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What happens if an EV's motor is stalled, i.e. prevented from rotating while attempting to accelerate? Can this result in over-current damage to the batteries, motor, or speed controller circuits etc.

I know that for normal hobby DC motors, this scenario can be extremely harmful and will almost certainly result in severe (often catastrophic) damage to the motor or controller circuits, if the stall condition isn’t promptly resolved.

How is this risk mitigated in an electric vehicle?

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  • $\begingroup$ The motor driver should be designed to handle this by limiting current. It certainly would be on something as full blown as an electric vehicle's motor driver. $\endgroup$
    – DKNguyen
    May 26 at 2:53
  • $\begingroup$ Most modern cars don't use normal hobby motors, they use synchronous reluctance or "SynRM" motors. They output much less power in stall conditions than normal DC motors. As others have said, the motor controllers also probably include protection features. $\endgroup$
    – Emily Conn
    Jun 22 at 17:41

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A stalled motor's cooling fan is not turning, at the same time its windings are dissipating lots of electrical power into heat. The heat ruins the insulation on the wire in the windings, and the windings then get short-circuited. This lets still more current flow through the motor, and the drive circuits themselves then fail due to excessive power dissipation in them.

This locked armature condition can be protected against by monitoring armature RPM as a function of drive power. High drive power + no RPM = locked armature which condition causes the driver circuit to fold back i.e., to reduce the drive power automatically to a value that is known to be survivable by a motor with no fan cooling.

It is also common to put thermistors on the drive circuit transistor heat sinks and on the inside of the motor casing, to generate a power shutdown command if either set of components gets too hot for any reason.

Alternatively, it is easy to implement a current sense circuit on the driver board which automatically limits the drive current to a level which will not overtemp the output transistors.

Note in this context that each of these error condition detectors is also used in audio amplifiers of modern design; in fact, you will almost always find at least two entirely independent power interrupt systems in audio amps to redundantly catch any error condition that could arise during operation. One important difference between the first generation of solid-state audio amps designed in the mid-1960's (which were notoriously prone to blowing up) and all later designs is the presence of more than one error condition detector circuit.

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    $\begingroup$ Minor correction: you don't know that the cooling mechanism, if any, depends on the motor not stalling. The motor could be fluid-cooled, with the fluid circulation controlled with a separate pump. $\endgroup$
    – Carl Witthoft
    May 26 at 12:51

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