# Torque output from an electric motor, when a force is applied to it's movemet

What happens to an electrical motor when a force is applied against it's movement?, an opposite force large enough to lower the rotational speed of the rotor to 0, or even larger. Will the motor continue applying a torque-force? will it have the same output torque than when it's spinning? And if the movement is stoped, but the voltage stays constant on the motor, will it burn up?

• What type of motor?
– user16324
Sep 15, 2016 at 11:11

If a force is applied against rotation, (induction motor for example) slip will increase, causing the current will increase, resulting in a greater magnetic force in order to drive the load.

If the same motor is loaded down to such a degree that the rotor cannot maintain near synchronous (minus slip) speed, and there is no overload device, then yes, it will burn up as the current will only be limited by the winding resistance without any counter EMF. You don't have to get it to zero speed to burn it up, just to the point that it falls below the design synchronous speed minus the slip.

The torque is a different beast. That depends on the motor and the method of control.

In a squirrel cage induction AC motor, across the line, maximum possible deliverable torque occurs at the design speed (synchronous minus slip).

The maximum deliverable torque is typically about 200% of the rated torque. Induction motors are also typically designed for at least 110% starting torque. The torque will drop off to zero at synchronous speed.

With a squirrel cage induction AC motor on a typical vector drive nowadays, the torque can be maintained pretty much constant over the entire speed range without damage to the motor (though a blower might be required).

A DC motor from zero to base rated speed will operate in a constant torque mode, and full torque can be delivered from zero speed. Again, a blower is required. Past base speed (field weakening), the motor will be operating in a constant horsepower mode; the deliverable torque will drop off as the speed increases above base in a linear relationship.

• Oh wow thanks a lot for this answer, it's really helpfull. Sep 17, 2016 at 21:40
• DO you know if there's any way, or any type of electrical motor, that can have a Stall torque, meaning a torque produced while having a its output rotational speed lowered or zero, without damaging the motor? like with our arm we can hold a cup in the air, having the constant force output to hold the cup in its place, and we can even add water to the cup, increasing the necesary force output from our arm, without damaging the arm. Is this possible for any type of electric actuation? Sep 17, 2016 at 21:49
• Yes, a DC motor can easily, and an AC induction motor on a modern day vector drive can as well. In both cases, external cooling may be required. Sep 19, 2016 at 10:29