VFD and motor starter

[reyamkram]

I came across, a VFD connected to a motor starter, that is the first time I have seen this. I thought when you have a VFD you do not need a motor starter. The Line of side, is connected to the line side of the VFD, The load of the side of the VFD( T1-T2-T3) is connected to the line side of the motor starter, then the load side of the motor starter (T1-T2-T3) is connected to the motor. Can I remove the motor starter, and run the motor from the VFD?? Any Input will be very help. Thank You.

 

[kwired]

There could be rare instances where you might do that, so more information about the equipment involved may help determine if there is good reason to do this.

Otherwise as a general rule about all you put in between drive and a motor is line reactors and possibly overload sensing equipment.

If you do have any switching device it should have aux contacts to open the run command of the drive whenever the switching device is opened. Many drives may not mind so much having the output circuit opened while running but won't like having the load suddenly closed into an active output.

 

[Besoeker]

I came across, a VFD connected to a motor starter, that is the first time I have seen this. I thought when you have a VFD you do not need a motor starter. The Line of side, is connected to the line side of the VFD, The load of the side of the VFD( T1-T2-T3) is connected to the line side of the motor starter, then the load side of the motor starter (T1-T2-T3) is connected to the motor. Can I remove the motor starter, and run the motor from the VFD?? Any Input will be very help. Thank You.

VFD?

 

[Besoeker]

I came across, a VFD connected to a motor starter, that is the first time I have seen this. I thought when you have a VFD you do not need a motor starter. The Line of side, is connected to the line side of the VFD, The load of the side of the VFD( T1-T2-T3) is connected to the line side of the motor starter, then the load side of the motor starter (T1-T2-T3) is connected to the motor. Can I remove the motor starter, and run the motor from the VFD?? Any Input will be very help. Thank You.

You can. If you keep, it you must ensure that there is an auxilliary contact of the contactor in the VFD enable circuit so that the VSD cannot be started with the contactor open.

We supplied VFDs mainly into the petrochem and water industries. As far as I can recall, all had a contactor between the VFD and the motor. This was specified/mandatory for a hard-wired emergency stop with a physical disconnect of the motor.

 

[Jraef]

There could be rare instances where you might do that, so more information about the equipment involved may help determine if there is good reason to do this.

Otherwise as a general rule about all you put in between drive and a motor is line reactors and possibly overload sensing equipment.

If you do have any switching device it should have aux contacts to open the run command of the drive whenever the switching device is opened. Many drives may not mind so much having the output circuit opened while running but won't like having the load suddenly closed into an active output.

:thumbsup:

It generally boils down to one of three issues:
1) There is a safety issue that requires a contactor downstream, in which case hopefully someone did it as kwired said with regard to an aux contact shutting down the VFDs Run command.

2) Whomever did this was unfamiliar with how VFDs function and thought it was necessary.

3) It's a really old VFD that does not provide motor running overload protection, and combined with #2, resulted in a complete starter, not just an OL relay.

 

[just the cowboy]

Line side of VFD

Line side of VFD

You will see them on the line side of the VFD as a safety cutoff/e-stop. They are found on the line side incase something goes wrong inside of the VFD and it don't stop conducting.

 

[Aleman]

I see plenty of machines and systems made with safety contactors that remove power from all drives and motor starters. So that's one way to do it.
Safety applications would be the only reason to have a starter feeding a VFD.

 

[kwired]

I see plenty of machines and systems made with safety contactors that remove power from all drives and motor starters. So that's one way to do it.
Safety applications would be the only reason to have a starter feeding a VFD.

Makes sense, but OP's was on load side of drive.

 

[Jraef]

Makes sense, but OP's was on load side of drive.

Still the same issue though. In SIL 3 rated safety systems before the advent of STO options (and for some, even after), you need(ed) to have a contactor in series to need the safety redundancy standards, and in Europe its very common to put them on the load side of the VFD, between the drive and the motor. The reason is, putting a contactor on the line side adds stress to the pre-charge circuit of the VFD if they are used every time you shut down. On the load side, all you need do is ensure that the VFD output cannot be on when the contactor opens or closes.

 

[Aleman]

Makes sense, but OP's was on load side of drive.

Looks like I need to read more carefully! But I have seen this on the load side too, done for safety reasons. Not a common practice but as
long as the safety circuit shuts down the drive signal it 'might' make for a better safety, with the output completely isolated. Kind of like opening
a local disconnect on a motor connected to a drive but automated.

 

[Jraef]

from a safety standpoint, it doesn't really matter, the contactor could be on either side. The reason it's often done on the load side is because of the pre-charge circuit.

Take a look at the diagram below. just before the DC link choke, there is a resistor symbol with a contact in parallel. That's the pre-charge circuit.

When you first energize a VFD, the capacitors want to charge themselves instantly, so they pull current at the available fault current rate albeit for only a small fraction of a second. But that can cause damage to the caps, the diodes and everything else. So a current limiting resistor is used to limit that current inrush. That resistor however cannot be left in series, otherwise it would burn out. So a contact is used to bypass it a second after first being energized. There are other ways to accomplish this, but this is the most common.

If you energize a VFD once per day, every day of the year for 10 years, you use that pre-charge resistor 3,650 times. Most mfrs size/rate those components for about 6,000 operations, so lots of fudge factor there. But if you open a contactor on the line side 8 times per day, you exceed that component design life in a little over 2 years. That's why as a general rule, you DON'T want to use a contactor or starter AHEAD of a drive if you can avoid it. So in safety applications where something that has an air gap is required, it's better to put it on the load side, then tie an aux contact of the contactor to the enable circuit of the VFD so that the drive cannot have any output if the contactor is open; i.e. the drive is disabled. In most contactors, a N.O aux contact will change state before the main contacts do, so by wiring a N.O. aux contact of the contactor to the Enable circuit, when you remove power from the contactor coil, the aux contact opens first, the drive output ceases, then the main contacts open.

 

[meternerd]

Are you sure it's not a Soft Start? They typically have a bypass contactor that cuts in when it's up to speed. But that would be across the Line and Load of the soft start. They also can have an isolation contactor to remove line side voltage when stopped. Just a thought.

 

[RossS]

I hope folks are still monitoring this thread.

There is another possible reason that wasn't mentioned above, cost savings. I have a renovation project where many of the smallish to mid-sized motors are at/near end of life. But the MCC is much newer and in very good condition. We are replacing motors and generally providing VFD control. But we don't have enough existing breaker space to move them to a panelboard, so we want to continue feeding them from the MCC (all the starters in question are Type 1).

My first question was the simple one, can you tie the motor starter output to the VFD input and expect it to work okay? This thread suggests you can. The better question is, should you? If you do it, does it matter whether the HOA is in Hand or Auto, or will either work just as well? If it does matter, what happens if someone, who doesn't know better, changes the position of the HOA? Is this a good idea, or should the starter be replaced with a safety switch?

Thanks to all...

 

[petersonra]

I hope folks are still monitoring this thread.

There is another possible reason that wasn't mentioned above, cost savings. I have a renovation project where many of the smallish to mid-sized motors are at/near end of life. But the MCC is much newer and in very good condition. We are replacing motors and generally providing VFD control. But we don't have enough existing breaker space to move them to a panelboard, so we want to continue feeding them from the MCC (all the starters in question are Type 1).

My first question was the simple one, can you tie the motor starter output to the VFD input and expect it to work okay? This thread suggests you can. The better question is, should you? If you do it, does it matter whether the HOA is in Hand or Auto, or will either work just as well? If it does matter, what happens if someone, who doesn't know better, changes the position of the HOA? Is this a good idea, or should the starter be replaced with a safety switch?

Thanks to all...

The bucket already has either a switch or CB that feeds the existing contactor. One could remove the contactor and HOA switch, pilot lights, transformers, etc and basically convert the bucket into a feeder.

You could also just rip out the bucket and replace it. You can often get two feeder breakers in a small bucket this way. Field labor is insanely expensive, and this approach might also be more cost effective.