Knowledge You Can Build On — Est. 1998
Inverter?
Don Thompson - Cutler Ridge, Florida
>I am not involved with electronics any more, but thirty years ago, I knew an inverter as a device used to convert direct current into alternating current. Quite useful for running a ham radio rig (or whatever) off of a car battery.
I am totally confused as to how an inverter relates to controlling lathe speed. Would some one be so kind as to 'splain?
Re: Inverter?
Rodney in Maypearl, TX
>Don,
My electrical expertise makes me fully qualified to plug in the toaster, but I have heard that the inverter allows the motor to deliver better torque throughout it's range, esp. low rpm.
Re: Inverter?
Wood Spinner
>Possibly it should be called a CONVERTER.
As on my lathe it converts 220 single phase to 220 3 phase. It then has a motor controll for speed controll. 3 phase is best for motor controll due to the fact that it allows more torque at low speeds than a direct curent motor.
Good turning and be safe
Bill in Casa Grande, AZ.
Re: Inverter?
Kent in Emmaus, PA
>The key is that the inverter used to drive a lathe has a variable-frequency AC output. Changing the frequency changes the speed of the AC motor.
I haven't seen a schematic, but I would expect that in practice, the lathe controller takes 60Hz AC, rectifies it to DC, then converts the DC to variable-frequency AC. I guess you could call it a "reverter."
(Yes, I know I'm ignoring the difference between single phase and three phase, but the principle is the same).
Re: Inverter?
Fred in NC
>Variable Frequency Drive (VFD) is, in my opinion, the more accurate name for this device.
Re: Inverter?
vicmarc.fan
>I dont know if you know about a graph of the sine wave, but the phase of AC in the USA is 60 cycles. This AC sine wave goes through 60 cycles a second to give us the power we use in our homes. If you streach out the amplitude of the cycle to say 30 cycles a second a motor will run at half the speed it did on 60 cycles. And so on till 0 cycles the motor will stop. Most controllers have a roestat to let you change this phase angle and are programable to do a lot more that I wont bore you with. On a DC system the controller is just the reostat which controlls the voltage, the higher the voltage the more RPM, the lower the voltge the lower the RPM.
Like I say there is a lot more to this but a AC controller controlls phase and a DC controller controlls voltage.
Ron
Re: Inverter?
Kent in Emmaus, PA
>"a AC controller controls phase"
Frequency, not phase.
Reostat=variable resistor
Leo Van Der Loo
>
Re: Inverter?
Fred in NC
>Not always.... my DC controller is a chopper design, it runs at a constant voltage, and the torque does not decrease even at the lowest speeds. It has an encoder in the motor (tachometer, if you want to call it that) that feeds back to the controller to maintain the set speed. My setup can go down to 60 RPM with full torque !!!!
Cheap DC controllers do vary the voltage, and are useless at slow speeds. That is why such cheap ontrollers have a higher minimum speed, such as 500 RPM. At anything less than full rated voltage, the HP decreases. Not very useful for turning wood, where we need slow speed and high torque for large diameter pieces.
Aha! Thanks for the explanation.
Don Thompson - Cutler Ridge, Florida
>I get it (more or less). I did not know that the fancy lathes are 3-phase, too.
Is this right:
AC->DC rectifier circuit
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DC->3-phase AC Variable Frequency Oscillator
