Would VFD or soft start work on 3 phase where power supply is not large enough for starting the motor

Thread Starter

joolsh20

Joined Jun 20, 2017
8
I am looking for a bit of advice, before I go off spending lots of money which may not solve a problem. I know if you put enough money into a project anything is possible, but I am looking for a cost effective way of solving the issue using the parts I have.

I have a diving compressor with electronic control panel. The panel and motor are 3 phase and the motor is 7.5kva 14 amp motor.
The control panel has a star delta starter configuration.
My issue is my intended power supply is not suitable (which was a tractor pto generator, but the old voltage regulator has burn out).
This leaves me with a dilemma.

My other options are:
1: Buy a much larger generator which can handle the torque for starting the motor (I know these motors can take 300% or running power), but this option can get very costly due to the generator size and this would need to be housed somewhere secure
2: Get a new new PTO generator (cheaper option) but my tractor is only 18-20hp, so maximum output would be 10Kva. But this would mean I can move the generator easily and I have a tractor!
3: I can scrap the 3 phase motor and simply bolt of a petrol engine, but this defeats the point of have a control panel with with auto start/stop and automatic drains etc...
4: Replace the 3 phase motor with a single phase with a higher Horse power (this means the control panel will be no good as it is wired for 3 phase)

I was hoping a variable frequency drive or soft starts my help with a lower power generator or tractor pto which would only produce 10kva!
Any advice would be greatfully taken
 

GetDeviceInfo

Joined Jun 7, 2009
1,879
if the motor won’t start, laying into it with what the generator has to offer, a VFD won’t provide that extra umf. Is not replacing the regulator your ‘cheapest’ option.
 

Thread Starter

joolsh20

Joined Jun 20, 2017
8
if the motor won’t start, laying into it with what the generator has to offer, a VFD won’t provide that extra umf. Is not replacing the regulator your ‘cheapest’ option.
The regulator is an old 1960/1970 wound transformer type. Even if I replaced it or even bought a new pto generator, I am not sure if the tractor and pto combination would power the start up current required. I know once the motor is running it will be fine. I dont know how much a VFD would help.
 

GetDeviceInfo

Joined Jun 7, 2009
1,879
isnt the VFD creating a sag lower than the source. Surely it could create a start longer than the source, but to what end. This might be a situation where a start transformer which provides increased current could be useful, but impractical, maybe. As the generator has similar characteristics, combined with a star/delta start, if it can run the motor at nameplate, it should have no problem starting. If it can’t run nameplate, it’s a lost cause. Guess what I’m saying is that a sagging supply is a soft start in its own.
 
Last edited:

dwdohm

Joined Mar 8, 2019
14
A VFD can help start a "variable torque" load, such as a rotary compressor, by limiting the inrush current on start where friction is not as large a factor as inertia in the power required to spin up the compressor from stop.
If the dive compressor is a positive displacement design (such as a piston compressor), even with an "compression unloader", it would represent a "constant torque" load and the VFD will not likely provide sufficient "break away" torque to start the compressor.
Since the primary power source is a tractor with PTO, I don't understand the reluctance to replace the unit with an engine driven compressor with autostart and other control features.
 

prepka

Joined Oct 5, 2020
13
I am looking for a bit of advice, before I go off spending lots of money which may not solve a problem. I know if you put enough money into a project anything is possible, but I am looking for a cost effective way of solving the issue using the parts I have.

I have a diving compressor with electronic control panel. The panel and motor are 3 phase and the motor is 7.5kva 14 amp motor.
The control panel has a star delta starter configuration.
My issue is my intended power supply is not suitable (which was a tractor pto generator, but the old voltage regulator has burn out).
This leaves me with a dilemma.

My other options are:
1: Buy a much larger generator which can handle the torque for starting the motor (I know these motors can take 300% or running power), but this option can get very costly due to the generator size and this would need to be housed somewhere secure
2: Get a new new PTO generator (cheaper option) but my tractor is only 18-20hp, so maximum output would be 10Kva. But this would mean I can move the generator easily and I have a tractor!
3: I can scrap the 3 phase motor and simply bolt of a petrol engine, but this defeats the point of have a control panel with with auto start/stop and automatic drains etc...
4: Replace the 3 phase motor with a single phase with a higher Horse power (this means the control panel will be no good as it is wired for 3 phase)

I was hoping a variable frequency drive or soft starts my help with a lower power generator or tractor pto which would only produce 10kva!
Any advice would be greatfully taken
Have you thought about directly driving the compressor unit directly from the PTO?
All you would need to is remove the pully and mount a coupling to the compressor shaft if the tactor has enough torque provided it is a diesel as petrol ones in this range likely to stall the engine. Might be best to decrease the ratio with belts and pulleys if you are using a garden tactor. You mention this tractor is from the sixties. You may not be able to find the PTO attachment due to age, but the well digging and trenching attachments of the farm tractors of this era used v- belt pulleys for this reason. You don't mention which tractor you have but I have seen tractors with air pumps mounted to farm tractors PTO for airing up tires on tractor and seeders etc. with pulleys to pump. I have seen some of those cheap Asian made motors have locked rotor current requirements in the 8-10X range just on the mains line. Until you know what the LRA of the motor is you can't properly size a generator to drive it. You can repair the tractor a d try adding three resistors and a three pole relay to bypass them once the pump is running between the controller and the pump. As long as you use the right resistors and wattage to.limit the leak current to a level below what the regulator can supply so you won't s.one it again. . a VF D may not fix the problem as they usually have capacitor banks and usually don't have enough in rush limiting to run from a generator as they are intended to run from the mains and they are the most expensive option in my opinion. If the pump has pulleys, a smaller motor with a smaller pulley on it may be another option but will drive pump slower increasing the fill time significantly.
 
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LowQCab

Joined Nov 6, 2012
1,110
""My issue is my intended power supply is not suitable,
(which was a tractor pto generator, but the old voltage regulator has burn out).""


I would replace the Voltage-Regulator on the Generator,
otherwise the Generator cold wind-up as wasted scrap-metal.
On the other hand, you may not need any Voltage-Regulation at all.

There is no indication provided as to how the Voltage-Regulator works,
other than "uses a Transformer",
I would be very interested to learn how it works.

If it is a simple Transformer, there's not very much that can fail.

https://www.ecmweb.com/basics/article/20897066/the-basics-of-transformer-voltage-regulation
https://circuitglobe.com/regulating-transformer.html

Your PTO Generator may work acceptably well without the "Voltage-Regulator".
You may simply need to set the Loaded / Running Voltage by changing the Tractor's RPM.

The Compressor already has an automatic "Soft-Start" built-in, ( Star-Delta-Starter ),
so the Start-Up Current Spike shouldn't be too extreme.

The inertia of the Tractor-Engine should handle the Motor Start-Up without
too much effort or drama,
not withstanding the shrieking Belts, pay no attention to them.
A ~25% Voltage sag for a half-second during Start-Up, probably won't cause any issues,
and like-wise,
a ~25% rise in output Voltage during "off" periods shouldn't be a problem either.

Quite often, 3-Phase Motors will list an acceptable "Voltage-Range"
on the Specifications-Plate, such as, 200 to 250 Volts.

Does your Tractor-Engine have a Governor ?, that would be a big bonus.
( If it's a Diesel-Engine, it will definitely have a Governor )
The Power supplied to the Compressor does not have to be
"exactly" 50hz, or 60hz,
but the frequency will affect its "Output-Volume-Capacity" to some degree.
The Frequency will be exactly determined by Engine-RPM, and PTO-Belt-Pulley-Ratio.

It's better to have the Voltage and Frequency slightly high, rather than slightly low.
Too low of an input Voltage/Frequency may cause the Motor to over-heat over time.
Too high of an input Voltage/Frequency will cause the same problem,
but to a lesser degree.
.
.
.
 
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