ECN Forum
Posted By: Alan Belson 3 phase compressor motor - 05/27/05 07:52 AM
I just got an ancient 3-phase air compressor for a renovation project. Built 2 20 1946. Compressor would be about 5cfm free air, as the piston swept volume is around 100cc. The present starter is not original & has been set at 3A. I have the original which is a crude 3-phase DOL toggle-switch. The problem is that the brass motor plate is marked only with-
(translated from the french).
4 pole 3ph 50hz cos.p 0.8
210/364 volts. 1400 rpm.
The 'Amps' square is blank- looks like it's been rubbed off. Motor was wired Wye.
Is there a way to determine a safe setting for motor overloads on a new starter?
The makers have long vanished.
I intend to fit a new pressure switch, safety valve and starter.
Posted By: Larry Fine Re: 3 phase compressor motor - 05/27/05 01:28 PM
Have a horse-power rating?
Posted By: Alan Belson Re: 3 phase compressor motor - 05/27/05 05:39 PM
No, there are no other indications of power.
Comparing modern compressors, 5 cfm would seem to require 1.0 hp approx to 120psi. What's throwing me is the size of the induction ('asynchrone') motor frame, it's 7" diameter and over 10" long, about the size of a modern 3hp unit, although I know motors tended to be larger then.
Posted By: mxslick Re: 3 phase compressor motor - 05/27/05 05:41 PM
Mr. Belson:

One method I've used with success is as follows:

If belt-driven, remove the belt from the compressor.

Using a clamp-on amp meter, measure the running current with no load on each phase.
If the meter has a peak-hold feature (my Fluke 36 does) also measure the peak draw on startup with no load.

Repeat the same steps with the belt on and the compressor running. Record those readings. If there is no unloader valve (releases pressure in the cylinder(s) when the motor stops, you'll hear the hiss at stop if it has one), then it would be good to repeat the measurements with the tank full.

Set your overloads to 125% of the highest running (not peak!) readings. If you have nuisance trips, try 135% of highest running amps. (The setting should roughly equal the peak current +10%/-30%, not a concrete rule but seems to work out that way.)

Also look carefully at the overload trip curves, some have longer trip times to allow for hard starting motors to ramp up without tripping.

I used this method to set the overloads on a pipe organ starter I designed and so far it has worked out perfectly. The only trips have been due to phase loss and one case of dry bearings.

Larry: Horsepower ratings are usually of little or no use in compressor applications, especially with modern units. Check out Sears' "5HP" compressor which will plug into a std. 15amp circuit. [Linked Image] For other motors, you've got the right idea as most HP ratings seem to fall within a standard current range. :thumbsup:

A bit OT, but in my pipe organ application, when does the blower draw more current? When the organ is idle (lots of backpressure) or when playing "full stops" (basically with little or no back pressure)? The answer may surprise you.
Posted By: chipmunk Re: 3 phase compressor motor - 05/27/05 06:08 PM
From other blowers, I would suggest when everything's closed the current would be lower. I can't remember the exact reason for this, although i remember it badly affects vacuum cleaners inasmuch they use the airflow for cooling. This probably doesn't apply to organs though.
Posted By: mxslick Re: 3 phase compressor motor - 05/31/05 05:32 AM
Chipmunk nailed it right off.... the organ blowers draw less current when the backpressure is high (the organ's not playing) and rises quite rapidly as more stops are used.

It has to do a lot with the design of the blower's impeller, certain designs place less of a load on the motor with higher back (or "static") pressure. The same organ blowers cannot be operated at all without being connected to the air distribution system or it will cause a severe overload. (I have heard of an instance where a 50hp organ blower was run with no air system connected, it took out a 2000 amp main breaker in no time flat.)

The starter I designed will also not allow the main (25hp) blower to restart until it has spun down to a stop, which can also cause an overload trip. (Time delay to accept start command after a stop command.)

Most organ blower motors don't rely on the airstream for cooling, they are either open frame or shaft mounted fan cooled.

A look though Grainger's catalog in the blower section will give a lot of insight as to different impeller designs and static pressure restrictions.

IIRC some water pumps follow the same principles.
Posted By: Alan Belson Re: 3 phase compressor motor - 05/31/05 02:12 PM
Thanks for your valuable advice. I've done some partial disassembly. There is an unloader valve, but it's not like any I've seen, the actuator works off the pressure-switch mechanism, so its vent is into the switch housing, which is coated in oil but perfectly preserved! The motor/pump is inline, direct drive, so I'll get unloaded readings with the cylinder head off. I'll post before and after/detail pics when it's renovated.
Thanks again,
Posted By: Trumpy Re: 3 phase compressor motor - 06/04/05 08:20 AM

There are some pictures of the gear that Alan is dealing with down in the Electrical Nostalgia Area.
To see these photo's click here .

[Linked Image]
© ECN Electrical Forums