Flux vector drive, motor, back drive

[W@ttson]

All I just thought of this situation:

1) suppose you have two flux vector drives each has a dynamic braking resistor and a respective motor. Both motors are coupled to some machinery. Only one motor operates at a time, the other just goes for the ride. Then they alternate, next cycle the second motor does the work. When the two motors are coupled like this and the second motor is being back driven, assuming the second drive is not enabled will there be any rise in the DC bus voltage and subsequently turning on of the dynamic braking transistor and use of the second motors dynamic braking resistor?


2) suppose the same situation as about but the second motor is a reduced size motor with a gear box with reduction in front of it, say 2:1, such that if the main motor is running at 1175rpm the second is being back driven at 2350rpm, does anything change with the scenario?

 

[synchro]

When the two motors are coupled like this and the second motor is being back driven, assuming the second drive is not enabled will there be any rise in the DC bus voltage and subsequently turning on of the dynamic braking transistor and use of the second motors dynamic braking resistor?

If the second drive is not enabled and its IGBT output devices are not switching, then there will not be any synchronous rectification of a regenerated AC voltage from a mechanically driven second motor. Therefore it should not cause a rise in the second drive's DC bus voltage.

 

[W@ttson]

Is it at all likely that the voltage output on the back driven motor could cause any damage to the drive output stage?

 

[winnie]

An induction motor cannot regenerate without a source of 'excitation', something to create the magnetic field.

In the scenario you describe, with an induction motor connected to a disabled drive, you will not get regeneration and will not get a rise in bus voltage.

The likelihood that you will see regeneration issues comes from the chance of a drive programming error leaving both systems energized at the same time, trying to run at two different speeds.

In other words, as you've described things you have a mechanical system that is capable of having regeneration issues, but you've described a programmed configuration that avoids regeneration. The failure risk is the risk of a programming error.

One possible solution is to join the DC busses of the two inverters. In this case any regeneration from one drive will supply the other drive, rather then heating the braking resistor.

Jon

 

[topgone]

On the management side, if this is a theoretical problem, I'd rethink my operational design and have both motor drives equipped with clutches to disengage when not used. Too many rotating parts are put in the system and you may not be able to avail of the services of any of the drives should both get worn out at the same time.

 

[Jraef]

Is it at all likely that the voltage output on the back driven motor could cause any damage to the drive output stage?

There might be a very small amount of residual magnetism in the frame of the non-energized motor that in theory COULD create a very low voltage on the terminals of the disabled drive. But it will not create any measurable DC bus voltage if the transistors are off.

I’ve done many “back drive” applications where you have a large motor rotating a drum on the machine and a smaller motor counter-rotating another inner drum (de-stemmers in the wine industry), we tie the DC busses together to avoid regen issues as Winnie said.

 

[W@ttson]

An induction motor cannot regenerate without a source of 'excitation', something to create the magnetic field.

In the scenario you describe, with an induction motor connected to a disabled drive, you will not get regeneration and will not get a rise in bus voltage.

The likelihood that you will see regeneration issues comes from the chance of a drive programming error leaving both systems energized at the same time, trying to run at two different speeds.

In other words, as you've described things you have a mechanical system that is capable of having regeneration issues, but you've described a programmed configuration that avoids regeneration. The failure risk is the risk of a programming error.

One possible solution is to join the DC busses of the two inverters. In this case any regeneration from one drive will supply the other drive, rather then heating the braking resistor.

Jon

I have a system just like that! It has 3 motors, with special drives modules that share a DC bus.

 

[W@ttson]

On the management side, if this is a theoretical problem, I'd rethink my operational design and have both motor drives equipped with clutches to disengage when not used. Too many rotating parts are put in the system and you may not be able to avail of the services of any of the drives should both get worn out at the same time.

understandable. Space is of the essence so we looking to forego clutches and alleviate the possibility of one failing/maintenance.

 

[W@ttson]

There might be a very small amount of residual magnetism in the frame of the non-energized motor that in theory COULD create a very low voltage on the terminals of the disabled drive. But it will not create any measurable DC bus voltage if the transistors are off.

I’ve done many “back drive” applications where you have a large motor rotating a drum on the machine and a smaller motor counter-rotating another inner drum (de-stemmers in the wine industry), we tie the DC busses together to avoid regen issues as Winnie said.

very interesting, thank you