OT - VFD and coasting motor with large inertia load

If a six lead motor (the "typical" U1,V1,W1, U2,V2,W2 terminals) is connected to a heavy rotating cylinder, and the drive that supplies power to the motor is using the coast to a stop method, does this present a possible problem of the motor acting as a generator and returning a damaging amount of power back to the drives' output transistors? The motor load is on low friction bearings, and it takes forever to stop using the coast method.
This question of possible problems comes from a customer, and I
do not think it is a problem because I have used coast to stop
in many different applications, but I do not have the technical
expertise to explain WHY this is not a problem. Can someone enlighten me?

As long as the output transistors are off there is no danger of putting more energy on the bus. There should be little or no voltage generated by the motor in this case so you shouldn't need to worry about overvoltage issues.

So the short answer is, no, this shouldn't be an issue.

Keith

Thanks for the reply, Keith. I have measured feedback voltage from motors that are being driven by the load, and the voltage is usually small, but why? When you disconnect power from a motor and the inertia of the load keeps the motor shaft turning at relatively high rpm, why is measured voltage so low and inconsequential?

When you remove power from an AC induction motor you no longer have the power needed to induce a magnetic field in the rotor. Without the rotor field there will be no voltage generated in the stator. If the motors were permanent magnet motors, where the rotor magnetic field is always there, you would measure the much higher voltages you expect.

The small amount of voltage you do measure is a result of the residual magnetism of the rotor. Being exposed to magnetic fields during normal operation imparts a small amount of permanent magnetism to the rotor.

Keith

thanks again, Keith. that explains it!

Even more OT... when I was in college, one of our profs recounted a story about a plant he'd worked in. Seems they set up a winder to reel up sheet metal (steel, Al, I forget what... ) comming off a line onto a spool. So the thing was built and installed and wired and they tested it with a empty mandrel. Worked fine... turned it on, the motor spun up, changed speeds, great. They turned it off and everything COASTED to a stop.

So the next day it reeled up a bunch of product. The spool was full so the motor turned off... and the rotating spool tore the motor from it's mounts and threw it through a concrete block wall. Seems that the motor shaft had a worm gear that that drove the spooler. Empty, the motor inertia was greater than the mandrel. Full... uh... oops!

Just finished a conveyor tripper job in a power house that had separate motors driving the two axles, each with its own gearbox.

Unannounced, the gearboxes were designed so that they locked up when back driven and this was with motors that had backside safety brakes. Not smart!!

Each axle has its own VFD controlling motor speed and direction with one being the lead and the other being the follower.

Needless to say, it was a bit tricky setting these drives up so that they both started at precisely the same time. Otherwise, the late starter would simply lock up and slide the wheels.

After consultation with the tripper and gearbox manufacturers, it was decided that the best way to run this was to use a precision speed regulator on the lead axle and an open loop speed regulator on the follower axle with max possible overload torque---in this case about 300% nameplate. That way if the follower gearbox locks up, the motor will eventually develop enough torque to crack it loose and resume normal rotation.

Definitely not elegant, in fact, downright ugly, but sometimes its up to the drive guy to "fix" other design blunders. Guess our work is worth something, after all!