Dynamic Braking

Yep, you've stumbled on the fundamental of a dynamic brake. The momentum of the rotor and whatever it spins generates the back EMF of the electric motor. By shorting out the motor, instead of waiting for bearing friction and wind resistance to stop the motor you briefly continue to heat the wiring with the mechanical stored energy of inertia. If you connect a capacitor instead of a short circuit then energy gets stored on the capacitor. But unless some steering circuitry gets added, the capacitor enegy will get dumped back into the motor, etc.

Hi redfred;

I thought that high-power resistors were used for controlling the braking. Is that right?

Correct me if I'm wrong. It is my understanding that there are two types of braking, dynamic and regenerative. In the former, the motor/generator output goes to the resistors that is wasted in the form of heat. In the latter, power is fed back into the power grid. Regenerative braking only works on DC motors. A generator produces power when a magnetic field is present as in permanent magnets. An AC motor can't function as a generator (alternator) without some source of excitation voltage.

I only make these statements from a laymans view and I may be wrong. Please enlighten me.

Ronseto,

You are mostly correct. Dynamic breaking all mechanical energy is lost and dissipated by resistors. Dynamic breaking can be used all the way to a full stop. In regenerative braking mechanical energy gets regenerated into the grid or power source. But a 3 phase AC synchronous motor can be used as a 3 phase AC generator. The rotor magnetic field(s) gets externally driven by a DC power source with capabilities of changing the current and therefore magnetic fields of the rotor. Regenerative braking cannot be used all the way to a full stop.

Good catch.

Thanks Redfred for the education. I learned more from you than if I were reading a textbook for hours.

The third type is the metadyne.

"But unless some steering circuitry gets added, the capacitor enegy will get dumped back into the motor, etc."

By steering circuitry do you mean some sort of resistance to control the amount of braking?

Steering circuitry in the form of diodes and transistor switches. For without these controls the power comes out of the motor into the capacitor until the rotor inertia makes the motor back EMF to drop below the capacitor voltage. Then the capacitor starts to accelerate the mass until the capacitor voltage drops below the motor back EMF. This oscillation of energy drops some on each translation from wire resistance losses, core losses and other mechanical losses until eventually the energy stored in the capacitor is insufficient to exceed static friction and the wires just dissipate heat.

Depending on your motor and application you can also employ 'DC injection electronic braking' where you disconnect the AC supply to the motor and inject DC across one of the windings to stop the motor faster. We employed this for a custom-built fabrication saw table (3ph Delta wound) as an additional safety feature that operates every time the saw is stopped and it works very well (I designed and built the system myself).

Google 'dc injection brake' or similar for more information.

It should be noted that connecting a resistor across the terminals only works for a capacitor run motor, a universal (AC/DC) motor, or a wound rotor induction motor (with the right accessories) because in order for a motor to become a generator, the magnetic field must be maintained. The caps will do that in a cap-run 1 phase motor. But if you just apply a resistor across the output of a 3 phase motor or a 1 phase shaded pole motor, it will not do anything. By the way, this is NOT considered "dynamic" braking, there is nothing dynamic about it. The braking energy is decreasing with speed and kinetic energy is irrelevant, whereas in a true dynamic brake, the braking energy is constant and the higher the load inertia, the more total braking energy you actually have, so it is "dynamic" in that respect. So in short, "resistance braking" is different from "dynamic braking" and is only suitable for a limited number of motors with low inertia loads (on alternate Tuesdays with a full moon...)

There are many multiple ways to brake an AC motor, it all depends first on what KIND of AC motor you have.

"Dynamic braking" generally refers to using a VFD or some other electronic power supply on a 3 phase motor to continuously excite the motor windings into being a generator (actually an alternator) and bleeding the resultant energy into resistors as heat.

A subset of that is Line Regenerative Braking that pumps the energy taken from the motor into the AC supply source. Another subset involves having multiple motors on the same machine and using the braking energy of one for other motors that are still operating.

DC Injection braking is where you pump DC into one set of windings of an AC motor and create a stationary magnetic field, which in turns creates a counter-rotating field in the rotor and stops the motor quickly. Again, this is not dynamic because the braking enegy is decreasing as the speed does, plus the motor rotor is having to absorb all that energy internally. So it is limited in that aspect, you can only brake occasionally.

Resistive braking, as mentioned above, has very limited applications on very specific types of AC motors, i.e. 1 phase cap-run and some types or would rotor specialty motors.

There is also a method called PLUGGING which applies DC to the AC motor.This is sometimes used in elevators to slow the doors in the final few inches of travel.Also instrument chart recorders sometimes employ this method, as it is very economical to implement.