Is there a problem cutting the power to an operating VFD?

[kustomizer]

I am wanting to use a VFD to control a 3 phase, 3hp motor, I have a single phase motor I want to be running at the same time. I want the pair to run for a given amount of time, lets say 30 minutes, then both shut off. I was thinking I could put a spring wound timer ahead of the VFD where

I would wind up the timer which would start the single phase ( fractional hp ) motor and power up the VFD, then I would start the VFD. When the timer wound down it would cut the power to both.

My concern is as to whether or not this would damage the VFD or if there is a better way to do this?

 

[jim rozen]

If you wanted to shut down the VFD controlled motor more gracefully you could put a 120 volt relay alongside the single phase motor and use the relay contact to operate a low voltage control line on the VFD. The other issue is the drive may go into fault mode when it would be powered up directly via the timer switch, as most VFDs won't go directly to run mode, unless they're configured deliberately that way, in the parameter set.

Others will be more knowledgeable, but I doubt a direct power-off to the drive will harm it.

 

[Gumbydammit]

I second what thermite said. The software in the VFD "brakes" the motor and reduces the EMF from the collapsing electrical field in the coils. (The way I understand it).

Good luck
Gumby

 

[rons]

One of my drives has a built in inhibit jumper. If the jumper is removed then that connection can be brought out to an external make/break.

 

[jim rozen]

That would be a low-power 'signal', ....
/QUOTE]

A contact closure is how that's done with most vfd units. No need for any external power at all.

 

[JST]

Most if not all modern VFDs have undervoltage dropout, so that when the bus voltage gets to be too low, they will shut down. There should be no problem cutting power.

The question is what is wrong with stopping the VFD the normal way?

 

[BOB-OO]

Generally it's not an issue. Personally, I would insure the VFD is not actively transferring power to motor(run mode & moving) prior to cutting power because its just good practice. That said from experience, I have inadvertently cause many VFD's to lose power during operation and never damaged one. The well known brand-Fanuc, Mitsu, Emerson, Yaskawa - are extremely reliable and have lots of ways to protect themselves when wired correctly, I can't speak to cheap Chinese stuff as I've never used them.

 

[kustomizer]

Most if not all modern VFDs have undervoltage dropout, so that when the bus voltage gets to be too low, they will shut down. There should be no problem cutting power.

The question is what is wrong with stopping the VFD the normal way?

I want to start the machine, load it then have it run for a certain amount of time then stop by itself

 

[Clive603]

Look for a DIN rail mount timer relay or two.

Built for that sort of job. Use one on the VFD controls and another, operating a little later on the contactor on the VFD power line. Or find a combined, multi function one.

This sort of thing :- https://cpc.farnell.com/europa-comp...lu-VrsYoG0d8OElrZSl3cLbslofWtdUhoCWAAQAvD_BwE

UK link but plenty of similar on your side of the pond.

Both dial setting, like that one, and electronic. Umpteen different versions of switching contacts. That one is simple sinlge pole. I have a bigger one in the might be useful rescue box with about 8 contacts and delay settings a well as a simple timer.

Generally cheap enough that futzing isn't wort it.

Clive

 

[JST]

I want to start the machine, load it then have it run for a certain amount of time then stop by itself

And just why does that not allow using the normal start/stop?

The VFD does not know or care if it is "you" hitting the switch, or a timer.

 

[rons]

You should use a digital input to shut down the VFD under it's own power. The same signal can be used to turn a switch on the VFD input.

 

[SomeoneSomewhere]

It's quite common in older & smaller machines (before safe-torque-off functionality became common) for physically disconnecting the input to a VFD with mechanical contactors to be the way to shut the machine down if an e-stop was pressed or a guard opened. It's not going to damage the drive.

You should avoid powering the drive up more than every few minutes, though, as the inrush currents are substantial, especially if the inrush thermistors (NTCs) haven't cooled from the last startup.

 

[JST]

Same to a lesser extent for those having a resistor and relay to shunt it. Too many starts in a row can toast the resistor, which is normally undersized for wattage, because it relies on it's mass to prevent heating in the short time it is carrying current.

 

[Garwood]

I run my air compressors with VFD's. Instead of switching the VFD's on and off, they just stay on and I open/close the run/stop signal wires on the VFD.

 

[SomeoneSomewhere]

Yeah, it's not the preferred solution. If you can switch the input signal to the VFD, do that.

But for safety applications where you really need the VFD to stop powering the motor even if there's a hardware or software fault, well, cutting the power isn't going to kill it.

 

[kustomizer]

My trouble is stopping the VFD and the single phase motor at a certain time

 

[jim rozen]

My trouble is stopping the VFD and the single phase motor at a certain time

1) put a small 120 volt coil relay in parallel with your single phase motor power leads - the ones that are energized through the timer switch.

2) use one normally open relay contact to select 'run' via the vfd logic lines. Read the manual to to program one of the inputs on the VFD to do this. You will need NO other parts to do this. Two low voltage control leads to the relay, and two coil wires to the relay.

 

[rons]

Post the manual for the VFD. Then somebody can list what steps need to be performed.

 

[kustomizer]

Post the manual for the VFD. Then somebody can list what steps need to be performed.

I will have to dig around a bit to find it, somewhere in the back of a 40 foot overseas container, If I remember it is a K&B?
We are having some more winter here, I will likely wait for some warmer days so my hands don't get stuck to everything I have to move to get to it.

 

[Mark Rand]

The other way to do it is with a contactor on the VFD output. That has got the disadvantage that it would need to be three pole.

Presumably you are planning to use the same single pole contactor to switch both the single phase motor and the VFD?