Capacitor inverter compressor

Tj Findlay

Hello, can anyone help educate me here?
I have a 2 hp compressor is only small oil free and it's mounted to my ute
It's had a 750 running wattage. To power it i plugged it into a 1000w pure sine wave inverter.
It would start 2-3 times but I was pushing everything to its peak and wanted to help the system along so I cut the power cord and put 35 pf starter capacitor I between the inverter and the compressor. The compressor wold run when plugged into the mains but not the inverter. The inverter blew a couple of transistors witch I've since replaced. And bought a bigger inverter. I'm worried ill damage this one as well, my aim is to use the capacitor for the initial start up wattage and the have the power flow over the top of the capacitor for running. Can anyone tell me what I'm doing wrong? All components are ratted and Are comparable.

Cariboocoot

Re: Capacitor inverter compressor

Welcome to the forum.

Take the capacitor out as that's not how starting caps work. You really can't just guess at a size and put it in-line. A starting cap would be on the start windings of the motor, and it probably already has one. Where you put that one would be the location for a running cap, and that's a whole other issue. If it isn't right it will actually make the PF worse and increase the strain on the inverter rather than relieve it.

Chances are you're not going to be able to improve that compressor easily. It would come down to opening the motor and getting at what's inside combined with some careful measurements of current, Voltage, and PF while being powered from a source that won't blow out on you (the grid).

Something that draws 750 Watts running is probably going to have quite an enormous start surge. Over 2kW I'd hazard. Better if the compressor has a pressure-relief start (so the motor does have to start against the air pressure already stored). Your best bet is to get a 2kW sine inverter and make sure it has good, heavy wires (like 4/0) on the DC side. That way the brief high draw when it starts won't cause problems with Voltage drop on the input. While running it will be the same 750 Watts as before, and that will be an easy load on a 2kW inverter (about 63 Amps on 12 VDC).

I have a similar compressor which draws 1.2 kW when running. It has start cap and run cap and relief valve and still uses a large amount of power. When cold my Honda 2000 gen won't start it. The Outback 3500 inverter has no trouble at all. Compressing air is simply a big power consumer.

mike95490

Re: Capacitor inverter compressor

2 HP is actually close to 1500W of power. Either they overstate the horsepower or something else is wrong. My well pump is 1/2 hp, and it consumes 1,000w according to the control panel of my inverter. So I'm suspicious of the 750W rating.

Cariboocoot

Re: Capacitor inverter compressor

mike95490 wrote: »

2 HP is actually close to 1500W of power. Either they overstate the horsepower or something else is wrong. My well pump is 1/2 hp, and it consumes 1,000w according to the control panel of my inverter. So I'm suspicious of the 750W rating.

It's the old motor power dodge: mine claims to be 3.5 HP. Stall the shaft and it probably is. In use? About 1.5

Or put simply: they lie to impress. :roll:

waynefromnscanada

Re: Capacitor inverter compressor

Hi TJ, sorry to hear of your problems. Unfortunately almost all these "small" compressors are notoriously hard to get started, so the future doesn't look good on getting it operating on your inverter. Likewise unfortunately, it is common practice for manufacturers and retailers to hugely exaggerate the HP ratings of household/handyman compressors. Is there an electrical information plate or sticker on the unit? Example, if it states 8 amps running, it will be roughly 1/2 Hp OUTPUT, likewise, 10 amps is typical of a 3/4 Hp OUTPUT motor, and 12 amp would be roughly 1 Hp OUTPUT (assuming 120 volt motors). If the actual motor has a "hump" on the top or side, that is probably the start capacitor. If it has 2 "humps", that usually indicates both a start capacitor and a run capacitor. No "humps" at all, usually indicates the motor has no capacitors - - usually, because sometimes they hide a capacitor inside the motor, under the rear end cap. IF it has a start capacitor, it will be in series with the START winding, and is only in circuit until the motor comes up to speed. IF it has a "Run" capacitor, it is usually of much smaller capacitive value than the Start capacitor, and is continuously in series with the "start" winding, and always carries a reduced current.

westbranch

Re: Capacitor inverter compressor

My Makita compressor has a 10 amp run rating and it uses that when it is in 70F temps, just cool it down to the 40's and it will almost stall out a 3000w genset , inverter type and non inverter... really makes them work ... when it gets up to temp it is fine...

Tj Findlay

Re: Capacitor inverter compressor

westbranch wrote: »

My Makita compressor has a 10 amp run rating and it uses that when it is in 70F temps, just cool it down to the 40's and it will almost stall out a 3000w genset , inverter type and non inverter... really makes them work ... when it gets up to temp it is fine...

Thanks for the advice guys, I'm pretty keen to get this working without buying a new compressor or another damn inverter so ill remove the current capacitor and have a look inside the compressor to see what I can "beef" up to help everything along. As it is I have to remove the air filter open the pressure release and have all three battered in parallel connected to the inverter just for it to turn over...

mike95490

Re: Capacitor inverter compressor

You might get a Kill-A-Watt meter, and use it to read the "Power Factor" of your running compressor. Then you can fine tune the RUN cap (if so equipped) and the Starter cap. And you are sure it's a pure sine inverter ?
And BE SURE you are not mixing up the start and run caps, start caps are only designed to be engaged for a few seconds, then a centripetal switch on the motor, cuts them out.

Make sure your battery cables are larger then average, you are consuming a lot of amps trying to start the motor, and it all
has to come out of the batteries. Good connections, good fuses, all factors to get the motor to spin up.

Tj Findlay

Re: Capacitor inverter compressor

I'm sure it's a pure sine inverter, but I've bought a 1500w modified sine inverter be cause it only a compressor not a tv or a anything a square wave should interfere with. Am I right in doing this? As told by Waynefromcanada it only has a starter cap inside the unit. Wired into the outer windings. It's a 20pu farad cap so I switched it with a 35pu capacitor and removed the one I put on the AC side of the inverter. I'm yet to try this. But I think I test it with the mains and a few sercut beakers first. lol I'm shooting in the dark at the moment.

Cariboocoot

Re: Capacitor inverter compressor

Tj Findlay wrote: »

I'm sure it's a pure sine inverter, but I've bought a 1500w modified sine inverter be cause it only a compressor not a tv or a anything a square wave should interfere with. Am I right in doing this? As told by Waynefromcanada it only has a starter cap inside the unit. Wired into the outer windings. It's a 20pu farad cap so I switched it with a 35pu capacitor and removed the one I put on the AC side of the inverter. I'm yet to try this. But I think I test it with the mains and a few sercut beakers first. lol I'm shooting in the dark at the moment.

Wrong way 'round.
Motors prefer sine wave and don't like square wave much, whereas any good TV/computer/et cetera will be able to filter out the massive 'distortion' of MSW and run without any trouble.

Changing the starting cap will not necessarily improve things; it may have the best choice in there already. It's a matter of getting the current sine wave demand lined up with the Voltage sine wave supply. The cap charges then discharges through the motor windings, bringing the two 'waves' in-line.

Definitely test it on the mains rather than risk an inverter.

waynefromnscanada

Re: Capacitor inverter compressor

Tj Findlay wrote: »

it only has a starter cap inside the unit. Wired into the outer windings. It's a 20pu farad cap so I switched it with a 35pu capacitor

Most likely changing the value of the original factory installed capacitor will make things worse, as it's value is picked to create the greatest starting torque. Changing the value of the coil, or the capacitor will throw that off. Since you already have it in there, go ahead and try it just in case there might be something strange going on with the original, but expect it to be not as good as the old set-up. Expect it to result in the motor drawing more, not less current when trying to start, and expect it to have less starting torque.
And yes, although MSW inverters will start and run motors, induction motors draw more power, and run hotter on MSW. Induction motors definitely prefer Pure Sine power.

GreatBallofFire

Re: Capacitor inverter compressor

Cariboocoot wrote: »

Welcome to the forum.



Something that draws 750 Watts running is probably going to have quite an enormous start surge. Over 2kW I'd hazard. Better if the compressor has a pressure-relief start (so the motor does have to start against the air pressure already stored). Your best bet is to get a 2kW sine inverter and make sure it has good, heavy wires (like 4/0) on the DC side. That way the brief high draw when it starts won't cause problems with Voltage drop on the input. While running it will be the same 750 Watts as before, and that will be an easy load on a 2kW inverter (about 63 Amps on 12 VDC).

I have a similar compressor which draws 1.2 kW when running. It has start cap and run cap and relief valve and still uses a large amount of power. When cold my Honda 2000 gen won't start it. The Outback 3500 inverter has no trouble at all. Compressing air is simply a big power consumer.

This is extremely good advice right here. Adding a pressure relief bypass valve can help you out a lot. Don't spend too much time changing to various caps, it will probably make things worse. A simple T-fitting with a valve for a bypass will help you more than any caps will.

-Mark