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Safe max speed for typical motors in VFD use?

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  • #31
    All my VFDs except my ABB 3hp unit have been bought second hand, the 1hp Siemens Vector Drive unit running the Myford lathe cost me £45 when I bought 2 for £90, working ex-industrial control stuff, that was over 10 years ago, the other runs my Rapidor Manchester Power Hacksaw.

    I've since bought 3 more 1hp 440 series Siemens Vector Drives NOS or S/H light use and never paid over £95, the motors I buy new because I have a good industrial supplier locally and it's just not worth messing with crap motors, I tried that a couple of times and I'm not too slow a learner to realise that second hand motors can be like Mystery Metal, often a usable only as a hammer, anvil or door stop ;-)

    If the question had been "What can I do with a cheap Chinese VFD and a mystery motor?" my answer would have been "Hold a door open", but none of the references proving what I'm doing every day in my workshop to be impossible had the caveat "But you could do it if you aren't cheap and buy decent kit" and that's my point, 3 phase motors with good VFDs can do more than you might expect.

    If you're cheap you'll have to kiss a lot of Frogs to find your Prince, but don't assume that there isn't a good way to quickly and easily do what you think is impossible
    Last edited by Magicniner; 01-07-2018, 11:21 AM.
    If you benefit from the Dunning-Kruger Effect you may not even know it ;-)


    • #32
      I recently gave away 2 x 5.5kw 480v ABB ACH550's to another member. Very nice VFDs, and they also put out "sine wave" (or a heck of a lot closer then a normal vfd). We used them for data center HVAC pump control in redundant and time-shared modes. I was going to use them on my dual speed dual voltage 7.5hp lathe motor but it has weird problems when connected in high voltage mode and that shows as poor current balance even in low voltage connection (but it runs great ). I suspect one of the 16 windings are mislabeled from the factory, but that's a big time sink to sort out and easier to change out the motor and make a custom pulley for drive/brake. Project priority #8765, so I moved on, for now.

      I collect free motors specially three phase. Amazing how many I have accumulated over the years, and they do get used or traded. Slightly different to just motors, yesterday I got 3 Baldor 1/2 hp dental lathes and one Redwing. 2 bad switches, one with tape on the shaft (some idiot tried to put a straight bore attachment without realizing they are tapered shafts), and one with bad bearings. Few months ago I got 2 x 10hp SINGLE phase motors.

      That diagram above is a bit misleading. Yes, it make the point but the current cannot be read as constant to wb like they show as comparison to the other curves.
      Last edited by lakeside53; 01-07-2018, 11:51 AM.


      • #33
        You had me there! I read a curly w as Omega but not the one in standard fonts ;-)
        If you benefit from the Dunning-Kruger Effect you may not even know it ;-)


        • #34
          About 10 years ago, I thought it would be a good idea to wind an induction motor to run at a nominal 8 VAC so it could work with a 12V battery and custom made VFD. I used a single phase 2/3 HP 120 VAC pump motor, removed the windings, and rewound it with #17 AWG magnet wire on the 36 slot stator, making a 12 pole 3 phase motor. I made a simple "modified sine wave" VFD using a PIC and MOSFETs and I was able to get it to run using only 12 VDC. However, I was at that time not so savvy about VFDs and MOSFET drives, and when I took it to a motor shop to demonstrate it, there was a puff of magic smoke and some sparks and the MOSFETs were toast.

          My intention was to get some old 5HP three phase motors, rewind them for low voltage and high pole count, and overclock them up to 6x (360 Hz) to get 30 HP, enough to use for an EV. I had a Fuji/GE 2 HP VFD that I bought on eBay for about $60, and I used it on single phase 240 VAC with two 120V-12V transformers, to power the motor. IIRC I was able to go up to 4x (240 Hz) at which point the motor was spinning a fan at about 2400 RPM.

          I didn't have a video camera at that time, or I would have made a video. It seemed that there was no problem running at 240 Hz, and I think it might have worked at 360 Hz, but my test setup was not very solid and I felt it too risky to continue. I started to build a device to measure torque by driving a 3 HP treadmill motor as a generator with a switched resistive load, but I lost interest and put my toys away (but still have them). As I learned more, I came to know that the laminations of standard 50/60 Hz motors became very inefficient at higher frequencies, so I would probably not get any more than 2x-3x useful HP, without a custom motor using very thin silicon steel stator laminations, and also possibly a custom rotor with a different "skew" and slot count.

          Until now I had never heard of the argument that a motor would stall above 2x-3x because of the rate of magnetization/demagnetization. Power transformers are made with similar laminated cores, and they can be run on frequencies of at least 1000-2000 Hz, but of course an induction motor involves a rotating field and a rotor that spins with a certain amount of slip. Maybe a two pole motor will stall at 2.5x (9000 RPM), but higher pole counts may allow much more overclocking.

          Paul , P S Technology, Inc. and MrTibbs
          USA Maryland 21030