braking system for 55kw motor.


Thread Starter

yogesh m.tapaswi

we have 55KW AC motor on surface grinder without drive. we want to have braking for it (without drive) Of the DC injection breaking and method of
reverse rotation which one is better. Will there be any problem to the motor if made reverse by contacter when rotating in fwd direction?
You are better off using a DC injection style of brake rather than a reversing starter (plugging). A simple starter can be rigged with a diode bridge and resistors to get the proper dc voltage for stopping power. Better than that if you were to look into softstarts Baldor has one with dc injection braking as an option.
good luck
Definitely use DC injection method. The reverse rotation will result in excessive amperage on winding of motor and will overheat. Also this will cause overloads to trip and also contacts to weld. I had an experience with a miswired bypass contactor on a softstart where this happened.

Phil Corso, PE

Responding to Yogesh Tapswi's Mon, Dec 31, 2001, 11:02am query:

A) Some background for the uninitiated.
Overlooking mechanical braking methods, there are five usual methods to electrically brake an induction (asynchronous) motor. Mr. Tapswi mentioned two: dynamic (dc), and plugging (ac reversal.) The former, called dynamic-braking requires disconnection of the ac supply from the stator, and then application of a suitable low-voltage dc supply. The latter, called plugging, is accomplished by reversing two of the stator leads until the motor is nearly stopped. BTW, the other three methods are: regenerative-braking; rheostatic-braking; and eddy-current braking.

B) Dynamic Braking Method.
Pro: it is possible to adjust the dc and therefore the braking effort; motor heating is less; and the motor cannot reverse because torque at zero speed is zero.
Con: needs a dc source, hence higher cost; and unfortunately maximum stopping torque occurs near zero speed meaning a longer time to stop.

C) Plugging Method.
Pro: lower cost since circuitry is less complicated; and the motor will take less time to stop than the dc method because maximum stopping torque is nearly available at the moment the ac supply is reversed.
Con: the motor can speed up in reverse direction (unless a zero-speed switch is used to disconnect all power); and motor heating is

I hope that the above is useful to you. If you need application information, please contact me.

Phil Corso, PE
(Boca Raton, FL)

Phil Corso, PE

Attn: Messrs D. Bacon & R. Chavi

Please note that the Pro/Con list shown in my response on Mon, Dec 31, 2001, 5:11pm omitted a key point... braking upon loss of power. Braking reliability of either the dc or ac methods discussed is low if there is not a alternate supply.

The best method to accomplish this would be the dc approach, but only if it can be supplied from an alternate source.

I recall one technique for a very small small motor (<5 Hp) using two phases of the disconnected motor to supply a single-phase bridge connected to the third phase. I don't remember details except that each of the motor's three phase windings were accessible. I also recall, that even though there was some braking effort, expected performance was disappointing.

And now for a "rest of the story" story:

The rescue ship shown in the movie "Perfect Storm" was named after the world's largest ocean-going tug, the USCG Tamaroa. Unfortunately, it was sunk three times: in WW II; in the Korean war; and in the late '50s dry while in drydock. The third time was caused by an irate coastguardsman who scuttled the drydock.

And now the rest of the story... After refurbishment was complete, the Captain of the ship committed suicide. Needless to say, as part of the refurbished team, I declined to go on sea trials.

Phil Corso, PE
(Boca Raton, FL)
Responding to "Unknown's" Wed, Jan 2, 2002, 1:39pm, comments on the subject:

Your observation of excessive plugging current (often thought to be 2 times starting inrush) is a fallacy.

For simplicity, rated current at design speed is essentially determined by an impedance comprised of the both the stator and rotor reactances and resistances. The equivalent rotor resistance varies as the inverse of slip speed. Therefore, at rated speed, the reflected rotor resistance is R' = R/s, where s = slip speed at rated load. For example, if s = 5%then R' = R/0.05. But, when two stator leads are interchanged for plugging the relative slip speed is not s, but instead 2-s. So now R' = R/1.95 for the example given. (An aside: slip speed at standstill is 1). When the inductive parameters are included, the resultant impedance will yield a plugging current that is more likely to be some multiple of full load current... I will even concede that it could even equal starting inrush current, but never 2 times inrush current.

Regarding your comment about welded contacts, I believe that contact chatter is the more probable cause, especially if the overload devices operated.

Phil Corso, PE
(Boca Raton, FL)