Omron 3G3HV-B4370-CUE motor regenerative energy problem

[ROAMER]

Hi all!

My first post here.

I have above mentioned inverter that I tried installing on a 30kW motor for stone cutting machine. 

Original problem is that when machine is cutting it is inducing garbage back into the line, causing all lights to flicker. This is due to linear movement of the cutting blade. Since rotation is converted to linear forward / backward motion, load on the motor keeps oscillating. As one of the solutions for this was reccommended, install a VSD. 

VSD was sourced from a friend that had one as a spare, so that is what I'm stuck with for now.

When trying to run the motor, it starts and ramps up to 37 Hz (for a 50Hz target speed) and then following fault comes on:

Main circuit overvoltage
(OV)
The DC voltage of the main
circuit exceeded the
overvoltage detection level
(approx. 400 V for 200-V class
models and approx. 800 V for
400-V class models).
The deceleration time setting is too short and regenerative
energy from the motor is excessive.
→ Increase the deceleration time or connect the Braking Resistor
Unit.
A surge is imposed when the phase advance capacitor is
switched.
→ Insert an AC reactor into the power input side of the Inverter.
The voltage of power supply to the Inverter is too high.
→ Lower the voltage within the rated power supply voltage.

 

I can also see that kW reading on the drive is also going into negative reading. So drive probably can not absorb that much energy from the motor as it's load is variying, also going into braking due to zig zag motion of the blade.

This drive doesn't have provision for braking resistor so I can not try that. 

My questions are:

Is there any alternative for this problem? Solution wihout the resistor?

If anybody has enough experience, will drive of this size be able to cope with this problem if another model with braking resistor is chosen? Or bigger drive need to be installed?

At 35Hz, we tried cutting, and there was no lights flickering. So we are sort of half way to the solution.

 

Thanks!!!

[marke]

It sounds to me as though the "power" demand of the motor is very erratic with both positive and negative power flow where the motor is both driving the cutter and breaking the cutter (slowing down). This can be due to a concentric load where the cutter system is not properly balanced.

Where there is an issue with the balance of the load and the motor is both driving the load and the load is driving the motor, then yes I would expect to see voltage disturbances on the supply. The magnitude of these disturbances is a function of the supply impedance.

Adding a VFD will not fix this problem, and can make it worse due to the inability of the VFD to feed energy from the load back into the supply.
Adding braking resisters may help to stop energy being fed back into the DC bus, and this may keep the machine running, but you now have considerable energy being wasted as heat.

An active front end drive will prevent the motor from tripping on over-voltage, but will still enable the energy to be fed back into the supply and in addition, will add common mode switching current to the network and potentially cause additional issues.

 

Do I have a solution?  No. If the load current is unstable, a VFD will probably make it worse and trip.

Slowing the cutter down, will reduce the load and you could get down to a load where it does not affect the supply, but the work rate will be severely reduced.

[ROAMER]

Thanks for the reply marke!

I am coming to pretty much same conclusion.

Maybe only solution will be to install VFD (oversize it) with DC resistor and cope with energy loss.

Or maybe put some sort of reactive filter on the line. But then I guess it will have to be tuned somehow to frequency of this energy sloshing around. I did not find any ready made solution. And I don't have enough expertise on the matter to design myself. 

Another thing that comes to my mind maybe isolation transformer? Delta star or star delta? Would that help maybe? 

[marke]

The best way to flatten the load demand, is to add a large flywheel. Any device you add to try to flatten the load will not work well.

You may be able to change the way the saw works perhaps, but a reciprocating saw is going to have a modulating torque demand and this will cause an equal variation in the electrical demand.

A flywheel acts as a mechanical capacitor to reduce the torque variations on the motor.

Best regards,

Mark.

[ROAMER]

Thanks Mark.

Machine already has 400kg flywheel. But obviously this is not enough.

Thanks again for your advice!!!