How long would you run a VFD after issuing a stop command?

[BigBill53]

Hi,
I've come across a situation in a pumping station and was wondering if I could get a few opinions.

Basically, we have a VFD that has a line contactor supplying it power. The idea is that after issuing a stop command to the drive we turn off the contactor.

Some people here are of the opinion that we can turn off the contactor pretty much straight away. Others are of the opinion that we should wait 30 minutes or so, to allow the VFD to stay powered and run the fan to cool it down.

Personally, I don't get why you would wait any more than the time it takes for the motor to stop. I can't see why you might want to wait for the VFD's to cool down. Aren't they designed to run that way in the first place?

 

[itsmoked]

You can can drop the contactor as soon as the motor stops. Further cooling should not be required. Most drives I see kill the fan instantly if the stop command is issued though it's often settable.

If you drop the contactor before the motor stops the motor will no longer be controlled to a stop. That might cause problems.

I would be wary of powering off VFDs in wet humid areas for any length of time. Being ON keeps them dryer.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DickDV]

Cycling input power on a VFD is very stressful to its power components and should be avoided whenever possible. Surely, cycling should be limited to not more frequent than once per hour.

 

[LionelHutz]

I'm with DickDV, if you just must do this then I would be very careful of the duty cycle. I've seen many badly implimented control schemes with line contactors that have killed the VFD.

 

[BigBill53]

DickDV, could you elaborate a bit more on this please? Is the main reason its stressful to cycle power on a VFD because of the rapid charging of the capacitors?

Unfortunately I can't do much about the line contactors cycling - it was a customer request. At certain times of the year there is so much lightning around this area that they want the drives to be disconnected when they are not in use. And then when they need to run they absolutely have to run, so I can't limit the number of starts per hour.

What's worse is that these VFDs are really just acting as very expensive soft starters. This is what happens when you get people who aren't electrical engineers making decisions.

 

[Skogsgurra]

Some drives are not built for AC input cycling - there is no current limiting in the charge circuit. Some *can* be cycled, but need time to cool precharge resistor. Some can be cycled as you like. The latter type has electronic (switch mode or thyristors) input current limiting.

Then, you have to think about reaction time when you need power from the motor. If the drive is stopped, but not switched off, you can jog as much as you like. If it is also switched off every time you stop - then frequent jogging will usually not be possible.

As always - you have to understand the applicatio, the customer's expectations and the limitations of the hardware.

Gunnar Englund

http://www.gke.org

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

 

[jraef]

The issue of having line isolation contactors is valid in lightning prone areas, we used to do that all the time in Florida. But still, cycling should be as limited as possible unless the VFD mfr specifically says it's OK, as per what Skogsgura said above; not all power structure designs are created equal.

Cooling is also another issue, again because all are not equal. I know for a fact that some designs specifically rely upon the cooling fans running for a long time after shutting down. This again is something that should be checked with a design engineer from the specific VFD mfr.

There are no safe generalizations that make this scheme universally acceptable.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[ozmosis]

I agree with all previous.
To add to jreaf's post, some VFD's are designed whereby the cooling fan blows through the electronics as well as over the heatsink whereas some blow air over the heatsink only. The latter generally have bigger heatsinks and more focus on heat dissipation than the others that are physically small but are very prone to hotspots if the fan would suddenly stop.
Also, I would say that the risk of lightning strikes compared to the risk of uncontrolled power off/on of the mains supply needs to be looked at. Rapid Off/On of the mains supply can be very detrimental to the life of a VFD.

 

[DickDV]

The stress on power-up is largely a matter of managing the inrush current to the DC bus capacitors.

I would think that a properly-designed lightning suppression network ahead of the drive would be a better option that frequent power cycling. Whether the inrush current is limited by precharge resistor, scr rectifiers, or other means, the system is not generally designed for frequent cycling. As skogs mentions, you may find a manufacturer that offers a beefed-up design for this purpose but I don't believe I have ever seen such.