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DC brake on an AC motor
My freewheeling fans are causing premature bearing failures in the gear reducers.
By Hilton Lawson on 30 August, 2006 - 12:25 pm

My problem is that I have several motors on cooling tower fans that drive the fans through a gear reducer (5.5:1). During the summer months we need the additional cooling of the fans, during the winter months the fans ore off. The problem comes during the winter months because the fans turn slowly from the rising heat. The gear reducer bearings are lubricated with oil slingers and the slow rotation causes the upper bearings to run without oil thus causing premature bearing failure. Four of the motors are 30 hp and one is 100 hp, all are induction motors started with across-the-line 3 phase starters @480vac.

My plan is to apply a small DC current to two of the phases when the motors are stopped to act as a holding brake. My concern is that the motors will never come to a complete stop on their own and the regenerative effect will blow my DC circuit. My plans are to start a 10 minute timer from a starter auxillary contact that will energize a curcuit consisting of a 1kv full wave bridge, a load resistor, and a grounded shunt resistor. the purpose of the load resistor is to limit the current to the motor windings, the purpose of the shunt resistor is to limit the back emf from the regenerative effect.

Has anyone seen such a circuit in action? Are there other considerations that I have missed? Keep in mind that this is a very wet application and there are chlorine based chemicals present so any type of mechanical brake is out of the question.

By Steve Myres, PE on 30 August, 2006 - 11:49 pm

How about a bolt through the fan cowl for winter operation? The fan will turn till the blade hits the bolt and stop. If you want to interlock the starter, put a prox that sees the bolt head when in place and won't energize the starter for that fan.

Or instead of a bolt, you could use a solenoid, unless that's too much like a mechanical brake for corrosion purposes.

I am not sure that a DC current on one winding of the stator will work very well due to the fact that the rotor will not have any particular magnetic orientation with which to apply braking torque against.

1 out of 1 members thought this post was helpful...

Responding to H. Lawson's Aug 30, 12:25pm dilemma... I've never tried this but interlocking implementation should be far less complicated than the system you propose:

Rather than the DC source (I don't understand the 1kV bridge comment), resistors, grounding, etc, install a 3-phase variable output transformer, like a Variac. Connect it so that phase rotation opposes motor rotation. Input voltage to the transformer should be the same as the motor's. Adjust transformer output in 10% steps, of the motor's nominal rating.

The most difficult part is to obtain a current that will stop rotation without unduly heating the winding, especially if the motors' are fan cooled. I suggest the following:

For each of the steps noted above, determine the rate of temperature-rise by carcass temperature method, or preferably, by winding resistance measurement.

If you need additional detail, contact me!

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

By Curt Wuollet on 31 August, 2006 - 12:54 am

It'd be easier to simply start the fans once a day or so to keep the bearings oiled. I'm not sure that you can get enough DC braking long term without prohibitive current. Of course, keeping the motors warm (and dry) is a good idea anyway.



Won't work. DC injection braking works by creating a stationary field in the stator. When the moving rotor passes through this field, it creates its own counter rotating field that opposes the direction of rotation, AND limits current in the stator. Once you come to a stop, no more counter rotating field, no more braking torque. Plus, not you no longer have the counter EMF acting upon your stator circuit so the current spikes, possibly damaging the windings. You current limiting resistor will work to prevent that, but it will also prevent any useful braking torque at the same time!

Most DC Injection Brakes have what is called an "anti-windmilling" mode of operation. This means they are interlocked with the motor starter to brake PRIOR to starting the starter instead of after stopping. This solves the windmilling problem of your across-the-line starting causing the motors to damage the fans, but it still cannot be used as a holding brake for more than a few seconds.


1 out of 1 members thought this post was helpful...

While researching some of the responses to H.Lawson's Aug 20, 12:25pm query... I found a 1974 article describing a solution to your dilemma! Some 20- 60Hp machines were involved. Following are the advantages cited:

o Low voltage DC injection to only one phase.
o All components located at the MCC.
o Eliminated the frequent daily 'bumping' or 'jogging' operations.
o Reduced wear & tear on electrical components.
o Eliminated the need to dispatch personnel to the motor site.
o Included a novel, but simple interlock between DC and AC circuits.

If interested contact me!

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

1 out of 1 members thought this post was helpful...

For Hilton Lawson... I duplicated the anti-windmilling system described in my Sep 11 response. Although 1-ph instead of 3-ph, one fan was "windmilled" by a second one. The applied DC was 10% of nominal. It was successful.

If you want details contact me!

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