3 Phase compressor, single ph service: What options?

[Fl_Key_Guy]

Let me see if I have this correct:
Y'all are saying that the Mariner can run just fine on a faster (about 3600rpm) 5hp motor at 1200 rpm ccw rotation of the pump. Therefore, the the 7.5 hp that it usually comes with is just overkill or normally runs a Mariner faster. Is that correct?

I never got an answer as to how fast the Bauer Mariner usually spins and could not find that on line. The Yachtpro runs at 1200 rpm but I don't know if that is one of Brownie's modifications from the stock Mariner?

I also noticed that the gas powered Capitano has a gas engine the same size as the electric motor. Usually a gas motor is significantly higher in named horsepower than the electric version for the same compressor. For example, a 1hp electric is about equal to a 4 hp gas. That Bauer would use the same size surprises me. Maybe they are just real conservative with their electric sizing?

 

[Fl_Key_Guy]

I see Derek was responding as I was writing my last post (I didn't see it before writing). Thanks for the excellent tutorial on motors, Derek!

I can't seem to get much of any info on the pump. If I can't even get stock pump speed, I sure won't find a torque curve!

I did find plenty on Mariner II at this site:
BAUER

 

[pescador775]

Let me see if I have this correct:
Y'all are saying that the Mariner can run just fine on a faster (about 3600rpm) 5hp motor at 1200 rpm ccw rotation of the pump. Therefore, the the 7.5 hp that it usually comes with is just overkill or normally runs a Mariner faster. Is that correct?

No, no one said that. Among our collective remarks we said that the Mariner when powered by the Leeson 5 hp motor can generate 5.8 cfm @ 3500+ psi @ 1100 rpm and furthermore, that the oil pump will produce rated press under those conditions. All this is true if you do not deviate from the advice given by Craig. This includes the specific components, rotational speed and CCW rotation of the motor shaft when viewed head on. This rotation is necessary because of the design of the compressor fan and has nothing to do with the oil pump which is omnidirectional on the Mariner I. It has also been stated that the motor is not expected to draw enough current to trip a 30 amp circuit breaker.

Dereks explanation is excellent but is of most use to equipment wonks like me and some others. I am sure that we all appreciate his contributions.
Pesky

 

[pescador775]

A quick note about slippage. The rotational speed of the Leeson is actually 3600 rpm. The slippage speed is the nominal rated speed or 3450.

 

[Scuba Duck]

If the Leeson motor delivers 5 HP at 3450 RPM, you should find out what RPM 5 HP of load equates to on the compressor block (torque/HP curves), and size the pullys accordingly.

One more thing to think about before you go and purchase the 5 HP motor.

If there is room to mount the VFD directly on the cabinate of the compressor, there is something very interesting you can do.

You could use the main motor contactor in the control panel to power the VFD with single phase, and then the power the motor directly from the VFD. This way the control circuit in the control panel would have the regular 60 HZ 240 volts so that it performed as designed.

You could then use the soft start circuitry that comes standard in most every VFD. Instead of the VFD flipping on power like a switch, it slowly ramps up the frequency to the desired value (user programmable rate), therefore eliminating the traditional starting surge. Remember the current is related to the slip. When a motor first starts, its not moving and the slip is enormous, causing the familiar starting surge and flickering lights.

No starting surge, power it off the dryer outlet, keep the 7.5 HP 3 phase motor. Turn it at whatever speed you want. The slower you turn it, the less current it will draw.

 

[pescador775]

I'm not too worried about surge with the Leeson although one should never say never. Modern circuit breakers should be able to handle a surge of about 10X their nominal rating due to the instantaneous power feature engineered into the switches. This is done in consideration of the very subject that we have been discussing.

Of course, the pulley size is crucial. Craig has actually done this with a Mariner and specified the components in a previous post which lends more confidence that my assumptions and simple calcs are for real.

 

[Fl_Key_Guy]

Okay, excellent!

I'm trying for a VFD first, as Derek suggests- not only because of the several advantages, but also so I don't have to wrestle that humongous motor that is hanging underneath the pump and will not come out easily. Every VFD spec I've looked at has me drawing roughly twice the amps needed by the motor when doing a phase conversion with it, but I can live with that.

If I can't get a VFD at a price I can tolerate, I'll either spring for the Marathon Electric 7.5 hp or one of several 5 hp offerings that run at 3450+/- and change the pulley to give me 1150- 1200rpms on the pump.

Now if I go to 5 hp, are you saying the pressure should be dropped to about 3500? (That was a specific question I asked that either wasn't answered or I missed the answer.)

 

[Fl_Key_Guy]

By the way, although I didn't find much for specifications on the Mariner, the Mariner II requires 4kw which is just a little over 5 hp, notwithstanding that a 7.5 is used. Speed is 1270. The gas version runs on a Honda 9hp which doesn't have the butt of a 7.5 electric..

 

[Scuba Duck]

Most American stuff is rated in HP. The Europeans and the rest of the world rates stuff using the metric system and KW. They also use 50 Hz. So the 1270 RPM motor at 4KW is probably designed to turn at 1500 RPM with no load (the same motor would turn at 1800 rpm for 60 Hz bus frequency).

Keep looking on EBAY for VFDs. Every once in a while, someone buys too many of them for a big job, and is just trying to get rid of them. I picked up one for a 1/2 HP application for $30. Too small for your application, but it was cheap considering all the stuff it can do. The enclosure I put it in cost way more than that.

If your motor is not really delivering 7.5 HP (as you mentioned) then a VFD rated for 7 HP might work for you. As you slow the output frequency down, the compressor will apply less load, and the delivered power of your motor, and current through your motor, will also decrease.

A similar device, although I don't think they are as practical as a VFD, is comething called a rotary 3 phase converter. Its essentially a motor/generator combination that makes 3 phase. You don't have any of the electronic controls (soft start, speed control, etc) that you have in a VFD, but they cost less. Also available on Ebay.

 

[Fl_Key_Guy]

I was looking at the 60hz spec and it is the pump that they stated was designed at 1270 rpm, not the motor- sorry about the confusion.

I had a PM that identified the Mariner as 1300 rpm stock, and a chart might be coming.

At 4kw needed for the pump, and using a NEMA B minimum efficiency motor (84%), you would need a 4762 watt motor which is about 6.5 hp (if you get your horsepower from amps and not the "manufacturer's rating", and using 230 volts).

However, the "4kw" in the Bauer block specification is probably rounded up, and probably is based on maximum pressure rating. Hence, Craig's conversion is a workable option although near the motor's limits. With the widely varying voltages and brownouts here in "the islands, Mon", I might be pushing the envelope with a 5hp motor.

The VFD looks like the best way to go , but will probably generate a lot of heat which will need to be accounted for to ensure its lifespan.

Again, thanks for the help!