Motor Problem

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techie2
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Motor Problem

Post by techie2 » Tue Nov 01, 2005 7:25 pm

I am not a motor expert. Hoping that someone who visits may work on motors enough to know an answer to this question..

I am helping a friend who has a problem with his front loading washer. I believe an ASCO Model 8005. The motor does not run at all when connected to the washer.

Here is the info on the motor nameplate:
ASKO Art. no 80 582 64
MCA 61/64 - 148/AC
14400 RPM 2.2A 540W
208/240 V 60 HZ

The brushes are nearly worn out, we know. I metered the field coil and read .5 ohms. Measured the armature (thru the brushes)and read 2.5 ohms. The motor has a tachometer, or so it appears. All wires go to a connector..

My question is this: Is .5 ohms too low for the field coil? Also (without a schematic handy) I wonder if perhaps this motor is supposed to operate with the field coil in series with the armature? When bench testing it in shunt (using a heavy variac) the current soars after about 30VAC and I wouldn't think of running it at 220V. Any comments will be appreciated.

PS: If you tell me to take this post to an appliance forum, please give me any good ones you've found.. Thanks...

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Chris Smith
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Re: Motor Problem

Post by Chris Smith » Tue Nov 01, 2005 7:35 pm

Test all your field windings to GROUND, for a short.

Same goes for your armature windins.

Also check for interconectivity between the two, there should be none, especially when all the leads are disconnected, and tested.

Your ohm readings are not out of the ball park, for 60hz.

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techie2
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Re: Motor Problem

Post by techie2 » Tue Nov 01, 2005 7:41 pm

Thanks Chris.

I did check between armature and field terminals, but did not think to check to the motor case.

Do you think that field should be in series with the armature? In shunt (parallel) the current goes so high that it pops a 15 amp fuse on my test panel..

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Chris Smith
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Re: Motor Problem

Post by Chris Smith » Tue Nov 01, 2005 7:57 pm

Brushes?
Series windings?
Most likely not?
Its hard to guess, you should be able to ohm it and find out the wire connections.

Id check the "to ground" first.

15 amps is a short.

Either your windings are shorting directly to ground, or they are shorting to each other, making their path very short, and non "inductive" as in the magnetic field/ impedance etc,.... just a dead short.

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techie2
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Re: Motor Problem

Post by techie2 » Tue Nov 01, 2005 8:38 pm

Well, I can't check it tonight. I took it back to him. I told him I do think it is defective.

You mention "ground" a lot. When I tested this motor on the bench I did not have the case grounded. (shame on me!) But I do have a GFCI on the line to all of my outlets on my workbench.

So, I am just "fishing".. I really thought that .5 ohms on that field windings seemed low. I realize that the impedance to 60 HZ will be much higher. I agree with you. A motor rated for 2.2 amps should never soar to 15-20 amps.. unless there is a short between windings or whatever..

A replacement for it is $180, so I wanted to be really sure I was correct in my testing methods.

It will run with power applied to only the armature.

It seems to be the field windings that are in trouble.

I guess I will tell him to go ahead and order a replacement motor.


Thanks

rshayes
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Re: Motor Problem

Post by rshayes » Wed Nov 02, 2005 12:44 am

That is a rather unusual motor. First, note the RPM rating. Most AC motors operate at or near synchronous speed or a sub-multiple of it. For 60 Hz, synchronous speed is 3600 RPM. Most AC motors work in the 1750 (induction) to 1800 (synchronous) speed range. These depend on generating a rotating magnetic field inside the motor by using two field windings with a phase shifting capacitor. The maximum speed that the field can rotate is 3600 RPM.

Also, AC motors do not usually have brushes.

This motor is rated at 14400 RPM, considerably faster than the synchronous speed.

This sounds more like a "universal" motor. This is basically a DC motor (with brushes and a commutator), usually with a series field. It will also operate on AC, since the field current reverses at the same time as the armature current. This causes the shaft to rotate in the same direction for both half cycles of the AC power.

The universal motor is not limited by the frequency of the AC line, and speeds in the 10,000 RPM range are not uncommon. Early models of the Dremel motor tools used universal motors instead of permanent magnet motors, and ran at speeds in the 10,000 RPM range.

Most universal motors were relatively small. This one is unusual in that it is somewhere around 1/2 horsepower and is operating on 220 volts.

If it is a universal motor, then the field would be connected in series with the armature. The line voltage would be divided between the field and the armature when the motor is operating properly.

Try replacing the brushes, since these are usually the first thing to fail. Check it on the Variac with a series field connection. The current will probably be much lower once the motor starts running and the speed will tend to go up under no load conditions.

Remember to tie the motor down before testing it. Don't just hold it in your hand.

<small>[ November 02, 2005, 08:05 AM: Message edited by: stephen ]</small>

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jwax
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Re: Motor Problem

Post by jwax » Wed Nov 02, 2005 3:22 am

At $180, I'd get a quote from a few local motor rewinders- a complete rebuild should be way less than that!

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Externet
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Re: Motor Problem

Post by Externet » Wed Nov 02, 2005 7:59 am

Does the shaft turn by hand?
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Will
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Re: Motor Problem

Post by Will » Wed Nov 02, 2005 10:14 am

0.5 ohms is too low for the field coil - That infers a current of 240 amps at 120 volts. The field current is always steady at the initial stable current because back emf does not affect the field current. That would make it seems likely that your field coils are shorted or grounded. If you say that the current was 15 A when you switched it on then you need to determine whether that current was in the field or in the armature. Zero field current would cause excessive current in the armature because no back emf (The generating effect of the armature coils operating in an electric field) I think it unlikely that this was a series wound motor because a washing machine implies a fairly stable load current (That might cause an argument) Series wound motors are usually for highly variable torque loads such as portable drilling machines i.e. when increased load slows down the machine, back emf decreases hence armature current increases (Which increases torque). Since some or all of the armature current is routed through the field coil which also increases the torque - The net effect being that the torque increases per the square of the armature current.
BB

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techie2
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Re: Motor Problem

Post by techie2 » Wed Nov 02, 2005 1:49 pm

OK.. Thought you guys would want to know the final outcome on this one..

Stephen had some really good points, and had or came closest to having the right answer.

1. I stated that the motor spins, with just armature current.

2. I stated that the brushes are worn and in need of replacement.

3. In testing I should have tied the armature & field coil in series.. (14400 RPM -- must be used as a "universal motor"....

4. Instead I had tried tying the field coils in parallel.. WRONG! Too much current, blows fuse.

5. Cleaned the communtators with very find emory paper...

6. The guy put it back on his washer, just of a quick test.. It ran. But he noticed right away a big unballance in the machine on spin cycle, and realized that some other parts are bad.. We strongly believe that the reason the motor was failed was due to bad brushes, and the vibration probably jarred them enough that they were not making good contact with the communtator bars...

So he only needs to spend $40.00 for brushes. (They come in the holder, can't seperate.. Great for DIY but hard on the pocket book.)

And he needs some rubber baby buggy bumpers or whatever to keep the drum assembly from shaking..

I think I saved him $140.... :)

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Re: Motor Problem

Post by Dean Huster » Wed Nov 02, 2005 6:33 pm

For a lot of us, we're confounded that this is not the typical 2-speed induction motor. A similar thread appeared on another forum in the UK and they seem to have a lot of universal motors in their washers (note the original 208/240v nameplate specification in the OP) and I was not used to that. With all the electronics going into common household appliances these days, using a universal motor would be a good choice for some variable speed applications such as this, but as users, we need to be aware of this change and the fact that brushes are now going to be a maintenance item that we've not had to deal with before. We always blew that off on the $25 Sears blender and just bought a new one. Not so with a $400 - $1000 washer!

Dean
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Robert Reed
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Re: Motor Problem

Post by Robert Reed » Wed Nov 02, 2005 9:22 pm

Tech
You definately were on the right track putting the feild windings back in series connections. The big key here is you mentioned a tach device. These are generaly used with series motors to control speed via triac control. It is common in europe to use this system being as how they operate from 230VAC and seem to be more enegy conscios than we Americans. Your stated speed (14.4 K rpm) is pretty standard for this setup, with the electronics controlling the speed down to whatever.Universal motors are becoming very common in the household and all of the beefier woodworking routers today have 3 1/4 HP motors in them that draw 15 amps and more, and run at speeds of 22,000 rpm or higher. As far as winding resistance checks--forget it, as 90% of the ohmeters won't give an accurate check. Lets just say their should be some resistance usually less than 1 ohm. A spinning motor presents a much higher impedance to line voltage than a simple DC ohmeter will indicate.Generally shorts (other than to ground) will only be indicated on a motor shops "growler" You can probably thank your home breaker panel for tripping and saving your motor from testing it in paralel.

<small>[ November 02, 2005, 09:24 PM: Message edited by: ROBERT REED ]</small>

rshayes
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Re: Motor Problem

Post by rshayes » Wed Nov 02, 2005 9:22 pm

The ASKO Model 8005 appears to be made in Sweden. A universal motor might be a very reasonable choice in Europe, since it would operate nearly as well on 50 Hz power as it would on 60 Hz power. If the speed was controlled by feedback, rather than by the line frequency, it might also be possible to design a unit that would work with 110 and 220 volt lines. The controller might be as simple as a phase controlled SCR. That might also explain a 220 volt motor in a 110 volt unit.

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techie2
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Re: Motor Problem

Post by techie2 » Thu Nov 03, 2005 7:54 am

Jwax:
I talked to a man at a motor repair/rebuild shop. He said they haven't worked washing machine motors in years. And it appeared that this one is in some way pressed together, not bolted. I think that we are in the age of throw-away motors just like throw-away everything else... UGH! :)

Stephen & others:
My friend found the schematic for the washer. However I will probably not see it unless he has further trouble. He owns a gym where I go and I don't see him at home. It is interesting how this unit works. I am going by HIS discription... The motor has to rotate the drum one direction and then back the other, repeatedly on the WASH cycle. Best we can figure from his meter readings on the plug that goes to the motor is that the AC feeding the brushes is being flipped back and forth in phase to accomlish this action. He said the spin cycle is VERY fast and seems to step up a couple of times.. I am speculating that the contol circuit reads the tach, and as the speed of the motor/drum picks up it keeps adjusting the voltage or duty cycle until the control unit is "happy" with the maximum speed that it should attain... My guess is that this helps prevent overloading the motor when it is trying to get a heavy drum of wet clothes to spin.. It could be using a TRIAC or SCR. This is all speculation on my part.

<small>[ November 03, 2005, 07:58 AM: Message edited by: techie2 ]</small>

Robert Reed
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Re: Motor Problem

Post by Robert Reed » Thu Nov 03, 2005 5:34 pm

Techie
The back and forth motion of the drum is called the 'agitate cycle' and is usually accomplished by mechanical means. The transmission comes into play for various cycles.
The speed control on these units are performed by a marvelous chip dedicated for that purpose.The tacho generator on the shaft reports rpm back to the chip. This is then compared to a set point that has been preselected by analog or digital means depending on the particular chip employed. The set point is generated from the cycle timer. when the set point voltage agrees with the processed tacho voltage the motor stops hunting and stays at that speed. But these chips do more than just that. They look at line voltage and sometimes motor current and sum these with the tacho out put to arrive at a voltage which is then summed with the set point voltage. This voltage is compared with a ramp voltage generated internal to the chip and synced with the zero crossing point on the line voltage. When the ramp voltage reaches the set point value, it operates a trigger circuit which in turn fires a triac in phase control fashion. This in turn detemines speed of motor operation. Te only way SCR's would work in this circuit is two in inverse parralel, hardly done these days since the advent of 'alternistor' type triacs (a much higherDv/Dt rating) These chips also sense overload conditions and provide soft start too. Most of these chips are manufactured by Melexis (a european company)

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