For engines when we increase the load on them while maintaining the rpm they start consuming more fuel.

But what happens for an electric motor when we increase the load on it.

Suppose 3 cases then:

  1. What happens when a motor is running unloaded and we start increasing the load on it.
  2. What happens when a motor is loaded with "x" load and running on 50% of its capacity and then we start increasing the load on it.
  3. What happens when the motor is running on its full capacity carry a particular load and then we increase the load on it.

It would be of great help you explain current, voltage and resistance or any other factor involved in it as I am a mechanical engineer so I have little less knowledge about electric motors.

  • 1
    \$\begingroup\$ if overloaded, engines also start to slow down and eventualy stall \$\endgroup\$ – jsotola Oct 31 '20 at 4:54
  • 2
    \$\begingroup\$ If you increase the load on an electric motor, it also uses more "fuel" (electric power). \$\endgroup\$ – Peter Bennett Oct 31 '20 at 4:56
  • \$\begingroup\$ Many sites explain simply how motors work. \$\endgroup\$ – Solar Mike Oct 31 '20 at 4:58
  • \$\begingroup\$ All 3 cases : the motor will draw more current, as well as slow down a little. If you add too much load it stops and draws its stall current. \$\endgroup\$ – user_1818839 Oct 31 '20 at 11:10
  • \$\begingroup\$ What is stall current? If anybody can explain as I am from the mechanical domain so I have no idea about it \$\endgroup\$ – Tushar Dubey Nov 1 '20 at 5:46

It depends on whether the motor is being controlled or not.

Assuming it is not being controlled at a constant speed, then really the same thing happens in all three cases.

As the load is increased, the motor will slow down, and the torque will increase. Current into the motor will also increase.

If the load is increased above the maximum, the motor may overheat after a while. If the load is increased far above the maximum, then the motor may stall and/or trip a circuit breaker due to high current.

Motors with electronic control can maintain constant speed even when the load varies.

Efficient induction motors running directly from utility power can typically produce much more torque than the full load torque. But they will also consume high current in the process. The peak torque occurs at relatively high speed for efficient motors.

Certain applications use less efficient induction motors which are designed to have maximum torque occur at stall speed. I guess these work better for applications where the load changes rapidly and violently (such as rock crushers and punch presses). The more you slow it down the harder it fights back, and it never gives up even when it is stopped.

The more efficient motors will give more and more torque as you load them up but at some point they reach peak torque, and once you slow the motor down below that speed, it gets easier and easier to resist the motor and make it stop altogether. Stall current is still very high even though no work is being done. Best not to stall this type of motor.


The relatively simple types of motors that operate without an electronic control unit are electromagnetically self regulated. The DC motors tend to operate at a speed determined by the applied voltage. AC motors operated at a speed that is determine by the applied frequency but requires the voltage that is appropriate for the frequency. When the load is increased, the speed drops or tends to drop. That causes an electromagnetic imbalance that results in increase current. The increased current causes the motor to develop more torque. The balance is restored when the torque supplied by the motor matches the torque required to maintain the speed of the load.

Every motor has a speed vs. torque capability curve. Every load has a speed vs. torque demand curve. The motor will operate at the intersection of the two curves. The various types of motors have differently shaped torque vs. speed capability curves, but their general nature is for the curve to slope downward from high torque at zero speed to zero torque high speed, with torque on the vertical axis and speed on the horizontal axis.

Electronically controlled motors are much the same, but the controllers adjust and limit the operation along an infinite number number of essentially vertical torque vs. speed curves that can be produced for a range of operating speeds. The controller limits the maximum torque by limiting the current.

That is the general concept for all electromagnetic motors. The detailed theory of operation differs among the many types of motors and control schemes, but the overall concept is generally the same for all of them.

See curves below for squirrel cage induction motor (SCIM), wound rotor induction motor (WRIM), series DC, shunt DC, permanent magnet DC (PMDC) synchronous and electronic speed controlled motors. Note that the some curves cross the maximum safe current line indicating that the current must be limited during starting.

enter image description here


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