I know that low voltage damages AC induction motors and usually under voltage protection relays are used to trip the motor in such conditions. But the theory behind that is confusing to me. After searching on the internet I found the below answer (still not clear enough for me.)

P=VI, motors are manufactured to deliver the power P. Since V reduces, to keep P constant, the current gets increased and damages the motor.

If this statement is correct supplying low voltage to a bulb should result in lighting it more brighter.

In a motor, the magnetic flux is the component which creates the rotational force, and magnetic flux depends on the amount of current flowing in the wire (not the voltage.) So my fact is if a high current flows through the stator coils is should create the required magnetic field and motor should work.

I know that I'm understanding something wrong here. Please, I hope an electrical engineer or some expert can help me understand the theory behind it.


In AC induction motors peak torque, that which is used to accelerate it from a stop or to recover from a step change in load, is related to the square of the voltage change. So if the voltage is 80% of normal, the peak torque capability of the motor will be only 64% of normal; at 70% voltage, you are down to less than half of the available peak torque. That means the motor is more easily able to stall and even if that doesn't happen, the slip of the motor will increase so if it pulls the same current, it will do less work, meaning the efficiency becomes poor and more of the energy consumed just becomes heat in the motor. Or if doing the same work, it will pull more current and current comes with heat. the latter scenario, more current, leads to hopefully tripping an overload protective device but the lower efficiency may increase the motor heating effects even though the current is low enough to not affect the protective device.

If the load on the motor has decreased as the voltage dropped, or the motor was originally grossly over sized, that may not be a problem. But generally a motor is selected to provide just enough power as was necessary.

  • \$\begingroup\$ Exactly answered the point of my question, It's all a matter of efficiency. Thanks a lot. \$\endgroup\$ – danial weaber Sep 19 '17 at 7:50

The motors generate a voltage that reduces the current with increasing RPM like any generator. So the stall or start current is only limited by the DC resistance of the coil.(s) as some have starter windings too.

If the voltage is not high enough to cause enough rotation then the stall current of 300% to 10x is enough to create more heat than at full RPM and rated current.

Industrial motors are often design for low surge of 300% while starting cap type induction motors boost current until it reaches a certain inertial RPM threshold then switch off the starter cap or by some other means.

  • \$\begingroup\$ Usually, we set 10%(plus or minus) threshold in PFR to trip the motor in under or overvoltage conditions. So is it effective only when starting the motor? Let's assume a motor rated 400v once it is started can still rotate at 350v. So if the motor started successfully with 400v and if the voltage drop to 350v is it safe to operate it. Is this under voltage protection only required when starting the motor? \$\endgroup\$ – danial weaber Sep 18 '17 at 4:57
  • \$\begingroup\$ Please explain to me how the voltage affects the rotation, as I can understand the current is the factor which creates the magnetic field to generate the required torque. When a high current flow why doesn't the motor start while it's having a high current hence a large magnetic flux. \$\endgroup\$ – danial weaber Sep 18 '17 at 5:05
  • \$\begingroup\$ electronics.stackexchange.com/questions/294449/… \$\endgroup\$ – Tony Stewart EE75 Sep 18 '17 at 5:15
  • \$\begingroup\$ Thanks a lot, sir, I think I understood it now. The load-torque-speed characteristics curve explained it. \$\endgroup\$ – danial weaber Sep 18 '17 at 5:38

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