# How does an AC motor during dynamic braking act as a generator?

#### Train tech

Joined Mar 10, 2024
9
the title was supposed to be - How does an AC motor during dynamic braking act as a generator?

I will explain my understanding, please explain where i am going wrong.

To create a braking force DC is supplied to the stationary windings once the AC is cut off. This creates a constant magnetic field, inducing a current on the spinning rotor, creating drag and slowing the rotor.

The constant magnetic field created can be used to create a generator, by using the kinetic energy of the rotor. A generator requires a rotating coil in the centre of the magnetic field.

Does that mean the rotor has a coil and brushes so during braking the current created can be utilised? Does anyone have a link to video that explains it?

Also, what’s the difference between dynamic braking and DC injection braking? I feel I might be getting the two confused.

#### schmitt trigger

Joined Jul 12, 2010
925
Yes, two different mechanisms.

DC injection in an induction motor, creates a fixed magnetic field creating a holding torque.
All the mechanical energy is dissipated as heat on the squirrel cage rotor and wasted. This wasted energy will rapidly and significantly overheat the motor, therefore it should only be applied momentarily.

Dynamic braking applies a sinusoidal waveform whose frequency is below the synchronous speed, causing the motor to behave as an induction generator and pump the braking energy back into the motor drive.
Of course this energy can also destroy the motor drive’s electronics if not dissipated somewhere. It can be dissipated with shunt resistors, or used to recharge batteries. This latter method is the best, but also the most complex.

Joined Jul 18, 2013
28,780
DC injection braking works due to the current induced into the rotor shorted turns, (squirrel cage), where the induced current is attracted to the applied stator DC.
usually applied at lower RPM's.

#### Train tech

Joined Mar 10, 2024
9
Yes, two different mechanisms.

DC injection in an induction motor, creates a fixed magnetic field creating a holding torque.
All the mechanical energy is dissipated as heat on the squirrel cage rotor and wasted. This wasted energy will rapidly and significantly overheat the motor, therefore it should only be applied momentarily.

Dynamic braking applies a sinusoidal waveform whose frequency is below the synchronous speed, causing the motor to behave as an induction generator and pump the braking energy back into the motor drive.
Of course this energy can also destroy the motor drive’s electronics if not dissipated somewhere. It can be dissipated with shunt resistors, or used to recharge batteries. This latter method is the best, but also the most complex.
If possible please can you explain dynamic braking in further detail? What exactly happens to switch from a motor to acting as a generator?

#### schmitt trigger

Joined Jul 12, 2010
925
From Wikipedia, bolded words by me.

An induction generator or asynchronous generator is a type of alternating current electrical generator that uses the principles of induction motors to produce electric power. Induction generators operate by mechanically turning their rotors faster than synchronous speed. A regular AC induction motor usually can be used as a generator, without any internal modifications.

If you lower the applied frequency to a value lower than what it would be required for the rotor to turn at that synchronous speed, it becomes a generator.
If you Google the term, there are plenty of drawings and diagrams explaining the concept.

#### MisterBill2

Joined Jan 23, 2018
19,055
Max's explanation in post #3 is totally correct, but not very detailed. For induction motors, which is the kind his explanation applies directly to, there is the external rotating field and the generated magnetic field due to current flowing in the shorted turns "squirrel cage winding. When a stationary field is applied the same rotor windings generate current that produces an opposing magnetic field, which causes the braking torque. The excess power is converted to heat by flowing thru the low resistance winding. An induction motor will also produce some breaking torque simply by short circuiting the line connection immediately after removing the AC power . At least some induction motors will work that way.

Joined Jul 18, 2013
28,780
An induction motor will also produce some breaking torque simply by short circuiting the line connection immediately after removing the AC power . At least some induction motors will work that way.
It is rare to see an induction motor generate.
I have seen at least on installation application that used an immediate contactor reversing of the phases in order to brake very quickly.
Still in use 40yrs later.

#### MisterBill2

Joined Jan 23, 2018
19,055
It is rare to see an induction motor generate.
I have seen at least on installation application that used an immediate contactor reversing of the phases in order to brake very quickly.
Still in use 40yrs later.
Driving an induction motor above synchronous speedis claimed to make it generate usable power. (unverified)

#### nsaspook

Joined Aug 27, 2009
13,443
Driving an induction motor above synchronous speedis claimed to make it generate usable power. (unverified)
The cheap wind turbine generator guys use them. It's sort of a hack to use a 3-phase motor (with residual magnetism on startup) as a single phase induction generator with a wildly varying frequency and voltage output. That output is converted to DC for battery charging and to run inverters for stable AC.

and there is the grid connected versions.
https://www.mathworks.com/help/sps/powersys/ref/windturbineinductiongeneratorphasortype.html

https://www.mathworks.com/help/sps/...inedoublyfedinductiongeneratorphasortype.html

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#### MisterBill2

Joined Jan 23, 2018
19,055
I never meant to imply that it was simple or easy. But quite a few of the electromechanical processes can work in both directions. also, some LEDs can produce a voltage from light. Others do not.
(I never investigated the physics of that.)