# What determines RPM speed of Electric motor?

Discussion in 'Sensors and Actuators' started by supak111, Apr 27, 2013.

1. ### supak111★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

334
4
Apr 29, 2012
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)

2. ### Rleo6965

585
9
Jan 22, 2012
DC motor will go to smoke.

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,838
Jan 21, 2010
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.

kinda. But these two things don't really tell you anything.

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.

The motor would arc over inside and probably not even turn.

The results would be substantially the same.

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.

4. ### duke37

5,364
772
Jan 9, 2011
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.

5. ### supak111★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

334
4
Apr 29, 2012
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.

6. ### quantumtangles

152
3
Dec 19, 2012
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: Apr 28, 2013
7. ### supak111★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

334
4
Apr 29, 2012
Does anyone know of any motor that can keep speeding up? Basically not have a rpm limit?

8. ### duke37

5,364
772
Jan 9, 2011
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.

9. ### supak111★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

334
4
Apr 29, 2012
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.

10. ### duke37

5,364
772
Jan 9, 2011
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.

11. ### Six_Shooter

98
0
Nov 16, 2012
EVERY motor ever made or will be made has this property. The failure may not be completely
mechanical, but there is a point where a motor is just not capable of remaining in a working condition.

I'm not sure what it is that you're trying to do here.

In theory a motor that could just simply keep gaining RPM would have moving parts that weigh nothing, and could never actually be attached to something to use that RPM. Once the motor is loaded it changes the mechanical and electrical failure points.

12. ### john monks

693
2
Mar 9, 2012
Supak111, a series wound motor is like what you are describing. It has a field winding connected in series with the windings in the rotor. If the motor is run without a load the field becomes weaker as the motor increases in RPM. This causes a decrease in counter EMF and the motor tends to run even faster. If the motor was ideal the RPM would approach relativistic velocities. Motors on trains are sometimes series would because the train needs the increase in RPM as the train gains speed and needs the high field when the train is starting from zero speed. And this type of motor should never be run without a load because it can fly apart and kill somebody.

13. ### JMW

90
3
Jan 30, 2012
Shunt wound motors after starting and then losing the field, will speed up to self destruction. The field provides a back EMF that acts as brake.

14. ### supak111★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

334
4
Apr 29, 2012
Ok I will look into shunt wound dc motor and series wound motor, if they aren't one and the same and find our more as to how they work.

And yea I was simply asking if there is such a motor that would keep speeding up. Lorentz force theoretically allows for something to just keep speeding up so why shouldn't there be an electric motor that dose the same. Seems that in most motors wounding gets to a point where they make as much current as they use up so that always seems to be the limiting factor right?