[1ctoolfool]

my 7hp compressor causes enough of a voltage drop sometimes to kick off the mini mill power intterupt alarm. machine must be re-started to clear. since summer has started and AC is on it has become an annoying problem.

What can I do to stabilize the brief (1sec?) voltage drop?
Isolation transformer? or is there another product designed to do this?
thanks,
Joe V.

[Al_The_Man]

If the motor is 3 phase, you can buy soft-starts, about the size of a large contactor, I am not sure of the implications of using with a compressor, probabally have to make sure it is unloaded when starting.
Most of the electrical supply houses carry them. But it should take care of the high inrush.
If it is 1ph cap. start motor, you options are limited as these motors do not like soft starting etc. You can however take steps to make sure it is off load when starting.
Al.

[Geof]

Another way to reduce the starting load is put a centrifugal pulley on the motor. This allows the motor to come up to about 75% of full speed before the clutch picks up and starts driving the pulley. I think this compressor caused voltage drop problem was discussed before and someone posted a link to this type of pulley.

[1ctoolfool]

thanks for the info, I also thought about putting a smaller drive pulley which would de-rate the compressor but I could live with that.
the compressor is single phase 7hp,
don't know the inrush current exaclty but it's close to 40 amps.
thanks
joev

[Al_The_Man]

The other way is to provide a service independantly for the compressor direct from the panel. If your incomming service is sized large enough the volt drop will only occur on the compressor line.
Al.

[Geof]

You have a rotary compressor? I believe I recall you posting about getting a rotary a few months back.

Maybe you could look into getting an unloader valve so the compressor runs all the time but justs goes off load instead of shutting off the motor. I have two rotaries that work this way; the unloader blocks the inlet to the compressor so it cannot draw any air in. If there is no air going in there is no load on the motor because the compressor is not compressing. I have no idea if these can be used on all makes of rotary but you could maybe find someone who knows about them.

[gar]

070601-1543 EST USA

You need to provide more information.

Before my questions here is some useful information.

The ideas of both Geof and Al are good.

An isolation transformer will do nothing for you. A sola regulating transformer feeding the CNC would be helpful but you do not want to pay for it. You need to make some voltage measurments to have quantitative data to work with. If your mini mill has an internal transformer with adjustable taps you may need to change these.

1. Are you 3 or 1 phase?
2. What voltages do the various loads work at?
3. What is your main service amp rating?
4. What is the voltage at the service entrance with nearly no load? With the mill on? With mill off and air conditioner on? Most stuff off but pump on? Most stuff off while piump is starting?
5. Describe the circuits from the main panel to the various large loads?

.