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Sherlock Ohms

Unexpected Perpetual Motion

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RFI-EMI-GUY
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Silver
Re: Motor over-run effects
RFI-EMI-GUY   5/5/2015 7:57:09 PM
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Our 1999 is still on the road running with well over 100K miles. It is surprising to me as so much is wrong with it. Thankfully it is a third vehicle in the household.

William K.
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Re: Motor over-run effects
William K.   5/5/2015 7:53:51 PM
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Thanks, but since the problem went away I have letit slide. Presently the vehicle is ten model years old and while it still is working fine it may at almost any time suffer some very expensive failure. The timing belt on my 2001 PT Cruiser failed at 58,000 miles, resulting in quite a bit of engine damage. And such a small engine compartment that it was quite a challenge to do anything. My thinking is that we were much better off in the sixties, as far as cars go. But few agree with me.

RFI-EMI-GUY
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Silver
Re: Motor over-run effects
RFI-EMI-GUY   5/5/2015 7:34:55 PM
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Just in case you consult the Haynes manual for the resistance values for the multifunction switch. Beware, they were incorrect, at least for a 1999 model year. I bought the new non-returnable $185 switch and found the exact same readings with the new one (as with the original). Upon contacting Haynes they sent me an addendum to correct the manual. If you need to replace the switch, I may be able to help you with a spare switch at a much reduced price. It is for a 1999, so check the model numbers. If you are wondering the body control manual (BCM) reads an analog voltage from the multifunction switch to determine what speed or mode the wipers should be running. The MF Switch has a resistor array in it.

William K.
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Platinum
Re: Motor over-run effects
William K.   5/5/2015 5:22:15 PM
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RFI-Guy, I had a similar problem with a 2005 Dogde Caravan. The wipers would only run at high speed, no matter what the switch setting. Since it was raining at the time that was only a small problem. And days later when I went to investigate it, the problem was gone. Thanks for the clue about what the problem might be.

RFI-EMI-GUY
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Silver
Re: Motor over-run effects
RFI-EMI-GUY   11/14/2014 3:06:05 PM
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I found myself troubleshooting the windsheild wiper system on a Plymouth Voyager. The variable speed wiper system would not work. Turns out after buying a $185 multifunction switch, that the root cause was the wipers would not park properly. Unparked, the variable speed would not start.

Why did they not park? Because Chrysler used the spare contact on the power switch relay to ground the back EMF of the motor to ground to brake the motor.

A relay used in this manner will fail eventually because the relay contacts have very little current to burn through surface corrosion.

Cabe Atwell
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Blogger
Re: UNEXPECTED PERPETUAL MOTION
Cabe Atwell   3/26/2014 4:44:53 PM
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That's why every time I work inside my PC I disconnect the power supply from the wall outlet and then hit the computer's power button to discharge the residual electrical current.

William K.
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Platinum
Re: UNEXPECTED PERPETUAL MOTION
William K.   1/27/2014 4:05:23 PM
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John R, it is certainly reasonable that spinning one of the PM brushless motors would generate a serious voltage, and that certainly could be a shocking hazard, no doubt. In fact, I know that I have seen manufacturers warnings on drive systems that cautioned against back-driving the motors, since it would generate voltages that would damage the control system. Some VS and servo drives do specify a resistor ro handle that power, since without some place for the power to go the voltage certainly will rise. 

And there is definitely no reason to consider a stepper, since, in addition to the reasons that you described, qucikly closing a fluid valve can lead to very large pressure spikes that can do quite a bit of damage.

 

 

John_Reed
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Iron
Re: Motor over-run effects
John_Reed   1/27/2014 2:51:23 PM
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I hve found that applying  DC to a spinning induction motor brakes it rapidly. Applying a stationary magnetic field will produce current in the squirrel cage and perhaps some hysteresis losses in the iron as well. It should be possible with the ECM motors having permanent magnet rotors to simply switch in a resistor load to brake the motor. Perhaps some manufacturer will make this an option, or one might modify the motor to make the windings accessible to external cuircuitry. 

John_Reed
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Iron
Re: The real perpetual motion
John_Reed   1/27/2014 2:41:24 PM
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The term "perpetual motion" was meant to be facetious.

John_Reed
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Iron
Re: UNEXPECTED PERPETUAL MOTION
John_Reed   1/27/2014 2:37:00 PM
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I did this job about 12 years ago. A few relays sufficed. In fact I could have simply put in a three position switch (open-off-close). As the water in the tank was critical in case of a plant fire I wanted some additional protections though. The circuit also included an emergency stop switch and the motor/actuator had overtravel limit switches which were wired to kill power in case the first limits failed.

A stepper would not have been a good match for this application as speed control wasn't required. The gear reduction ratio was tremendous, thousands of motor revolutions to completely open or close the valve. 

A few years later I had another stored energy problem. A customer had designed a machine for conditioning torque rods. These were steel and came in diameters of 1/2 to 3/4 inches. Conditioning consisted of twisting each rod somewht past its elastic yield point. The machine had a 5 hp motor and needed speed control, so a Hitachi variable speed drive was added. The VFD had no problem controlling the motor as the rods were twisted, but when motion was reversed energy was fed back to the VFD from the rod. This showed up as a voltage spike within the VFD which tripped it out. The sudden release of torque from the motor allowed the machine to spin backward out of control. The solution was to add an energy dissipating power resistor.

Recently I was given a 1 hp Genteq X13 ECM motor which had failed. I was unfamiliar with this type and opened it up. Inside was an electronics package which connected to the motor windings. The rotor had a permanent magnet with a number of poles. The windings comprise  a three-phase motor or generator. Spinning the shaft produced nearly 400 volts at 360 Hz. from the windings. 

It seems wise to take some time to understand the capabilities of new types of motors so that potential hazards can be avoided. For instance a system which could spin an ECM motor with failed electronic components or internal shorts  might make electrical connections hazardous even if there was no source of electrical power connected. 

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