Workshop Electrics


Postby JonnyH » Thu Oct 14, 2010 6:06 pm

All single phase 240 volt feel free to ask for more info.
Crabtree household consumer unit split load was run from an 100amp fuse outside, replaced approx 5 years ago with a 65amp and wire tagged! Hob actually draws 32amp.

Main concern is one machine with 1 1/2hp non cap start motor continually blowing a 13 amp fuse 1 in every 3 startups, hot or cold. Run from consumer unit with 20amp RCD. It never trips the RCD.
Welder on lowest setting in to same socket will trip same RCD at inapropriate moments spending more time actually resetting RCD.

Just as a matter of interest i have an old 3hp English induction motor, non cap start that blows a 13amp fuse every time, bridged it starts up and runs no problem with no effect on any of the sockets plugged into.

Same but different circuit RCD's feed separate sockets.
3hp motor when running will trip the RCD when turned offor upon startup.
1hp motor with a low current DRO plus a US transformer is now starting to trip another RCD.

I know i have a Bosch fridge that causes problems on the house side, even so cannot remember the last time anything tripped an RCD.

Any ideas what to do - starters, 16 and or 32amp industrial sockets etc.
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Postby ericmark » Sat Oct 16, 2010 3:16 am

There are many options to reduce the start current of motors including mechanical by removing the load.
Soft start and inverters have now transformed the motor control scene and allow three phase motors to be run from a single phase supply.
Most old motors have a start winding either capacitor or centrifugal switch controlled but not all. Commutators and powered rotators are used in drills and vacuum cleaners and also many items needing variable speed.
The information given is not enough to give a detailed answer but of course one has to remember under HSE rules persons working as electricians are responsible for what they do and although there are no fixed rules as to qualifications required to work without the knowledge would leave oneself open to prosecution.

RCD protection is completely different to overload protection and from your post it would seem you have many faults and lack of knowledge and the only valid advice has to be to get some one who does know what they are doing to correct your problems.
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Postby JonnyH » Mon Oct 18, 2010 11:26 am

Thanks for the reply.

Its all single phase 240V one RCD to every socket.
Though a 1 1/2hp motor would only use 4.68amp running, the startup way exceeds 13amp from the 1965 motor. Its run off an RCD 20A and confirms what was mentioned at the time, maybe a starter would sort.

Split load consumer unit was replaced a number of years ago and left with known problems, ("its safer than it was") he didnt come back and died 5 years ago.

From memory there is a difference between neutral and live voltage, more evident off peak live running close to 256V, peak around 240/245V.
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Postby ericmark » Tue Oct 19, 2010 9:10 am

Costs are some times odd due to quantity of items being made and although in general we can say use an inverter or soft-start without listing items and checking prices for all different options one can easy recommend an uneconomic option.

This is not really a DIY job and you need some one on site to work out what you need.
ericmark
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