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Nurples (Military) (OP)
15 Sep 09 11:53
I'm currently deployed to Iraq with the US Army and I'm working on hooking up an air compressor. I am not an electrician, but I have a little knowledge.

Here are the specifications:

Brand: Speedaire
Model: 1WD76
49CFM @ 175 PSI

Pump speed: 1035
Pump model: 1WD22

We are trying to run the air compressor on a 30Kw generator that is producing 3PH 110V power at 60HZ. If my math is correct, it should take a little under 14KW to run this motor. This generator is plenty big enough to run this compressor.

Here's the issue: When the air tank is empty (no load on the motor) the air compressor runs great. It builds up the air tank to 175psi. and shuts down correctly. When the air pressure is lowered and the motor attempts to turn back on, it struggles to rotate at all. It may rotate a few times and then stops. We have taken the air pressure down to 75psi and the motor still will not start up.

We've also wired a 100A breaker in line with the compressor. This breaker blows when we try to start the compressor. Without the breaker in line, the compressor attempts to start, but turns really slow or doesn't turn at all when under a load. The data plate on the compressor says 65A, so I'm not sure why we are blowing a 100A breaker.

I've done a lot of reading and haven't found answers to my questions yet. Some of that may be that I don't know exactly what I'm looking for.

I have read that you can rewire some motors to get more torque out of them to handle heavier loads. The cover on the motor gives options to reconfigure the power rating for high voltage (460V) but getting this type of voltage isn't an option for us right now.

Any suggestions on this issue? Am I missing something simple? This motor came with the setup, so it should be large enough for the compressor. I guess my only real problem is that the motor will not start up under a load.

Thanks in advance!

SSG Babcock
Motor Sergeant
E Co/3-10 GSAB
COB Speicher, Iraq
Compositepro (Chemical)
15 Sep 09 12:06
It sounds like your unloader valve is not working. Compressors will not start against full pressure. When they stop you will hear a hissing sound for a few seconds. This is the air pressure at the compressor outlet being vented. A check valve prevents the tank from venting. When the compressor starts there should be no pressure at its outlet and this unloading valve closes. This gives the motor a chance to get up to speed before it has to provide much torque.
rbulsara (Electrical)
15 Sep 09 12:23
You cannot run a 208/230 or 460V motor on a 115V supply or generator.

You need a transformer to step up the voltage to correct motor voltage either 230 or 460. The motor windings needs to be connected for the correct voltage also.

Also see if your generator can be reconnected to produce 208V or 230V.

Rafiq Bulsara

waross (Electrical)
15 Sep 09 12:50
The current rating (65) looks like it is wired for 208/230 Volts.
Are you sure that the generator is not 120/208 Volts, rather than 110 Volts?
The last time that I saw a 3 phase 110 Volt generator was about 25 years ago and it was very old then.
But, on a 110 volt supply, the motor may not draw enough current to trip the 100 Amp breaker.
Your motor will draw over 300 amps when starting but the thermal lag on the breaker should permit this.
I believe that yourproblem is mechanical. Those are the text book symptoms of a faulty unloader valve. Fix the unloader or install one. You may build an unloader from a check valve, a solenoid dump valve and a time delay relay.

"Why not the best?"
Jimmy Carter

Nurples (Military) (OP)
15 Sep 09 12:54
Compositepro -- As far as the unloading valves, I don't hear it releasing any air when it kicks off. I'll have to check into this.

rbulsara -- This is where it's a little confusing to me.  The power from any of the lugs (L1, L2, L3) to Netural (L0) is 110V (+/- a few volts depending on what we set it at). Then from L1 to L2 I get the 208V and the same for L2 to L3 and L3 to L1.  This is where the 3Phase 208V comes into play as I understand.
Nurples (Military) (OP)
15 Sep 09 12:58
waross -- Yes, I may be stating the voltage wrong.  I'd have to look at the generator in order to tell (it's located down the road).  I do in fact know it say 208V and when testing the lugs as stated above, I get around 110~120V from L1,L2,L3 to L0 and get 208V +/- when going from L1~L2~L3...  So I'm pretty sure the voltage is correct.
waross (Electrical)
15 Sep 09 13:09
If you have access and permission, tweak the Automatic Voltage Regulator up to 127/220 Volts. If not possible, it should still work.
The voltage will be 120/208.
A compressor that size will not start against back pressure. Fix the unloader.  

"Why not the best?"
Jimmy Carter

rbulsara (Electrical)
15 Sep 09 13:57
OK. Your voltage is 208V 3 phase.

I agree with Bill that you have to unload the compressor when starting.  

Also I would use 150A breaker, not that it is your first problem as your gen boggs down even without the breaker.

Rafiq Bulsara

jcarrr (Electrical)
15 Sep 09 14:53
I suggest that you not change the voltage settings.  As soon as the large starting load is presented to the generator, the exciter quickly shifts to maximum current, no matter where the voltage is set.  Changing the voltage or motor configuration will not make the generator produce more power than it does now.  The engine is probably the limiting factor.  If the generator needs adjusting, it will be adjustment of the stability control or the roll off control.  If either or both are set improperly for your circumstances it will decrease the ability of the genset to start loads that are a large fraction of the rated capacity.  Increase the stability and start the voltage rolloff at a higher RPM.  Read the manual.

Addition voltage under normal operating conditions will mostly serve to damage loads connected to the generator.

The electric motor will be most happy when the voltage to the motor lugs is exactly as the label specifies.  Either excessive voltage or reduced voltage will cause the current to increase.  The resulting current increases the heat load on both the motor and generator unnecessarily, reduces the capacity of the installation to carry additional loads and is the moral equivalent of pouring fuel on the ground.

Fix the unloader.  Worst case, if the check valve is holding, install a valve to vent the compressor so you can start it by manually opening the valve and then closing it after everything is turning,
jraef (Electrical)
15 Sep 09 15:59
The unloader will not make any noise if the compression chamber is not charged. The typical operating mode is that it's a solenoid valve that is spring-held open to atmosphere and energized to close after the motor is at speed (via timer), so as soon as you turn the motor off it will unload the compression chamber and remain open until you start it again.  What happens a lot is that corrosion builds up on the solenoid shaft and it sticks closed even though the solenoid coil is de-energized. Removing it and cleaning it usually solves the problem.

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flandrax (Electrical)
15 Sep 09 19:53
i would also take a look at the check valve. The valve could be stuck open allowing whatever pressure you have in the tank to be present on top of the piston.  
Nurples (Military) (OP)
17 Sep 09 11:11
Thanks for all the information guys... I've worked on the system and it does have an unloader and it does work.  When the pump runs, it builds up the air pressure to 175lbs and then shuts down and you hear the air coming out of the unloader.  

The problem seems to be that the generator doesn't have the initial power required to start the motor fast enough.  When the circuit is closed to start the compressor without any air in the system, the motor starts to turn slowly and gets to normal speeds (almost like a soft start).  When the motor starts up the generator bogs down dramatically.  After 3~5 seconds, the generator regains speed and the pump runs fine.

I believe the problem to be that the generator isn't strong enough to handle the initial start up of the motor.  Since the generator bogs down, the output wattage drops causing the motor to start slowly.  And, since the motor is starting slowly, the air pressure in the lines build up before the motor can get to running speeds.

I intend on hooking up a 30KW generator to try to get a faster startup from the motor.

waross -- Above mentioned that the motor would pull in excess of 300 Amps... If my math is correct, that's around 50KW of power.  This seems like a HUGE draw for such a small motor.
electricpete (Electrical)
17 Sep 09 11:28


Above mentioned that the motor would pull in excess of 300 Amps... If my math is correct, that's around 50KW of power.  This seems like a HUGE draw for such a small motor.
No, it's about right. Starting current is typically around 5-7 times full load amps.  But if you want to try to convert it to watts you would have to consider the power factor is low... down around 0.2 during start.

It certainly may be the case that the motor is dragging down the generator during start.  However please double check another thign - This is a dual voltage motor... that means there is more than one way to connect it.  ** You need to make sure your connections are correct for the voltage you're using..  Otherwise if you connect winding groups in series for 460 when they're supposed to be in parallel for 230vac, your motor will "think" it's getting half voltage.

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electricpete (Electrical)
17 Sep 09 11:32
How many leads does your motor have?

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Nurples (Military) (OP)
17 Sep 09 11:39
I want to say it has 6 leads, but I can't remember off hand.  I'll go take a look in a bit and reply with an exact count for you.  

I know the motor is dual voltage, but if it was wired for 460V, what would the effect be during normal operation if it was only getting 208V?  Would it run slower? The motor seems to be operating at a normal speed when it's running.  It just has issues starting when their is air in the tank.  Even 50LBS of air in the tank stops the motor when it's not already running.
electricpete (Electrical)
17 Sep 09 11:51
If it is wired for 460vac and supplied with 208vac, the torque curve is shifted down by a factor (208/460)^2 < 1/4 (or lower if voltage droops during start).  This makes it much more challenging for the motor to start.... will take longer to get up to speed and draw more current for that period.   It may not get anywhere near close to normal full speed if the torque drawn by the load is greater than motor breakdown torque which is <25% what it should be due to miswiring.  

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electricpete (Electrical)
17 Sep 09 11:53


and draw more current for that period
What I meant to say is the duration of starting current is longer since it takes longer for motor to start.  Magnitude of starting current would be lower than normal starting current.

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waross (Electrical)
17 Sep 09 13:29
Some unloaders close almost instantly. If the motor doesn't spin up normally these unloaders will cause problems starting.
Is this a turbo charged engine? Turbos sometimes have problems with motor starting. If the initial load is very light, the generator will bog down until the turbo spins up.
In your case:
1> It's probably OK but check the unloader for the third or fourth time.
And in no particular order:
2> Larger generator.
3> Add a dump valve with a slight time delay to unload the compressor until the motor is up to speed.
4> Add a capacity chamber so that the back pressure does not build up as fast.

"Why not the best?"
Jimmy Carter

edison123 (Electrical)
17 Sep 09 22:10
Can you measure and post here the actual voltages across the motor terminals during starting (without air in the receiver tank) ? May be the generator AVR is not fast enough to maintain the voltage during the starting inrush. Or there is a high voltage drop in the supply cables.


Nurples (Military) (OP)
20 Sep 09 6:22
Update:  I've wired the compressor into a 30KW generator and everything seems to work normally.  Only issue now is that when the compressor starts up, it sometimes trips the "Short Circuit" fault, which turns off the power.  We can manually override this each time, it's just a pain.  

Thanks to everyone for your help and suggestions.

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