VFD for powering a car lift

[Guineafowl]

TL;DR

My 3-phase car lift, currently powered by a rotary converter, is struggling to lift at full capacity. Would a VFD improve things?

Power supply to the 7.5kW rotary is 1ph, 240Vac. Supply voltage drop at full load is within 5%.

The rotary outputs (roughly) 415Vac 3ph to the screw-type car lift, capacity 2.7 tonne.

The lift motor is a 3.5kW, 50Hz, 4-pole, asynchronous induction motor with a power factor of just 0.65. Phase continuity and insulation resistance are good, and the windings look like new. Bearings replaced.

The set-up works just fine, except when lifting my friend’s RR sport, weighing around 2.6 tonne. The motor often stalls and we have to drop the car down and start again. Note that the same motor size is specified for the larger, 3.2-tonne version of the lift.

Voltage drop between rotary and motor is minimal. However, at the motor, when lifting my 2-tonne car, one phase drops from 415V to 350V. The others are not so bad. The motor is pulling 9.6 - 13.5 amps per phase, higher than its plated FLA of 11A.

Options:
1. Larger rotary converter. VERY Expensive and not justifiable for one particular car.
2. Gear the drive pulleys down. The motor pulley is very small, and the screw pulley very large. Space is limited and belt contact is already borderline.
3. 6-pole motor. Expensive.
4. 3ph-3ph VFD between rotary and lift motor. A friend on an engineering forum has offered a 7.5kW one for a price that makes it worth a shot.

My hope is that the VFD will help with the inherent phase imbalance of the rotary, perhaps also with the poor power factor of the motor. It offers V/F control and torque boost as well. Finally, I’d like to experiment with speed control, on the basis that, roughly speaking, operating a little under 50Hz will be in the ‘constant torque’ range of the motor, while slowing down the mechanism to reduce loading.

Link the the VFD manual: https://www.sakobpq.com/resource/attachments/e0b4d2b3f3b946e7827e3247c770f867_17.pdf

Thoughts appreciated.

 

[Averagesupernova]

VFDs can be configured to be powered with single phase. Seems a bit silly to run a VFD off of a rotary phase convertor. The VFD will have a tendency to do the same thing that you are seeing now and that is one phase voltage sagging.
-
How is your current rotary convertor set up? Do you have capacitors wired between the manufactured leg and either of the others? How is this convertor started? Are you using a transformer to step up the voltage after the convertor or are you relying on the windings in the idler motor to function as an autotransformer? A well set up convertor should do what you are asking it to.
-
I would not feed the main power for your lift anything except the line frequency it was designed for. The three phase motor that actually does the lifting may get along just fine but control circuits will not.

 

[Guineafowl]

VFDs can be configured to be powered with single phase. Seems a bit silly to run a VFD off of a rotary phase convertor. The VFD will have a tendency to do the same thing that you are seeing now and that is one phase voltage sagging.

This one is a 3ph 400V in and out model. Can’t be changed. Cheap enough for a punt on this problem; if no go, I can use it for something else. It’s not worth buying a 1ph 230V in, 3ph 230V out model for this one car - it would have to be specced a lot higher than the 3.5kW, given the lift application with its nearly locked-rotor starts.

The rotary is a commercially-made one for which I don’t have the wiring diagram. All the capacitors are in spec and there is a beefy 240V-415V transformer at the front end. The idler motor is a huge cast iron 7.5kW running in 415V delta (for star-delta starting in other applications).

I think there might be two main problems:
1. The rotary output is not perfect 3ph - unbalanced and perhaps not 120deg apart?
My hope is, the VFD will iron out all this and produce a ‘better’ output. I assume if the phases are unbalanced, this has an effect on the rotating mag field, and torque.

2. The lift motor is very small for the quoted power - no cooling fan or fins, only 0.65 pf, 30% duty cycle.
The VFD may help with that awful power factor. As you point out, the converter should easily do the job, but perhaps the reactive current is loading it down too much.

Edit: one last point I should make, is that good VFD and phase converter makers often ask on their websites that the customer ring them before ordering, if the unit is to be powering a car lift. So it seems to be a special case, perhaps because of the hard starts.

 

[Guineafowl]

Motor plate, for interest: