Compressor HP Vs KW

[Afterimage]

I spotted a 2.5HP compressor in Aldi,but upon closer inspection turned out to be less than 1.8KW,my compressor is 2HP but has 2.2KW rating and I've seen other compressors with 3HP that have a 2.2KW rating.

I was under the impression that 1HP was roughly equivalent to 1KW,which would mean that the Aldi compressor was closer to 1.5HP than the claimed 2.5HP - or am I missing something?

 

[madkayaker]

1hp is 0.75KW (i think)

 

[piman]

Hello Mick,

I'll confirm MK's figure, strictly speaking 746watts = 1 h.p. but three quarters is a good estimate.

Alec

 

[series2]

Hi,
I went to the shed to have a look, as I'm running 2 2HP compressors off a 13amp fused spur from the garage. All it says on the compressors is 2HP, no power rating.

I googled the conversion and found this calculator seems 1HP is as previously said, approx 0.75Kw.

So, theoretically at least, you can run upto 4HP from a 13amp supply.

 

[piman]

Hello Series2,

the problem with inductive loads like motors is that you cannot estimate the current by dividing the Kw by the voltage. A 1 KW motor say at 240 volts would seem to need about 4.2 amps but at the average power factor of 0.8 you need to multiply the current by 125% so over 5 amps so 13 amps will be pushed to run much over 2.2KW (say 3 H.P.)

Alec

 

[series2]

I appreciate the maths behind it (just wasn't in the mood to do it). I knew when I installed my current setup there may be an issue with the supply rating at 13A, but was utilising the existing feed to the shed, where there was originally one 2HP compressor and a light bulb.

I figured that as the compressors only run about 30% of the time, and standard BS1363 fuses will run at 125% for 30 mins (if not more) & the compressors both have unloader valves, therefore I might just get away with it! There will also be a voltage drop en route to the shed, which will work in my favour!

All seems to be well, as I have just painted a car. The compressors worked hard that day.

In the real world, I should have a minimum 16A supply but that would mean digging up 30 feet of garden to replace the cable and fitting a new consumer unit in the garage, and uprating the breaker in the house. Part P etc.etc.

 

[madkayaker]

voltage drop will work agent you as voltage down=current up

 

[series2]

I=v/r

 

[series2]

Please don't make me do the maths

 

[piman]

Hello Series2,

" I=v|r",
if only electricity were that simple, what happens with motors is that with a lower voltage their speed drops and the current increases because the generated voltage that opposes the supply voltage decreases and the current rises.

To understand that is to think of when the motor is first switched on, there is a huge rush of current (7 to 10 times full load current is what I was taught) which is the result of the impedance of the windings divided into the supply voltage. (normal ohm's law). But as the motor speeds up it's also a generator and the induced current opposes the supply and the supply current drops off.

All very interesting and very confusing.

Incidentally, "Part P etc.etc. ".

Why does that remind me of the H. & S. E., very well intended but no allowance for experience\common sense.



Alec

 

[series2]

Sorry, I'm not trying to over simplify, To be honest I don't really know where I'm going with this

I think it was because of the original post, I actually thought about my own installation, which was temporary about 7 years ago. Last year I replaced one worn out compressor with 2, and because it worked when I plugged it in, I never really gave it anymore thought.

I suppose I should have done, as I do know about the relevant equations, power factors, etc. (I used to be bench test engineer for Rand).

Now I have thought about it, the fuse will blow next time I switch it on!

 

[madkayaker]

what type of fuse

 

[series2]

It's connected via a swiched fused spur from the garage ring, with a 13a fuse in it.

 

[madkayaker]

not the best! but wouldn't recommend changing it to any thing bigger. does it have RCD if not fit one ASAP you can get a fuse spur unit for £20 from screwfix

 

[piman]

Hello MK,

I don't quite understand this current (sorry) obsession with RCDs? At one time they were required for outside power points as Joe Bloggs was likely to chop through his extension lead while cutting the hedge and then pick up the live severed end of the cable.
For a fixed appliance, like a compressor and someone who is obviously aware of the hazards etc, why?

Alec

 

[series2]

Whole garage on own RCD protected consumer unit. Feed to shed comes from garage 30A ring Via FCU, 2.5mm arctic cable in buried conduit to shed.

I Know it's not ideal, But it was only temporary. Garden now established over conduit, she who must be obeyed loves garden, so I can't dig it up!

Conduit also carries 3/4" airline back to the garage, that's not a good idea either!

 

[madkayaker]

all circuits under 17th edition for use by non electrically qualified persons up to and including 32A regardless of load type must be RCD protected

 

[R Kraft]

If the motors that you use over there, are like mine.
The motor pulls three to five times the running current while starting.
Robert

 

[madkayaker]

same this side of the pond

 

[series2]

A wired in compressor doesn't need one, as far as I'm aware. Not sure about CEE form sockets, but domestic standard 13a always.

IMHO they are a good idea as they can save lives, not everyone treats elastictrickery with the repect it deserves.

I think, if I remember correctly, there is something in the regs about if an appliance is plugged in and could be used in a high risk area, eg bathroom, garden or kitchen it must be rcd protected.

When I worked at Rand's, every bench had its own, and they were tested daily. Saved me from at least one shock a week!