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VFD - Minimum Speed

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    #16
    Originally posted by Jraef View Post
    The concept of “restarting” the motor is flawed, but the rest of your perception is fairly correct. There is no issue with the VFD running at lower frequencies, it’s all about the motor. If the motor cooling is done via fans on the motor shaft (ie TEFC), then the fans move less air at lower frequencies, yet the current in the motor might be the same, depending on the machine function (which is likely why Besoeker was asking).

    Given that you said the motor current is going UP as the speed goes down, that’s a sign that this is exactly the type of application where motor cooling is going to be problematic for you.

    But we don’t know anything about your motor; someone may have anticipated this and selected a motor capable of this, especially if they selected a good quality “inverter duty” motor. Look up your motor specifications and specifically look for the motor’s “turn down ratio”. So for example if you are running at 10Hz on a motor designed for 60Hz, that is s 6:1 turn down ratio. If you want to run it slower than that, it needs a higher ratio. So a 10:1 turn down ratio means you can run it at 6Hz, a 100:1 ratio means you can run it at 0.6Hz etc. If your motor was NOT selected as inverter duty and does not state a turn down ratio, I typically tell people to assume no better than 4:1, so 15Hz as the slowest speed. You say you are already running at 10Hz, so hopefully this is not the case and someone knew what they were doing.

    Post back motor nameplate data data if you want more help with that, including make and model.
    Here are a couple photos of the motor nameplate!

    Attached Files

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      #17
      Originally posted by Russs57 View Post
      Hmm, some interesting comments. In my experience wound rotor motors are seldom less than 5 HP and may be as high as 15,000 HP. Perhaps confusion with a (commutated) series wound motor? I would also say few modern AC to AC VFD's us SCR's. Of course it is dangerous to make such generalizations. Not trying to nit pick here as much as understand others experience as we all see different things depending on our jobs.

      Jody, in a nutshell, I would say the majority of modern 3 phase AC to AC VFD's are like this; You bring the input AC to six diodes where it is turned into DC with AC ripple voltage. The resulting DC voltage will be almost 1.41 times the input AC. The diodes will only conduct near the peak of the sine wave of the AC input voltage. You pass that voltage off to a capacitor bank (which may also have an inductor) to get rid of the AC ripple and store energy so you can power the load for the part of the sine wave in which the input diodes aren't conducting.

      Assuming you have adequate capacitance this means the DC bus voltage is nearly a constant, typically around 650-670 for 460/480 VAC input.

      Next this DC voltage is "pulsed" to the motor by IGBT's. These pulses take place at the carrier frequency of the drive. This could be a few kilohertz or as high as 15khz.

      When you want a lower motor speed you send narrower pulses to the motor and when you want more RPM the pulses are "on" for a greater length of time. These pulses are square waves but because the motor windings are basically an inductor the resulting current draw is close to sinusoidal.

      Now this is rather simplified and there are different types of drives with different control methods.
      This is down the line i was thinking for the VFD aspect of this subject. Then the motor cooling factor was brought in which is equally as important. These motors have exterior fans on them and as the the nameplate states the gear ratio is 214:1. I know this isn't the turn down ratio, but I guess the nameplate gives the information fort the gearbox too.

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        #18
        Originally posted by Jody Boehs View Post

        This is down the line i was thinking for the VFD aspect of this subject. Then the motor cooling factor was brought in which is equally as important. These motors have exterior fans on them and as the the nameplate states the gear ratio is 214:1. I know this isn't the turn down ratio, but I guess the nameplate gives the information fort the gearbox too.
        Yes, that's common nameplate data when it is sold as a "gearmotor" unit.

        So as I interpret that nameplate, the final shaft speed is 3 RPM, with a 214:1 gear ratio, which means the motor shaft speed would be just 642RPM, so it must be a 10 pole motor (720RPM sync speed, 11% slip). So when you say they want to turn it down to under 10%, you mean they want the work shaft speed to be under 0.3 RPM? Must be an extruder then...

        If the motor has a separate blower then, they must have understood the application. I'd watch this really carefully though, that 2HP 10 pole motor is NOT going to be on the shelf anywhere! If it burns up, your machine will be down for weeks.
        __________________________________________________ ____________________________
        Many people are shocked when they discover I am not a good electrician...

        I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

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          #19
          Originally posted by Jraef View Post

          Yes, that's common nameplate data when it is sold as a "gearmotor" unit.

          So as I interpret that nameplate, the final shaft speed is 3 RPM, with a 214:1 gear ratio, which means the motor shaft speed would be just 642RPM, so it must be a 10 pole motor (720RPM sync speed, 11% slip). So when you say they want to turn it down to under 10%, you mean they want the work shaft speed to be under 0.3 RPM? Must be an extruder then...

          If the motor has a separate blower then, they must have understood the application. I'd watch this really carefully though, that 2HP 10 pole motor is NOT going to be on the shelf anywhere! If it burns up, your machine will be down for weeks.
          It is 8 RPM. So that would be 1,712 Motor RPM. The external fans I was referring to are mounted to the motor shaft.

          There is also a possibility that 100% would be running at 90Hz. I'll have to verify that when the machine is between runs though. The individual that programmed those drives has done that before.

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            #20
            That company is still in business. Might be worth giving them a call. Motor looks old, old enough that I'm inclined to doubt it was intended for use with a VFD (it wasn't clear to me if the machine always had a VFD).

            I remember a well known cooling tower manufacturer that would quote tower efficiency numbers based on running the fan motors at a hertz that was too low for the splash lubrication to work in the gear reducers. Sometimes the problem can be other than your area of focus.

            FWIW at 90 hertz you would only have (at best) 67% of the motor's rated torque at your disposal.

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