Random electric motor question

[noparlpf]

So despite having briefly been a physics 100/101 tutor I have basically no idea how electricity works. :smalltongue: My question is what happens to an electric motor if you supply it with a higher voltage power source? Will it just turn faster, or is it going to heat up more than it's designed to handle and melt some of the metal parts/ignite some of the nonmetal parts?

[Gnoman]

A lot of things can happen. None of those things are good. There will be smoke, and probably fire. Lots of fire. There might even be an explosion.

Consider plugging two air conditioners into one wall outlet. The outlet will get hot, then the circuit breaker will trip to keep your house from burning down. This is the result of an excessive current draw. The exact same thing will happen if you shove too much voltage into anything, including a motor.

To explain this, we need to look at Ohm's Law, one of the most basic electrical concepts.

I=E/R, which can be stated as Current equals voltage divided by resistance. The resistance of a motor is largely fixed, so increasing the voltage will result in an increase of current. Current is the actual movement of electrons, which makes the motor spin.

[Dodom]

Will it just turn faster, or is it going to heat up more than it's designed to handle
Yes.

It'll do both. Turn faster, and then break. If the voltage is only a little higher than recommended, you may still be within its safety margin.


It's not uncommon to burn motors with excessive current, that happens without exceeding the recommended voltage too:
A stopped motor has a fairly weak resistance, while when in motion it generates impedence which counters the voltage. If a motor jams, it has little impedence and not much resistance to work with, so it lets a lot of current through. If it's not turned off or unjammed quickly, it will burn.

In my experience, a regular motor won't get hot enough to melt. They have plastic parts and bearing grease which will smoke and stink up the place, and the thing is most likely broken at that point, and if there's flammable material nearby it really is dangerous. There's probably some videos on what happens next if you leave it on, but people tend to hurry and unplug it once they see it smoking!

[The Great Wyrm]

It depends on the type of motor. Some motor types (for example, synchronous AC motors and stepper motors) have speed that isn't determined by voltage.

[Gray Mage]

A DC motor's speed is proportional to its current (and therefore voltage), although not only that. Synchronous motors' speed is proportional to the frequency of the current, not it's magnitude. As a DC motor, a stepper motor will be faster towards the next step with more current applied.

In most eletrical motor control uses the reference or input is considered to be the voltage, with a maximum safe value.

A DC motor will also not usually have significant influences from reactances, as the suply cab be considered constant most times.

[factotum]

As a DC motor, a stepper motor will be faster towards the next step with more current applied.


That's not really how a stepper motor works? The motor controller will energise the next "step" at a specific time, and it's that timing that determines how fast the motor turns, not how much current you supply to it. The current may well allow the motor to generate more torque, but that's another thing entirely.

Re: synchronous motors having a fixed speed they will spin at--same applies to some other common motor types you'll encounter. For instance, an electric fan usually makes use of a shaded pole induction motor, since they're designed to spin continuously for a very long time and using that type of motor means they don't have to worry about wear and tear on the commutator that you'd get in a DC motor. This type of motor will always spin near, but not quite at, synchronous speed, regardless of how much current you supply.

[Gray Mage]

That's not really how a stepper motor works? The motor controller will energise the next "step" at a specific time, and it's that timing that determines how fast the motor turns, not how much current you supply to it. The current may well allow the motor to generate more torque, but that's another thing entirely.


Yes, it will go at the specific time no matter the magnitude, however with more current the time needed to reach the next step will be shorter.

[factotum]

Yes, it will go at the specific time no matter the magnitude, however with more current the time needed to reach the next step will be shorter.

Which means the motor could *theoretically* spin faster, but it won't in reality because its speed is entirely controlled by the motor controller telling it how fast to move along each step, which is the point I was making.

[Gray Mage]

Which means the motor could *theoretically* spin faster, but it won't in reality because its speed is entirely controlled by the motor controller telling it how fast to move along each step, which is the point I was making.

Sure, but there is still a proportional relation with the voltage and the angular speed, which is my point.

[ellindsey]

It will depend on the type of motor being used, and the type of control circuit driving it.

A brushed DC motor will only act as a pure resistive load when stalled. When turning, a DC motor generates a back EMF which increases as the speed of the motor increases. As the motor speed increases, the current drawn by the motor will decrease. Eventually the motor will reach a point where it won't increase in speed any more, drawing just enough current to overcome the friction in the motor and the load being driven by the motor. If you increase the voltage driving the motor, the unloaded speed of the motor will also increase. The current drawn by the motor will also increase, but the degree to which it does is complex to calculate and depends a lot on what kind of load the motor is driving. Other things that may happen are increased arcing and erosion of the brushes, bearing wear and overheating, and possibly even centrifugal disintegration of the rotor if you speed it up too much.

A synchronous AC motor, on the other hand, will have a speed determined by the frequency of the AC voltage driving it. Increasing the voltage will not speed the motor up, but will cause it to have more torque and be better at holding that speed under load.

Stepper motors turn at a speed determined by the rate at which their control circuits have been commanded to step. Most stepper motor applications have the motor turning at a predetermined rate, not connected to any physical feedback, so increasing the voltage will have no effect on the motor speed, unless you also command the stepper driver to turn the motor faster. It will however increase the motor torque, the higher voltage will allow you to better overcome the resistance and inductance of the motor windings, so you will be able to drive heavier loads and have a better ability to hold position against load.

How well the motor is able to handle this will depend on the motor and how much overhead it was designed with. Some industrial motors are heavily overengineered, intended to run decades in service, and will be able to operate at a higher voltage at the cost of a shorter lifespan. Cheap motors, or ones engineered to be just barely at the edge of what they can survive already, may fly apart or catch fire instantly if you run them at higher than their design voltage.