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DC brake for AC motor
DC brake for AC async. motors
By M.Ghaffari on 25 July, 2006 - 3:17 pm

Who can help me about the theory of DC brake (DC voltage connected to two phase) for induction motors?

thank you.

By Dennis Patterson on 26 July, 2006 - 8:58 am

Well if the DC was isolated then to a certain point it would brake the drive.

But why you would do this is beyond me. You could damage the windings by not evenly loading the dc across 3 phases, and for how long would you do this? if you are trying to brake the drive, then im assuming its on a VSD anyway, if so then use the DC braking feature of the VSD or use a resistor bank, which should also be an available control from the VSD. typically you dont DC brake until the drive has nearly halted, if you want to brake from full speed then the resistive braking should be used me thinks.

By M.Ghaffari on 29 July, 2006 - 2:28 pm

Thank you for your comments. Really there are some brake chopper in our machine to stop the 3phase induction motors I like to know more about it do you have some information?

Mr. Ghaffari,

Is this brake connected the shaft of your 3 Phase Induction motor? How are you driving the induction motor i.e. Simple starter or VFD (Variable freq. Drive)? We use DC brakes in some applications for Emergency Stop situations the motors in our case are driven via VFD units. The power supplies are simple 90VDC rectifiers. I'm not sure what type/brand rectifier you are using. Please explain a little more about your application.


By M.Ghaffari on 5 August, 2006 - 11:52 am

Dear behrouz
The brake is not connected to shaft actually it is not an electromagnetic Dc brake this unit apply a Dc voltage between two phases after the main 3 phase voltage cut off and it stops the motor. I awnt to know how much the voltage is and how we can calculated required voltage.
thank you

I am by no means an expert on DC Brake Units but I think a place to begin for calculating the voltage requirement for dc injection would be the motor winding Resistance and the rated motor current. I think with these parameters you can calculate the voltage applied to the windings. Try simulating it using a spice software. good luck


By Anonymous on 15 June, 2007 - 11:33 pm

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By Phil Corso, PE on 12 August, 2006 - 8:42 pm
1 out of 1 members thought this post was helpful...

Responding to M.Ghaffari's Aug 5, 11:52 am query... the DC current
magnitude can be found in the List Archive as Thread #:

Phil Corso, PE {Boca Raton, FL, USA}
[] (

The theory behind DC injection braking is that if a DC current in injected into a 3-Phase AC winding, with the rotor still in motion, a pulsating Torque will be produced, in effect bringing the rotor to stand-still.

If you are using a variable speed drive, it's easier to connect a resistance across the terminals of the braking chopper (basically a power transistor circuit), of the variable speed drive. Tthe drive manual can tell you where to connect the resistor. The wattage and the value of resistance will depend on the drive rating and the inertia of the load. Many AC drives also have the feature of DC current injection to produce braking. But it is not as efficient as Dynamic Braking (connecting resistor).

Usually you have to set the parameters for "current" and "time". Current can be equal to the rated current, and time may be 2 to 3 seconds.

If you are not using a variable speed AC drive, and your load requires braking, then there are two ways: by Plugging, which means reversing the supply,or by DC current injection into any two terminals of the AC motor.

I have handled many motors which have operated for years with this type of braking, without damage to the windings. Class F insulation for winding is recommended.

For such cases, the DC current can be equal to the rated current, and the time can vary from 2 to 3 seconds. In fact some experts suggest 150% rated current for effective DC injection braking. If the motor is rated for 415 Volts, a DC voltage of 110V can be used for the purpose. And most important, a high wattage resistance must be connected in series with the DC source to limit the current, since the motor inductance is virtually a short circuit for the DC source.

I remember reading some excellent material in ABB website on electric braking. Look in their "Technology" section.

By purushotam kumar panchal on 24 April, 2012 - 7:22 am

D.C. brake works on torque basis. in our brakes torque will made by springs tension, ie springs generate torque for stop motor free rpm. when d.c.current removed from the brake, brake torque will be works & when we start motor current will flow in brake by D.C. circuit, that time brake works for release torque than motor running freely & working.