Another electrical question.

[JohnD]

Prompted by another thread here (Rings Radials RCBOs and I'm lost - General Discussions on anything - Sideways Technologies ) I have a parallel question.

My garage runs off an antiquated 'consumer unit', a cast iron box on the wall with three porcelain fuses inside and a large lever on the side to turn  it on or off .  The fuses carry a length of fuse wire over the porcelain hump, each end wound around a screw that you tighten.  Familiar from  my youth, but vastly out of date, I fear    My compressor runs off one of these, via a ring circuit (I think!) to the second garage.    Earlier this year, the compressor was more and more reluctant to start, until it refused.     I diagnosed a failed starting condensor (Diagnosed!!  It was a wild guess!) and I replaced it with another of the same rating and size, and all seemed well.   But in the last ten  days or so, trying to start the compressor has blown the same fuse in the fuse  box.

The card of fuse wire that I have (and  fear I  may not  be able to replace!) has 10,  15 and 30 Amp wire wound on it.   Having measured the blown wire thickness I replaced it with 10A.    15 would seem unwise,  but has the Electrical Engineering Dept. of Sideways  University any advice please? 

John                                          

 

[Steve 13-60]

Hi John, what is the watt rating of the compressor?

I=W/V which will give you the basic current rating but it will draw more on start up for a few seconds.

Use 230 for volts as they dropped it from 240 some years ago.

[zetecspit]

1 hour ago, JohnD said:

 

The card of fuse wire that I have (and  fear I  may not  be able to replace!) has 10,  15 and 30 Amp wire wound on it.   Having measured the blown wire thickness I replaced it with 10A.    15 would seem unwise,  but has the Electrical Engineering Dept. of Sideways  University any advice please? 

John                                          

 

Why is 15A unwise? The fuse is there to protect the wire frm fusebox to socket, and I doubt you have 1mm (ie thin lighting cable) probably 2.5mm twin and earth, and in a radial that is rated at 20A, in a ring 32A. (assuming it is not buried in insulation!)

I would expect any half decent compressor to run at 10A or over. Fuses are more resistent to short term high current at startup, but not invincible. 

[mattius]

I had this issue with the old porcelain fuse board and the miles of dodgy wiring attached to it.

I couldn't even turn on the compressor.

According to my father in law (retired spark), the compressor puts a large load on the circuit when starting and that was probably exceeding the current capacity of the wire and hence suitably sized fuse wire.

I would be a bit wary of increasing the fuse wire too much without checking the capacity of the wiring in your garage, the fuse (RCD in modern times), is dictated by the wiring and it's configuration. For instance modern day 2.5mm T&E can do roughly 24A in a radial or 32A in a ring configuration.

I ripped out the whole lot and wired a new ring main ( don't do this it's slightly illegal, but I have a father in law who checked it over and gave an approving nod ).

[JohnD]

Thank you, to all the dons of Electrical Engineering!

Steve 13-60,  the specs label on the compressor says " 240V  12.5A  2.2kW"   although I=W/V gives  9.1A, or 9.6 if only 230V.
zetec, I've always thought it unwise to put a nail in place of a fuse (as it were!) in case a less replaceable  part of the circuit melted.

Mattius, Yes, I thought that a starting electric motor briefly draws a large current and that the capacitor was to 'top up' the mains supply and prevent that being overloaded.  Please tell me if I'm wrong! 

On the last, the new capacitor has an M8 bolt bonded into the end furthest from the connectors.   I've assumed this was to mount it. but the OE capacitor was held in place by two zip strips, so I haven't used the bolt.  Should it be earthed??

The compressor, or rather the fuse box, has behaved  since I last replaced the fuse wire.    So maybe it was just one of those things.

John

 

 

[zetecspit]

1 hour ago, JohnD said:

Thank you, to all the dons of Electrical Engineering!

zetec, I've always thought it unwise to put a nail in place of a fuse (as it were!) in case a less replaceable  part of the circuit melted.

 

 

 

Yep, that is true. 

As stated, the fuse or MCB (breaker) or RCBO (combined breaker with RCD, the BEST protection for a circuit) is there to protect the fixed circuit wiring, not there to protect appliances. If your garage is wired in 2.5mm T+E (probable but check) you can safely up the fusewire to 15A and still have a huge safety margin. 

Be aware that circuits clipped to a wall/joist etc can safely carry more current than the same wire buried in plaster  or worse insulation. I expect a garage to have the wires all surface clipped. 

FWIW I have a 180A mig. It should be connected up using a 16A plug. However, for the last 5 years I have been using a std 3pin plug with the 13A fuse replaced by a bolt. On full power it used to occasionally pop the fuse, so this was my temp fix to finish a job. The mig will also trip the garage 32A RCBO in the house conumer unit. I really ought to uprate that to a D type RCBO. BTW 30mA RCBOs or RCDs are essential for a garage, the danger of getting a nasty shock are simply too great to not have one, even if you do get the occasional nuisance trip. 

(and yes, this stuff is part of my day job)

Re the reason a compressor or indeed any motor has a high startup current, it is simple. As the motor starts turning, it also acts as a generator, producing "back emf" (voltage opposing the mains supply)  This reduces the effective voltage across the motor, and therefore the current draw, down to the running current. When teh motor starts there is no back emf, so a much larger current can flow.  I measured an electric rad fan a while ago, 8A when running, but startup was 16A. (a lot of fan adverts quote a power that is calulated from teh startup current, which is very misleading. Saying that the big Spal fan I just fitted to my Sprint draws about 14A when running, not measured the startup current but it doesn't blow a 25A fuse, a 20A was not up to the job)

Edited November 28 by zetecspit

[JohnD]

Thanks, Clive!    A comprehensive reply!

But while you no doubt can say what wiring is by looking at it, I can't!   The guy who wired the second garage did fit an RCB, so no doubt the circuit is as you prescribe.  But I'll leave the 10A fuse wire in there while the compressor keeps starting OK!

JOhn

[zetecspit]

John, even 1.5mm T+E (often used for lighting circuits) will carry 16A all day long if it isn't buried in insulation. That is why I expect a 15A fuse will not pose any risk at all.

For checking, this link is handy

CSA	Overall Dimensions	Diameter of each Live/	Number of strands	Approx Measurement across
		Neutral copper core strand		bundle of strands (‘diameter’)
1mm²	7.8mm x 4.25mm	1.13mm	1 (solid core)
1.5mm²	8.2mm x 5mm	1.38mm	1 (solid core)
2.5mm²	10.3mm x 6mm	1.78mm	1 (solid core)
4mm²	11.9mm x 6.25mm	0.85mm	7	2.56mm
6mm²	13.5mm x 7mm	1.04mm	7	3.13mm

 

 

 

Edited November 28 by zetecspit

[DeTRacted]

11 hours ago, JohnD said:

I thought that a starting electric motor briefly draws a large current and that the capacitor was to 'top up' the mains supply and prevent that being overloaded.  Please tell me if I'm wrong! 

Not quite right John 

A single-phase motor actually has two windings because it needs a field which is rotating relative to the rotor in order to start up.  The 'run' winding is connected direct to the supply but if this alone is powered at start-up, the magnetic poles are alternating but are always in the same place so there is no force to pull the rotor round and It just sits there buzzing and drawing a large current due to the lack of back-emf.

A second  'start' or "auxilliary" winding is connected to the supply through a capacitor. Since current in a capacitor leads applied voltage by 90 degrees it makes a second phase so there are two windings powered about 90 degrees apart and the rotor 'sees' a magnetic pole which is moving around every 1/4 cycle of the supply. The rotor wants to align with the pole so this starts the motor spinning.  In some motors when the rotation reaches around 75% of synchronous speed, a centrifugal switch opens and the start winding is no longer powered.  Other motors have no switch and so power the start winding all the time at a lower current; this gives lower starting torque but better efficiency when running by providing power-factor correction. 

 

Edited November 28 by DeTRacted
typo

[JohnD]

Another most helpful answer!

And on starting the compressor does "sit there buzzing" until it gets going, sometimes for a second or two.    If that is too long, then without Back EMF, the high current blows the fuse?  Would a differently specked (larger?) capacitor help?   It blew the fuse again this PM.

John

[DeTRacted]

 

It sounds from your original post that the thing used to work OK without blowing the fuse and without sitting buzzing for a couple of seconds on startup, John.   If the fuse-blowing behaviour is new,  perhaps something still isn't right  - either that new capacitor is faulty or could there be something mechanical stopping the motor from rotating easily?

[JohnD]

It doesn't really "buzz", but "chug" as if it is trying to turn over. It's a direct drive compression, no drive belt, so can't see if it turns while it does that.

I replaced the original capacitor with one of the same rating, 60 microFarads.    Would one with a higher rating help, do you think?  The pump rating is 12.5A 2.2kW.

John

[DeTRacted]

You shouldn't increase the capacitor value arbitrarily as it changes the operating conditions of the motor and can cause it to overheat.   To know the right value you need to know a lot more about the motor - It depends whether the capacitor is just a starting one or start-and-run ie. does the motor have a centrifugal switch or is the capacitor in circuit all the time .   60uF sounds as though it might be a start-and-run capacitor;  as a rule of thumb a start capacitor would typically be more than double that value for a 3HP motor. 

Electrolytic capacitors are used for starting alone as they are physically small for a given value, cheaper,  and are only passing current for a short time. Start-and-run capacitors are not usually electrolytic since they pass current all the time so need to be more robust and are larger.  Metallised paper or Polypropylene types are common for that.    

[Nick Jones]

2 hours ago, JohnD said:

It doesn't really "buzz", but "chug" as if it is trying to turn over. It's a direct drive compression, no drive belt, so can't see if it turns while it does that.

I replaced the original capacitor with one of the same rating, 60 microFarads.    Would one with a higher rating help, do you think?  The pump rating is 12.5A 2.2kW.

John

Does sound a bit like a capacitor issue.  Do you have anything to measure capacitance? Would be interesting to see what the present incumbent measures at?

Also worth checking all connections are clean and tight as a bit of volt-drop can have similar effect.

[JohnD]

Thanks, Nick!  I'll add that to the 'todo' list!

John