Higher horse power from single phase motor 10

[jemjack]

I have a compressor which requires 5 hp, here in Spain the highest power single phase motor I can buy is 4 hp. At the moment I have a 3 hp motor driving it, but it is underpowered and the motor overheats and cuts out.

Can I put 2, 3hp motors together with drive belts to the compressor so that I have 6 hp available, or would the two motors not like working together?

Or can I gear the compressor down, ie it should run at 1300 rpm. At the moment it is geared down by its pulley to run at 1000 rpm on the 3 hp motor. But according to my thoughts if I bought a 4 hp motor and run the compressor at 1000 rpm, that would be the same as running the compressor with 5 hp at 1300 rpm. Am I correct in this?

The voltage here should be 220 volts, it is about 210 volts. So presumably a 5 hp uk motor at 240 volts would not be a good idea?

There is no possibility of 3 phase.

Any help here would be most gratefully received

Jeremy Joel

 

[itsmoked]

If you are limited to 3HP motors, are you not limited by the starting currents? If you are limited by the starting current, then two or more motors or larger motors are all going to cause problems. Like all the businesses or homes sharing your transformer dimming out every time you start your compressor.

What are you doing with your compressor? Sometimes you can store the energy in a tank. Say, you are running impact wrenches. Often these are used intensely for a minute and the compressor can not keep up. But if you have a large air tank your compressor can do all the work before-hand over a longer period than the use periods.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[wayne440]

Be aware that some compressors have a minimum speed below which they will not lubricate properly. If I understand correctly, the limitation is the size of readily available single phase motors, not the capacity of your service.

What is the application? You might be able to reduce the peak horsepower requirement by lowering the compressor cut off pressure. It takes a lot more power to make 200psi air than it does to make 125psi air.

BTW, at 210V 50Hz, a 240V 60Hz motor might be a better choice than a 50Hz 240v "UK motor".

 

[jemjack]

Hi ,

Thank you both for your posts, the compressor is a high pressure breathing air compressor, up to 200 ats, but I do not use it above 150 ats. Used for filling air cylinders for scuba diving.

1 option I don’t think that starting currents would be a problem, as the compressor starts with no load, so I could start one motor, driving the other motor and the compressor, and then the second motor. Would this be reasonable to do?

2 option It appears that the compressor is quite happy to run at 1000 rpm instead of the correct rpm of 1300, so using a 4 hp motor would be ok if my assumption is correct, that 5 hp at 1300 is the same as 4 hp at 1000 rpm. But is this assumption correct?

Thanks

Jemjack

 

[TheBlacksmith]

Not exactly. One horsepower is 33,000 ft lbs per minute. Reducing the rpms reduces the amount of work done in a given time interval. One horsepower is one horsepower, whether at 10 rpm or 10,000 rpm.

 

[Skogsgurra]

I think that your first idea is OK (Can I put 2, 3hp motors together with drive belts to the compressor so that I have 6 hp available, or would the two motors not like working together?).

I also think that the starting current could be OK - if you don't see any problem now, and start unloaded. Use one breaker for the two and just try it out. You will later need separate thermal protection and all that. Probably a heavier breaker and also bigger fuses or whatever.

The motors will work happily together. At least if they are the same brand and type.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jemjack]

Hi The Blacksmith,

Is what I meant was that if the compressor requires 5 hp to drive it at 1300 rpm, would a 4 Hp motor be happy (not overheat) if it drove the same load at 1000 rpm?

Thank you Skogsgurra So it would not be a good idea to buy a 3hp motor from Spain, rated at 220 volts 50 hz to use with my existing uk 3 hp motor which is 240 volts at 50 hz?

Thanks and best wishes

Jemjack

 

[waross]

As Gunnar points out, the motors must be matched. The safest way is to use two identical motors.
An induction motor runs a little slower than synchronous speed. As the motor load is increased the speed drops slightly and the motor current increases slightly. BUT different motors have different speed torque curves. In NEMA land, with the most common synchronous speed of 1800 RPM) we have motors rated at 1760 RPM, 1750 RPM, 1740 RPM and other speeds for less common designs.
I am sure that you will have similar speed ratings in IEC land.
Two motors with the same rated speed will, theoretically, share the load in proportion to their horsepower ratings.
Caveat: Manufacturing tolerances, motors may not run exactly at the rated speed. I would allow 10% or 20% oversize to allow for deviations between motors and slight variations in belt ratios. (A worn vee belt will ride deeper in the groove and may affect the drive ratio slightly.
Reduced speed: A compressor depends on the flywheel effect to smooth out the torque pulsations. Reducing the speed greatly reduces the flywheel effect. Motor heating from a varying load depends on RMS horsepower, not Mean or average horsepower.
If reducing the speed results in less flywheel effect and greater torque variations, the RMS horsepower demands may cause overheating. (See the Cowern Papers for a discussion of RMS horsepower.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Compositepro]

If you change the pulley ratio and operate the compressor at lower rpm the horsepower requirement will go down. Half speed will use half power. It will take twice as long to fill a tank.

 

[zlatkodo]

Hi, jemjack ,
Rewindshop can do for you a redesign of three-phase motor 7,5 kW to single phase motor 5.5 kW.
Also you can use a three-phase motor of 7.5 kW (Wye connected) and connect to the single phase voltage. I recommend the following wiring :see illustration
C1 is run capacitor, C2 is start capacitor.
Zlatkodo

 

[jraef]

Have you thought of using a VFD and a 3 phase motor? A VFD can inherently be used as a phase converter, taking in a 1 phase supply and putting out 3 phase to the motor. A side benefit to this is the issue of starting current causing a severe voltage drop on a single phase service can be reduced or eliminated. You don't have to change the speed, but you can set it up to ramp slowly to avoid excessive current draw on start-up. That might come in really handy for SCUBA tank compressors that may end up being used in places where you don't know the service conditions in advance.

You will need to size the VFD for 2X the motor FLC rating if you are feeding it with single phase power because of a couple of component sizing issues. So if for example your 230V 5HP 3 phase motor is 14A FLC, you need a VFD rated for 28A. Beyond that it's easy, the VFD becomes your motor controller so you can eliminate the starter to save money, and the soft starting will not only prevent voltage drop on starting but also save on wear and tear of the mechanical components on the compressor.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[jemjack]

Hi everyone who wrote in to help me with my problem horse power for the compressor, You have given me plenty of food for thought, so now I should be able to solve the problem one way or another.

Thanks very much,

jemjack