmore-WON-T-Agitate-but-WILL-spin-and-pump-Motor-hums-when-it-should-agitate
So I'm using short wires with alligator clips to connect the capacitors. Tommorrow I'm going to solder them together and finish the repair.
I'm glad I didn't throw away the original capacitor, and extra glad I was able to find it...
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On 03/03/2014 08:58 PM, Home Guy wrote:

Hope it stays working.
I have seen capacitors lose value too, but again that is very rare.
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Home Guy:
If bumping the washer caused it to work properly, I expect your capacitors are OK, but the connections are dirty. I don't believe you have a slipping belt. If you did, your laundry room would be full of smoke and you'd smell burning rubber.
I expect you can find the correct capacitor online for under $10. You may have to import it from the States, but that's no big deal, I import stuff all the time and I can tell you how to fill out the Canadian Border Services B3 form, and the procedure to follow to get it across the border. However, keep in mind that even a $10 item will cost $10 to 20 in shipping costs.
The easiest way to understand how a start capacitor works is to understand that there's no such thing as capacitor start or split phase electric motors that use 3 phase electric power. With three phase electric power you simply arrange the poles around the stator 120 degrees apart and you create a rotating magnetic field.
With 120 volt single phase power, you can normally only create an oscillating magnetic field. To create a rotating magnetic field, you have to use some kind of trick to create a rotating magnetic field for the rotor to follow.
The way this is done with a capacitor start motor is to have two identical windings wired in parallel and arranged at a 90 degree spacing around the stator. So you have North pole of winding 1, South pole of Winding 2, South pole of Winding 1 and North pole of winding 2 all spaced 90 degrees from each other around the stator.
Now, we simply wire a capacitor in series with ONE of those windings.
With a resistor, the current is highest when the AC voltage sine wave is highest, and the current is instantaneously zero when the applied AC voltage sine wave is zero.
With a capacitor in series with the start winding, things are totally different. With a capacitor, the current out of the capacitor (and hence through the start winding) is highest when the RATE OF CHANGE IN VOLTAGE of the applied voltage sine wave is highest, and that occurs on the applied sine wave when the instantaneous voltage is ZERO volts. That is when the voltage is changing from a positive voltage to a negative voltage and vice versa. That's 90 degrees out of phase with the winding that doesn't have a capacitor.
Similarily, with a capacitor in the start winding, the current out of the capacitor (and hence through the start winding) is at a minimum when the applied voltage sine wave is at a maximum or a minimum. That is when one plate of the capacitor is fully energized and the voltage on it is starting to drop, the instantaneous current out of the capacitor will be zero. That's 90 degrees out of phase with the winding that doesn't have a capacitor.
Consequently with the start and run winding wired in parallel, the capacitor in the start winding will have it's current sine wave 90 degrees out of phase with the run winding, and as a result, the magnetic field of the start winding occurs 90 degrees sooner or later than the run winding. And, that's true even though both windings have the same voltage sine wave applied to them.
So, the capacitance of the capacitor you use is important. You can just take one from one motor and use it in another.
There is also another kind of induction motor that doesn't use a capacitor. Instead, in a "split phase" motor, winding #1 will be made with a small number of coils of thick copper wire, and winding #2 will be made with a large number of coils of thin copper wire. Because of the difference in resistance of these coils, they have different resulting impedances, and that results in one coil developing it's magnetic field earlier than the other, and that's what creates the illusion of a rotating magnetic field for the rotor to follow.
In both capacitor start motors and split phase motors, the motor is perfectly happy to turn in the reverse direction if you simply reverse the polarity of one of the windings. So, if you switch the wires going to Winding #1, the motor will start with the same torque, and it'll reach the same operating speed, only it'll be turning backwards. Never reverse the polarity of both windings in an induction motor. That will cause a singularity in the space time continuum with the result that you finish doing the switch before you began, and hence the motor turns in it's original direction again.
Maytag washing machines have a reversing relay that automatically switches the wires going to the run winding. That relay is controlled by the timer, and it reverses the terminals to the run winding so that the motor spins in one direction during the wash cycle (where the agitator oscillates) and in the reverse direction during the spin cycles (where the wash basket spins). Most washing machines use this reversible characteristic of induction motors to reverse the direction of rotation of the motor from the wash cycle to the spin cycle. In Maytag's case the motor turns a belt that causes a pulley to rotate up or down a helical shaft. When the pully rides up the shaft it engages clutches in the transmission that lock the transmission up so that the spinning of the pully directly results in the spinning of the wash basket. And, when the motor reverses direction, that pulley turns the opposite way and rides down the helical shaft to unlock the transmission and allow the rotation of the pulley to cause the oscillation of the agitator.
In a case like this, I think your best bet is to got to a factory authorized Kenmore repair depot to buy your parts. It's true that you'll pay more for each part, but you also get all the expert technical support tossed in free of charge. So, you spend a bit more on parts, but you save all the labour by doing it yourself under the tuteledge of a Kenmore appliance repairman.
--
nestork


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On Tue, 4 Mar 2014 03:58:52 +0100, nestork

That makes a lot of sense, but somehow I never had any smoke or any burning rubber smell. For a while, when it was time for the spin cycle, I guess I would hear a noise and I would go there and push the basket with my hand to get it started. Either that or I'd just go check on the machine and find it stuck not yet having spun. I think it often started the 2nd spin cycle on its own.
When that was getting less reliable, I tried to tighten the belt but did a bad job. The basket still woulnd't start on its own, but I was able to push start it again.
A few loads later, I got a jack handle from on old GM car and used that as a better lever to tighten the belt and it's been fine for about 10 or more loads now.
I don't understand it but one can't always assume theory is reality -- well maybe it somehow went quickly** from working well to having a belt so loose it didn't rub, but still grabbed on when I pushed it!! --
and it will only take a minute or two to see if the motor really is not turning.
Mine is a Sears Whirlpool fwiw. It always agitated properly and did everything but spin. I was kidding about tightening it again in 30 years. I think some day I will need a new belt.
**I've been away for a couple months at a time. Maybe it loosened then.
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No, I was thinking of a washer. You didn't say before that it was direct drive.
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On 03/03/2014 03:28 PM, Home Guy wrote:

Motor start capacitors are internally fused ...occasionally the link can break but the contacts still touch and will measure OK...but not work under load.
I worked on industrial equipment for 38 years that had such capacitors in them...but it's a condition I've only seen a few times.
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Are you sure it wasn't running all the time, but the belt was slipping. That can sound like a hum too iirc.
Before I tightened it, I would give the edge of the drum a shove and that would start it on the spin cycle. (I think it's a sub-cycle really. Why is it called a cycle?)
On my Whirlpool I have to tighten the belt every 30 years, but some belts might loosen more quickly.

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On Monday, March 3, 2014 3:28:49 PM UTC-6, Home Guy wrote: Does the cycle difference spin vs wash)depend on the direction of motor rotation? It's been 20+ years since I worked on a Kenmore washing machine.
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Ok - So it looks like using the two capacitors in parallel is increasing the starting torque of the motor and all is well(for now). But something is/was/has changed and I wouldn't feel too comfortable.
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snipped-for-privacy@sbcglobal.net wrote:

Paralleling them creates a cap. value of sum of two.
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On 3/3/2014 11:56 PM, Tony Hwang wrote:

Lets see if any math nerds out there: The hippopotomus cap is the sum of the farads of the other two hides.
Very obscure and dificult to follow hints at a very old math joke.
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