Running a motor at overvoltage

[Sparkyman]

I've looked in FAQ and tried Search but this must have been covered before so if one of you kind people could point me to the answer it would be appreciated.

What exactly happens if you connect a 110V motor (say a small fan) to a 240V supply, and why?

Thanks

 

[electricpete]

The iron will saturate, increasing core heating.

Hot core may lead to stator winding insulation damage.

You are also challenging the insulation rating, although I think at the voltages you mention that wouldn't be a problem.

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[itsmoked]

The motor will not spin any faster.

I can't see how it "may" anything... But rather "will" something.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

What kind of a motor is it?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[shipsaway]

Insulation breakdown will cause the motor to fail. Heat is the major factor, for every 10deg rise in temp above the rated ambient you decrease the insulation strength by 1/2. Also if there are any controls in the circuit; automatic thermal trips etc. they'll overheat aswell with the overvoltage and become an open circuit.

 

[waross]

Short answer, the motor will fail quickly from heat caused by a disproportionate increase in current. You can probably count the life of the motor in minutes on your fingers.
Note, when you double the voltage on a resistor the proportionate increase in current is 400%. With a motor the increase will be disproportionately higher.
respectfully

 

[Skogsgurra]

I still do not know what motor it is. There are motors that can live through a 100+ % overvoltage for short periods. Nothing was said by the OP for how long the overvoltage was to be applied.

First split second:
An universal motor will rev up and perhaps explode from centrifugal forces, perhaps not. A shaded pole motor will start and run. A three-phase motor will start and run. A split phace capacitor motor will start and run. The latter three will run very close to sych speed.

Few seconds from start:
Most protective devices like motor protection and fuses trip.

Whithin a minute or two:
Embedded thermal protection trips.

After that:
Ashes and shrapnels. Not really because of the halving of life per ten degrees temperature increase but simply because everything gets way too hot and the universal motor probably cannot run at such a high speed for a long time.


Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

Check out Figure 1 in this pdf from EASA, the Electrical Apparatus Service Assoc. It shows that although current initially drops with an over voltage of a few percent (following ohm's law), it rapidly increases with voltage increase, at what appears to be a logarithmic rate. That chart stops at just 15% over voltage, but you could extrapolate from it that the motor current would be off the chart at 200% voltage. Not only that, but look at the linear relationship to torque and then think about your mechanical drive train components! There's your shrapnel potential right there.

http://www.easa.com/mo/TN-pdfs/TN08-397.pdf

Gunnar,
Shrapnel is already plural.

 

[electricpete]

The only mechanical concern I see is during acceleration, since the motor matches the machine torque at steady state (I'm assuming the load is matched and we're not relying on the motor to reach breakdown torque to limit mechanical stress on components). And assuming that the motor rotor has higher inertia than the load (such as most pumps), the bulk of the accelerating torque goes to accelerating the motor rotor and doesn't even pass to the load. The torque speed curve increases roughly with voltage squared, but would track below that due to saturation.

Interesting link:

http://www.easa.com/mo/TN-pdfs/

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[itsmoked]

Of course the OP is probably referring to one of those little black 23W motors for a 3/4HP condenser fan.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electricpete]

My assumption is induction motor as in the link of jraef.

But as gunnar mentioned, we don't know.

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[electricpete]

keith - what does 23W mean? Not 23 watts I hope.

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[sreid]

I once got 220 on a 110 small shaded pole motor and there was a spark and a snap right away. Dead Motor.

 

[oldfieldguy]

Practical experience with a cooling fan (1/6 HP) rated for 120 v single phase, connected inadvertently to a 240-volt source: excellent acceleration, ran fine for about three minutes, then POOF! Visual inspection revealed burnt and fused windings.

old field guy

 

[aolalde]

Current is limited by impedance (Z, the combined winding resistance and reactance)
Z = R+ jX

were R<<X

I = V/Z

Double voltage saturates the magnetic circuit, the core permeability “mu” approaches zero, and the reluctance of the magnetic circuits approaches infinity then the reactance X approaches zero. As a result, the impedance is almost same as the winding resistance Z = ~ R

I= V/R
The energy dissipated in the winding ( watts); W= I^2*R, will increase with the squared ratio of the new high current, leading to a premature insulation failure.

 

[jraef]

Good explanation.



"... premature insulation failure."

That kind of sums up my life...

 

[sreid]

Yes, like "The car died due to an electrical failure. A connecting rod cut the main battery cable."

 

[itsmoked]

Sounds like my buddy's diesel Olds.

He was pulling a steep grade with the car fully loaded.
1) A connecting rod broke.
2) It flew around and cut an inch wide slot out of the oil pan.
3) All the oil vented out the pan onto to road.
4) Then the rod punched thru the block.
5) All the coolant vented into the pan and out the same slot.
6) Then the rod came around and jammed in a thick web member.
7) The drive train stopped instantly. Leaving 45 feet of skid marks.
8) The drive train stopping that suddenly broke the cam shaft in half near the center.
9) The cam shaft launched forward shoving the sprocket thru the cam chain cover.
10) The camshaft also proceeded further in to the back of the water pump destroying it.
11) The sudden stop gutted the automatic transmission.
12) The gutted transmission sheared 3 teeth off the ring gear.

The CHP couldn't even push the car off the road. (hwy 17 at Lexington Reservoir)

I guess I could say the car had an coolant failure.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

I hope you fixed it for him!


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Sometimes I only open my mouth to swap feet...

 

[MikeHalloran]

Great idea, Smoked.

The Olds Diesel is an excellent analogy for an electric motor run on increased voltage.


Same result; wallet depletion.


Mike Halloran
Pembroke Pines, FL, USA