Line or load contactor

[MikiBg]

Hi all!
By your experience, which is a better solution for VFD controlled motor outgoing:
1. With line contactor (upstream of VFD). Some say that it's good for VFD to be disconnected from power supply if it is out of operation for long time (e.g. standby pump unit)
2. With load contactor (downstream of VFD). Some say that it's better to isolate motor from VFD when it's not running.
Be free to advise.

 

[Skogsgurra]

RTFM.
For quite e lot of applications, its it not good at all to switch the motor.
See no reason why a VFD couldn't be connected to grid forever. That is how most of them are used.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

But of the two choices, line would be slightly more beneficial. Load does nothing for you as far as protecting the drive. Where is a transient going to come from on the motor side if the motor is not running? Load side contactors are sometimes required for personnel safety on machinery against inadvertent movements during routine maintenance or operation tasks. But where is the danger on a pump? You have to open and lock the disconnect means to work on it anyway.

The down side to a line contactor is that if it does need to come on and off frequently, it stresses the VFD's DC bus pre-charge circuit, which van lead to premature failure.

"Will work for (the memory of) salami"

 

[controlsdude]

concern 1
Problems with VFD failures is the cycling off/on of the vfd.

concern 2
If you worried with personal safety just buy a drive with safe off torque feature and dont use an output contactor. Again if someone is working on motor, then they need to have the disconnect shut off.
All VFD disconnects need the disconnect aux contact wired into the vfd enable or wired to plc input then this contact used in PLC program to turn off the enable.

 

[MikiBg]

Thanks for the ironic answer, Gunnar. Maybe "by your experience" doesn't always has to comply with "by the manual". If does, why do we need this forum, anyway? We just need to read the manuals.
Within catalogs for Altivar drives, you can find both devices into schemes. A guy for Schneider told me, some years ago, that load contactor is a good solution for safe isolation of motor, because it prevents potential risk of "leakage" from drive to motor, even if drive is not running. Maybe load contactor with control scheme enabling contactor closing before drive start and contactor opening just after the drive stops is a good solution? I don't quite understand the purpose of line contactor when CB is used as upstream protection, specially if remote disconnection of the drive from the line isn't requested.

 

[jraef]

Your "guy from Schneider" was regurgitating something that had to do with machine safety concerns in a manufacturing environment, for example when you might have a VFD running a feed conveyor and someone is working on a DIFFERENT piece of machinery associated with that conveyor and you want to ensure that NOTHING can cause the conveyor to operate while that work is performed. Now we just use the Safe Torque Off feature in modern drives for that.

But again, there is ZERO application for this sort of scheme in a pump. Don't waste your time.

"Will work for (the memory of) salami"

 

[itsmoked]

It's not a great idea to un-power a VFD that is not operating. The bulk capacitors in DC bus start "unforming" every time power is removed. Further, the bleed resistor can provide some tiny amount of heat to help prevent condensation in the electronics.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

That too, although it's generally accepted that it takes about a year of being left unpowered for the capacitors to need reforming.

"Will work for (the memory of) salami"

 

[Skogsgurra]

I did not mean to be ironic. Some VFDs have express rules in the manual and some don't. So, it is a good idea to actually RTFM. That is what TFM is there for and the general opinion should not overrule what the manufacturer says in his manual.

I had one case where opening the motor circuit made the inverter explode (1200 kW smoke stack fan where high steam pressure inadvertently took out everything, including fan motor breaker) when the motor was switched off during full operation near rated load. This happened during commissioning and everything hadn't been tested (or thought out in detail). The breaker has now been removed.

As I said before, I see no reason to disconnect a motor from the VFD unless there are several motors and a safety switch or thermal disconnect for each motor and there are several motors, like a conveyor with many driven rolls.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[controlsdude]

Output Contactors
Before Safe off Torque on drives
Only seen this in a design where they were used as Estop. (only if you allow the mechanical to coast to stop is done safely, otherwise you have to ramp the motor down first, then thru a time delay open these contactors).
You never let the contactor open for any other reason besides estop. Like was said before you run the change of destroying the drive.

Of course you had a circuit breaker before the drive to interrupt power.

 

[MikiBg]

Thank you for feedback.

 

[ozmosis]

MikiBg
I think you need to qualify the definition of isolation for the motor and drive.
It is common practice to isolate the motor for maintenance reasons (and usually lock off the isolator). This would not typically be a contactor, but a manual switch/isolator. It is typically positioned between the VFD and the motor but the expected practice is to power off the VFD before opening the circuit to the motor. This often happens with pumps and fans alike.
There are drive companies who say it is ok to switch on the output but this is not the same as saying is is good practice. It isn't. If you open a circuit between a drive and motor when it is running, then this is quite bad practice and depending on the design of the VFD, it will either stop and alarm (the VFD will 'see' a very high impedance) or go bang.It may keep running but this is also not good. The reason is, if the isolator is then closed into the output of the VFD the motor will demand a very high current (it is the same as DOL) and likely trip the VFD, at best.
So, isolating the motor is common practice when providing maintenance on the motor/load and is usual the isolator is visible to the person working on the load.
A contactor on the output, unless integrated into a drive bypass system with mechancial interlocks and logic to prevent incorrect opening/closing of the supply, is unusual and probably not recommended.

Addressing your specific comments:
1. With line contactor (upstream of VFD). Some say that it's good for VFD to be disconnected from power supply if it is out of operation for long time (e.g. standby pump unit)
As already mentioned, if you are leaving the VFD without power for >12 months or so, it means the caps will possibly need gentle persuasion to function properly and the occasional power-up is often beneficial. This would be the only reason you would need consider not bringing a VFD back on supply without any thought about how long it has been out of action. Otherwise, it's good to keep power on the drive and then you can at least see the keypad in case any specific errors are visible
2. With load contactor (downstream of VFD). Some say that it's better to isolate motor from VFD when it's not running
I think you need to understand from the 'some people' what they mean by "better". Better for what?

 

[jraef]

Better for contactor suppliers?

"Will work for (the memory of) salami"

 

[davidbeach]

Good points from ozmosis; one thing to add - most disconnect switches can be ordered with aux contacts that open fast and close slow. These aux contacts get wired into the drive controls so that the drive shuts off before the power contacts open and won't restart until the motor is back in the circuit.

 

[LionelHutz]

Like others have posted, just randomly disconnecting and connecting the motor to the output of a VFD is a great way to blow up the VFD. The disconnect switch David mentions with late make/early break aux contacts tied into the drive enable is your best bet if it just "must" be done.

Cycling a line contactor on and off isn't a great idea either. I'd want to see the same late make/early break aux contacts wired to the VFD enable on that contactor as well. Some kind of timing circuit would also be a good idea to ensure there is a minimum open time of say 5 minutes before closing again.