I recently saw a 120V single phase in/out VFD forsale and it got me thinking. If you wanted to moderately slow down a single phase motor it would seem this might work OK. I presume you would need to start the motor at max speed and never run too slow or the starter would kicked it. My application is a small Beuhler lapper that runs too fast for my likes but if I could slow it down to 35% of top speed that would work great. Any thoughts?
Check again ... it may have been 120 single in/240 three out.
Slowing a single phase motor is impractical for two separate reasons.
1) the start switch would likely cut in (if it isn't timed). 35% of synch speed wqould surely cut it in.
2) Since the single phase motor has NO INHERENT TORQUE at all*, it relies on inertia to carry it through the "zeroes" of the incoming line current. At slow speeds the inertia involves a lot less energy, and the motor power would drop like a stone. Might not be enough for you.
* without the start circuit to induce a phase difference, the single-phase motor just sits and hums.
When turning, the rotatin is "synched" to the static alternations of magnetic field some what similarly to "pumping" a swing. But if the motor is stalled, and the start circuit does NOT cut in, there is no torque.
A 3phase motor has inherent torque at zero RPM.
>Check again ... it may have been 120 single in/240 three out.
I initially wondered the same thing so I called and confirmed with the seller that it was 120V single phase in/out VFD.
>the start switch would likely cut in (if it isn't
>timed). 35% of synch speed would surely cut it
I worried about that initially also. I tested it and it appeared that the starter did not kick in until the speed had dropped to about 20% of the running speed.
>Since the single phase motor has NO INHERENT
>TORQUE at all*, it relies on inertia to carry it
>through the "zeroes" of the incoming line
>current. At slow speeds the inertia involves a
>lot less energy, and the motor power would drop >like a stone. Might not be enough for you.
Not enough power might be the case. I was also concerned that any major increase in loading of the lapper might slow the speed down enough to cause the started to kick in. The 8" brass lapping wheel has pretty high inertia and the lapper spins for 30+ seconds after powering down so that might help it get though the "zeroes".
If the starter did engage, do you have any thought what might happen and if permament damage might result?
The seller wants about $30 for the converter, so it might be an interesting experiment provided I did not blow anything up.
Thanks for the insight.
1) You cannot use these 1 phase output VFDs on capacitor start 1 phase motors. The capacitor looks like a short to the transistors so the rapid di/dt (Delta (change) I (current) over Delta t (time)) can cause them to turn on by themselves out of sequence and damage them. The VFD's output also has a lot of harmonics in it, which rapidly overheats the capacitors as well. It's usually a race to see which one lets out the magic smoke first. If you research it, you will see that you can only use these VFDs on what are called PSC (Permanent Split Capacitor) motors and Shaded Pole motors, neither of which are the type you would find on a machine tool.
2) The start switch is a problem even for starting at full speed. Unless the VFD is over sized for about 4x the motor rating, it will ALWAYS ramp even though you set the ramp time to 0.1 seconds. That's because the VFD components will be limited to 150% current at best, maybe even lower if it's a cheap drive. So when you attempt to start the motor too fast and the motor wants to pull more current (6x FLA) than the VFD components are capable of, the VFD will artificially limit current by extending the the acceleration ramp to protect itself. That then means the motor starting capacitors will be in the circuit too long, in which case refer back to item 1 regarding capacitor damage.
Bottom line, you could easily damage an expensive motor with your bargain drive. I wouldn't risk it.
Thanks for the advise - I'll skip the 120V VFD.