15 amp power lines

[Bob Willson]

I recently installed 15 amp power to my tool cupboard so that I could hide my compressor in there. It used to work fine when it was plunked in the middle of my workshop but it has been blowing its condensers lately, and at $30 a pop I don't want to replace them again.

The details are:
The line used to be 5 metres long and worked OK.
I extended the line by 7 metres to put it in the tool cupboard.
The wire I used is stranded copper wire with the following dimensions
Positive wire: 5 strands of .65mm dia
Negative wire: 7 strands of .65 mm dia
Earth wire: 7 strands of .65 mm dia
The compressor also has a 2 metre 15amp lead on it
The motor is 240v 3 hp (or so I am told, the plate is illegible) and (supposedly) draws about 40 amps on startup. :(

The condensers are 108-140 MFD 220v.

The question is, are the wires heavy enough to support this load and enable the condensers to do their work without melting?
When they blow, they make a reasonable sized bang and the room fills with smoke. :eek:

If they aren't heavy enough, what size should I be using for a run of that length?

[Barry_White]

Bob

A couple of questions.

Is the 15 Amp socket permantly wired with flat Twin and Earth cable or are you using round flexible cable.

The twin flat cable should be what is called 2.5mm² and is rated at 23amps.

The cable you have stated is uneven between the positive and the negative. You say one has 5 strands and the other has 7 strands. Both should be the same which should be 7 x 0.67

It should be protected with either a 20 amp circuit breaker or a 15 amp fuse.

[Bob Willson]

Hi Barry
The cable is permanently wired flat cable.
I just stripped another bit of the red wire and it is also 7 strands. Sorry, must have broken a couple of wires when I did it the first time.

So this cable is not good enough then? .325 * .325 *(Pi) 3.14 = .33 mm
.33 * 7 = 2.3 sq mm

[Barry_White]

Hi Bob

That is 2.5mm² cable and it should be capable of carrying the current. 2.5mm² comes as 7 x 0.67 or 1 x 1.78. I think stranded cable will actually carry a slightly higher rating.

What size is your fuse or circuit breaker.

Not sure what could be blowing the capacitors, maybe need to get the motor looked at.

[gatiep]

Bob,

What cables feed the board. Maybe there are lighter cables or too long lengths for the area etc which causes a voltage drop. Take a voltage reading at the motor with it running. I am sure that you'll find a huge voltage drop and that is what is causing the problem. It is quite common with contractors using long undersized extention leads on compressors.

:)

[Dan]

Maybe having the compressor hidden inside a cupboard is causing heat to build up (the cooling fins are there for a reason).

[Bob Willson]

The circuit breaker is 20 amp.

Damn! I was hoping you weren't going to say that about the motor.

How is the motor actually checked?
Is this something I can do myself with a meggameter?
I assume that you are thinking that part of the motor windings is blown thereby putting extra load on the rest of it and thereby increasing the amperage draw drastically?

[Bob Willson]

I wrote that last posting about two hours ago before I went to sleep to get ready for work tonight but then got distracted and forgot to press the send button.

Dan, it's a big cupboard. about 3m * 2 m. It is the coolest place in the house. :)

Gatiep: Hmmmmm. I will try that. That is what I was asking before, whether or not the cable was thick enough for a run of that length; I just didn't know how to test it.

[Barry_White]

Bob

Not sure about testing motors, may be worth taking it to someone who fixes motors.

I think Joe was talking about what cable size to your sub board if you have one or does the 15 Amp circuit go back to the main switch board.

Is the run 7 metres or 12 metres. Even a 12 metre run is not excessive for a 15 amp circuit if it comes off the main board.

If it is coming of a sub board it depends on how far that run is and what size cable that runs to it.

My main board is out in the paddock on the transformer pole. My run to the sub board is in 6mm² cable and the run to the sub board is 43 metres. Admittedly my 15 Amp circuit is only 2 metres from the sub board, but I am going to put another 15 amp circuit in that will be 8 metres from the sub board but I think I will run it in 4mm².

[gatiep]

Test the voltage at the switch closest to the motor with a multimeter before switching on, then again with the motor running. I suspect that somewhere in the circuit there may be some non standard wiring, which with the extra few meters of 2.5 mm 2 that u have added has made the drop too much for the motorcapacitor to be comfortable with. Ofcourse you could just run it with the previous setup that you had when it did run good. If it keeps working there, it proves the point. How about phoning a sparky mate?


:)

[Bob Willson]

How about phoning a sparky mate?

Used to have one next door but he moved. Is a plumber OK? I still got one of those. :)

I can't really run it in the old place as it was the middle of our garage and I had to move it too often.
I will try a voltage measurement before and after as you suggest.
Gotta skeddadle off to work now. :( 3 years before I retire. :D)

[gatiep]

Bazza, you are 100% in with what I am thinking. Like in your case for instance, if you checked to the sub board from the plug all would seem ok, but if you only had 2.5 istead of 6 mm2 running from the paddock, it could've caused a problem.Most these things work out well be retracing one's steps.....a process of elimination.

I bought a new saw and a mig welder a while ago.Everytime I switched one of them on they would trip the rcd. I eventually connected the circuit to the input side of the rcd, so that it didn't go thru rcd at all. However, every time I switched one of these it tripped the rcd further down the circuit. Aaah faulty rcd....altho its further down and shouldn't trip. Replace rcd....same result. Suddenly I realised that it didn't trip if the fluoros in the shed weren't on. I had a sparky spend some time here, after he told me on the phone that I was talking nonsense. He tried things, swapped things, gave up and left, telling me that it is not possible. A second sparky had a go, no success, meggared the saw and the welder. Gave up and left. Both put it in the too hard basket. When I found out that did not trip when the lights were in circuit as well ( ie not just switched off but both live and neutral removed out of circuit ), I started disconnecting one fitting at a time. Lo n behold, one fitting had a leak which caused all the nonsense. Removed that one from the circuits, connected up as all should be and no problems. Often people buy a new machine, have rcd's tripping, blame the new machine, which in fact it just accsentuated a problem that existed before .

Anyway, back to Bob's problem. Eliminate.

:)

[Kev Y.]

Bob, I think that maybe you should re-think what dan said.. the COOLING fins are on the motor for a reason.. to get rid of heat build up in the motor circuit.

They cool the motor by passing air over them. locking the compressor away will only lead to heat build up.

IF you really really have to have it in a cupboard, at least drill a few hole top and bottom to assist with some kind of ventilation flow to remove the heat.

Just humour us :rolleyes: , place the compressor out in the workshop again and see if the problem still exists

[johnno402002]

Hi Bob,

To check the voltage drop, measure the voltage at your 15 Amp point with a voltmeter when the compressor is off, then measure it again after the compressor has started and is running normally. The difference should be not much more than 5%. If your cables are big enough, but you still have a big voltage drop, check all connections between you motor and where the street wires connect to your house for signs of burning. A prime candidate on a compressor is the pressure switch.

Capacitors are susceptable to harmonics on the power supply. When you lengthened the circuit, you may have set up a tuned path to your capacitor for these harmonic currents. These harmonic currents heat up the capacitor. The source of harmonics can be quite difficult to find, but they can be from inside your shed ( fluro light ballasts), your house, or your street. One cure is bigger cables to supply the load.

Doing the above will mean you will be working with live exposed electricity. It will kill you. If you are not experienced doing this, get someone who is to check it with you. I'm not being unnecessarily alarmist, I've seen the dead bodies, and the burnt ones.

Hope this helps,

regards,

John

[gatiep]

Very sound advice John.

:)

[Bob Willson]

Just humour us , place the compressor out in the workshop again and see if the problem still exists

If it does, then it will cost me $30 to find this out. :(

If the voltage drop test doesn't work then I will try it outside again.

I will test about 1 or 2 pm (when I wake up )

[Sturdee]

Bob,


I don't think the cupboard is the problem. As you know I placed mine in a cupboard in order to soundproof it and my cupboard is smaller and I don't have any problems.

I think it is either the compressor or your new wiring. Do you have any other 15 amp plugs on a different circuit? If so try running the compressor from that for a while and thus ascertain if it is the plug and associated wiring or the compressor.



Peter.

[ED T]

I would have expected that capacitor to be 400V, not the 220 you stated.

It is only overvoltage which kills capacitors.

Ed T

[Bob Willson]

OK
I measured the voltage at the outlet and got 237 volts. That is OK, 240 is only a nominal voltage anyway.
Plugged the new condenser in and turned on the power switch for the compressor. It ran for about 10 seconds while I was getting the probes in place for the voltage readings and then the condenser spat electrolyte and started smoking.
If , as ED T says the condenser is meant to be 400 volts then that would explain it. Would it make a difference if there were two condensers sharing the same load? Can they split the voltage between them and get 400 volts that way?

Just phoned Qld Trade Tools and they assure me that the 200volt condenser is correct for this machine. It is beginning to look like I may need a new motor, or at least this one rewound. :(

[johnno402002]

Bob,

Another thought, perhaps the start switch on the motor has stuck, leaving the start capacitor in circuit after it should have been switched off. Still, ten seconds is not all that long, it should handle that.

Did the compressor sound normal during the ten seconds?

regards,

John

[Kev Y.]

If it does, then it will cost me $30 to find this out. :(

If the voltage drop test doesn't work then I will try it outside again.

I will test about 1 or 2 pm (when I wake up )


OOPPS :o :o :o

[Markw]

Bob,
Just a quick thought, is this a real compressor - one with a belt and possibly 2 stages or one of those direct drive things?

I had one of those horrible direct drive things first and wouldn't give you 2 cents for it. It too blew up (condensor) but was caused by the motor being overloaded by a gauling compressor piston. Shound't have bought crap - new compressor is 2 stage belt drive - cost a s##t load but was worth it.

[ED T]

Johnno is on the right track I think. CHECK THE SWITCH.

If you parallel up two identical capacitors, the resultant will take same voltage with twice the capacitance.

Put them in series and you get twice the rated voltage, but half the capacitance.

Ed T

[Bob Willson]

First off, two things:
The capacitors are not equal. The first one is marked a 16 uf 500 volts and the other is 108 - 140 MFD and is 200 volts.
Next, this is a "proper" compressor. twin cylinders, belt driven.

OK, I took off the belt guard and removed the belts to see what the condition of everthing was like. I next removed the top of the switch. This allowed the switch to pop up into the 'ON' position. (Knocking it down turns it off.)
I then ran the motor by pluging into the 15 amp line and using the 15 amp socket switch to turn the motor on/off.
I ran it for 10 seconds and tested the heat build up in the capacitors. NIL
I ran it for 60 seconds and tested again. NIL
VERY slight heating of the motor itself, well within specs I should think.
These tests were done with the belt OFF.
So, where do I go from here?
PS even though the capacitor had leaked electrolyte it still seemed to work fine. Maybe I won't need to spend that extra $30. :)

[ED T]

Johnno was not talking about the on/off switch, but the one which brings in the capacitor during the start up.

You probably have a capacitor start, induction run motor. These are the most common in the 2 to 3hp single phase range of motors.

Single phase motors develop b all torque until they get up near their design operating speed. So starting a compressor is tough. So the capacitor is designed to boost the torque until it gets up to speed. Then it is supposed to cut out. The switch which does this is a centrifugal one on older motors, attached to the shaft. Newer motors may have an electronic one.

Johnno is suggesting that this switch may not be working.

Your no load test proves nothing, because the motor is not loaded.

See following for lecture on motors


http://www.iprocessmart.com/leeson/leeson_singlephase_article.htm


Ed T

[outback]

This may be resereceting the dead a bit, but as Bob hasn't yet posted his latest findings, may I very humbly ask if the replacemet capacitors are the correct type, that is start or run types, in the correct place?

[soundman]

One thing has occured to me.
Is the dump valve working. This is the valve that dumps the head preasure when the motor stops. It should be part of the presure cut out switch.
If it isnt the motor may be trying to start under load and failing to properly start causing excess load on the capicitor.
Did you say the motor ran OK with the belts off?
A compressor motor should be at its lowest load at the beginning of the run and the highest load at the end.
Can you measure the current drawn with a clamp meter?
There are a variety of motor types It is probably a good thing to find out what type the motor is.
I have a trade tools compressor & I don't think it has on speed contacts I think its just a simple phase shift induction motor, ie capacitor in circuit all the time.

I'd also be enclined to trace the path of the existing wiring.
If It was me I'd be running 4sq mm all the way bach to the board. With recent changes in the regs a lot of sparkies would be inclined that way too.

Afterall this is realy sparkies work.

cheers

[Harry72]

Hey guys I've got 6sq mm cable X 25m run to my shed, what would be the max safe amp loading, would it handle a 35amp load conituosly?(3hpTS/3hpdusty/2hp comp/2hp evap aircon + 9 fluro lights could all be running at the same time)

[Barry_White]

Harry

6mm² cable will carry 38amps. You would need to calculate the amperage of each machine to see what your maximum load would be. Just check what the compliance plates say on each motor bearing in mind that if the dusty, airconditioner and the saw all started at the same time there will be a significant load on it at once.

A 6mm² circuit should be protected by a 32amp circuit breaker or HRC fuse or a 25 amp rewireable fuse.

There are so many factors involved in this such as where the cable is running either in free air or in conduit or in insulation how long the run is, if it is buried underground, how many sub-circuits off it which will have derating factors applied to it.

To calculate the maximum demand is as follows.

Lights - 75% of the connected load
GPO's - 1000w for the first out let and 400w for each additional outlet
Airconditioners - Full connected load
Motors - Full load of the highest rated motor and 50% of the full load of the remainder.

This is only a very tiny bit of how the load is to be calculated and probably the average electriction would not sit down and calculate it out accurately.

[Harry72]

6mm² cable will carry 38amps. You would need to calculate the amperage of each machine to see what your maximum load would be. Just check what the compliance plates say on each motor bearing in mind that if the dusty, airconditioner and the saw all started at the same time there will be a significant load on it at once.
Yeah thats what I thought, I calculated by adding up the watts of each, 3hp TS is 2250w,3hp dusty 2250w, comp is 2hp 1500w, aircon is 1.5hp but I allowed 1500w, the 9 lights are 18w ea unsure on lights? so I allowed 500w.
So that adds to 8000w /by 240= 33.3amp would that be correct(or near).

Am I right in thinking the dusty/aircon is a continous draw, the TS would be a cascading draw(changing draw between cuts etc)and the comp is a ramping draw?
This condition would only ever happen in summer(aircon), but the TS/dusty will be on a regular basis together, the comp will only run occasionly as I use it to remove saw dust from the machines as Im machining.

I intend to have 3x 15amp PO(1 ts,1 dusty, 1 spare), 5 GPO's, dual circuit on the lights 3 center and 6 outer lights.
If this draw is too much for continous work, would it be advisable to run a second 4sqmm supply for the 5 GPO's(3 will be doubles)?

[Barry_White]

Why not run 10mm² cable. It is rated at 50amps and requires a 40amp circuit breaker or 32amp fuse.

[Harry72]

Because a 6mm cable has already been run, but not connected.
What sort of $$$ is 10mm worth, would it be cheaper than running an extra 4mm and associated fittings?

[Barry_White]

Harry

Starting to get a bit too complicated. Are you doing this yourself or do you have a sparky. If you have a sparky he should be able to work it out for you.

If you are doing it yourself I would suggest you go and buy a copy of AS3000 SAA Wiring Rules from Standards Association of Australia.

I would also suggest if you are not sure of what you are doing I would get a Sparky.

Remember One FLASH:eek: and you are ASH.:(

[Harry72]

Yeah I got a sparky, just trying to get as much back ground on the subject as I can.
The sparky knows whats required(he's freshly trained, and he works under his father who has many years)but he wont know what I'll need in the future unless I tell him!
Thats why Im asking here, where many have WWmachinery expirance in workshop layouts of power supplys.

[Barry_White]

Harry

According to the SAA Wiring Rules you can only have a maximum of 4 15amp points on 6mm cable.

You could run your 4mm cable and put your GPO's and your lighting on that circuit.

[Harry72]

So the extra 4mm will be the go then I'll call me sparky tomorow, discuss what I need.
He wants some built'ins for his daughter... Ahh mates rates are the best rates!
Thanks for your help Bazza!

[Bob Willson]

This may be resurecting the dead a bit, but as Bob hasn't yet posted his latest findings, may I very humbly ask if the replacemet capacitors are the correct type, that is start or run types, in the correct place?

Hi Outback
Yes, the replacement caps were the correct type etc.

It may be that, as has been suggested, there is something wrong inside the motor. If so, it will have to wait a while as it likely cost a few hundred dollars to fix everything up, and I don't want to spend the money on it right now. In a few weeks I will probably take the whole compressor back to Queensland Trade Tools and get them to look at it for me. :(

[outback]

No probs Bob, Just a thought, I had a motor a while back whic I suspected had a dicky capacitor, did some swapping around with replacements, started her up, and had the smoke thing happpening, same as you described, so I thought just maybe you had done similar to me. :o

[Schtoo]

Harry, I might be a bit rusty here but...

Run a second 6mm, stick both into a small sub board in the shed.

IIRC, 6mm can be parralleled as a submain.

Check with the fresh fella, I been off the tools for long enough that I can't recall ALL the details off the top of my head. I'd need a few months to be back up to speed.