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I don't know why, where or how I picked this up, but I believe that if you power an electric motor while blocking it, and thus stopping it from moving, you'll damage it. I'm thinking of the following type of motor, and possibly servo motors:

Simple DIY motor

Is it indeed true that holding the shaft in place while powering it will damage it? And if so, why? And could this be prevented by simple measures? (Like putting a resistor in front of it.)

Is the "damaging process" any different in servos, or do they get damaged in the same way?

Peter Mortensen
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Daniël van den Berg
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  • Depends on if the motor is build so that it breaks or not... – PlasmaHH Dec 13 '16 at 12:05
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    @PlasmaHH could you elaborate that? How should it be built to (not) break, and given an arbitrary motor, how would it be possible to tell if it will or not? – Daniël van den Berg Dec 13 '16 at 12:07
  • One thing left unmentioned is that a stalled motor will heat up only one or two rotor coils and commutator segments so even if the current was limited to the safe operating current it would still be localised more than in a turning (even slowly) motor. Most motors also use the rotation to move the air around and transfer heat to the surroundings. – KalleMP Dec 13 '16 at 13:16
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    Another concern with badly designed or manufactured low cost reduction geared (servo) motors is that the high speed gear sets are sometimes made of weak (plastic) materials and not able to withstand the full stall torque. – KalleMP Dec 13 '16 at 13:25
  • The duplicate should have gone the other way, this is a better question and answer than the supposed duplicate. –  Dec 20 '17 at 13:57

6 Answers6

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Simply stopping a motor from turning does not by itself hurt it. Think about it. The motor is stopped when you first apply power, and nothing gets hurt.

Most motors are designed so that the mechanical forces from the maximum torque won't hurt the motor.

The reason some motors shouldn't be stalled with full voltage applied is heat. All the electrical power going into the motor goes to heating the motor. Many motors can be over-heated this way. Designing them to be able to dissipate the heat from maximum applied voltage while stalled would require making other parameters less desirable. In many cases that's not worth it.

What makes stalling a motor even worse is that's when it draws the most current. So not only is more electrical power being dumped into the motor, but a higher fraction of that is turned into heat in the motor at low speed.

To a pretty good first approximation, think of a motor as a resistor in series with a voltage source. The resistor is just the DC resistance of the windings. The voltage source represents the motor acting as a generator as it turns. The voltage is proportional to speed, and opposes the applied voltage when the motor is turning due to this applied voltage. A stalled motor therefore only looks like a resistor. As the motor speeds up, that resistor is still there, but less voltage is effectively applied to it, thereby causing it to draw less current.

Sophisticated motor controllers either model the motor's internal temperature, or measure it outright. This includes tracking how much of the power delivered to the motor is transferred away by the rotating shaft, and how much is dissipated by the motor as heat. When the motor gets too hot, the drive power is reduced to avoid damage.

Since the damage is due to heat, stalling a motor under full power is OK for at least "short" intervals. How short "short" is depends on the design of the motor, and is something that good datasheets give you guidance on.

Olin Lathrop
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    +1, aside from just answering the question this is one of the most accessible descriptions of motor basics I've seen. – R.. GitHub STOP HELPING ICE Dec 13 '16 at 16:37
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    This, plus the cooling fan (if present) is driven by the same shaft that you've stalled, so it's not even getting cooled as effectively. – AaronD Dec 13 '16 at 19:21
  • You mean you haven't seen a DC motor with enough stall power to rip its own internals apart? I have. – Joshua Dec 14 '16 at 03:12
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    @Joshua: That's why I said "most" motors... – Olin Lathrop Dec 14 '16 at 11:30
  • It would be good to mention the gearboxes are usually the elements most prone to breaking should the actuator is not designed to operate when stalled (e.g. plastic gearboxes). Ripping windings or magnets due to poor design when stalled due to the high ertorque, that's so rare I'd consider it impossible. – Mister Mystère Jan 05 '17 at 11:19
  • Old answer, but it should also be noted that a stalled motor, due to the lack of that back EMF voltage source, will also pull much greater current from whatever supply or motor driver it's hooked up to. In many cases, stalling a motor for an extended time will fry the driver circuitry before significantly damaging the motor, as motor drivers aren't typically designed to withstand the stall current of the motor for more than a brief time. – Hearth Sep 21 '22 at 15:19
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An increased torque causes an increased current to flow: -

enter image description here

With no torque you get the no-load current and maximum speed. With full stalling torque you get a stalled rotor and maximum current.

Either end produces zero mechanical output so all the electrical power taken is burnt in the motor.

Clearly, with a constant voltage applied, the power burnt is very much smaller when the rotor produces no electrical torque than when the rotor is stalled. This is because the current is much smaller.

Hence the motor gets hotter when stalled.

if you power an electric motor while blocking it, and thus stopping it from moving, you'll damage it.

Yes, this can happen and for the above reason.

Is the "damaging process" any different in servos, or do they get damaged in the same way?

If you mean a servo like this: -

enter image description here

Then, it is less likely to be a problem because the servo amplifier will current limit at a more reasonable value (most cheap RC servos etc.) just to protect itself. But you cannot generally rule it out.

Could this be prevented by simple measures? (Like putting a resistor in front of it.)

Using a resistor is possible but it limits the current at a vital point - when the motor is beginning to rotate from a rest position. The current taken should be the stall current and, in normal operation this lasts for a few hundred milli seconds for a small motor but, if you put a resistor in series, full stall current (and hence full stall torque) is not available and the motor will take longer to accelerate to normal running. In a servo motor this can be crucial - response times are extended and this might impart sluggish behavior to the thing that is being controlled.

Active current limit circuits could be used that allow for brief periods of full stall current but then start to reduce current if the period lasts too long. But, now you are into a level of sophistication that may not work in some applications. For instance, if the motor is stalled and the current backed-off, what do you do when the demand signal changes - do you instantly restore full current and make the stall situation just as it was when not using such a control method?

Andy aka
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Most motor designs (including the classic brushed DC motor) will draw less current the faster it's spinning (in the intended direction). So, if the shaft is blocked, the current will be high. This current can be referred to as the "starting current" or "blocked rotor current". In some designs, the motor is not supposed to withstand the starting current indefinitely: it's supposed to be rotating at a speed in which its current draw will be lower. This expected current draw may be called "steady state current" in these designs.

If too much current goes through a motor, it may heat up. This can cause all sorts of trouble, but probably the most common cause of failure is the failure of the insulation of the windings because of heating.

FrancoVS
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There are several ways to break a motor, three of the of them being current related, over torque physically breaking something, over current demagnetising the field magnets, and over-temperature melting plastic or burning insulation.

Stalling a motor makes the current increase above the rated current, increasing the torque above rated, and increasing the power dissipation above what it's designed to deal with.

Whether the extra torque breaks something depends on how much extra current flows. For most motors, it would be quite difficult to get enough current flowing to break something if you stay inside the rated voltage. Serious abuse, overvoltage with stall would be needed.

The same goes for demagnetisation. Staying within the rated voltage should mean the current stays at safe levels, even when stalled.

Whether the extra power dissipation melts something depends on how long you hold it stalled. It will be more possible to damage a motor this way.

Neil_UK
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The current that flows through a motor is primarily determined by the back EMF that the motor develops. The back EMF is proportional to the speed of the rotor. Therefore, if you prevent the rotor from turning, a large amount of current flows through the windings — this is called the "stall current" of the motor.

Most motors are not designed to handle stall current indefinitely; the windings will overheat and the insulation will be damaged when the temperature gets high enough.

Dave Tweed
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The simple answer is it depends on how much current can flow while the motor is stopped / stalled. The current limit in this situation is the resistance of the winding's and any brushes, etc.The total circuit resistance is more likely to be adequate for current limit on smaller motors .Larger motors are more likely to need some additional form of current limit .

If the current is too high the temperature rises and either burns out the winding's and or melts any plastics, etc., causing the motor to short circuit or jam. Temporary stall's can usually be tolerated. Mechanical parts can also break, but as long as the servo is not forced backwards whilst driving forwards this is usually not an issue.

Autistic
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Spoon
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