Motor With VFD Slows Down When Frequency Above 150

[SteveHGraham]

The saga of the belt grinder continues.

My new motor arrived today. It's a 3HP motor, and my Teco VFD is rated for 3HP. I wired it up quick and dirty to check it and make sure it runs. It appears to be fine, but there is an issue with the VFD. When you go above 150 Hz, the VFD decides you need to slow down, so it starts decelerating. Any ideas as to what causes this?

I can't actually claim to need 167 Hz (5000 FPM), but I would like the grinder to do what it's supposed to.

Wondering if it's just the dubious twist-connector wiring I did to get it going. I do not plan to leave it like that, but I needed to make sure the motor functioned.

[SteveHGraham]

Knowing I was likely to be nuked from orbit, I posted the same question at PM, and they became quite agitated. Apparently, when you run a motor at a really high frequency, it acts as a low-pass filter. They also seemed to think it might, you know, blow up.

Sounds like I need a 6" drive pulley.

[BadDog]

Generally, 120 Hz is the max recommended.

[SteveHGraham]

I'm going to squeeze 120 out of it, and if it fries, it fries.

[Magicniner]

There's some potentially relevant info in some of the posts in this thread -

http://www.model-engineer.co.uk/forums/ ... 108360&p=2

- Nick

[SteveHGraham]

Interesting stuff.

One of the major problems with forum people, even at the exalted PM, is that they will speak with great authority about things they don't know much about.

[spro]

being on the receiving end?

[SteveHGraham]

Well, I trusted someone other people look up to, and it appears that while he was doing his best, he was not quite on target.

[BadDog]

I wonder if he was talking just about just it's potential for over-clocking? Low rpm motor have more poles and are generally heavier and it is generally true that they are better built than high rpm motors just to facilitate the extra stuff and weight of the larger armature. So, if you had a final drive to reach the same SFPM, I could see where maybe basing on the low rpm motor is the better option and ultimately produce a wider range, partly due to the extra mass keeping things smoother. I'm just winging it, but that's what comes to my mind based my limited knowledge.

[Magicniner]

John Stevenson's posts there are what I was directing you to, he is a very experienced engineer and does a lot of motor work for a living, he doesn't do BS or guess
Additionally I run a 1Hp motor at up to 7000rpm in my benchtop CNC mill and it's perfectly happy at that, but you need a modern Inverter Rated Motor as older motors are a bit hit and miss as to how well they run on inverters, you can sometimes tweak performance by juggling parameter settings but at the end of the day a good new motor is often cheaper than half a dozen old ones and the time spent to find they perform less than optimally.
"Recommended Speeds" for motors are like "Dry Clean Only" labels on clothes, the manufacturers simply haven't tested anything else and don't intend to

- Nick

[WJH]

The Teco VFD has an auto detect feature that automatically configures different settings that are particular to your motor. Don't remember what they are, but it may be worth a try, might get a few more hz out of it.

[SteveHGraham]

I think the big lesson here is not to let people who make knives teach you about tools.