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selfish request... need brain power

Posted: Mon Mar 24, 2014 6:08 pm
by Baffled
I'm putting this here because I know there are a lot of very smart mofo's that browse this sub-forum. It has vaguely to do with silencers because I've used this tool to build more than one. :lol:

It's the VFD on my mill. :? A few years back, I bought a Hitachi VFD to convert my step pulley machine into a variable speed machine. I loved it from day one, and would encourage anyone to do a similar conversion.

A problem has cropped up. It no longer outputs a stable frequency. I have the display set to freq, and when I dial a speed, the hertz jitters up 10, down 5, up 12, down 3. Obviously the cutting speed is going nuts while this is going on.

I first noticed it about 6 months ago, but it's getting worse and worse.

Unit is a 240V, 1P in, 3P out, to a Leeson 3HP motor. The Leeson motor was a high-grade job, bought it when I bought the VFD. Before I trash this one and buy another, anyone have a guess as to where the fault might lie? I can do basic troubleshooting, but nothing exotic. Basically, any way to tell if it's the VFD, or the motor? My gut says VFD.

Thanks...

Re: selfish request... need brain power

Posted: Mon Mar 24, 2014 6:50 pm
by Stan1
Electrical noise? See if you lost a ground wire connection to the shielding on the control or power cables. The shield grounds are only connected on one end, right? I'm guessing a broken ground since it was working for a long time.

Please post the resolution. Thanks

Fwiw I have a couple of the Hitachi VFDs. They have been great.

Re: selfish request... need brain power

Posted: Mon Mar 24, 2014 7:13 pm
by Capt. Link.
Baffled wrote: A problem has cropped up. It no longer outputs a stable frequency. I have the display set to freq, and when I dial a speed, the hertz jitters up 10, down 5, up 12, down 3. Obviously the cutting speed is going nuts while this is going on.

I first noticed it about 6 months ago, but it's getting worse and worse.

Unit is a 240V, 1P in, 3P out, to a Leeson 3HP motor. The Leeson motor was a high-grade job, bought it when I bought the VFD. Before I trash this one and buy another, anyone have a guess as to where the fault might lie? I can do basic troubleshooting, but nothing exotic. Basically, any way to tell if it's the VFD, or the motor? My gut says VFD.

Thanks...
I would bet VFD as well.Use a good digital volt meter and record voltage to the three phases between and to ground as a reference.Use a clamp on ammeter to record draw on each leed and total to help exclude the motor.Compare your findings to placards and listed spec of motor and VFD.It could need minor repair or adjustment.I would say a capacitor is about to fail or a bearing is about to fall out.

Re: selfish request... need brain power

Posted: Mon Mar 24, 2014 8:11 pm
by bakerjw
All motor drives operate with the same principle.

Voltage comes in be it single, double or 3 phase.
It gets rectified into a capacitor bank. On a 220V input, it will store 150VDC on each capacitor giving 300VDC across the pair.
The output control circuitry uses 3 sets of FETs to pulse width modulate the DC from the capacitors to the each output phase.

As mentioned, you can measure each phase to ground and see what is different between them, but keep in mind that it is pulse width modulated AC and not what AC normally looks like on an oscilloscope. It is something like this:
Image

So if phase 1 and phase 3 give a certain reading that is different than phase 2, then you may have isolated part of the problem.

If you can gain access to the capacitor bank, check the DC measurement. It should be around 150VDC. Then also check AC voltage. Checking AC voltage will tell you if a capacitor has started to get leaky.

I would guess that there are some poor connections and possibly cold solder joints. Does it behave like this when it is cold or only after warming up? It could be thermal which would indicate cold solder joints and possibly poor heat sink thermal conduction. Radio Shack sells heat sink compound.

Post some picks of the internals and I can point out some of the places to check.

ABOVE ALL, BE CAREFUL. Measure the voltage on capacitors (the big round blue or silver things) before touching anything inside. They should bleed off but they'll knock you on your ass if they are charged.

Re: selfish request... need brain power

Posted: Tue Mar 25, 2014 9:13 am
by Baffled
Thanks, guys, this is a good start.

The behavior happens regardless of the temperature. I've never noticed this thing even getting vaguely warm even after hours of work. Seems like it's built like a tank. I would agree that the capacitor(s) might be going bad, given the ^^ excellent description of how they are involved in creating the 3P power.

The capacitors are right in front. I'm aware of their danger, these suckers are big, and will power the unit's cooling fan for a good 20 seconds after power off.

The other thought I had was the on-board potentiometer. If it's going bad, that'd be an easy fix.

Re: selfish request... need brain power

Posted: Tue Mar 25, 2014 6:05 pm
by bofcchmw
i too would have to say that based on what you are seeing that an electrolytic capacitor is possibly failing. there may be no sign of it failing, or the end may show signs of bulging or even be cracked, but they do dry out and fail causing all kinds of strange issues and instability until they completely fail.

with the on-board potentiometer, use a analog meter (has a needle) and watch as you adjust it from one limit to the other, it should be a smooth movement, with no jumping around.

Re: selfish request... need brain power

Posted: Tue Mar 25, 2014 6:54 pm
by bakerjw
Welcome to ST.

Re: selfish request... need brain power

Posted: Fri Mar 28, 2014 9:05 am
by Baffled
I haven't pursued this yet, but before I do, will replacement of the capacitors be possible? In the sense that they will be something I can buy from Digi-Key or Newark... they hopefully won't be some weird exotic Hitachi part.

I guess I just need to get off my rear and trouble shoot.

Re: selfish request... need brain power

Posted: Fri Mar 28, 2014 2:40 pm
by bakerjw
Most motor drives that I've worked with have capacitors with screw terminals. That gives a better mechanical connection to the buss bar. Although it's what I've seen, that doesn't make it a rule.

The biggest issue is getting the right size both physically and in capacitance. You can always solder wires onto them and attach them to buss bars if that's how they are connected.

There are many suppliers out there.

Re: selfish request... need brain power

Posted: Fri Mar 28, 2014 3:11 pm
by danb35
It's unlikely that capacitors will be a custom part. Semiconductors, sure, but passives not so much.

Re: selfish request... need brain power

Posted: Fri Mar 28, 2014 5:27 pm
by Capt. Link.
The screw terminals are low resistance connections for the high currents that can be present.
Discreet components like large high voltage high value caps may be one of the most custom of any parts made.I have a few that are very rare and expensive running into the multiple thousand range.This being of recent construction I'm sure they may be available for a lesser amount.Let us know it could just as likely be a tiny cap on a regulation board that cost .50 cent.