[Shop-talk] air compressor problem

[johnny b]

Season’s Greetings to one and all.

After all of these years of listening, I now have a request from the 
back bench.

I have an air compressor problem.

The compressor is an Ingersoll Rand SS5N5, which is a single stage, two 
cylinder, 5hp electric driven compressor unit on an 80 gallon vertical 
tank. It is rated at 135 psi max., 18.1 cfm @ 90 psi.

I use it in a home shop, mostly light duty.Some weeks, it doesn’t run at 
all.

Originally, I noticed that occasionally the compressor would not come up 
to speed on a restart after tank pressure dropped.When this happened I 
could turn it off and wait a while, then turned it back on and it would 
run normally.In the last month or two, it has gotten to where the 
compressor would run normally only on the first startup of the day.After 
that, it wouldn’t come up to speed.

Today, I started working on it.

I started the compressor with an empty tank.Air temperature in the shop 
was about 60 degrees.

Pump rpm was 990:motor was drawing about 14 amps (I’m using a Craftsman 
clamp on type meter).

At about 60 psi, rpm had dropped to 960, and amps increased to 20; motor 
was noticeably working harder.

At 100 psi, rpm was at 870 and amps were at 22.5.

At 120 psi, rpm was about 800 and amps were at 23.5.At this point the 
belt was beginning to slip so I turned the compressor off.Data plate 
rating for this motor is 22 amps.

I immediately removed the tube from the compressor head to the check 
valve and restarted the compressor.

With no load, rpm was 1001 and amp draw was 13.It ran this way for 
several minutes with no sign of problems.

The temperature at the bearings on both the motor and the pump was 80-85 
degrees.

The motor was replaced in June with a comparable new unit.The pressure 
switch assembly was replaced at the same time.The oil was changed about 
2 months ago.It has always used IR Synthetic Compressor Oil.

Today, I removed the check valve and it was clean and operated 
freely.The unloader valve sounds as if it is working correctly.

So, what else can I check?

Thanks for listening,

johnnyb