OT: Ever used a 3phase induction motor as a brake?

[Sliver]

The application is on a ball mill inching drive.

The current setup uses a 75hp 575V 1800 rpm motor to position the mill for lining up the access doors for removal and mill entry.
The procedure is to engage an inching 'clutch' which meshes in the forward direction only and drive the 4800hp sync mill motor to align the doors a certain distance past top dead center. The balls in the mill then are all piled up on the forward side and to relieve the stored energy the mill is allowed to roll back through a friction brake that needs to be released in a very controlled manner to prevent overspeeding the inching drive.
This procedure is very tricky the first few times and if the stored energy is not properly released, we have had the mill shift by 6" or so with people inside. They were not happy.
A suggestion has been forwarded from another plant that uses the inching drive motor to control the roll back. Once the mill is rotated past the position required the motor is reversed (they use reversing starter) and the motor acts as a brake to bring the ball charge down to the bottom in a controlled fashion.
My question is if the drive has the tourque to push the load does it have the ability to hold back the overspeeding torque?
What would happen if the backspinning motor was allowed to accidently freewheel for several seconds before the reversing power was applied? If we lose the motor on fuses or overloads then you would have to be quick on the manual brake I think before the motor and reducer explode.
Anyone see or hear of a setup like this?

thanks,
Brian.

 

[mrm37]

We use drives to start and stop machines but nothing with that hp. We can controll ramp up or ramp down speeds. They also work as brakes at 100% torque in a few cases. We use all AB products and the slc analog in/out card actually controls a drive.

 

[kamenges]

The short answer is yes, a 3-phase motor that is overdriven will apply reverse torque at the shaft. This is required in 4-quadrant AC drive applications. The fact that you won't control the motor frequency or voltage doesn't matter.

This isn't a perfect representation but a motors torque/speed curve continues past the zero torque point where shaft speed matches electrical speed and continues into the reverse torque area. If you fix the zero-torque point and rotate the torque/speed curve 180 degrees about the zero-torque point you will have some idea of what to expect.
You also have drive train inefficiency working for you as you lower, which may help. You already brought up some inportant points. You want to make sure you use a spring-set brake driven by the motor monitor circuitry to make sure you know when the motor is in control of the load. You may also be able to use the existing brake to take some of the load off the motor by controlling pressure to the brake during lowering. That will tend to keep the motor current down and decrease the chance of a blown fuse/tripped overload.
A stand-alone mootr speed monitor may be useful in this case also. Use it to set the brake if things get out of hand.

Keith

 

[Sliver]

Thanks

Keith and mrm,

Thanks for the quick response.
I realized that a VFD can be used to brake an oversped load but it dumps the extra power into the DC bus and may require braking resistors to disipate the surplus energy. Just curious but where does the generated power go in my case. Back onto the grid?
Keith, I think you are right on spot with your advice.
The inching drive brake works only in the reverse direction. It is applied before jogging the inching drive and released by means of a large handwheel to allow roll back.
So the procedure would be to set the brake and jog inching drive to past center as we do now. Then start releasing the brake until the mill just begins to roll back with one hand and with the other jog the drive in reverse, but if it starts to overspeed release the jog button and apply the brake. May require two guys.
I'll have to think about it.

thanks again,

Brian.

Just reread the notes for the reversing inching drive. They use an ammetter on the motor to help detect when the load has been centered. It could also be used to help in braking application. This is sounding better all the time.

 

[kamenges]

Orioginally posted by Sliver:

Just curious but where does the generated power go in my case. Back onto the grid?

Yes, it goes back on the grid. I would suspect a certain amount is burned up as heat in the motor but no more than when the motor is producing driving torque.
I was a little vague in my original post. The motor on a VFD example was just there to highlight that an AC motor that is overdriven will act as a generator and it will happen whether the motor is connected to a drive or to the AC line.

You may want to consider automating the braking function if you can. Granted, as long as the operator is paying attention to the system you can't beat it for monitoring. But I would suspect that electronic control of the brake would be more consistent.

Keith

 

[Sliver]

I will look into the option of an electrically operated brake.
Thanks for the insight Keith,

Brian.

 

[JohnW]

Just curious but where does the generated power go in my case. Back onto the grid?

This is how asynchronous AC generators such as wind generators work, the generator is started as a motor and as the propeller starts to overspeed the motor it starts generating power which goes into the grid. We have a test rig that uses a VFD as a dynamic load for motors and if we overspeed that we start generating. You could use a VFD to brake your mill but it would have to be a regenerative drive where the power is dissipated back to the grid instead of the DC link.