Wiggly amps, NVRs and motors?

[Cornish JackParticipant @cornishjack]

Yet again, I join the ME supplicants

Recently re-installed my Dore-Westbury mill after some 15+ years in storage/relocation etc. Opened the, already connected, NVR box to re-route the wires and, (eejit!!) didn’t note the method of fixing the innards to the case. Rather than generate ‘magic smoke’ by getting it wrong, I dug out another (Crabtree) NVR and wired that in instead. First problem – when the motor feed wires were removed from their armoured sheath, there were TWO BROWN leads and an earth. I used my expertise (took a guess) and wired them via a piece of chocolate strip. Plugged in, the first switch-on produced motor rotation (in the correct direction!!) followed very quickly by a stop. Changed blown 5AMP fuse (eejit!) for a 13 amp similar and tried again. Same result – 3 second run, then stop. Several attempts to restart failed. About two hours later tried again and instant start (hooray!!) followed by the same type stop. With long pauses between attempts, this sequence can be repeated and the stop APPEARS to originate in the Crabtree unit – hearing not too clever, so can’t be sure. Soooo …., wiggly amp gurus – any suggested cures, please?

Rgds

Bill

PS – Suggestion of bursts of 3 seconds of milling interspersed with 2 hour intervals isn’t quite what I’m after!!

B

[Cornish JackParticipant @cornishjack]

[David Clark 13Participant @davidclark13]

Hi There

I doubt it would change anything but reverse the two brown wires.

If you only have two wires, that is all you can do.

There is always a reason afuse blows.

Uprating it to 13 amp or just changing it is not a good idea.

You need to find out why it is blowing.

Is the motor getting hot?

Is it getting up to speed?

Can you get somoen with good ears to listen for a centrifugal switch changing over internally?

Sounds like that may be the problem or possibly the capacitor breaking down after a while although that sounds unlikely as it is sending the motor in the right direction.

regards David

[Gordon WParticipant @gordonw]

I am no elect. guru, but since the m/c has been laid around for years, have you checked for odd bits? Things like dead earwigs ? Also is it really dry?

[Steve GarnettParticipant @stevegarnett62550]

Sounds possibly like the thermal trip in the Crabtree NVR is getting upset, but there’s almost certainly a reason for this that isn’t to do with the switch. So you have to be a bit careful here, as there are a number of other possibilities you can’t exclude. One of the more common ones is an earth leakage fault in a motor that hasn’t run for a long time – certainly a distinct possibility because fuses are blowing. Excess current flowing to ground would certainly warm up the thermal trip, and that could easily, in an old switch, take a good while to reset itself.

 

The switches themselves are pretty unlikely to be the cause of this sort of trouble, and in this instance, it sounds rather more like it’s doing a sensible protection job, rather than being at fault. I’d say that your first action should be to get that motor checked carefully.

[Les Jones 1Participant @lesjones1]

Hi Cornish Jack,

The 5 amp fuse blowing could just be due to the high initial start current of the motor. Are you sure that the thermal overload trips in the NVR are a suitable rating for the motor. (What is the rating of the motor and what is the current rating of the trips in the NVR ?) Have you checked that the spindle turn freely ?

Les.

[NJHParticipant @njh]

Hi Cornish Jack

 

I agree with Steve here. This is not a situation for “poke and hope”. I suggest you get an electrician to check this out – it may cost a few bob but the alternative might cost infinitely more!

 

Regards

 

Norman

Edited By NJH on 30/06/2011 14:52:27

[Cornish JackParticipant @cornishjack]

Many thanks to all for the prompt replies.

DC1 – will try the wire swop later – can’t get my head around that causing the trip. The original fuse blowing was, I suspect, as Les Jones suggests. Motor doesn’t get hot – probably not running long enough. It does get up to what I reckon is operating speed. Might have to persuade SWMBO into the workshop to listen for me – that WILL cause trouble, ‘cos she’s a tidy person!!

Gordon W – Gave it a good blast with the air line and nothing untoward appeared.

Steve Garnett – Sounds reasonable but do you know if these thermal trip devices are more sensitive than a distribution board RCB? Queried ‘cos said RCB DOESN’T trip.

Les Jones – Yes, the motor turns quite freely. As to the NVR rating, the rating plate is inside the box and near impossible to see, never mind read – tiny writing!!

NJH – Could well be the ultimate solution – will proceed very cautiously, using all available cover!!

I have a couple more motors spare , so that might have to be considered.

Thank you again

Rgds

Bill

[Les Jones 1Participant @lesjones1]

Hi Cornish Jack,

The RCD detects a different type of fault to a thermal overload.

The RCD detects leakage current to earth. The normal domestic type trips at about 30 mA and does so quickly. The thermal trip trips if the current is too high over quite a long period of time (several tens of seconds.) It is designed not to trip with the high starting current which only occurs for a short time. If the thermal trips in the NVR are designed for say a 1/4 HP motor then if it was used with say a 1 HP motor they would probably trip even though there was no fault.

Les.

[Steve GarnettParticipant @stevegarnett62550]

Well, with the additional information that the RCD doesn’t trip, but the thermal trip does, and takes ages to reset, I’d say that trying a different NVR contactor might be the thing to do. On the face of it (and obviously without seeing it), the thermal trip appears to have gotten itself to be rather sensitive!

[Cornish JackParticipant @cornishjack]

Thank you Les and Steve.

Update – Have managed to check the NVR and it IS that which is tripping. I tried DC1’s suggestion re. swapping the motor leads and the motor still runs and in the correct direction … definitely a ‘nil comprende’ on that!!

Combining you gentlemen’s info and advice, I think the answer, or, at least, a first step, will be to change NVRs. Last time in the shed, I had another look at the original NVR and realised that I was trying to re-install the innards 90 degs out of kilter!! It now installs correctly and I should be able to re-instate the power feed as it was when it last worked properly. Fingers crossed and, if successful, will let you know. Again, thank you.

Rgds

Bill

[Cornish JackParticipant @cornishjack]

Update to the update.

Gentlemen, thank you – your remote diagnosis would appear to be spot on. I have just rewired the original NVR and reconnected to the motor. Switch on, press the green and all working as advertised!!

It would be nice to think that I understood the technicalities involved but my understanding of electrics came to an abrupt halt at my first electrics lesson at Air Sig’s School, when our instructor opened with “There are TWO types of current flow – Conventional, Positive to Negatve and Electron, Negative to Positive” Being a bit of a dullard, I had to ask, “Which is correct?” Response was “I can’t tell you!!” Thereafter, wiggly amps remained a closed book!

Again, thank you

Rgds

Bill

 

[Steve GarnettParticipant @stevegarnett62550]

Well I’m glad for you that it’s fixed – but judging from what you are saying, it may well be a good thing that you didn’t progress much further with electronics. If you had trouble with basic electron flow, what on earth would you have made of the concept of holes (the absence of electrons) as charge carriers? This is the basic explanation of what happens in those wiggly devices with the pre-programmed smoke. The primary design objective with these things incidentally, regardless of what you are using them for, is to contain the smoke!

[Author]

Posts