i have about 5 ac induction motors, 10 to 50hp, which start across the line, for safety reasons i need to fast stop them, some have mechanical brakes already, but they don't work fast enough.

I have found two options so far:
1: dc injenction braking unit
rejected for the following reasons
- heats up motor, the stopping energy is absorbed into motor
- cemf, these units seem to have atleast a 1 or 2 second delay, before applying full braking torque, because the have to wait for the regenerative voltage produced by the motor to decay or risk being overvoltaged.

2:ac drive with dynamic braking
under consideration
-very expensive, not great reliability at 600V, need to replace old motors with new ones or use open core output reactors to protect old motors

anyone have any suggestions?

 

IHMO you can try an "dc injection" old-style,
suppling, with a tri-phase tranformer, 200 Vac to one phase of your motor with a contactor. I've do this many times to stop quickly motors without problem for the motors or for the gear reductions

380Vac 220Vac
| | | | |
| | | | |
\ \ \ KM1 \ \KM2
| |___|___________| |
| | |_______________|
| | |
MOTOR

I think that you'll test this app. in few minuts
with a plc to drive the contactor and regulate the braking time, a tri-phase transformer 380-220
1000VA and an 1,5 HP induction motor

 

For really fast stopping you need really big mechanical brakes. You may also wish to consider a brake/clutch. Since safety considerations are involved a brake/clutch system is the best way to go. These are quite reliable and are extensively used on mechanical punch presses from small to huge to instantly stop high inertia loads. Clutch/brake monitoring controls are also available to monitor the mechanical health of the system and prevent a start if should the brake not be able to stop the system. If safety is involved, dont rely on DC injection, inverters, regeneration, or even reverse plugging the motor.

 

Ooops!!!
A little mistake!!!!!!!
The 220V to supply at the motor must be rectified with a a tri-phase rectifier bridge!!!

> IHMO you can try an "dc injection" old-style,
> suppling, with a tri-phase tranformer, 200 Vac to one phase of your motor with a contactor. I've do this many times to stop quickly motors without problem for the motors or for the gear reductions
>
> 380Vac 220V rectified
> | | | | |
> | | | | |
> \ \ \ KM1 \ \KM2
> | |___|___________| |
> | | |_______________|
> | | |
> MOTOR
>
> I think that you'll test this app. in few minuts
> with a plc to drive the contactor and regulate the braking time, a tri-phase transformer 380-220
> 1000VA and an 1,5 HP induction motor

 

Respnding to Paul Hrynyshn's Tue, Jan 7, 11:29pm query:

First answer the following questions:

1) Are you interested in safety-first, or cost-first?

2) Can you assure that a source of ac will always available?

3) Have you 'searched' the List archives?

Regards, Phil Corso, PE (Boca Raton, FL) [[email protected]] {[email protected]}

 

It's cheap, easy and effective.
Essentially a reduced voltage starter in reverse. Here we have 480V motors, so to brake them we apply ~80-100V to the motor in opposite phase. A 600-240 transformer brings our 480 down to 192 V then a 480-240 transformer brings us to 96VAC. Use a timer and a reversing contactor setup. It isn't super fast, but it's pretty good, and easy to implement, and easy on the motors.
Good Luck.
Paul Burke
[email protected]

 

Responding to Paul Burke's Thu, Jan 16, 5:29pm, reply:

While the approach will solve the problem, I must caution against its arbitrary use. AC plug-to-stop could, in the long run, have a deleterious effect because of an unanticipated increase in duty-cycle. Thus, NEMA type, insulation class, cooling method, inertia, all play an important role in the motor's life expectancy. A generalized discussion follows.

During acceleration, the kinetic energy is stored in the rotor as heat. Subsequently, at normal speed the heat transfer across the airgap into the stator is a minimum because of the flow of cooling air. This is the case for self-ventilated or forced-ventilated motors. However, when plug-stopped, the heat energy stored n the rotor could be a much as 4 times that of the normal acceleration case. Then, while at standstill the heat is transferred into the stator and its insulation experiences an abnormal temperature-rise. The effect is cumulative, like putting straws on a camel's back.

The positive side of Paul's low voltage approach is that the negative side-effects (excuse the pun) are somewhat ameliorated.

Regards, Phil Corso, PE
(Boca Raton, FL)
[[email protected]] {[email protected]}