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# What determines RPM speed of Electric motor?

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 04-27-2013, 09:17 PM
What determines RPM speed of Electric motor?

From my research its the windings in the motor and voltage.

So my question is, can you keep increasing the speed of the motor by constantly increasing the voltage BUT lowering the amps?

So say running a small toy 12v dc motor with a car ignition coil? Coil has lot of volts but not much amps.

I would start the motor with 12v so that it gets to its 12,000rpm its operating speed, then once upto top RPMs, I would disconnect the 12v and connect the 30,000v from the coil to power the motor.

What would happen?

1. Motor would continue to speed up past 12,000 rpm?

2. Or slow down because there isn't enough amps from the coil?

(please ignore that the windings in the motor would spark from lack of insulation, this question is theoretical)

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 04-28-2013, 12:25 AM
DC motor will go to smoke.

Know your enemy before you conquer.
That is.
Know those components before you repair.

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 04-28-2013, 12:45 AM
Quote:
 Originally Posted by supak111 What determines RPM speed of Electric motor?
For a non-synchronous motor, the speed of the motor is determined by the input power and the losses. The speed increases until the losses (mostly friction in an unloaded motor) balance the available power.

Quote:
 From my research its the windings in the motor and voltage.
kinda. But these two things don't really tell you anything.

Quote:
 So my question is, can you keep increasing the speed of the motor by constantly increasing the voltage BUT lowering the amps?
You can't raise the voltage and reduce the current at the same time. Raising the voltage will increase the current.

If you somehow *could* raise the voltage and lower the current in the correct proportions to maintain the power input, the motor would turn at the same speed.

Quote:
 So say running a small toy 12v dc motor with a car ignition coil? Coil has lot of volts but not much amps.
The motor would arc over inside and probably not even turn.

Quote:
 I would start the motor with 12v so that it gets to its 12,000rpm its operating speed, then once upto top RPMs, I would disconnect the 12v and connect the 30,000v from the coil to power the motor.
The results would be substantially the same.

Quote:
 What would happen?
Quote:
 1. Motor would continue to speed up past 12,000 rpm? 2. Or slow down because there isn't enough amps from the coil? (please ignore that the windings in the motor would spark from lack of insulation, this question is theoretical)
OK, if it's theoretical, and if the coil can now supply infinite current, the motor would speed up, friction would cause it to get hotter and hotter. Then it would melt.

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 04-28-2013, 09:37 AM
For an unloaded DC brush motor, there will be a voltage and current which will be needed at any speed. The speed will be roughly proportional to voltage and the current will depend on the torque. The torque will rise with speed due to wind resistance.

Running at extreem high speeds and voltages will damage the motor with arc over, resistance heating or rotor disintegration.

An ignition coil will give out pulses, not suitable for driving a motor. You would need to rectify and smooth the output, an igniyion coil has a high resistace and would not be able to supply much current, hence much voltage.

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 04-28-2013, 10:31 AM
Say I wanted to do this to a simple homopolar motor? Nothing to burn out, simple design, so whats needed to keep it speeding up?

Basically I am wondering if its possible to make any kind of electric device/motor that will KEEP speeding up just as long as you keep adding more electricity.

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 04-28-2013, 11:45 AM
Dont know about you, but I really want to see this motor being vaporised.

I would'nt get drunk and go to a tattoo parlour. But if someone else did, I would go along to watch.

Mutatis mutandis, I wont do this to my own motor... but I want to watch.

Last edited by quantumtangles; 04-28-2013 at 11:58 AM..

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 05-02-2013, 09:08 AM
Does anyone know of any motor that can keep speeding up? Basically not have a rpm limit?

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 05-02-2013, 09:49 AM
I suggest you go to Iran and get details of their centrifuges. Dynamic stabilisation is a problem.
You will need air or magnetic bearings and a very strong rotor that will not explode under the centrifugal forces.

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 05-02-2013, 10:01 AM
So there is no motor even theoretical one that can keep speeding up to its destruction as long as there is power applied to it?

Uranium centrifuges appear to be 90k rpm, that still doesn't mean that they have to control it from spinning out of control.

I was wondering if there is anything that can theoretically spin to it destruction. Meaning: a motor that would just keep gaining rpm, until it reaches its physical max and fall apart.

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 05-02-2013, 11:34 AM
The faster you spin, the more the centrifugal force. There is a limit to the stress for all materials. Perhaps a carbon fibre wound rotor would be best?

A car dynamo became obsolete partly beacause its speed is limited, run them too fast and the windings fly off. The alternator has the winding wound over the shaft and constained by steel pole pieces so can run much faster.

If you make big one, gravity can help, you could call it a pulsar.

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