Reducing start up current on air compressor for off grid

BB. · March 2017

Does it look like you are getting a better starting/running current profile (soft start)?

Watch the temperature of the electric motor... Most (some, many, most?) VFDs use square wave current of some sort and the motors can run hot.

-Bill

mcgivor · March 2017

BB. said:

Does it look like you are getting a better starting/running current profile (soft start)?

Watch the temperature of the electric motor... Most (some, many, most?) VFDs use square wave current of some sort and the motors can run hot.

-Bill

Bill has an interesting point there, years ago we had a company that supplied us with VFD's but failed to take into consideration the insulation rating of the motors, they were rated at 600V, the motors needed to have a 1000V rating, subsequently the motors failed due to insulation breakdown of the windings, over time. Was an expensive mistake which cost them the replacement of over 30 motors in the 5-20Hp range. The drives can actually exceed the rated voltage in the form of spikes, not trying to scare or discourage you, just some sideline information, FWIW.

oil pan 4 · March 2017

It sounds like this ABB drive uses modified sine wave. I will put my fluke123 on it to see how bad it looks. But I'm pretty sure this free gear box motor I got from work was on a VFD and ran probably between 20 and 24hrs a day.
I have built up welding machines and have gotten pretty good at reducing transient spikes. I was going to put 250VAC MOVs between each phase going to the motor.

With single phase motors I have been able to greatly reduce starting amps. The key is to disconnect the motor from the load.

For example I finally tested putting the go cart clutch on on the 2.5hp 2 pole motor. It reduced 45 amps down to 23 amps. That simple $50 centrifugal go cart clutch is the cheapest most effective one step method to startup inrush reduction.

oil pan 4 · March 2017

My 2hp 2 pole motor and magnetic clutch are here. I will be trying to get that going this weekend.
My gustimate is the 2 pole motor will have an free spin full power starting inrush of around 15 amps. I think resistor starting the free spinning motor will reduce it to around 10 amps.
Slam engaging the electromagnetic clutch onto the compressor? Who knows, my guess is between 10 and 15 amps.

oil pan 4 · March 2017

I got the motor and compressor on the base plate and found I needed a 56 inch drive belt.
Which is much bigger than what's normally used on compressors.
Everything else i have built uses 45 to 48 inch belts for simplicity and it's the easiest size to find. I ordered one from Tyler TX for $13.

oil pan 4 · March 2017

I got my electromagnetic clutch and 4 pole motor powered compressor going. It's just hot wired together at the moment for testing.
I ended up welding the motor foot to the motor body, the bolts were not holding the motor stable.
Energizing the clutch then full power starting the compressor hits 18 amps. This is like starting a normal compressor with hard pulleys. 45 amps with the original 2 pole 2.5hp motor down to 18 amps with a 2 horse 4 pole motor is pretty good.
If I apply full power to the motor and clutch at the same time a curious thing happens. The motor only hits about 12 amps. Just that little bit of slip on start up makes a huge difference.
Free spin starting the 2hp 4 pole motor at full power hits about about 9 to 10 amps.
Free spin resistor starting the motor hits about 7 amps.
Engaging the clutch on the motor while running at full speed only peaks at about 7 amps.
Shut off at 120psi amps are at 7.5.
Unloading the compressor drops amps to 4.5 and can be used to cool the heads off before shutdown.

On my 1hp 4 pole powered compressor with the go cart clutch I was able to fit the 6 inch pulley on it. And use a standard 45 inch belt.
Full power starting this compressor with hard pulleys it was hitting up to 15 amps.
With the go cart clutch with 6 inch pulley welded on there it's hitting 10 amps on start up.
If I resistor start it, give it about one tenth of a second on resistors then hit it with full power it can drop to 8 amps.

It's working a lot better than I ever though it would.

dennis461 · March 2017

Make sure you apply for a patent. Call George Foreman at Invent Help!

oil pan 4 · March 2017

I didn't invent anything.
The purpose of this post was to show that reduced power motor starting is possible for most off grid electric motor driven eqipment.