two speed motor with VFD

I've got a 50 HP boring mill with a 2 speed (900/1740) motor that I want to brake. It is not possible to put a mechanical brake on the thing. The duty cycle of an AC motor brake is no where near enough, at least the ones I have found. If anyone knows of one with a duty cycle that can handle frequent start/stops thats the way I would go. The local motor shop wants me to put a VFD on the motor. The problem is I need the 50 HP at 900 RPM so I need to run the motor in both speed ranges, without stopping first. Is it possible to do this?

If you will look at the motor nameplate and post the motor horsepower for each speed range it will give us an idea as to what type motor you have.

If the low speed horsepower is 1/2 of the high speed horsepower then you have a constant torque motor.
If the low speed horsepower is the same as the high speed horsepower then you have a constant horsepower motor.

If the motor is a constant torque (low speed hp is 1/2 of high speed hp) then you can use a VFD easily. All you need to do is connect the motor or high speed and wire to the VFD. Whenever you need low speed all you do is set the the VFD to 50% speed (30 Hz). A remote potentiometer makes this very easy. The largest problems you will have is to install the VFD in such a manner as to allow you to use the orginal machine controls. You would need to post an electrical schematic before anyone could help with this.

When you speak of an AC motor brake, what type brake are you speaking of? Is this a brake added to the end of the motor? or are you speaking of a DC injection Brake?

I am familiar with DC injection brakes but have never used one on a motor as large as yours and I have not investigated duty cycles but this may be an option.

Bruce Norton
Kingsport, Tn

A Spindle drive (as in lathes) would be what you are looking for. They work with dual speed motors and have regen braking built in. But the only ones I know anything at all about (and only in passing at that) is the Yaskawa drives and they tend to use their own motors for a matched set.

Toolnut - thanks for the input.

It is a constant HP motor (50/50 on the tag). The mill has a 12 speed gearbox, 3x4, plus the two speed motor for 24 speeds. I need the 50 HP at 900 RPM since it splits the speed ranges. The 900 RPM's is not all at the lower 12 speeds. The brake I was looking at was a DC injection brake. I looked at Baldor and Emerson, the duty cycles were like 15 seconds every hour, nowhere near what is needed for a boring mill.

The braking capability is really what I am after. The ability to infinitely adjust speed is of some benefit. To get more top end RPM would be nice but the motor runs at 4x spindle RPM so it would have to go 2740 to be an improvement at the spindle, 900 RPM max now.

I am also worried about running the old, 1951, motor on a VFD.

Thanks

Tim

I forgot - machine controls and logic are not a problem. I've done several drive updates, but all with DC.