dave
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| posted on 7/7/13 at 09:36 AM |
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BIG Compressor
I have been offered a 200 litre compressor at a good price as it requires 3 phase power. Would it be possible to remove the 3 phase motor and attach a
single phase. the motor on the compressor is 5.5 HP.
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Litemoth
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| posted on 7/7/13 at 09:56 AM |
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From memory, I think the biggest motor you can reasonably run on single phase is 3hp. You could of course run two motors (stagger the starts to avoid
tripping ?) or use a 3ph motor (and change the pulleys/ratio - if it's too much hard work for the motor)
It maybe that it'll just take a bit longer to fill but the cfm will be down.
Not worth the hassle though really.
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designer
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| posted on 7/7/13 at 11:34 AM |
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Ditto, think 5.5Hp is too much for single phase.
As at your local electric motor specialist.
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Theshed
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| posted on 7/7/13 at 12:17 PM |
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3hp is not the limit for single phase - many showers are 8-10KW (0.75KW = 1hp approx)
3hp is the just below limit for running off a 3 pin plug (13amp x 235volts = approx 3KW = 4hp)
If you uses a 5.5hp single phase motor you would need something like a cooker socket and wiring. I am spending the day fighting with the wiring of my
Partners 7.5KW kiln located in her shed at the bottom of the garden. I have needed huge cable etc but it works ok. I have had to upgrade the cable to
12mm2.
I would suggest that you keep the existing motor and use a phase converter - 3 phase motors run so much better.
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dave
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| posted on 7/7/13 at 05:27 PM |
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I was thinking about a 3hp motor running from a 16amp plug & socket or hard wired. my main concern is will a 3 hp motor shift a 20cfm compressor
pump ?
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mark chandler
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| posted on 7/7/13 at 05:30 PM |
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I replaced the3 phase 3hp motor on my two post lift with a single phase 4hp motor, it has no problem and the initial start is very hard when loaded
with a car, it cost just under £200
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robocog
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| posted on 7/7/13 at 06:52 PM |
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Motors also take a LOT more current for a brief period during startup
I have just replaced my compressor with a much bigger older belt drive unit
The Brook Crompton motor on it states 12.5 amp (so I would assume its circa 3 hp)
Not sure if this is surge or when its running
Interestingly the motor has grease nipples at the front and back and also has a 3 phaze rating on it as well as the single phaze on it's
plate
It was tripping the garages 16a MCB and I replaced it for a type c motor rated MCB
It now makes the garage flourescent lights dim and flicker when it runs :-(
(but at least it does run and is a shed load quieter than the direct drive unit that I had)
Can't recall what size wire I used to feed the garage but may need to go up a size
Regards
Rob
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Litemoth
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| posted on 7/7/13 at 07:18 PM |
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quote:
Originally posted by dave
I was thinking about a 3hp motor running from a 16amp plug & socket or hard wired. my main concern is will a 3 hp motor shift a 20cfm compressor
pump ?
Yes but you may have to change the pulleys to change the ratio to take some of the load off. If it's a big piston pump then it'll struggle
to start. The 'cost' of this conversion will be that the cfm will be lower of course.
A 3ph 5.5hp motor is a powerful unit and isn't going to be replaced by a single phase 3 - 4hp unit
I think a single phase 3hp draws about 17A and a 5hp will be around 28A but during startup it will be significantly more than this depending on pump
resistance.
A replacement motor above 3hp will cost quite a bit as will a inverter to drive the motor you have.
In my humble...Unless you use a lot of air, a modern vee-twin 14cfm pump matched with a 3hp motor is a good economic choice....on a 50litre or
200litre tank.
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daviep
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| posted on 7/7/13 at 08:06 PM |
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quote:
Originally posted by Litemoth
quote:
Originally posted by dave
I was thinking about a 3hp motor running from a 16amp plug & socket or hard wired. my main concern is will a 3 hp motor shift a 20cfm compressor
pump ?
Yes but you may have to change the pulleys to change the ratio to take some of the load off. If it's a big piston pump then it'll struggle
to start. The 'cost' of this conversion will be that the cfm will be lower of course.
A 3ph 5.5hp motor is a powerful unit and isn't going to be replaced by a single phase 3 - 4hp unit
I think a single phase 3hp draws about 17A and a 5hp will be around 28A but during startup it will be significantly more than this depending on pump
resistance.
A replacement motor above 3hp will cost quite a bit as will a inverter to drive the motor you have.
In my humble...Unless you use a lot of air, a modern vee-twin 14cfm pump matched with a 3hp motor is a good economic choice....on a 50litre or
200litre tank.
Maybe want to check your maths?
3hp = 2.25Kw which would pull 9.4amps at 240v
5hp = 3.75Kw which would pull 15.6amps at 240v
Cheers
Davie
“A truly great library contains something in it to offend everyone.”
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Litemoth
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| posted on 7/7/13 at 09:35 PM |
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quote:
Originally posted by daviep
Maybe want to check your maths?
3hp = 2.25Kw which would pull 9.4amps at 240v
5hp = 3.75Kw which would pull 15.6amps at 240v
Cheers
Davie
Ummm, Thanks Davie...I didn't do the maths 'exactly' but used my memory of tables used many moons ago...
But:
Current drawn = (HP x 746)/(V x Eff x pf)
I don't see factored in your calculation:
Efficiency
Power Factor
Temperature
Inertia and accelerating torque
Slip
plus...the momentary current 'draw' on a cold morning start-up will be about double of the normal operating draw.
...if only it were as simple as you imply.
I didn't come down with the last shower of rain....
by the way....I'm mechanically biased and believe electricity to be the unfathomable work of the devil.
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SteveWalker
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| posted on 7/7/13 at 10:43 PM |
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Assuming an induction motor, the start-up current is anywhere between 6 and 10 times the running current. It only lasts a short while though.
Appropriately rated fuses or breakers withstand the extra load for long enough and the cables don't have time to become overheated.
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dinosaurjuice
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| posted on 8/7/13 at 07:21 AM |
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quote:
Originally posted by daviep
quote:
Originally posted by Litemoth
quote:
Originally posted by dave
I was thinking about a 3hp motor running from a 16amp plug & socket or hard wired. my main concern is will a 3 hp motor shift a 20cfm compressor
pump ?
Yes but you may have to change the pulleys to change the ratio to take some of the load off. If it's a big piston pump then it'll struggle
to start. The 'cost' of this conversion will be that the cfm will be lower of course.
A 3ph 5.5hp motor is a powerful unit and isn't going to be replaced by a single phase 3 - 4hp unit
I think a single phase 3hp draws about 17A and a 5hp will be around 28A but during startup it will be significantly more than this depending on pump
resistance.
A replacement motor above 3hp will cost quite a bit as will a inverter to drive the motor you have.
In my humble...Unless you use a lot of air, a modern vee-twin 14cfm pump matched with a 3hp motor is a good economic choice....on a 50litre or
200litre tank.
Maybe want to check your maths?
3hp = 2.25Kw which would pull 9.4amps at 240v
5hp = 3.75Kw which would pull 15.6amps at 240v
Cheers
Davie
15-17 amps is definitely right for a heavily loaded 3hp motor.
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mark chandler
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| posted on 8/7/13 at 08:20 AM |
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My 4hp tripped a 16 amp MCD on startup, moved to 32amp which resolved things.
My consumer unit is in the garage, 6' away from the motor, it does make the house lights flicker.
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dave
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| posted on 8/7/13 at 03:52 PM |
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Problem solved
[img]
 
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MikeRJ
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| posted on 8/7/13 at 04:39 PM |
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quote:
Originally posted by dave
I was thinking about a 3hp motor running from a 16amp plug & socket or hard wired. my main concern is will a 3 hp motor shift a 20cfm compressor
pump ?
Quite simply, no. You can't expect a 3hp motor to do the same work as a 5.5hp one.
You could use a 3hp motor and change the pulley sizes to reduce the compressor speed (and CFM) to bring down the power requirements.
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