Motor Speed Control (VFD - Variable Frequency Drive)

[EposLab]

Hello there!
I'm triggered in getting Full Control of my lathe's 100W Synchronous motor!
I am currently work only at 33rpm and I like to be able in use at Half Speed 33rpm (16rmp) , Standard 33 and 45rpm!
Any DIY or SALE recommendation welcome!

Thanks
Tasos
Epos Laboratory

[markrob]

Hi,

Way back, I modified an off the shelf 12Vdc car inverter (modified sine type) to become a poor man's VFD. I reverse engineered the schematic of a cheap 100W device and added controls to adjust frequency and voltage separately. As you lowered the frequency, you dialed the voltage back to keep the power to the motor constant. It worked well. Could have been further modified to do both V/F with one control. Not sure if they still sell inverters with this circuit, but I would not be surprised. These day's, pure sine inverters are pretty readily available and might be a good thing to hack. PM me if you want the details on what I did.

Mark

[EposLab]

Hi,

Way back, I modified an off the shelf 12Vdc car inverter (modified sine type) to become a poor man's VFD. I reverse engineered the schematic of a cheap 100W device and added controls to adjust frequency and voltage separately. As you lowered the frequency, you dialed the voltage back to keep the power to the motor constant. It worked well. Could have been further modified to do both V/F with one control. Not sure if they still sell inverters with this circuit, but I would not be surprised. These day's, pure sine inverters are pretty readily available and might be a good thing to hack. PM me if you want the details on what I did.

Mark

that would be nice if you can share these info as I already own an 12 to 230 inverter! I can give it a try! on the other hand I think pure sine wave is the only way!

[dmills]

Cheap way to try this (particularly in 120V land) is just to grab a butch power amp, slap it in bridge mode and hook up a signal genny. A honking great electrolytic in series with the motor is not a bad plan to avoid any DC offset issues. I did this once to get a tour out of trouble (They had a rider that said 120V, but turned up with a tone wheel organ that said 60Hz, one thing is easier to organise then the other at short notice....).

The type of motor will make quite a big difference here, shaded pole can have very low torque at low frequency, while a three phase machine will generally be fine (but you need three phases), only issue with three phase machines is the cooling fan becomes ineffective, but on a record lathe? Meh!

[markrob]

Hi,

I've attached two pdf's. One is a schematic for the inverter as I found it and the second shows my mods. Hopefully, it will make sense to you. I suspect that newer inverters are microprocessor controlled and may not be so easy to hack. Also, I note that you have an AC Synchronous motor. That may not do so well at running at half speed. In my case, the motor was a simple AC induction non-synchronous type and it ran fine at half speed (and reduced voltage). The modified sine is pretty crude and causes some excess heating in the motor due to the bad form factor of the current waveform. I didn't find that to be a big issue. It also likely has a bit more torque ripple when driven this way.

HW 150 Watt Inverter.pdf

Modified HW 150W Inverter.pdf

[EposLab]

Thanks for uploading the files! I will eximine and see what i can do!
The only way to control my sync motor is by frequency. So for half speed i need a stable 25hz pure sine wave supply (50hz region).

I was already thinking to midify the idler wheel but its not possible due to space issues (the wheel will crash on the chassis frame..).

[markrob]

Hi,

To be honest, I think the power amp signal generator suggested by dmills would be the easiest way to go. You might need to add a step up transformer to match the motor load to an 8 or 4 ohm audio power amp. There are some very low cost high power switch mode amps out there.

[Fela Borbone]

Hi,
 I'd like to emboss at half speed too, that seems a big improvement.
 Well, the best solution may be a mechanical reduction of speed, and let the motor work in the conditions it was designed for,but I cant figure out how to do it easily.
  About electronically, I have drived induction motors (not synchronical ones like yours) trough an old and cheap NOT ELECTRONIC  hallogen lamp transformers(In my country they are very common at flea markets), Whith a sine wave. Most common are 50w (or 50VA) but bigger can be found. You need two of them, one to be rectified and get the voltage for generating the wave and other for lifting voltage for the motor.So, one step down and other step up.
  For generating the wave, something like the TB6600 https://www.ebay.com/itm/202488241409  (Not tested by me, I use microcontroller and mosfet bridge, but this seems easier to use)
You may need a circuit to generate the clock signal (ne555 or alike, but better something quartz based).As you are using a synchro motor, you need no feedback.
It needs 4A for a 50w motor,but you will be using the motor well  under its power rating, so 4 amps is conservative. If your motor is 100w, you must search for something bigger.
  Despite a rectified 12V AC yields more than 12 DC,a  wave generated with a power supply of 12v. will yield  lower than 12V ac value. So after the second transformer's secondary  is less than 220.But, as you are aiming half speed, you should keep the Volts/Hertz ratio, so half speed, half voltage in theory. Practically, at the end, the heat in the motor is what really matters, so I take this rule not strictly and using 12volt supply for the first stage works for me on a 50w motor.
  A 470f capacitor at the 220 side of the output to motor  transformer helps with HF interferences. Short cables and screening is good idea too.
  Allways keep an eye in the motor and other parts heat.
  Have a nice day!
 
 
 

[Fela Borbone]

Note- TB6600 is for steppers so you have to connect just two of the four outputs, the motor only have one winding.

[EposLab]

 Well, the best solution may be a mechanical reduction of speed, and let the motor work in the conditions it was designed for

That was my first plan but it has space issues...
I was planing to modify the 78 rpm wheel of my master pro lathe but then it will crash on the chassis frame
Tye 33 wheel is just on limit!

[Gus]

Best solution for half speed mastering is to drive the platter via belt with an external servo motor .

https://www.lathetrolls.com/viewtopic.php?f=15&t=6212&p=46708&hilit=Presto+belt#p46708

[Fela Borbone]

+1 Yes, thats good idea!

[EposLab]

Indeed thats a nice idea but i prefer to experiment on the original motor!
So i manage to buy an Invertek Optidrive E2 (Single Phase output) from the fellow troll boryo.
Unfortunately the output signal is not Sine wave but PWM so there is enough space to experiment with!