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VFDs? How do the nominal 60Hz motors like the Variable freq?

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  • VFDs? How do the nominal 60Hz motors like the Variable freq?

    Further to a post below on idler motors and 3ph power:
    I have a 5hp idler started by a static converter (both obtained free with my old Sheldon), I can power a 3hp mill or my 2hp lathe with seemingly no reduction in capability from true 3ph power.
    I see VFD's recommended all of the time and have no personal experience with them but I wonder how the Mill (etc) motor really likes being fed a frequency for which it's B-H characteristics were not designed for.
    For example, 60Hz motors run at 50Hz will run hot and at reduced efficiency.
    Any experts out there have a comment?

  • #2
    Interesting info about power, motors and VFDs.

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    • #3
      I've had a VFD on my mill for a year or two, with no problem. This is a home shop, with home shop level use. As a practical matter, I doubt you'll run into trouble. If your usage is anything like mine, you spend far more time setting up, changing tools, and figuring out what you're doing that you do actually milling anything. What's the actual time you're running? Maybe 10%? The motor doesn't have time to heat up much.
      Try to make a living, not a killing. -- Utah Phillips
      Don't believe everything you know. -- Bumper sticker
      Everybody is ignorant, only on different subjects. -- Will Rogers
      There are lots of people who mistake their imagination for their memory. - Josh Billings
      Law of Logical Argument - Anything is possible if you don't know what you are talking about.
      Don't own anything you have to feed or paint. - Hood River Blackie


      • #4
        The important factor to a motor is the magnetic flux, volts/frequency. When the ratio is constant throughout the operating range, the motor is happy. For a 230v motor at 60 hz, the v/f ratio is 3.83. If you run the motor at 50 hz, the VFD will decrease the voltage to the motor to maintain the ratio of 3.83, or 191v.

        The heating problems with VFD run motors is mainly from lack of cooling at low speeds. If you run for long times at 10 hz or so, the motor could quickly overheat. The best solution to this is run at a higher speed and use the gears in the lathe to get the lowest range. This also gives you more torque at the spindle, since the VFD is constant torque, you don't get the toque advantage of the backgrears unless you change gears.


        • #5
          that was an interesting link provided by evan. their conclusion seemed to be that VFDs can definitely damage older motors not designed for VFDs.

          i used to work for a large bakery. we had a ton of motors of all sorts and types. i can definitely say that putting a "normal" motor on a VFD can overheat and destroy the motor in a short time. if you run the VFD within maybe 5% of the ratings for the motor, it should work, but if you plan on taking your 60Hz 3600RPM motor and running it at 10Hz and 600RPM for any length of time (or 100Hz for that matter) you will fry a non-VFD motor. we would always have new people (or people in a rush) that would put a standard motor on a VFD drive, and sure enough we would be replacing that motor the next week. these motors ran 24 hours a day, 5 days a week, so in a home shop environment you will probably get a little more life out of them.

          andy b.
          The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining


          • #6
            I dunno about the PDMA link. There's several items that doen't square with my experience.

            I agree that six step drives can cause major heating in plain vanilla induction motors without supplemental cooling. I've never enountered heating from plain vanilla motors run from up-to-date PWM drives when operated at full load above 45 hz. The problem of motor heating is a problem best addressed by motor insulation temp rating/load/motor RPM/enviroment analysis. It's even better solve it at the grass root level by bonding a NO 150 degree snap action switch to the stator iron and connecting it in series with a 6" biscuit fan tie wrapped to the motor's fan shroud to supplement the motor's built-in ventillation.

            As for Dv/Dt hazard to the insulation it's a bit less of a problem at 230 volts than 460. Simple EMI pi filters caan be made from a choke consisting of few turns of suitable insulated conductor conduction run through the eye of short lengths of iron pipe and a couple of 0.1 MFD caps. This is usally adequate to protect the most sensitive motor from recurrent voltage spikes. Here poor magnetic materials like iron pipe are superior in performance to sophisticated ferrites and silicon iron when their task of blotting energy from fast flux changes is considered.

            My nine years of experience in running plain vanilla induction (including Class "A" insulated motors up to 70 years old) on PWM VFD's at high load levels at all RPM's leads me to believe that much of the hoopla about mysterious heating and insulation degradation is mostly ghost stories. I think the stories are intended to either scare people into buying the expensive inverter rated motors or to inflate the cautioner's status as a guru dispensing unverifyable caution.

            The precautions necessary to prevent overheating a motor from heating from under-ventillation at low RPM is a snap action switch and a separate biscuit fan and to reduce potential insuluation degradation from inductive spikes a simpe crude pi filter. All very low tech and low in cost.

            You don't need no steenking $1000 motors to run most manual machine tools. Their duty cycle is usually very low - less than 40%.


            • #7
              That's probably right.

              The worst problem would be if the inverter were right at the motor (very high frequency output) or if it is at the right distance from the motor for a resonance in the RF area.

              Either one could lead to several thousand volts on the motor at RF frequencies. But I doubt they would get far.

              BUT, even at normal power frequencies, there is sometimes a problem with breakdown of insulation on the end turns, etc due to transients, so it IS possible.