How come this works

I thought you could not have switch or relay between VFD and load. I just wired a VFD to a lathe thru the original forward/reverse switch and it seems to work fine. What am I missing?
Bob

You can get away with a switch between the load and the vfd for a while but it will blow something in the vfd sooner or later if switched under load. Doesn't your VFD have reversing capabilities? Most do.

bob said:

I thought you could not have switch or relay between VFD and load. I just wired a VFD to a lathe thru the original forward/reverse switch and it seems to work fine. What am I missing?
Bob

Click to expand...

Your eyes are not good enough to see the incremental number of little microscopic holes you are punching in the silicon wafers of the transistors on the output of the VFD. Eventually, you will get so many holes that the transistor will turn into a full time insulator or conductor, which in either case means no more VFD.

What size is the motor, and what is the rating on the VFD?

If the VFD is oversized for the load motor, it will do better in
this regard.

Jim

Actually under rated, motor is 2hp cheap VFD 1 1/2. If you ramp up troo fast it faults

Umm, you maybe shouldn't do that too much. Switching the line between VFD and motor,
while the motor is under power, that is.

Jim

I don't plan to use it this way.

Well, I don't necessarily buy the "punching holes" idea.

The worst feature of that hookup is that it can dump a starting current load onto the VFD, probably overcurrent faulting it before the motor starts. If the VFD is large enough, and motor small enough, you won't notice.

Any VFD has high speed commuting (AKA commutating, or 'catch" diodes) diodes from each output to each supply rail. These conduct during part of each switching interval already, and are capable of "catching" the voltage and diverting current into the supplies.

A switch can stress those a bit more, but is not extremely likely to destroy a well-designed unit. They are not perfect, not is their turn-on instant. So some over-voltage is likely. But the IGBTs are over-rated for voltage to take care of at least 30 to 50% higher spikes.

I think the start current stress is probably worse than the voltage stress.

But since it works so much better to use the VFD for starting and reversing, why have a switch?

Yes, it will die.

Not a matter of WHY, or IF... it's a matter of WHEN. JST is right, it MAY survive it okay, if it's of very high quality and overrated... but I've seen guys blow even the best ones apart doing silly things.

Regardless of what you do now, the VFD's semiconductors WILL be weakened. If you'd like it to maintain useable worth, disconnect your forward/reverse switch, wire the VFD direct to motor. Connect wires to the forward/reverse switch, and connect those to the VFD's remote-control terminals.

Not only will you get the best of whatever-it-has-left, you'll also get dynamic braking and ramp-up functionality.


I'm trying to figure out why anyone would do this... ???

You need in-line chokes / reactors / coils (terminology differs between VFD makes) and a suitable VFD to get away with this. Just any VFD won't do. Although inline chokes can be got for most reputable makes these are normally only to allow relatively electronics boxes, such as CNC stuff, and the like to be switched once the VFD is up to speed. You loose pretty much all the VFD capabilities, effectively the box becomes a simple single to three phase converter but it can be a safe thing to do and allows you to use all your normal machine controls without fancy hook-ups. So far as I'm aware only 440 volt (nominal) output VFDs are made with this capability. You must use proper electrically operated contactor switches when doing this. The good 'ole American habit of grossly abusing a drum switch by using it to perform power switching rather than the circuit selection duties its intrinsically suited to simply will not do as the switch action isn't clean enough. Especially if its an old unit.

The drum switch also won't hack it when used with the VFD control inputs. Like all electronic gizmos VFD boxes like a nice clean switching action. If you want to keep the drum looks dump the innards and put proper micro switches inside. V3 ones are cheap, rated for millions of operations if good quality and creating the cams is an interesting machining job.

As always "I got away with it for mumble mumble years (so far) by dumb luck and abusing the safety factor." is very different to "You wouldn't normally do that but if you do it right, like this, all is OK."

My shop runs off a VFD generated three phase using normal machine controls but I bought a system specified to do this with VFD box and the necessary chokes et al.

Clive

it can matter whether the inverter is expected to deal with that when designed or not, but there isn't any inherent problem of blowing up the devices.

For example: I have what amounts to a 3 HP single phase VFD, actually a 2500W 120V inverter, in the shed out back. It runs off solar charged batteries.

I regularly run an air compressor and drill press from it, and other equipment such as angle grinder, hand corded drills, etc as needed. The normal machine controls are used, I just flip the device power switch when I want to turn on any device, and turn off the switch on the device when I am done. The inverter stays on as long as I am using tools.

The inverter is a direct output type, no transformer. This is really the same as one pole of a 3phase inverter, and this usage has caused no problems in 12 years of use, nor would I expect it to.

However, the inverter is designed for at least a 2:1 power ratio, IOW it can produce double power for short term. Some VFDs are like that, although 1.5x is possibly more common.

The switch is not the problem. The turn-on surge of the motor, which is essentially the locked rotor current, is the issue. It will cause an OC trip on the VFD if the inverter is not over-sized.

I always thought that the problem here was exceeding the reverse voltage breakdown
on the devices in the VFD when switching motors under load.

Either the reverse catching diodes, or the switching transistors themselves.

Basically you pay for two things in semiconductors: 1) large forward current capacity,
and 2) large reverse voltage tolerance.

I think most VFD manufacturers put the PRV ratings pretty close to the wire on this
stuff to save cost.

The purpose-built inverter, single phase, mentioned above, was *designed* for this
kind of operation and probably has robust transisitors and protection diodes in it.

Another trick done is to put very solid metal oxide varistor protection across the
actual switch contacts to snub inductive spikes that might get to the VFD.

Jim

They normally go one range past. 600V for 240V drives, 1200V for 480, etc.

Besides, the diodes will clip any transients to the supply lines. That action is quite positive, only depending on the speed of the diodes themselves, which might be an issue in some cases.

Current ratings are more of an issue.

Same basic type of devices in any inverter or drive, so..............

what if

Would it be safe to assume no damage to the drive or motor would occur if the reversing switch was never switched when energized. The VFD doesn't know what goes on with a dead output circuit. Of course all it takes is switching under even a partial load to damage the solid state goodies, so the rule no switches between VFD and load is a good one to abide by.

It would be safe to assume that...

Lloyd- it would be very safe to make that assumption. Not totally perfect, but very safe.

The caveat is simply because ANY contact, connection, conductor, etc., imparts a possible failure point... so there's a slight possibility that under a shutdown-reversal-restart sequence, that one contact doesn't fully 'make'. Most modern VFDs would sense this and fault out.

Most document sets indicate wether or not switching of the motor leads is acceptable, and if so, under what conditions. SOME of my Allen-Bradley drives have provision for detecting interlock switch states for motor-disconnection. This is sometimes used in motion-control applications, but sometimes it's simply used as an interlock-loop through a wiring harness connector, to prevent the drive from powering up when a drive assembly has been removed (like... a segmented conveyor or other material-handling system).

Yep, as long as you kept the switch from being operated with the motor in motion,
it would be fine to have it present. Either with the VFD off, or with the VFD on and
in "stop" mode, there's no output so there's no chance for damage by switching
then.

Jim