Variable Frequency Drives - info required.

[GeoffS]

I'm searching for a variable frequency drive (VFD) for a small, single phase AC motor.
The (geared) motor is part of a telescope drive which currently runs at 50Hz but needs to be driven at higher/lower speeds for different tracking rates.

Most of the VFD's I've researched tend to be single to 3 phase conversion and/or much higher power than is needed. The motor is 240V 50Hz and draws less than 1 amp.

Any recommendations? A DIY solution is OK is one is available.

[tecman]

There are plenty of single phase drives as you described.  Look at Grainger of Automationdirect.com.   A few hundred dollars will but one.

paul

[Uncle Vernon]

For milliamp stuff you could try fantech, anything over a couple of AMPs Danfoss is the way to go.

Danfoss trade prices are pretty good, you wont go wrong, they are quality all the way. Automationdirect.com.au carry the VLT stuff, not sure what their pricing is like I buy direct.

[GeoffS]

Someone on the astronomy forum posted this DIY solution.

Much cheaper than a VFD and looks like it would be pretty easy to build.

[Zero999]

Single phase motors often poorly at lower frequencies but in this application where no torque is required you're probably fine.

Here's some drives which are designed for single phase motors.
http://www.anaconsystems.com/text/opti_e2.html

But they're not cheap. You could try is looking for a cheap VFD which can be operated off load so you can connect the motor across two phases, leaving the third unused.

Regarding the circuit you linked to.

It should to perform PWM to reduce the voltage at lower frequencies otherwise the motor could overheat; a commercial VFD will do this and you should check that this design does it.

My guess is it wouldn't work with a 6VA transformer because the secondary voltage would've been too low. Mains transformers always have more turns than expected on the secondary to make up for the voltage losses under load. Even with an ideal 12V to 110V transformer and a 12V PWM sinewave, the maximum secondary voltage will be 78V which will reduce the torque somewhat. You'd probably get better results from a 9V transformer which will give nearer 110VAC with a 12V supply.

[NiHaoMike]

Mod a cheap power inverter or UPS.

[tecman]

Look for something available, such as:

http://www.automationdirect.com/adc/Shopping/Catalog/Drives/GS1_%28120_-z-_230_VAC_V-z-Hz_Control%29/GS1_Drive_Units_%28120_-z-_230_VAC%29

Around $ 100   Hard to beat

paul

[Zero999]

Look for something available, such as:

http://www.automationdirect.com/adc/Shopping/Catalog/Drives/GS1_%28120_-z-_230_VAC_V-z-Hz_Control%29/GS1_Drive_Units_%28120_-z-_230_VAC%29

Around $ 100   Hard to beat

Those inverters have a three phase output so they're not suitable unless they will work with one phase unloaded.

Someone on the astronomy forum posted this DIY solution.

On second thoughts, if you're going to run it from the mains, why not just get rid of the 12V transformer and run the whole thing off rectified mains?

You could use a tiny 3VA transformer or transformerless PSU for the MCU part and replace the IRL530s with suitably rated MOSFETs or IBGTs.

The only downside to this is it won't be isolated from the mains but the AC motor won't be anyway and neither will a VFD. The only time that would be important is for testing just use an isolation transformer. You could also drive the MOSFETs using a pulse transformer, if you want to keep the MCU isolated from the mains.