# Why does shorting/putting heavy load on a motor slow it/stop it?

I am doing a simple circuit which includes using a motor in theory (I am using a stepper-motor) as a break. One of the random fun-facts that I have known for a long time is how shorting a stepper motor (or putting very high load ) will cause high current and slower rotation (or sometimes stopping rotations all together).

However I do not understand how it works from an electrical/electromagnetic point of view. Where does that force come from?

• Mar 28, 2017 at 15:18
• The difference is that I am trying to understand the issue from a physics point-of-view Mar 28, 2017 at 15:20
• hyperphysics.phy-astr.gsu.edu/hbase/Forces/funfor.html#c3 Mar 28, 2017 at 15:48
• Torque is produced by current and the action reaction is reversible. RPM induce Back EMF reduce the voltage applied and thus the current drops with no load to just account for losses. Mar 28, 2017 at 15:51

Two concepts: conservation of energy, and reverse EMF (electro-motive force). When you move a magnet past a suitably oriented inductor coil connected to a load, the changing magnetic field induces a current in the inductor. The current in the inductor creates its own magnetic field. That newly generated magnetic field is oriented such that it produces a force that tries to accelerate or eject the magnet in approximately the opposite direction of its present motion. The kinetic energy absorbed from the magnet as it slows down will be proportional to the energy that heats the load on the inductor (and other system losses), thus conserving it.

Conversely, if you open the circuit, there is no place for current to flow, and thus no less reverse or back EMF produced.

• Back EMF is produced regardless of load. The moving magnetic field creates an electric field in the winding. The current is dictated by the load attached. Mar 28, 2017 at 16:27

When a motor is free-wheeling it turns into a generator.

By shorting out the motor you are applying a LARGE electrical load to the motor. If you want the generator to keep turning you need to apply a large torque.

Since all you have driving it is the inertia of the motor and whatever it was driving, this torque turns into a braking force.

You can think of this another way. Suppose you hook up a generator to a handle and wind the handle with nothing connected to the generator. The generator will turn quite easily.

Now attach an electrical load to the generator. You will now find it is harder to turn the handle at the same speed. Note: YOU have to supply the power to run the load with your arm.

When you short out the generator, you are affectively applying a VERY LARGE LOAD. Turning it by hand becomes REALLY difficult.

You just built a brake.

• My question was why this torque is formed in an electromagnatism-source. What do you call that force that apposes the turning and causes the breaking? Mar 28, 2017 at 15:10
• @ZaidAlShattle, as I said.. "braking force" Mar 28, 2017 at 15:11
• While I get the concept I tried looking up braking force but was unable to find the source of it, Is it from the back emf for example? Why does it become a break basically physics wise Mar 28, 2017 at 15:17
• @ZaidAlShattle Yes, the braking force is a result of the magnets in the motor trying to create a current when you spin it and the resistance and inductance of the coils trying to prevent it. Mar 28, 2017 at 16:00
• Thanks for the answer. I have upvoted the answer but I found hotpaw2's answer to be more towards what I need so I will be picking it as the best answer though. Thanks! Mar 28, 2017 at 16:25

All motors can operate as generators under the right conditions. Shorting the terminals is one of those conditions for some motors. A generator applies torque to whatever is turning it and mechanical energy from the source, rotating inertia in this case, is converted to electrical energy. If the motor is shorted, the energy is dissipated in whatever conductors are carrying the current including the motor windings.