Higher horse power from single phase motor
Higher horse power from single phase motor
(OP)
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





RE: Higher horse power from single phase motor
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
RE: Higher horse power from single phase motor
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".
RE: Higher horse power from single phase motor
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
RE: Higher horse power from single phase motor
RE: Higher horse power from single phase motor
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
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Higher horse power from single phase motor
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
RE: Higher horse power from single phase motor
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
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"Why not the best?"
Jimmy Carter
RE: Higher horse power from single phase motor
RE: Higher horse power from single phase motor
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
RE: Higher horse power from single phase motor
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.
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RE: Higher horse power from single phase motor
Thanks very much,
jemjack