Overloading induction motors

Thread Starter


Joined May 11, 2016
Hi friends
I am mechanical engineer working on axial fans .I've already read that these kinds of fans are driven by induction motors. Now I want to ask what will happen if we overload these motors( for example block the fan), Does the speed changes? if yes, Is there any limitation?


Joined Feb 5, 2010
Measure the ohms resistance of the motor coil.

Using 120 or 240 as voltage figure the current flow using ohms law.
This is the maximum current the motor can allow to flow if the fan stalls completely. If this results in a survivable rise of temp then your fan will be ok. If the heat build up is too much you will melt the fan.


Joined Nov 30, 2010
what will happen if we overload these motors( for example block the fan)
The best way to overload an axial fan is during the design. Put too much fan balde on the motor and you can get the motor to use too much current. After a proper design is finished, blocking the air flow causes the fan to lose its grip on the air. It can not inflict energy on the air stream and so the motor runs, "unloaded" with a blockage of the air.

If you design a motor to work into a certain pressure difference, it will use more power if you remove the pressure difference by running the fan in free air.

Glenn Holland

Joined Dec 26, 2014
If by "block the fan" do you mean block the airflow?
If the air flow is blocked at either the intake or the discharge, the fan will have less load and the motor will speed up.

As a common example, cover the intake of a vacuum cleaner and you will notice the motor speeds up, but slows down the intake is open.

If you are working on air handling machinery, there is a special branch of engineering for fans, blowers, and pumps and the pressure, volume, and power characteristics for each configuration are in shown in charts in accordance with "Fan Laws".


Joined Apr 4, 2016
When you block the airflow, the fan motor will probably be "underloaded". The motor speed won't change much as the speed of an induction motor depends on the supply frequency minus a "slip" which is fairly constant.

If the motor is mechanically overloaded then the slip will increase (slightly) and the motor will overheat. With a large overload, the motor will stall and burn out very quickly.

You can detect if the motor is overloaded by measuring the motor current which should not be more than stated on the plate. Or by measuring the RPM which also should not be less than stated. An induction motor should be protected by a thermal overload relay for reliability.