Is it safe to stop a spinning motor mechanically without cutting power to it? If not, why? Common sense tells me that it would be bad for the motor, but I don't exactly know why it would be.
A spinning motor has back EMF that reduces the current through the windings by opposing the applied voltage. A stalled motor has such no back EMF so the current through the windings is much higher (maybe 5:1 or 10:1 higher) than normal at full-load.
Most motors are designed to spin most of the time when power is applied (the exception being when they are starting up when they draw a lot of current and a lot of power is dissipated in copper losses. Cooling may be via an internal fan blade hidden under an end bell and attached to the shaft, and that is dependent on the shaft rotating.
There's nothing stopping you from designing a motor that does not overheat while stalled, in fact there are examples of such motors which were used to maintain the tension in some magnetic tape units and similar applications. If you do this with a regular motor however (without reducing the voltage or making other compensations) it's likely the motor will overheat and fail fairly quickly.
It is not safe (for the motor) to stall an induction motor powered from 50 Hz or 60 Hz mains. And it is not safe to stall a simple brushed DC motor powered directly from a DC voltage supply. Of course some motors have some type of built-in thermal limiting and will interrupt the current before being destroyed. So it depends somewhat on the motor. But I still wouldn't call it "safe."
But when the motor is controlled by an electronic speed control (inverter, VFD, etc), it may be possible to hold it in the stalled condition indefinitely, depending on the details of the speed controller and any sensors on the motor itself.
Stopping a motor by hand (literally) may not be safe for the hand or the person attached to it. It depends on how much inertia the system has and how reliable the torque limiting is. But that is not really an EE issue.
Stalling an energized rotating motor has both the short circuit energy being dissipated in the windings which is about 10x the rated power and the kinetic energy of the rotating mass which has stored energy to get up to speed over some time.
All this energy is much greater than the energy to spin up from start due to the counter force of the applied power and the braking power to overcome both effects.
This reminds me of the full load braking test on takeoff of a 777. Although the engines would be cut, the additional kinetic energy stored with a full load of fuel on a full speed aborted take off is one of the design requirements for the disc brakes.
It is not that it is impossible to do, but the windings would quickly over heat in seconds and trip the breaker within seconds, if selected properly. A stop current would be as high as the start current which is typically 10x +/-2x greater than the full load current at max generated power. But an external break with full power should either burn out the motor or trip the breaker before this happened .
It really depends on the type of the motor, and on the controller used to govern that motor's operation.
Suppose there's no controller: the motor is just connected to a voltage source.
- Stalling a mechanically commutated motor, like a universal or brushed DC motor, will overload the commutator contacts.
- Stalling an induction motor will overload the windings, since they are designed to operate in synchronous mode where back-EMF neutralizes some of the input voltage.
Suppose there's an open-loop controller: the controller doesn't know about motor's speed, but perhaps can sense the motor current.
If the controller's design incorporates motor protection function, then an induction motor will be safe: the controller will protect the windings from overheating.
With a mechanically commutated motor, the controller can't easily discern whether the current is distributed through all commutator segments, or is flowing through just one segment. Thus, in general, the controller doesn't have the means of detecting a stall and protecting the commutator.
More sophisticated controllers can detect commutation without any additional sensors on the motor itself, and would thus reduce the drive current upon detecting a stall.
Suppose there's a closed-loop controller that measures both motor rotational speed and the motor drive current.
- Such controllers can unconditionally protect the motor as long as you configure them correctly. It doesn't matter what the type of the motor is, as long as the controller supports that type.
Now note that we merely talked about the electrical aspect of things. There's also a mechanical aspect. If you take a motor with enough rotor inertia, and brake the output shaft hard enough, the output shaft will fail in torsion. This is has some SFX appeal when the motors or generators are at or above the fractional MVA capacity. When their output shaft shears off, it's hard not to notice. Shaking of the floor is rarely optional.
To exemplify the point made in the excellent answers already given regarding the additional power drawn by a stalled motor, here is an example of the compressor motor in our fridge starting up as captured by our home energy monitoring system:
Note the power spike of nearly 700W when the motor first starts (from the starting point of our total home power consumption of about 1.1kW to nearly 1.8kW) then falling off to a steady state of around 100W (around 1.2kW total consumption). However this monitoring system records at 1 second intervals and that spike represents the average power over a 1 second interval. Instantaneous power levels may therefore be (and likely are) much higher than 700W. Keeping this motor in a stalled condition would result in that large amount of power being dissipated in the motor itself since it would be unable to:-
- operate at its (much lower) designed operating power level
- transfer (the bulk of) that energy to the mechanical system (compressor)
The anticipated end result of which would (hopefully) be a thermal cutout in the motor tripping, or that power circuit's breaker tripping, or the power to the house being cut off when the fire department arrived to put out the fire.