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Thread: Inverters for AC motor speed control

  1. #11

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    Quote Originally Posted by ziloo View Post
    Suppose I have a lathe machine with a motor rated at 1 hp. I want to do some cutting at
    300 RPM on a steel bar without losing power. If I use a dimmer for this purpose, I have to
    shorten the duty cycle by adjusting the trigger voltage on the diac that will in turn reduce
    the power I can expect from the motor.

    How does the Inverter allow operating an AC motor at slow speed?
    Where does the "Inversion" happen?
    You'll need an inverter duty motor, for starters. First, you'll need non-integral cooling, because you'll need just as much cooling at low RPM as high RPM. Second, the winding insulation needs to be able to handle higher voltage (due to spurs).

    You can use a "dimmer" for this. But you'll need to modulate that dimmer control to maintain the desired RPM. If the motor is running too fast, you need to turn the control "down". And of course when it's running too slow, you have to turn the control "up." The dimmer doesn't know or care about the motor speed. You can get a PWM spindle controller that does this for you by measuring the speed of the motor with some kind of speed sensor, be it a DC generator, or more popular these days, a pulse coder.

    A Variable Frequency Drive, like Chuck (G) suggests, eliminates the need for speed feedback because the drive maintains the speed of the motor by controlling the motor phase. (It will use PWM too, but this doesn't matter to you.) This will be a cheaper and easier route to go. I would argue that the feedback method is better, because you can run at a wider RPM range, but for practical purposes you're better off with the VFD.

    Frequency and power are two entirely different things. They can be controlled independently, and in the case of the VFD, they are.

    The inversion happens twice. There are two important parts to am inverter based motor control. The first is the power supply section. It converts your incoming power to a high power DC. (And sometimes the other way round, but we don't care about that here. Think regenerative braking.) The other part is the drive amplifier section. It converts ("inverts") the DC power back into power for your motor. In theory this is AC which is why it's called an inverter. But in practice it is PWM DC. The motor windings actually convert it to a sort-of AC.

    I'm sleep deprived and frankly don't use theory much (and I'm not getting any younger). I normally think of a spindle drive as just that. A "black box" called a "spindle drive" (as opposed to a position drive). So I apologise if I got some theory wrong. For your purposes, you should think of it as a black box too. Find a drive that will meet your desired operating parameters, buy the motor that it recommends, and away you go.

    You could technically probably put a spindle drive on an existing spindle motor, but you really need to know what you're doing. And you're not likely to end up with the versatility you're looking for.

  2. #12

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    Quote Originally Posted by paul View Post
    The AC power is simulated at a lower frequency, hence lower speed, assuming the motor is some sort of induction motor that has a slip-speed relationship with line frequency to induce a magnetic field in the rotor. With such a motor the triac-based control is a very poor idea. Be specific about what you mean by "power" as it sounds like you mean torque. The same torque at a lower speed is less mechanical power.
    Thank you.

  3. #13
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    Thank you all for your brilliant comments!!!

    ziloo

  4. #14

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    It is not obvious as the what type of motor he has. An induction motor will not do well with a triac based controller. A brush type, AC/DC type motor will be some better but will tend to be jerky at slow speeds. A pulsed DC source is much better and smoother. Speed controllers for exercise machines work well.
    Dwight

  5. #15

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    Most exercise equipment that I've encountered uses DC motors. If course these are old derelicts that I find in the trash just for the motor and motor controller.

    These are a good way to retrofit some machine tools but depending on the size of your lathe typically don't have enough stall torque.

  6. #16
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    ************************************************** ********
    Let me start be saying that my "Editing Time for a Post" has shrunk to
    5 micro seconds and I keep getting logged out!!!!???

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    Quote Originally Posted by Dwight Elvey View Post
    It is not obvious as the what type of motor he has. ....
    My question was very general, because I was more inquiring about
    the RPM control. But along the way I found out a lot more through
    the responses.

    I hear that VFDs work much more efficiently with 3 phase AC motors.


    Quote Originally Posted by KC9UDX View Post
    Most exercise equipment that I've encountered uses DC motors.....
    Do they use PWM only or combined with VFD for controlling the RPM ....


    ziloo

  7. #17
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    Anyone remember what they used in their highschool shop classes? I started on a belt driven South Bend, then ascended to the hydraulically fed LeBlonde. Had to grind your own tool bits back then. Still have a few projects laying around from those days. If you showed some aptitude for metal working, it was possible to enroll in Chrysler's summer training program, and they paid you 90 per hour. Not bad for early 50's. Never had to worry about the electrical power, but was constantly reminded about doping the tail stock with white lead.

  8. #18

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    Quote Originally Posted by ziloo View Post
    Do they use PWM only or combined with VFD for controlling the RPM ....
    For all intents and purposes there's no frequency applied to a DC motor. Of course, being PWM there's technically a frequency, but it isn't for speed control, and in any case a DC motor won't respond that way.

    Quote Originally Posted by Agent Orange View Post
    Anyone remember what they used in their highschool shop classes? I started on a belt driven South Bend, then ascended to the hydraulically fed LeBlonde. Had to grind your own tool bits back then. Still have a few projects laying around from those days. If you showed some aptitude for metal working, it was possible to enroll in Chrysler's summer training program, and they paid you 90 per hour. Not bad for early 50's. Never had to worry about the electrical power, but was constantly reminded about doping the tail stock with white lead.
    I didn't take shop class in high school; I took the bare minimum for graduation and worked half days in a very large and prominent but now forgotten tool and die shop. (Most of our dies were run in your area.) At that point I spent a lot of time chipping out and cleaning Clausing lathes. In my career I didn't do a whole lot of lathe work. But I've worked on many lathes in a different capacity over the years. One thing about those old Leblonde and Warner-Swayzes: you don't really need electricity. If you're desperate, a Shovel Head will do the work.

  9. #19

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    Quote Originally Posted by ziloo View Post
    Hello folks,

    I don't seem to understand the explanations I hear regarding the use of
    Inverters for controlling the RPM of an AC motor. They say that lower
    RPMs are achieved without the loss of power from the motor. But, if we
    are playing around with PWM, are n't we changing the power input into
    the motor?

    ziloo
    I still don't know if it is a AC induction motor or an AC motor with brushes. If it has brushes, it won't respond to a variable frequency.
    If it is an induction motor it won't work right with a dimmer like controller.
    Which do you have?
    Dwight

  10. #20
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    I tend to think of brushed motors (not sliprings) as being "universal". I've got some fractional horse brushed motors (4 lead) that have DC nameplate ratings, but are called "SCR motor". Don't ask--I can't figure that one out myself. Apparently they're used on conveyor lines.

    Shaded-pole motors can be controlled quite well with a plain old triac or even varaic. The rotor will "slip" under load and then turn more slowly.

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