Reducing starting load on 3HP motors

[Tom Kitta]

Compressors do start rather quickly under load. When there was the big freeze in Calgary with temps in -30C I was in the garage.

I saw my compressor 5hp single phase motor start... it started a bit, chocked, started chocked and after 10s finally took off --- I was using the very last bits of power in the neighborhood.

 

[PaulL]

Compressors do start rather quickly under load. When there was the big freeze in Calgary with temps in -30C I was in the garage.

I saw my compressor 5hp single phase motor start... it started a bit, chocked, started chocked and after 10s finally took off --- I was using the very last bits of power in the neighborhood.

Yeah - starting from empty was fine, but it didn't like to start when already above 70-80 psi. The bigger VFD just plows through it up to speed and then settles at <2KW.

 

[trevj]

Yeah - starting from empty was fine, but it didn't like to start when already above 70-80 psi. The bigger VFD just plows through it up to speed and then settles at <2KW.

I take it your compressor does not have a check valve and bleed down set-up?

In theory, the set-up bleeds the air right out of the compressor side of the system, so it never starts against an existing load in the tank.

As an aside, cold oil in a compressor is a straight SOB. A heat tape on it, or chucking a heater to it a while before use in the cold, sure makes a huge difference to the starting!

 

[Doggggboy]

I take it your compressor does not have a check valve and bleed down set-up?

In theory, the set-up bleeds the air right out of the compressor side of the system, so it never starts against an existing load in the tank.

As an aside, cold oil in a compressor is a straight SOB. A heat tape on it, or chucking a heater to it a while before use in the cold, sure makes a huge difference to the starting!

I switched to full synthetic when my shop wasn't heated and thought it made a huge difference as well.

 

[PaulL]

I take it your compressor does not have a check valve and bleed down set-up?

That might be what this wee line is that goes from the pump to the switch? I suspect I have mis-reinstalled it, and forgot about coming back to it. What makes it turn on/off?

In theory, the set-up bleeds the air right out of the compressor side of the system, so it never starts against an existing load in the tank.

As an aside, cold oil in a compressor is a straight SOB. A heat tape on it, or chucking a heater to it a while before use in the cold, sure makes a huge difference to the starting!

It's certainly cold - it was maybe 4 degrees in the shop today. A bit more after the forge ran for a while.

 

[trevj]

That might be what this wee line is that goes from the pump to the switch? I suspect I have mis-reinstalled it, and forgot about coming back to it. What makes it turn on/off?

It's certainly cold - it was maybe 4 degrees in the shop today. A bit more after the forge ran for a while.

I'd suggest looking at the manual, if you can.

Most compressors and water pumps use a pretty similar switch with high and low pressure sections that can be adjusted for the pressures that you want it to turn on at (adjust higher or lower, depending on need) and on the high side, the Max pressure that you want in the tank. RTFM.

The air bleed and check valve systems I have run across, I dunno if they are used and common, or if I just happened across all of them in existence, but it had a check valve going in to the tank, after the compressor head, and the line had a very small bleed hole in it, so the air pressure in the compressor cylinder would bleed down to zero between cycles, which would allow the motor to get a bit of a start, before it encountered a load.

Per Doggggboy's post above, synthetic may help you too!

 

[PaulL]

Ah, that makes sense. That's not what this is. The motor starts up with the smaller VFD but goes under-current (ie, overdrawn) somewhere around 40Hz. But the larger VFD runs it fine and the motor isn't warming up, and steady state draw is under 1.5KW, which seems consistent with a 2HP motor.

 

[Tom Kitta]

Most compressors have at least unloading valve - in mine it is rather laud hiss after motor stops.

During -30C incident the shop was at +4C. Normally compressor starts without any issues. There simply was not enough power for the initial motor spike - i.e. the grind was maxed out by all people heating with electricity.

I also do not think most compressors start under load - they simply have a switch that turns load on after they have certain RPM - part of the unloading valve.

Now, I know this is a bit late - what if you *adjusted* unloader valve so it only kicks in when RPM of the motor is say 80% of top RPM (and also releases when it drops under 80%)? Would that be a solution to starting?

 

[PaulL]

Now, I know this is a bit late - what if you *adjusted* unloader valve so it only kicks in when RPM of the motor is say 80% of top RPM (and also releases when it drops under 80%)? Would that be a solution to starting?

That sounds like the right thing. That explains the air line from the pump to the control valve with its plunger seal.
I'll see if there's an adjustment for it.

 

[jcdammeyer]

An update: I replaced the 3.7HP motor on my compressor as well, with a 2HP motor that my motor tech recommended instead of going larger. I have a 4KW inverter on it, which sounds like too big, but the smaller inverter that drives my power hammer's 3HP motor isn't happy pushing the compressor at lower speeds. And in a fit of brilliance, past-self put away all the air things in one box when leaving Kitcherer 3 years ago, and all the air bits went back together on the first try! Magic!
So here's the shop with more of the ply and partitions up, the striking hammer, and the Anyang hammer, and my little propane forge. I actually got some work done in there, though nothing photo-worthy.

Next is to get a box built around the batteries.

Is this the shop on the island? I don't remember you having a forge here in Victoria.

 

[DavidR8]

When did VFDs suddenly become this cheap? $133 including shipping? Am I missing something?

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I picked up that unit a few years ago for $50

 

[Dabbler]

I have an awkward feeling I'll be replacing the motors with 3-phase plus a VFD

In the long run, a VFD is your best bet.

You can use a VFD on an existing non-inverter duty motor, but have to be aware you cannot abuse it and you need to take of heating issues at low speeds. A PC 80mm fan does wonders for cooling non-inverter 3PH motors, and does it with very low current draw. A low noise fan is preferable..

 

[PaulL]

Is this the shop on the island? I don't remember you having a forge here in Victoria.

Yes, this is the shop at the cabin. TMS syndrome. The first step is to admit "I have a problem with outbuildings".

 

[jcdammeyer]

Yes, this is the shop at the cabin. TMS syndrome. The first step is to admit "I have a problem with outbuildings".

Yes and the other 11 steps are from the main building to the outbuilding?

 

[Susquatch]

The first step is to admit "I have a problem with outbuildings".

It's ok for you to admit that, but I admit that I love outbuildings. I leave the door open and watch the wonders of nature unfold around me. A well made and properly fitted outbuilding is far superior to teeny tiny porcelain devices locked away inside a building.

 

[trevj]

It's ok for you to admit that, but I admit that I love outbuildings. I leave the door open and watch the wonders of nature unfold around me. A well made and properly fitted outbuilding is far superior to teeny tiny porcelain devices locked away inside a building.

....until it is -40deg C, and the wind is blowing ice pellets the size that should be used in drinks on the beach....

Rule number One, at the Moose Camp. Never go to the outhouse, without a Rifle!

 

[Susquatch]

until it is -40deg C, and the wind is blowing ice pellets the size that should be used in drinks on the beach....

I actually enjoy weather like that. I remember one year deer hunting that was like that. I went back to camp to find everyone else had packed up and left. I got my deer later that day sitting right on the side of the run in a cross wind blowing into my face. I could see about 2 feet. I just raised my rifle and pulled the trigger as it passed by.

Rule number One, at the Moose Camp. Never go to the outhouse, without a Rifle!

We have one hunting spot at camp that is actually called the "shi***r post" for just that reason.