Dynamic Braking an AC Motor

Safety should dictate whether you diy or use a commercial product. Here are some links you may find useful:

http://www.irctt.com/TFS/TFS_Application_Note.htm

http://www.joliettech.com/saftronics-vg10_ac-drive_sdbu_dynamic-brake-module.htm

http://www.softstartusa.com/dcinj.htm

Depending on the load, it will get hot. The energy has to dissapate somewhere.

You may want to contact IPC, Telema Berger, or Post-Glover to get resistor recommendations.

For dynamic braking you can connect a low-voltage DC to one phase-coil. It will excite the stator and the induced current in the rotor will brake the motor. The exciter current will be determined by the DC voltage and the Ohmic resistance of the phase coil (and the applied serial resistor). The higher the DC current is the higher the braking torque will be.

The simplest method is when you remove the power from the motor, you place a short across the motor, as the motor, while free wheeling acts for a short time as a generator, you short this output and "overload" the generator, cvausing it to stop....

For a 20HP motor you will need some hefty contacts.......

It is used in good quality hand tools to stop them running on after power is removed....

It usually stops a motor in 1/2 second or less....

I didn't think a 3-phase squirrel cage motor would regenerate, but a wound rotor would; if the rotating field is still energized. From my posts of late, it should surprise no one if I am wrong. I learn a lot by asking dumb questions that no one else has the nerve to ask.

And, if you single phase a 3-phase hoist motor under load, it is going to run away. Well, until something tells the brake to set. Now this part is off topic but we once had a 400 ton shear-leg crane with a DC motor that was powered by 440 input into the motor controller, and it would regenerate and pump back power into the utilities and get money back from them. Never did understand how that worked.

Even a squirrel cage motor will generate when the power is removed. This also needs a 3 way short via hefty contacts.....

Until the fields decay they regenerate alright. Capacitive braking is an option, but DC injection as mentioned previously is quite effective.

Technically, "Dynamic" Braking (a.k.a. DB) refers to using a power source that can keep the motor windings excited at frequencies lower than the rotational frequency and turn the AC induction motor into a generator, then take the regenerated power out to a resistor. It is dynamic in that the more kinetic energy there is in the load, the more braking energy you get, but it is a diminishing return. At some point it cannot finish the job because you are putting less load on it as it approaches zero speed. So with DB, you must finish the job with something else, i.e. DC injection or friction braking. The great part about DB is that the kinetic energy is moved out of the motor into the resistors. DB is usually a feature of VFDs, but there are stand-alone DB systems for non-VFD applications.

Similar to DB is regenerative Braking, where instead of pumping the regenerated energy into resistors, it is re-converted to AC power and fed into the line. Other than that, all the features of DB are similar.

DC Injection braking is where you use a DC source to inject DC onto the AC windings. The stationary DC field it sets up in the stator creates a counter-rotating magnetic field in the rotor which opposes the direction of rotation. The good part about this is that it requires far less hardware than DB if you don't have a VFD and it is inherently possible if you do have one. The down side is that the kinetic energy in the spinning load is forced into the rotor, so the braking cycle is just as hard (thermally) as starting the motor, making it more limited in scope.

So if you have a VFD, the "perfect" solution is to use BD to bring the speed down by 90% and DCIB to finish the job quickly. This is the fastest way to stop an AC motor.

A third type of braking is load braking, the process mentioned above of placing additional load on the motor, such as a resistor, that takes advantage of residual magnetism in the motor to create a limited amount of regenerated energy and bleed it into the resistor. This is NOT "dynamic" in that the amount of braking energy is not dependent upon the kinetic energy of the load, but rather the small amount of residual magnetism left in the motor when de-energized. Some manufacturers, when designing motors for small appliances and power tools where this kind of braking is desired, will specifically design in materials that will retain more magnetism after being powered down. A good example that many people have seen is newer portable circular saws that stop the blade in seconds when you let go of the trigger. But this kind of braking is unreliable on standard off-the-shelf AC motors. It might work, it might not. And ecause the materials that retain more magnetism tend to have more magnetic reluctance, they are one of the first things designed OUT of energy efficient motors. So if you have a newer high efficiency motor it is less likely that you will have enough residual magnetism to realize any kind of effective braking.

One absolute truth: no matter what technology you use, you CANNOT stop an AC motor electrically any faster than you can start it across-the-line (DOL). Keep that in mind. If the motor MUST stop faster than that, you must use friction braking.

Not only braking but motor reversals for applied loads also utilizes dc injection were rotational reverse motion less than 2secs or so was requirement . Usually power semiconductors are used for such applications

The other way not mentioned is "Plug Braking" which is phase reversal and a zero speed relay.