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jand
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Posted: Sun Apr 20th, 2008 00:34 |
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Hi Folks,
I have Kenmore dishwasher that keeps overheating motors. This is the reversing motor that also pumps water out. Anyway, I've had this machine for a long time and two years ago it overheated the motor for the first time and shut down. A year later and after a few more overheats I replaced the motor when it was overheating every time. I replaced it with an older motor that I still had. This one lasted about a year and for other reasons I replaced it with a new motor. The second motor (older model) overheated twice during that year. The newest motor, installed 6 weeks ago overheated twice during this week. I thought that it might be the motor relay that was not working properly so I replaced that today too. After careful installation of the relay I tried it and the motor is arcing and still not working properly.
I'm running out of ideas here. Has anyone seen this before?
The machine never overheated for 10 years. Lately every motor has been overheating.
Q. Could this be caused somehow by improper installation of the motor? (I didn't use the centering tool normally recommended, I eyeballed it, but I feel confident about the shim installation)
Q. Could this be caused by old house wiring? I live in an old house that has noticable voltage drops on that side of the house. The machine is a portable unit and I didn't have any overheating at other locations. This machine doesn't have any capacitors on the motor circuit BTW.
Q. Could this be a wiring harness or timer problem?
Thanks for any help you can provide,
Jan
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Samurai Appliance Repair Man
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Posted: Sun Apr 20th, 2008 11:41 |
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| This is an odd one-- haven't seen this problem as a repeating pattern like you're seeing. The fact that you're seeing this is more than one motor tells me something is wrong either with the power supply or with the pump assembly. Check voltage to the motor, should be nominally 120vac but no less than 110vac (10% less than 120vac-- this is the standard). A lower voltage supply will cause the motor to draw more current to compensate. Is something is binding in the pump assembly, this will too will cause the motor to draw more current. Higher current draw = more heat = shorter motor life.
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Keinokuorma
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Posted: Sun Apr 20th, 2008 21:55 |
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I'm thinking too that you may be dealing with a voltage issue.
Is the house wired all the way from a single fuse box, or do you have "sub boxes"? If one of those have a weak Neutral connection, it may be causing voltage imbalance between the live legs. Even with no sub boxes, if an area of the home has been wired from a multi phase circuit with common neutral, the neutral dropping out will cause trouble.
Last edited on Sun Apr 20th, 2008 21:57 by Keinokuorma
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jand
Apprentice Appliantologist
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Posted: Sun Apr 20th, 2008 23:08 |
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Hi,
Thanks for your posts. I reached the same conclusion myself. The times that I was there and observed it overheat, it occurred in the middle of a wash cycle. It had been running for some time and all of a sudden just overheated and quit. The house is about 100 years old, several rooms with just one outlet in them. Recently we had the service at the box upgraded for that particular circuit because we were unable to run the dishwasher and a room air conditioner at the same time without tripping the breaker. Thanks for the tip on the phase imbalance. I didn't think of that. I'll alert the electrician to look for that too. This dishwasher uses a newer, more powerful motor as a replacement than the original. My observation has been that the newer one overheats more often than the older model. I would conclude that it is more sensitive to the voltage drop issue. If it is indeed more powerful it would follow that it would accentuate a voltage drop problem more than the old model would.
Thanks for your insight. I think we're on the right track. I was really stumped.
Best Regards,
Jan
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jand
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Posted: Sun Apr 20th, 2008 23:10 |
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To answer your question, there are two boxes, one for each apartment in this house. So it appears that the service has been split.
Jan
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Keinokuorma
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Posted: Mon Apr 21st, 2008 00:38 |
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Well, do you have 240V or 208V service to the house? Because either way, if one of the boxes is dropping out the neutral, it would explain the machine working fine in other areas... ask the electrician to examine both boxes and all circuits under load. Can't really predict what you find in an ancient installation.
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RegUS_PatOff
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Posted: Mon Apr 21st, 2008 01:56 |
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so... have you measured the voltage at the dishwasher when it runs, and when it overheats... ?
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jand
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Posted: Mon Apr 21st, 2008 03:33 |
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No, haven't measured the voltage under operation. Since its arcing right now I'll have to replace the motor first and then I'll be able to measure.
Jan
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jambatt
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Posted: Mon Apr 21st, 2008 17:45 |
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After careful installation of the relay I tried it and the motor is arcing and still not working properly.
Are the brushes arcing or is the arcing occuring in the wiring connections??
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jand
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Posted: Tue Apr 22nd, 2008 04:19 |
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Hi Jambatt,
No brushes on this motor, its inductive. And I'm not sure exactly where the arcing occurred. We shut it off right away.
Jan
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jand
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Posted: Tue Apr 22nd, 2008 04:21 |
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Hi Kienokuorma,
208V? Never heard of it...can you elaborate? I'm pretty sure it is 240V service.
Jan
Last edited on Tue Apr 22nd, 2008 04:22 by jand
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RegUS_PatOff
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Posted: Tue Apr 22nd, 2008 04:46 |
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jand wrote: Hi Kienokuorma,
208V? Never heard of it...can you elaborate? I'm pretty sure it is 240V service.
Jan
jand, you don't have your location listed...
Keinokuorma
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Last edited on Tue Apr 22nd, 2008 04:50 by RegUS_PatOff
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K,man
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Posted: Tue Apr 22nd, 2008 04:48 |
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| never heard of a dishwasher on 240, it should be 120 the higher voltage will burn up the motor. ive seen bad transformers put out 280 for 240 and 160 for 120 burn up every motor in the home,you neeed to run a new 120v dedicated service to the dishwasher
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jand
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Posted: Tue Apr 22nd, 2008 16:27 |
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Location added, thanks.
K,man we are refering to the 240 coming to the house then each half going to a different apartment potentially (at 120V). The dishwasher is a 120V unit but as I've said in earlier posts the concern has been that due to the gauge of the house wiring, distance from the breaker box, and high current draw inherent in any dishwasher, that the voltage drop may be excessive, causing premature aging of the AC motor.
Jan
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Keinokuorma
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Posted: Tue Apr 22nd, 2008 21:10 |
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jand wrote: Hi Kienokuorma,
208V? Never heard of it...can you elaborate? I'm pretty sure it is 240V service.
Jan
The very basic service in the US, Canada, S-America too (I guess), and some non-American countries as well, would be 120V. That is, Live-to-Neutral, single phase service. And you're probably right, 240V split phase service is the most common after that.
To go further into the issue, here's some reading with incomprehendible hand sketches and overly technical jargon: http://applianceguru.com/forum9/9262.html
Anyway, what I'm trying to say, if you have any kind of multiple phase service to the home, and you get abnormal voltage readings and overheating/malfunctioning of appliances in one part of the home, but they work fine elsewhere, you may be dealing with a loose neutral wire somewhere in the chain. If the Neutral drops off, appliances wired on the L1 and L2 legs will divide the voltage by Ohm's law, proportionally to their power draw.
For an example, if you wire two 60W, 120V light bulbs in series, and power them directly from 240V, with no neutral, they will work fine. their "center tap" point will be 120V away from the potential of either live wire, because both bulbs have the same resistance and current draw. If you were to measure, you would have very little voltage difference from the center point to the Neutral wire.
60W @ 120V = 0,5A
120V : 0,5A = 240 Ohm
Two 240 Ohm bulbs in series would be 480 Ohms...
240V : 480 Ohm = 0,5A
voltage loss for each 240 Ohm bulb would be...
0,5A * 240 Ohm = 120V
Both bulbs work like intended, each 60W @ 120V, combined power 120W.
OTOH, if you use a 40W and 60W bulb the same way, or any mismatched pair, you will be dividing the voltage differently.
60W @ 120V = 0,5A
120V : 0,5A = 240 Ohm
40W @ 120V = 0,33A
120V : 0,33A = 360 Ohm
These mismatched bulbs in series would be 600 Ohms...
240V : 600 Ohm = 0,4A
Voltage loss for the different bulbs would be...
240 Ohm * 0,4A = 96V (for the 60W bulb)
360 Ohm * 0,4A = 144V (for the 40W bulb)
While the sum of their voltage losses would be 240V, their center tap potential would be sitting 24V off the Neutral wire potential.
Also, the 60W bulb is designed to work with 0,5A. At 0,4A it is working with 80% of the design current, and at 64% of design power (38,4W)
Respectively the 40W bulb would be working with 120% of its design current (20% excess) and with 144% of design power (44% excess, 57,6W).
Their combined power at this abnormal status would be 38,4W + 57,6W = 96W. Were they wired correctly with the Neutral wire, their power would be 40W + 60W = 100W.
So, the neutral dropping off will make the voltage to distribute unevenly, causing abnormal function and premature breakdown. It is often the less powerful device that takes the damage, like in my example, the 40W bulb will not fare too well with this much excess power, while the 60W bulb will glow dim, and just go out when the 40W one blows open.
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Keinokuorma
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Posted: Wed Apr 23rd, 2008 01:00 |
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OK, perhaps 240V is now actually more common. 208 is used in some cases and areas. Does one or both apartments in your house have 240V outlets for ranges and dryers, or is there 120V only?
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K,man
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Posted: Wed Apr 23rd, 2008 02:57 |
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| ahh so i see your concern now have had many a customer burn up radial arm saws using 100 ft drop cord... voltage drop more than 10% will burn up in a hurry
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Keinokuorma
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Posted: Wed Apr 23rd, 2008 10:22 |
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While some inductive motors can be adjusted by voltage and are designed for low voltage use, it isn't completely unknown that motors under harder load will heat up with low voltage. This is partly due to the diminished power, increased rotor slip, and weaker ventilation. This is very similar to constantly running a 3 phase delta designed motor in wye mode.
Last edited on Wed Apr 23rd, 2008 10:23 by Keinokuorma
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jand
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Posted: Wed Apr 23rd, 2008 17:02 |
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Hi Keinokourma,
There is no 240V service in the living areas. There is however a 240V dryer in the basement, also going to the same service. The stove is gas.
Thanks also, for your detailed descriptions of asymmetric voltage drops across 240V with your lightbulb example. I think that readers should clearly understand, so I'll reiterate here, that the phenomenon you described is for situations when the neutral is "dropping off" as you described, meaning disconnected or otherwise not holding a solid position. In the normal case the neutral is not allowed to float around, is solidly in the middle, and can tolerate asymmetric current draws in the two 120V halves without signifcant change in voltage for each half. Do you concur?
Jan
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RegUS_PatOff
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Posted: Wed Apr 23rd, 2008 18:48 |
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a few notes:
From the power company's local neighborhood transformer, there are (3) wires into each residential home.
L1 . . . Neutral . . . L2
|_ 120 _ | _ 120 _|
| _ _ _ 240 _ _ _ |
Between neutral and either of the L1 or L2, there is 120v.
The L1 and Neutral pair goes to half of the 120v devices in the house.
The L2 and Neutral pair goes to the other half of the 120v household devices.
L1 and L2 goes to any 240v devices in the house:
electric range, electric dryer, electric water heater, electric heating, ect.
During some storms, ect, the Neutral wire from the transformer can get disconnected from the house.
That can cause many problems including what Keinokuorma mentioned above
Last edited on Wed Apr 23rd, 2008 18:55 by RegUS_PatOff
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