Having trouble with 25h.p 208v 3phrase f.l.a 72 #3thhn feed air compressor motor.Unit builds to110psi amp draw is 98ea. leg without the belts motor draws 17amps ea. leg at 211v.compressor mech. claims the machine is o.k and belts have proper tension.Can the windings still be at fault without load I get 17amps,did check all connections.Besides megging the motor leads is there any other test. Just want to make sure it is the motor not machine. Thanks
compressor motor
- Thread starter GerryR
- Start date
- Status
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Assuming (Danger, Will Robinson!) that nothing is damaged or out of whack, it sounds like the motor is undersized for the load.
Maybe it's a replacement motor of the wrong speed, or wrong pulley ratio.
Also, what is the terminal voltage when the load current is high?
Maybe it's a replacement motor of the wrong speed, or wrong pulley ratio.
Also, what is the terminal voltage when the load current is high?
frenchelectrican
Senior Member
- Location
- Nord'est Wisconsin et Paris France
I am leaning more likey wrong pulley set up.
What size pully you have it on now ?? you should try the smaller pulley that will reduce the current a bit.
Merci,Marc
What size pully you have it on now ?? you should try the smaller pulley that will reduce the current a bit.
Merci,Marc
weressl
Esteemed Member
GerryR said:
Sacre bleu! Sounds like a typical compressor sizing job. Since the compressors are cycling on pressure, they are sized to overload at the peak of their loading curve.
No-load current looks OK.
What does it draw at 100PSI?
frenchelectrican
Senior Member
- Location
- Nord'est Wisconsin et Paris France
Weressl.,,
As you mention the cycling on the air compressors i know pretty good percentage small to med size compressor do have on / off cycle period by pressure switch.
But however with dual operation compressors that will change a bit on that one due it run two diffrent way by pressure switch to turn on / off or use the unloader valve on the intake port [ some done by thottle down the intake ports ] on the contounits run units.
However.,, for majotry of the compressor work on most of them. I useally pretty much dail in pretty good with right set up. Few case i end up changing the pully to get it right.
With correct set up with on/ off pressure switch cut off point the FLA should be near or on the max amparage rating.
I will give you a quick rundown spec on my compressor so you will get the idea real quick. { Mine have dual operation either on/off or cont run mode with 175 PSIG system in there }
15 HP @ 480 V 3? FLA 21.1 amp loaded [ just before the pressure switch cut off or on cont setting ]
Unloaded mode 12 amp
the FLA plate on the motor is 22 amp.
Merci,Marc
As you mention the cycling on the air compressors i know pretty good percentage small to med size compressor do have on / off cycle period by pressure switch.
But however with dual operation compressors that will change a bit on that one due it run two diffrent way by pressure switch to turn on / off or use the unloader valve on the intake port [ some done by thottle down the intake ports ] on the contounits run units.
However.,, for majotry of the compressor work on most of them. I useally pretty much dail in pretty good with right set up. Few case i end up changing the pully to get it right.
With correct set up with on/ off pressure switch cut off point the FLA should be near or on the max amparage rating.
I will give you a quick rundown spec on my compressor so you will get the idea real quick. { Mine have dual operation either on/off or cont run mode with 175 PSIG system in there }
15 HP @ 480 V 3? FLA 21.1 amp loaded [ just before the pressure switch cut off or on cont setting ]
Unloaded mode 12 amp
the FLA plate on the motor is 22 amp.
Merci,Marc
- Location
- Massachusetts
Has the compressor ever worked properly?
Is it brand new?
That would be where I would head.
Check with the manufacturer what RPM the compressor is designed to run at and verify it is not over driven.
I have run into this a few times with fans, people replace a motor and either can not remove the pulley from the old motor or they damage the pulley then they replace it with a larger one without thinking.
Is it brand new?
frenchelectrican said:
That would be where I would head.
Check with the manufacturer what RPM the compressor is designed to run at and verify it is not over driven.
I have run into this a few times with fans, people replace a motor and either can not remove the pulley from the old motor or they damage the pulley then they replace it with a larger one without thinking.
- Location
- Massachusetts
weressl said:
You don't use unloaders?
I don't know of any piston compressor that would start with full tank pressure in the cylinder.
weressl
Esteemed Member
iwire said:
Whether it is reciprocating, screw or centrifugal, unloaders are always in place except for very small application.
The compressors operate on a variable curve. They start out unloaded and then as the (back)pressure increases their load also does. By design the loading does not stop at FLA, but at some point beyond that. The cyclic loading assures that the compressor has time to cool off so that thermally it is not overtasked. A motor cycling load and contiuous load differs in hat the peak load in the former can be grater than in the latter.
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