vfd control

[ptonsparky]

Customer had me replace a small VFD that had failed. The equipment is used in the fall during potato harvest and is controlled via radio and an on site supervisor. While looking at another failed VFD in another building it became apparent that they are cycling the power to the VFDs via magnetic starters.

Drives, at least the smaller ones, seem to be able to carry on for a few seconds after power has been lost.

The first small drive is set up with a manually operated 2 wire start. I would like to set up a small relay that will detect when the drive first loses power, and initiate the Stop while the drive still has a bit of carryover. Not a big problem to do but is it worth the effort until I get a chance to revamp the entire control? Will it do any good, other than keep me busy for an hour or so?

They have 9 drives on another machine that are controlled the same way. All they are really after is smooth start of the equipment.

Off topic, the manager of this plant is smiling because during all economic downturns, the sales of Potato Chips go up.

 

[Jraef]

The problem is not turning them off by opening the contactor, the problem is turning them on when the Run command is enabled via that 2 wire control. That's because the microprocessor gets the signal to start firing the transistors before the bus is fully charged, so it can cause an unnecessary surge in the output current and strains all of the power components. So your solution is a good one, in fact most VFD owner's manuals will recommend it.

 

[Besoeker]

Customer had me replace a small VFD that had failed. The equipment is used in the fall during potato harvest and is controlled via radio and an on site supervisor. While looking at another failed VFD in another building it became apparent that they are cycling the power to the VFDs via magnetic starters.

By magnetic starter, I assume you mean what I would call a contactor. (I'm a Brit but that's not my fault.....)
Most drives have a relay output for drive ready (healthy) or at least one that can be programmed for this function. You could use that in series with an auxiliary contact of the main contactor to inhibit the drive from getting a run signal until it is ready.

We normally fit the contactor downstream from the inverter with just such an interlock to prevent the drive from being enabled with the contactor open. That way, the drive stays energized all the time and there is no start up delay while the pre-charge pumps up the DC link capacitor.

Isolation is usually by an upstream breaker or a fuse-switch.

They have 9 drives on another machine that are controlled the same way. All they are really after is smooth start of the equipment.

If that's all they need, soft-starters would have been cheaper, smaller, and more efficient, particularly if a by-pass contactor is fitted.

 

[ptonsparky]

I like the drive ready relay idea. That just made things even simpler. It will take more time on the road than to do the work, at least for the small drive.

 

[chaterpilar]

Of the 100 + drives i have installed, never used any contactor ( Input/Output).. the 24 Vdc logic input ( LI1) is what i always use.

The problem with contactor based startup is stress on capacitors especially if the motor has frequent start-stops.

cheers.

 

[Besoeker]

Of the 100 + drives i have installed, never used any contactor ( Input/Output).. the 24 Vdc logic input ( LI1) is what i always use.

Yes, you can do that of course.
We usually use a contactor. This mainly because of a safety requirement to provide a hard-wired emergency stop circuit giving a physical disconnect.

The problem with contactor based startup is stress on capacitors especially if the motor has frequent start-stops.
cheers.

This isn't normally a problem if the contactor is on the downstream side of the drive - the DC link capacitors remain charged.

 

[iwire]

Yes, you can do that of course.
We usually use a contactor. This mainly because of a safety requirement to provide a hard-wired emergency stop circuit giving a physical disconnect.

I thought that was required in most applications of VFDs.

This isn't normally a problem if the contactor is on the downstream side of the drive - the DC link capacitors remain charged.

And I thought that was a bad idea to place a contactor on the output side of a VFD.

 

[Besoeker]

IAnd I thought that was a bad idea to place a contactor on the output side of a VFD.

It's a view that I have seen before - sometimes by the VFD manufacturer.
The concern is that the drive could be enabled before the contactor is closed thus subjecting the drive to direct on line current. But it is a very simple issue to ensure that the drive isn't enabled before the contactor is closed.

 

[VinceS]

Requiring Output Contactors

Requiring Output Contactors

In the EU this reference exists.
"? EN954-1, Safety of Machinery ? safety related parts of a control system ?"

I believe, because of all the import/export goods, most US MFR's often consider offshore references and get them mixed into US Doc's. For example, in most all the sales pitches to sell safety relays EN954-1 is listed. Possibly an IEC issue for Europe, but not US. An output contactor is also suggested in NFPA 79 for a class 0 stop. I think 79 is a great reference but not enforceable like the NEC.

If short... output contractors may be a great idea in some eyes, but are not currently required, unless specified per design.

 

[weressl]

Customer had me replace a small VFD that had failed. The equipment is used in the fall during potato harvest and is controlled via radio and an on site supervisor. While looking at another failed VFD in another building it became apparent that they are cycling the power to the VFDs via magnetic starters.

Drives, at least the smaller ones, seem to be able to carry on for a few seconds after power has been lost.

The first small drive is set up with a manually operated 2 wire start. I would like to set up a small relay that will detect when the drive first loses power, and initiate the Stop while the drive still has a bit of carryover. Not a big problem to do but is it worth the effort until I get a chance to revamp the entire control? Will it do any good, other than keep me busy for an hour or so?

They have 9 drives on another machine that are controlled the same way. All they are really after is smooth start of the equipment.

Off topic, the manager of this plant is smiling because during all economic downturns, the sales of Potato Chips go up.

If the contactors are used as an e-stop, it is a hard thing to get around. Cycling the power on the ASD's is bad as Jraef explained, but it is also bad for the capacitors, it will send them South in short order. If it is possible, eliminate them.

An alternative to e-stop function would be magnetic brakes installed on the motors themselves. The E-stop pushbutton or relay could de-activate the breaks AND simultaneously open the two wire control. If the stop command failed in the ASD, the magnetic break will mechanically stop the motor from rotating and the ASD will go into a failure mode on overcurrent. It is still better than a contactor on either the input our output of the ASD and teh reliability is at least as good if not better than breaking the power to/from the ASD with a contactor.

 

[Besoeker]

If the contactors are used as an e-stop, it is a hard thing to get around. Cycling the power on the ASD's is bad as Jraef explained, but it is also bad for the capacitors, it will send them South in short order.

Not if the contactor is downline from the inverter.

An alternative to e-stop function would be magnetic brakes installed on the motors themselves. The E-stop pushbutton or relay could de-activate the breaks AND simultaneously open the two wire control. If the stop command failed in the ASD, the magnetic break will mechanically stop the motor from rotating and the ASD will go into a failure mode on overcurrent. It is still better than a contactor on either the input our output of the ASD and teh reliability is at least as good if not better than breaking the power to/from the ASD with a contactor.

If implemented correctly, shutting down the power to either the input or the output of a VFD has no detrimental effect.