A Back EMF Query

[GeeKay]

A friend of mine once observed a puzzling phenomenon, which he has never been able to properly explain. This event took place long ago when he was doing an apprenticeship in electrical engineering at a naval dockyard and it concerned a 'runaway' effect that occurred when he was part of a team testing large industrial motors. The cause of this effect, as he recalled, was associated with 'back EMF' and it (or something akin to it) caused the motor to speed up uncontrollably. The runaway effect was so powerful that the instructors had to pull the plug on the motor and/or physically bring the armature to rest before the motor tore itself apart. My friend is a self-employed electrical engineer, yet he confesses that he is still unable to understand why the above effect took place. Therefore, he (and I) would very much welcome an explanation as to its likely cause, or the underlying principles that brought it about. Many thanks.

[Danijel Gorupec]

When you switch off the magnetic field (by switching off the field current), a DC motor will perform the runaway.

 

A non-loaded DC motor (and similar types) will accelerate until its back-EMF (almost) equals to the supplied armature voltage. The back-EMF generated by a motor depends on its rotation speed and the strength of the magnetic field inside motor. If you decrease the field strength without decreasing the supplied armature voltage, the non-loaded motor will have to accelerate further to generate enough back-EMF.

 

Note however that the runaway will only happen if the motor is under light load (or non-loaded) because as magnetic field strength decreases, also the motor torque decreases.

[Enthalpy]

It happens with DC or universal motors whose excitation coil is in series with the rotor. Without a mechanical load, the rotor current is small, and so drops the excitation current and resulting field. As a consequence, the rotation speed increases, and is known to potentially explode the rotor.

 

It happened (...to professors) in my engineering school. The rotor didn't explode, but it jumped out of the bearings through the stator. Then, it accelerated on the ground and smashed a hole in a stone wall, whose repairs were still visible (and commented) years later.

 

Similar things can happen on a turbo-alternator if the electrical load is lost and the turbine still receives vapour. Some power plants with several turbo-alternators orient them to minimize damage (think of nuclear reactors) in such an event, older ones don't.

[GeeKay]

Many thanks for explaining in some detail the cause of this effect. Enthalpy: your recollection of the runaway rotor still has me chuckling