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VFD acting strange.

VFD acting strange.

VFD acting strange.

I have an Altivar 312 VFD running a 24" radial arm saw. (7.5hp) Besides just generally engaging one's fight-or-flight its got two problems.

1) The customer complained of a "bad smell" which I tracked down to a burnt-out braking resistor that charred the enclosure paint into a burn waffle pattern under it.


2) When turned ON the saw ramps up to (terror speed) sits there for about 4 seconds then ramps down to about to about a hundred RPM and sits there for about 4 seconds then ramps back up to speed. Constantly cycling this way. There's no speed control involved - you turn the saw ON and it accels up to the VFD parameter speed (10sec) and when you're done you hit stop and it decels (15 seconds) and stops.

The saw's normal ops are turn it ON power-feed it thru a bunch of aluminum, hit stop, unload the material, index the feed stock, screw around getting it all lined up and jigged in, and after 2~3 minutes the cycle is repeated. So a 3 minute start stop cycle. This has gone on for a year. I'm wondering if the resistor has always been threatened and finally died or if this bizarre cycling thrashed it. They indicated they smelled something funny tried to find it in the whole shop gave up and when they attempted to use the saw discovered this cycling behavior.

One last bit of info. When it does this cycling as soon as the saw gets to high speed the display switches from Hertz to "OVERBRAKING" fault then the slowdown occurs. What's causing this cycling?!

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD acting strange.

It might be going into drive overload (as opposed to motor overload) and it is programmed to respond to a drive overload by overriding the speed command in an attempt to shed load. Then when it decels, that is stressing the braking resistor. That entire strategy doesn’t really work on constant torque loads like saws though and should be disabled. But if it is overloading the VFD, the drive may be under sized for the application. Is this a single phase feed to a 3 phase motor? If so, was the VFD de-rated to compensate? De-rate on a component class drive like the 312 should probably be 65%, but people often skimp on that because someone told them otherwise and will do something like put in a 10HP drive for a 7-1/2HP motor. That’s not enough de-rate and the excess DC bus ripple causes over heating of the transistors which the drive interprets as a drive overload condition.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: VFD acting strange.

Bus capacitors getting weak tend to cause odd issues as well.

RE: VFD acting strange.

Yeah, kind of the same issue really. Loss of capacitance causes the DC bus ripple to increase, stressing the transistors.

"Overbraking Fault" is the French way of describing a DC Bus Overvoltage by the way, which is what would happen if decelerating without a braking resistor. My guess with regard to why it failed is that the resistor was not rated for enough watts for the duty cycle you described.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: VFD acting strange.

Hmmmm. The drive is fed 3Ø 208V and that was the first thing I checked with a meter. All three phases present. The blade turns effortlessly. There is no loading while it's doing this absurd cycling.

Thanks a bunch for the French translation! Wow.

I'm going to take over a slightly wrong brake res today and see if adding it has any effect on the cycling. I don't expect it will. I'm also going to hunt down the motor nameplate. I think it said 7.3hp but does that really make sense? Maybe it said 7.3kW which would definitely be a problem for this 5.5kW VFD. This whole thing was a factory job and is only about a year old.

The factory has done some really stupid cheap things like required two power supplies. One is 1Ø 120V for the controls and the other is the 3Ø 208V for the motor. With just the controls powered you could command the saw to start but it wouldn't because the VFD wasn't powered. Anytime later someone could turn ON the 3Ø and the blade would instantly spin up, all because the maker was too cheap to put in a control transformer to power the controls from the 3Ø. Installing a relay that leaves the 1Ø disconnected until the 3Ø is present was my first meeting with this pile.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD acting strange.

Ugh... that's a really bad design for something as dangerous as a saw!

5.5kW is actually 7.3HP (7.37 to be precise), so it's possible that they used a 5.5kW IEC motor and re-labeled it with a HP value for use here. I've seen that done.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: VFD acting strange.

And if they are running it at 60 Hz, 8.8 HP.

"Why not the best?"
Jimmy Carter

RE: VFD acting strange.

Got some more info and some corrected info.

Motor is 7.5HP 208/230/460 2-pole. 50/60Hz. Interestingly this is all on a PAPER STICKER.

Drive was set to a 60Hz motor. I set the ref a year ago to 50Hz to tone it down enough for the workers to approach it with only 'light' stun-baton encouragement.

I checked everything, wading thru the atrocious manual on a smart phone screen deciphering all the dumb-ass three-letter-7-segment abbreviations of every last parameter.

I decided the drive was bonkers as the brake resistor fried without them ever using the machine that day. Digging around, the drive insisted the incoming VAC was 262V even though it's actually only 241V. I believe the A2D converter circuitry has shifted for whatever reason and was deciding the DC bus voltage was too high and went into braking with the machine just sitting idle. This might also cause the galloping as the controller would decide "over voltage!" and brake the DC bus voltage down simply robbing the drive of the energy needed to run the saw blade. As soon as the DC bus voltage gets low enough the brake-resistor is disconnected and the bus voltage recovers, allowing re-accel back up to speed but just as it gets there the brake gets kicked back ON and the cycle just gallops along repeating that.

I pulled the drive and am searching for a non-Schnider replacement with sensorless vector. (They discontinued the drive anyway.)

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD acting strange.

Could be related to a VFD phenomenon called “DC bus pump-up”. If you have access to the A-B Knowledgebase you can download a tech note describing it, you can find it by googling that term and if you don’t have an account there, it’s free but you have to give them your email address for verification.

I’ve run into this twice now in just the last 6 months and since discovering it, I realize I had probably seen it many more times and didn’t understand what was happening. I was always attributing it to PFC caps or utility caps somewhere acting in resonance, but there are more things that can cause it. The main symptom though is just as you describe, a DC bus OV issue just by connecting it, even though the line voltage reads as normal.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

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