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VFD speed control fault

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John Haine29/05/2020 16:14:31
3265 forum posts
175 photos

I have an Altivar 28 "Square D" VFD driving my mill which has developed a fault in the +10V supply it makes available for the speed control pot. I had a previous problem where the control was very "jerky" that I traced to a faulty pot, but this one is different, in that the top frequency I can set the output to is ~60 Hz though the programmed limit is 75 Hz.

On checking the maximum voltage on the pot wiper I get ~ 8V, so I disconnected the "hot" end of the pot from the 10V output and measured the latter, getting about 9V. So the regulation on the 10V output seems to have gone wrong.

Has anyone encountered a problem such as this and managed to fix it please? Looking for ideas before I dive in. Might finish up just using an external power supply I guess...

old mart29/05/2020 16:20:14
1906 forum posts
151 photos

Assuming you are using a remote pot, can you remove it altogether and revert to the built in control?

Stuart Bridger29/05/2020 16:54:03
464 forum posts
26 photos

Bit of hack, but you could just tweak the max frequency to take account of the the 90% reference voltage?

Robert Atkinson 229/05/2020 17:17:31
751 forum posts
17 photos

This is most likely to be a failed or failing electrolytic capacitior. Less likely is a resistor failure. I assume there is no circuit available for the internals of the VFD so a bit of detective work wil be required.
Before you do ANY work on the VFD, Uplug it and leave for a least 10 minutes before touching anything. Disconnect it completley before attempting to open it up. You will have trace the connections and components on the 10V terminal. Look for buldged or leaking electrolytic capacitors r verheated resistors.

You could use an external 10V supply but the failing supply could still be used internally. Worst case thi could result in an uncontrolled overspeed condition.

Where are you located?

Robert G8RPI.

Edited By Robert Atkinson 2 on 29/05/2020 17:17:47

Michael Briggs29/05/2020 17:27:54
192 forum posts
9 photos

Hello John, the VFD on board 24 vdc supply normally used to energise digital inputs is rated at 100ma. I would use it with a 78L10 voltage requlator to get the required 10 vdc supply. Regards, Michael

John Haine29/05/2020 20:28:52
3265 forum posts
175 photos

Michael, that's a good suggestion. It's capable of supplying 100 mA so no shortage of current. I'm thinking anyway of controlling the speed from the box I'm building to also control the new power feed so could be combined. As a quick fix I could actually just supply the existing pot through a series dropper to get 10V max.

Robert, thanks for the cautions, after 50 years in electrical engineering I do tend to be quite careful. "There are bold pilots, and there are old pilots, but not very many old, bold pilots" as they say in a different context. As there is an internal digitally set maximum speed parameter, the unit is probably reasonably protected, and I suspect the 10V supply is provided to match the standard control interface so it could do without it.

John Haine29/05/2020 22:07:55
3265 forum posts
175 photos

Hmm - well the plot thickens! I thought I'd just check the 24V, found that with the motor switched off it was ~20V, with the motor on a bit lower and clearly fluctuating judging by the unstable display on my multimeter. Obviously something amis with the drive, I'll get the cover off tomorrow.

John Haine30/05/2020 20:22:42
3265 forum posts
175 photos

Had another check, definitely the low voltage supplies are fluctuating madly, not surprising it isn't working very well. I'm in the process of fitting a spare-ish inverter to the big mill, I'll see about having this one repaired. A couple of days ago I saw a post on here somewhere where someone mentioned a company offering a fixed-price VFD repair service - can anyone point me to it please?

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