Measuring Amperage Draw On A 240v Compressor Motor

[roblouvasz]

I used an AC clamp meter on one leg on my 240v 6hp Marathon compressor motor. While running it was reading around 16.5 amps. I checked the other wire while running and it read the same. So, does that make the amp draw 16.5 or 33.0 amps? I'm going to replace the contactor that came with it and replace it with a proper mag starter. By the way, I bought the compressor used and this is what it came with. I did start a thread about contactor vs. mag starter a while ago. I'm just now getting around to changing it out. Thanks.

 

[rlitman]

16.5a

 

[Gear Box]

My understanding of this is 16.5 amps is the draw- both 120 legs are out of phase 180 degrees.

 

[wyliesdiesels]

Its 16.5a.

You dont add both legs.

Having said that, motor starters/mag starters are sized based on HP NOT ampergae for motor loads.

16.5a is around 3HP not 6HP. Where did you get the 6HP figure? Off the tank? Manu. are notoriously famous for bloated HP ratings.

What does the nameplate on the motor say?

If spl, then need to size the HP based on FLA and NEC FLC table...

 

[rlitman]

Its 16.5a.

You dont add both legs.

Having said that, motor starters/mag starters are sized based on HP NOT ampergae for motor loads.

16.5A is around 3HP not 6HP. Where did you get the 6HP figure? Off the tank? Manu. are notoriously famous for bloated HP ratings.

What does the nameplate on the motor say?

If spl, then need to size the HP based on FLA and NEC FLC table...

The motor current will depend on the load. A 6HP motor will not develop 6HP without a load on it. So, the 16.5A could be because he was testing the current when it was pumping into an empty tank, or if it is actually a 6HP motor that he tested at the end of the cycle, perhaps the pulley ratio has the pump running too slowly.

 

[roblouvasz]

The motor nameplate says 24.5 FLA. I Measured the amps with the tank empty. I probably need to take the amperage reading during the whole pressure building cycle and then a start/stop cycle after the tank has built pressure. I'll do that tonite and post the results.

 

[roblouvasz]

OK. I measured 16.5 at start up. By the time it reached 170psi, the reading was 23.0 amps. I opened up a valve to decrease the pressure and when it turned on it was around 22-23.0 amps. BTW, the cut in pressure was 125psi which I think is too low. What should a cut in pressure be?

 

[American Locomotive]

Current measurements seem fine. 125 PSI is a fine cut-in pressure, as generally your air system should be regulated to 90 PSI.

 

[CGT80]

You need to know the max tank pressure, safety release valve pressure, the split between on and off for the switch, and know what your needs are.

If you have long runs, small hose, or multiple filters, then the pressure may drop at the point of use. The more pressure in the tank, the more reserve you have. 90 psi with no air flowing isn't the same as running a tool and trying to keep 90 psi.

Increasing the pressure will increase the load on the motor.

I have heard to multiply the name plate amperage by the service factor (1.15 on the motors that I have), and not to exceed that. My 5HP baldor motor is rated 28.5 amps. Newer 5hp baldor motors will be less. My compressor pulls 30.0 amps at 170 psi, which is where it is set to shut off. It pulls 165 amps to start and once running is pulling less than 30 amps until it gets closer to the shut off point. The pressure release valve is 175 psi. A couple times, the switch was set a tad high and it set off that valve. This pump was running at around 650 rpm with much less load on the motor, when I got it. I bumped it up to just over 800 rpm with a new pulley on the motor, to get maximum cfm.

 

[roblouvasz]

Everything is used at 90 psi or lower so I'll leave the cut in pressure as is. My main concern is getting the proper type/size of magnetic starter. An auto parts store near me that used to sell a lot of air compressors has a couple of NOS mag starters on clearance. The first is a Square D Class 8536 Type SCG-1 NEMA 1 which is only rated for a 3hp motor but is rated for 26.5 amps FLA. The other is a Furnas WSX5 230ip rated for 40 amps FLA. They're selling each for about 80.00 each. Any thoughts?

 

[American Locomotive]

The Furnas unit is rated for 7.5 HP

 

[Lelandwelds]

The more pressure in the tank, the more reserve you have. 90 psi with no air flowing isn't the same as running a tool and trying to keep 90 psi.

Increasing the pressure will increase the load on the motor.

You can always get some extra reserve by adding another tank. If you add the dry tank downstream of the filters, they cant hurt the air flow. If you stop the pump at a lower pressure, the electrical savings can really add up over the years.

 

[roblouvasz]

I've never had an issue with not enough air when spraying or using airtools, it just seems that my last compressor had a cut in at around 140psi and a cut off of 170. The pump is a Quincy 325 pressure lubed unit sitting on top of an 80 gal tank. My main concern is replacing the generic contactor with a proper mag starter. I'll look into getting the Furnas. Thanks for all of the replies.

 

[MBfreak]

The phase currents in a 3 phase system can be used to calculate the appearant power ( S, kVA) a follows U ( phase-phase )* Phase current * sqrt3.
In your case 240*16,5*1,73.
This is NOT te actual work (P, kW) that the motor produces

As a rule of thumb use the kVA rating multiplied by 0,88 and you get the kW figure , ie the mechanical work the motor cranks out.

No need to go further re kVA/kW, unless you are like me. I try to dig deep even in things I have little knowledge of. I live by the motto " If not understood, it will not work"

Best regards.

Ola

 

[johnnyradiant]

...

16.5a is around 3HP not 6HP. Where did you get the 6HP figure? Off the tank? Manu. are notoriously famous for bloated HP ratings.

What does the nameplate on the motor say?

...

Kinda funny how we all seem to know the sellers can magically come up with bloated #'s but yet we all seem to tolerate it or worse yet gobble it up. In anything from a simple light bulb all the way up the ladder.