20hp old air compressor starting options

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gorms400

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I am really just digging into this for my own knowledge.

Customer of the company I work at wanted to hook up an old ingersoll rand air compressor. It is 20hp, 9 lead 230/460v motor, compressor head has 3 pistons I believe. It is basically in a shed, over 500 feet from their main disconnect, 400a 208v 3 phase overhead service. Their power that runs all their equipment (large junkyard) is fed from a 200a overhead feeder off that main, which someone before me tapped off of to feed this pig. Traditional across the line mag starter, btw. They were decent enough to install buck boost xfmrs.

It would start and satisfy the pressure switch no problem from empty, but starting under load was no good, tripping overload. Voltage drop was terrible, around 180 while starting. Not sure if this was before or after the boost transformers.

Boss had idea of installing a new overhead feeder, 4/0 aluminum ASCR, biggest available supposedly. This is where I got involved in the installation. We ran the overhead, and temporarily bugged it onto the beginning of the 200A feeder to see if it works before configuring a permanent feed. It did seem to work, though not perfect. Motor takes a good 4-5 seconds to get up to full speed, after which it runs pretty good, around 55-60a at 208v at the starter (after boost xfmr). But starting current is over 200a and takes what seems like forever to get under 100a. It is about 5 seconds though.

Seems rough on the equipment IMHO, even if it is used. This compressor will be used every day, full 10+ hour shifts with frequent starts. I was amprobing the feeder and counted 5 starts in about 15 minutes.

Would a soft start or part winding starter help at all? I know compressors are tough starting and the voltage drop doesn't help. I also am not the executive decision maker, but would like to hear some ideas to make this thing run reliably. Smaller pulley on motor side is first thing that came to my mind.
 
gorms400 said:
I am really just digging into this for my own knowledge.

Customer of the company I work at wanted to hook up an old ingersoll rand air compressor. It is 20hp, 9 lead 230/460v motor, compressor head has 3 pistons I believe. It is basically in a shed, over 500 feet from their main disconnect, 400a 208v 3 phase overhead service. Their power that runs all their equipment (large junkyard) is fed from a 200a overhead feeder off that main, which someone before me tapped off of to feed this pig. Traditional across the line mag starter, btw. They were decent enough to install buck boost xfmrs.

It would start and satisfy the pressure switch no problem from empty, but starting under load was no good, tripping overload. Voltage drop was terrible, around 180 while starting. Not sure if this was before or after the boost transformers.

Boss had idea of installing a new overhead feeder, 4/0 aluminum ASCR, biggest available supposedly. This is where I got involved in the installation. We ran the overhead, and temporarily bugged it onto the beginning of the 200A feeder to see if it works before configuring a permanent feed. It did seem to work, though not perfect. Motor takes a good 4-5 seconds to get up to full speed, after which it runs pretty good, around 55-60a at 208v at the starter (after boost xfmr). But starting current is over 200a and takes what seems like forever to get under 100a. It is about 5 seconds though.

Seems rough on the equipment IMHO, even if it is used. This compressor will be used every day, full 10+ hour shifts with frequent starts. I was amprobing the feeder and counted 5 starts in about 15 minutes.

Would a soft start or part winding starter help at all? I know compressors are tough starting and the voltage drop doesn't help. I also am not the executive decision maker, but would like to hear some ideas to make this thing run reliably. Smaller pulley on motor side is first thing that came to my mind.
Click to expand...
. Compressors often have a bleeder valve that releases the pressure in the piston cylinders when it stops after coming up to pressure. Make sure that is working. It is on or very near the pressure switch.
 
gorms400 said:
... it runs pretty good, around 55-60a at 208v at the starter (after boost xfmr) ... Smaller pulley on motor side is first thing that came to my mind.
Click to expand...
If it runs well and the motor current is close to, but doesn't exceed, the nameplate FLA when the air pressure's at its maximum, the pulley ratio is correct.

You might move the boost transformers closer to the main disconnect, reducing current and voltage drop in the long feeder. If available, you might select a higher-voltage tap.
230v + 10% = 253 v
230v + 10% + (voltage drop at minimum current) = ... (you do the math)

A soft-starter wouldn't hurt, but seems unnecessary and expensive.
One step further along this line of thought: A VFD and a pressure transducer, configured so it starts just once per day, then varies the speed to precisely meet the demand.

Is this a "new" installation without preconceived notions? Install the compressor & motor close to the main disconnect and run a compressed-air pipe to the point of use instead of an electric-power cable.

And, of course, assure that the unloaders remain open until the motor's up to full speed. Compressors need not be tough to start if you don't start them under load.
 
20hp old air compressor starting options

I second the notion that you need to verify that the compressor is unloading properly. A properly unloaded compressor will not start any differently with the tank pressurized or not.

There are 2 common methods of unloading 1-a valve either on the pressure switch or centrifugally operated mounted on the end of the crankshaft dumps the pressure at the discharge and a check valve in the tank stops the unloader valve from emptying the tank. 2 - head unloaders hold the inlet valves open so no compression takes place. A solenoid valve is commonly used to pneumaticaly operate the head unloaders. A 20 HP compressor would be likely to use method 2.
 
gorms400 said:
I am really just digging into this for my own knowledge.

Customer of the company I work at wanted to hook up an old ingersoll rand air compressor. It is 20hp, 9 lead 230/460v motor, compressor head has 3 pistons I believe. It is basically in a shed, over 500 feet from their main disconnect, 400a 208v 3 phase overhead service. Their power that runs all their equipment (large junkyard) is fed from a 200a overhead feeder off that main, which someone before me tapped off of to feed this pig. Traditional across the line mag starter, btw. They were decent enough to install buck boost xfmrs.

It would start and satisfy the pressure switch no problem from empty, but starting under load was no good, tripping overload. Voltage drop was terrible, around 180 while starting. Not sure if this was before or after the boost transformers.

Boss had idea of installing a new overhead feeder, 4/0 aluminum ASCR, biggest available supposedly. This is where I got involved in the installation. We ran the overhead, and temporarily bugged it onto the beginning of the 200A feeder to see if it works before configuring a permanent feed. It did seem to work, though not perfect. Motor takes a good 4-5 seconds to get up to full speed, after which it runs pretty good, around 55-60a at 208v at the starter (after boost xfmr). But starting current is over 200a and takes what seems like forever to get under 100a. It is about 5 seconds though.

Seems rough on the equipment IMHO, even if it is used. This compressor will be used every day, full 10+ hour shifts with frequent starts. I was amprobing the feeder and counted 5 starts in about 15 minutes.

Would a soft start or part winding starter help at all? I know compressors are tough starting and the voltage drop doesn't help. I also am not the executive decision maker, but would like to hear some ideas to make this thing run reliably. Smaller pulley on motor side is first thing that came to my mind.
Click to expand...

Like others mentioned, the head pressure un-loaders MUST be working correctly. You should hear the head pressure relieve, at every off cycle. Your Three cylinder pump must unload ALL three heads properly. Those Ingersoll piston pumps have a output check valve built into each head, if I recall correctly. Those valve assemblies, in each of the three heads, must not leak, or some tank pressure will leak back into the head, after the system shuts down. Most compressors styles generally use one tank check valve, for all of the output lines from the heads, but Ingersolls were normally connected as described, a valve in each head. Rebuilding of the valves in the heads will cure this problem if it exists.

Second point about un-loaders and cycle times. Starting it every three minutes is excessive, especially on low voltage. The unit can be setup with a constant run system, and a start stop system. With a 3 position selector switch and a 3way, normally open solenoid valve, and the pneumatic head un-loaders . Then, in heavy load times it can be switched to constant run, where the motor stays running, and the solenoid and pressure switch determine when to apply load, by closing the un-loader valves. Normal start stop operation could be selected for less demanding times.

ConstantRun-StartStop Compressor Diagram.jpg

Running a 20HP 240/480V three jug pump on 208V, 500 ft away is a recipe for burnout. I would install a used tranformer at the head end to convert to 480V. This would reduce the 500' voltage drop problem significantly, and provide voltage much closer to what it was designed for.
Rewire the motor and starter for 480V, and replace the heater elements to suite. Add the constant run feature, if you really want to run it hard.
But in any case, make sure that the head valves are not leaking tank pressure.

MTW Ω
 
Never knew about the head valves, good info. I will have their mechanic look into it.

Stepping up to 480v was another option but the customer wants that as a last resort. One reason is cost, other is nothing they own is 480v, and I think they want to have the over head feeder as another backup. It's a strange place to work.
 
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