What causes a motor to burn out?

[mrkevelev]

How is motor speed regulated (lets say a PM DC motor)? Is the voltage kept constant, and current increased/decreased, or is current kept same and voltage increased/decreased or both? What about torque and speed? Will giving a motor too much voltage or current damage it? I'm trying to understand what changes and what stays the same. Another example is a light bulb. Let's say its a 120 V, 60 W bulb. So to increase the brightness would mean decreasing resistance, thus increasing current, and thus increasing power. But would giving it too much current blow the bulb or giving it too much voltage, or only the power? Again, I thought I heard once something like "a device only will draw the current it needs" but that part I don't understand. Thanks.

 

[russ_watters]

Again, I thought I heard once something like "a device only will draw the current it needs" but that part I don't understand. Thanks.

First, the phrase "burn out" almost literally means what it says. For the context of your question, the failures are overheating related. Light bulb filaments and motor insulation melt, causing open or short circuits (or one then the other).

As for why:
Different devices obey different rules. Light bulbs are essentially fixed resistors (change with temperature, but that's it). So to make them burn out - to make them overheat - you would increase the voltage, which causes the amperage to increase, which together causes the heat output (and thus temperature) to rise.

Motors are inductors and operate on different rules. And different motors operate differently. To be honest, I'm not certain of the working of permanent magnet motors, but I believe they behave somewhat similar to resistors in terms of increasing voltage increases power output (increasing RPM). Someone else can verify though...

What is more interesting to me is AC induction motors though. These motors operate at fixed RPM or die trying. "Resistance" (impedance) is a function of the inductance, which is a function of the torque applied against the motor. This means that at a fixed voltage and frequency, if you apply a brake to the motor the rpm will stay roughly constant and the amperage will go up and up and up until the motor overheats and destroys itself.

 

[.Scott]

The incandescent (old fashion) light bulb is a bit easier.
When you first apply a voltage, the filament is cold and has a low resistance. So it draws more current and quickly heats up.
As it heats, it becomes more resistive - because of the materials chosen for the filament. As the resistance goes up, the current goes down. Wattage is voltage time current, so the wattage also goes down as the filament heats up. This is how the bulb regulates the current.

For a hotter bulb, the filament is either thicker or shorter - making it less resistive and therefor set to draw more current.

Since brighter bulbs are hotter, the evaporate their filaments faster. As the filaments get thinner, you would thin they get dimmer - but what often happens is that they start to get thinner in one spot more than others. That makes that spot hotter than the rest - and so it evaporates material faster than the rest and ultimately breaks at that point.

 

[Borek]

How is motor speed regulated (lets say a PM DC motor)? Is the voltage kept constant, and current increased/decreased, or is current kept same and voltage increased/decreased or both?

To make things more complicated, in some motors you keep both the voltage and the current constant, but you pulse them and you get different power varying pulse width. Same approach can be used for LEDs and light bulbs (google PWM).

 

[Windadct]

Outside of the "traditional" failure modes, heat, bearings, etc - a more modern issue can be insulation failure due too Low Quality signal from an inverter or regulator / motor drive. Common on PMDC / BLDC motors. ( And I guess DC motors - but these are less common in long life high use applications)

The PWM signal (voltage) is a square wave and can be exaggerated by the cabling to the motor, leaving very high voltage spikes at the motor ( dV/dt) -- this leads to degradation of the insulation of the motors windings. They have Inverter grade cabling and motors for just this reason.

 

[sparkie]

I'm not all that in the know on DC motors, as I've mostly done work on larger type AC motors: 1/2 HP to 50 HP three phase motors, and tons of smaller single phase motors, as well as 150-200ish HP 4160V motors. AC motors themselves don't typically burn up, but their windings do. Here are a few of the main reasons they fail:

Environmental factors:
Dust/debris accumulation causing poor thermal dissipation of windings
Moisture accumulation from the environment, particularly in the motor junction box (commonly called a "peckerhead" )
Corrosive atmosphere causing early failure of winding lamination

Poor Installation/Design factors:
Skinned wires somewhere in the power circuit causing an arcing action and increased current and unbalanced power in the windings
Improperly configured overload protection
Improperly sized frequency drives and/or non-inverter duty rated motors (lower fan speeds on squirrel-cage motors don't dissipate heat as designed)
Overload/Contactor failure or an absence of proper circuit protection
Undersized motors circuits for the load (think about motors starting under a full load which isn't it isn't rated for)
Undersized/improperly derated or installed feeder conductors (such as running circuit conductors in separate circuits)

I didn't really touch on mechanical wear as it isn't a common problem. Almost always when a motor fails, especially repeatedly, there is a design issue that needs to be addressed in the system. The things listed above are just things I have witnessed in my experience, and the most common factors, and I have not really included mechanical factors as it is pretty uncommon to have a motor fail mechanically from wear before its designed life-span, but it does happen. Moving parts wear out. Hope this helps.

 

[Tom.G]

How is motor speed regulated (lets say a PM DC motor)?

Here are a couple excerpts from an earlier thread regarding a generator and/or motor. They apply directly when there is no load on the generator/motor.
When operating as a generator, the faster it turns the higher the output voltage.

induced voltage in a conductor in a magnetic field is proportional to rate-of-change of the magnetic field.

The motor aspect is that the speed is such that the induced rotor voltage is equal to that required counteract the the effective rotor voltage. Effective rotor voltage is the applied voltage minus the IR (resistive) loses in the winding and brush resistance.

 

[mrkevelev]

Good info here. Another related question: so if I know that a device is rated for an DC input voltage up to 12 V, and it doesn't mention current, as long as the voltage I give is 12 or less, then it shouldn't be damaged? What if I give it 12 V at a much higher current than it needs?

 

[russ_watters]

What if I give it 12 V at a much higher current than it needs?

You Can't. Current is demanded by the load, not forced by the supply.