Reducing Running Amperage of 5HP Electric Motor

You might get away with it. Might not.

Inrush current at start may doom you.

The motor doesn't know it's not going to be doing 5 HP worth of work when it starts.

Next?

thanks for the laugh

Any time.

I wasn't sure how the recovery was progressing, so I'm having one more for you before bedtime.

Yeah it can work, you just need the right combination....

http://www.ebay.com/itm/Schulz-Single-Stage-Air-Compressor-Pump-2-3-5-HP-Horsepower-FREE-Filter-/280828412003

Congratulations Solar Eagle - you found the compressor pump I am seriously considering purchasing. Schulz has unbelievable customer service - had TWO reps respond to my question about minimum RPM to run this pump at with a smaller motor and still maintain effective lubrication. Plus one of the reps gave me the phone # of one of their service techs.

Back to the original topic, the first responder to my CR4 inquiry may be on to something - inrush current may be too much for wiring that can only handle 240V & 30 Amps - even if I install a much smaller drive pulley than recommended for a 5HP motor. I suspect it depends upon the brand and construction of the motor and it may be that the only way to know is to actually buy the 5HP motor & see if it kicks out the circuit breaker.

Is this on a 1-phase or 3-phase system? For 5 hp, 1-phase is a bit unusual, but possible with some "farm duty" motors.

Circuit breakers will hold well past starting current for a short time....should not be a problem...You just need to make sure the motor is loaded over 50% ....75% would be ideal....too low and the motor may overheat....I would monitor the motor heat with an IR gun....but on a compressor it should cycle enough to work.....

Thank you Solar Eagle. Sounds like you have had some experience with subjects similar to this one. I think I'll size my drive pulley to drive this pump at 60 - 65% of the recommended speed for a full 5 HP. I can borrow an IR temperature gun, so when I get this all together I will watch it closely. Thanks again for all your suggestions.

Why not come back if you do this and let us know how it worked out?

Can you give some specs on the motors?

I have a "5 hp" Sanborn compressor that not only has a 20 amp 240V plug on it, but also gets used regularly on a 30 amp generator circuit no problem (generator is rated at 8,000W/12,000W starting.

The reason for the quotes is that a homeowner duty 5 hp compressor motor and a 5 hp TFC industrial unit at the same voltage can be night and day difference. List your desired motor type and specs and someone can probably give you a good idea.

The size of the pulley doesn't matter. It is the compatibility of the motor inrush current with the wiring that feeds it that will be the deal-breaker. At the moment of switch-on, the motor doesn't know whether it is driving a small pulley or a larger one, and the inrush current will be exactly the same whichever is fitted.

Buy the proper equipment for the job!

240v & 30 amps???? That is 7200 Watts or 9.6HP. I don't see a problem unless you have a lot of current draw on some other machines when it tries to start up.

Jim

The quick answer is "no". The motor doesn't determine the load, the thing it's driving (in this case the compressor) does. The size of the piston, how far it travels, and to some degree the pressure it's working against, are some of the things that determine how much work has to go into turning the shaft.

Since you haven't changed anything about the compressor, the motor still does the same amount of work. Changing the pulley size changes the rate at which the tank fills up, but you're still moving the same size piston and stroke.

You "might" try using an automatic pressure unloader to make it easier to start the motor, and filling the tank at a lower pressure and/or draw off a lower volume, but you cannot expect the same output if you use a lower horsepower motor, even if you change the pulley size.

how about if you paint the motor with shiny paint?

At Leyland Australia the saying was 'if it doesn't go, chrome it'

But seriously,

• make sure it starts unloaded - check the unloader valve.

• use aluminium pulleys

• use the minimum workable pressure setting

will give your idea a better chance of working.

also, run an ammeter over your current setup -

pun intended! pun intended!

Changing the pulley size actually makes a lot of difference.

It changes the rate at which the work that is dictated by those factors gets done, which is how load is calculated. There is a reason compressor pump manufacturers specify different pulley/HP combinations for different CFM ratings on a given pump. As you mention, the end result is a lower output for the lower power set up. Using a lower power setup for a more powerful motor just reduces overall motor load (and likely efficiency).

You could steal your neighbor's kid's mini-bike and use one of these.

Now, it's portable.

Will current be reduced by pole changing?.

Be cautious of the 5HP surplus motor. Motors rated for air compressors have defined limits on starts-per-hour and on duty cycle (on/off %). Starts-per-hour limitation begs you to have a larger tank capacity and longer motor-on-times for any given air consumption. Duty cycle limitation begs you to have shorter on-times so the heat from electrical inefficiencies can be dissipated during longer off-times. This tension between starts-per-hour and duty cycle will need you to model the air demand, the compressor capacity at chosen speed (relying on pulley choice), and the motor performance limits.

Graphing this for a few combinations should show you the Goldilocks Zone.
https://en.wikipedia.org/wiki/Goldilocks_principle

You could use a soft start unit. As long as the unloader valve is open during start up, the current draw will be minimal.

How about a VFD.

VFD/Soft starter may be expensive.

I'm not an engineer by any stretch of the imagination but just a dumb wrench turner interested in things. It would seem to me the amount of power to drive this isn't going to change. By running at a slower speed it will just take longer to build up to whatever pressure is set at. Reducing the set pressure will reduce the load on the motor?

Just to be safe, I've started another intensive course of the cure for you.

It's not a cure. It's a treatment.

By the way, last night's treatment was a smaller glass, but today you have the correct glass.

Here is your chance to get an education on capacitors. If the 5 hp motor unloaded draws, say, no more than 50 per cent of the ampacity of your wiring, you can solve your problem with sequential (trial and error) adding of capacitors and adjusting of sheave ratios starting with the smallest motor and largest compressor sheaves. It is important that the motor have a high temp circuit breaker built in, though, and your compressor pressure valve is set at the low end of the recommended range for the refrigerant.

But I always strive to find a route by which to run parallel conductors for full ampacity so that I do not have to de-rate anything.

If the op is in the US., the code does not allow parallel conductors for less than 1 awg size. He has to use the proper size wire.

As I recall, in NEC 310, the limiting size is 1/0 as the smallest size for the actual paralleling conductor, as opposed to the existing conductor awg. The requirement is built around 3 phase and common raceway or sheathing with other conductors, neutrals and/or grounds; in the present case I was visualizing a completely separate route for non-bundled paralleling conductors. There are exceptions, ganging of breaker requirements, etc., but it all eventually comes down to ampacity of the entire circuit, which can be complicated. Thanks for highlighting this point.

I was thinking 1/0 as I answered. Type O in the brain. I don't think there is an exception in the code for that, however.

If you are going to limit the motor current to the same as a 2HP motor then there is no advantage to installing a 5HP motor and instead is a waste of money.

Is this a 120VAC compressor?

What is the garage outlet current rating? 15A or 20A?

If the compressor outlet wiring is dedicated to serving the compressor only and it has 14 AWG copper which is rated at 15A and there are two current conductors with a ground it is possible to upgrade the outlet to 220VAC by installing a two-pole 20A, single-throw circuit breaker in the power distribution panel, taping the white conductor with back electrical tape in the outlet and at the distribution panel, and installing a 220VAC outlet.

This work should only be attempted by a experienced, competent person with knowledge of the applicable local electrical codes.

With all due respect, a 5HP compressor duty motor - at least in the "surplus category" is not that much more expensive than a 2 HP motor. My issue is it is a long way to the breaker box to also have to run very heavy gage wire to my garage. But eventually I will do so when more funds become available.

Upon further review it looks like I have been fretting about what appears to be bad advice in the Q&A section of the Northern Tool website for a 5HP IR 2 stage compressor. One purchaser was "told by the IR technician" to run a dedicated 60 amp circuit to his new compressor. So I naturally thought when I decided to upgrade my worn out air pump to a 5HP rated pump, that for the time being I would have to downsize the motor pulley if I were to purchase a compatible 5 HP motor; and run it at 2 or 2.5 HP equivalent. But further research shows other purchasers stating they have gotten by with a 30 amp circuit at full rated power.

A closer look at the nameplate on the motor I have been contemplating buying, shows it has a 19.5 amp draw - so where the IR Tech was saying a 60 amp circuit for their 5 HP 2 stage unit is very puzzling. No way am I buying wire rated for 60 amps that would be legally required if I used a 60 amp breaker, just to run a 5HP single phase 230V air compressor for "hobby use".

Along the same lines as what Solar Eagle, JNB, and JIMRAT suggested, I believe I just need to use 10-3 Romex with a 30 amp breaker and start off with the largest motor pulley to drive compressor at it's max. rpm and if the 30 amp breaker trips out, then I can probably just drop the speed down to 80% using a slightly smaller pulley and at least be running at 4 HP equivalent and still be in an efficient speed range.

Now for the weird part - I happen to have a 2HP Magnatek motor that says "115/230V" and "24/12" for the amperage on it's nameplate. That is no joke - the nameplate still looks brand new. Must be one hell of an inefficient 2HP motor!

Sorry, but you clearly do not know how any of this works.

"Must be one hell of an inefficient 2HP motor!"

Not really true.

Standard amperage for a 2HP single phase AC motor is:

24A @ 120VAC

12A @ 240VAC

Standard amperage for a 5HP single phase AC motor is:

56A @ 120VAC

28A @ 240VAC

Wire Rating

# 14 AWG CU NM (Romex) is rated at: 15 amps.

#12 AWG CU NM (Romex) is rated at: 20 amps.

# 10 AWG CU NM (Romex) is rated at: 30 amps.

These ratings can be found in the NEC Table Section 310.15 (B) (16).

Note that "Romex" type NM cable current rating is limited to 60C temperature exposure.

5 HP x 740 ( watts/HP ) = 3700w/240v gives us <16A. NEC tables give 28A @ 240v

I have seen new motors from China showing much higher A on their nameplates than the calculation gives. Now i know where it is coming from.

Can you please explain to me why the higher A rating?

Confused old man.