Andy Hall wrote:

I have a couple of related situations in my workshop.


The cyclone dust extractor has a 2kW single phase motor which is fired
up by a contactor, with that it turn operated by an impulse relay
controlled by an RF remote - i.e. I can have a little box on the table
saw and operate the extractor from there. Eventually I may do some
current detection stuff on the saw's motor and switch it that way.
At any rate, a 16A type B MCB was originally used for the dedicated
circuit for this. It had a tendency to trip, but didn't always. A
type C was fitted and resolved the problem.

I have a similar issue in mine if I plug a 3kVA transformer in too near
to the CU - it can trip the 20A MCB. In the middle of the circuit or on
a short extension however it is fine.

My saw table has three 4kW three phase motors. These are
individually switched in for saw, spindle and planer operations as
required. The saw will take up to 315mm blades, so the power is a help.

There is also a VFD (variable frequency drive) essentially an
electronics module, which is useful in several ways.

Firstly it ramps up rather than hard on (as it were) and thus reduces
the inrush current.

This soft start for motors can make a big difference to inrush. I think
this is pretty much a requirement for motors much above 3kW IIUC.

I think that he should be concerned about the mechanical arrangements
to drive the blade etc. This much power on a small blade could pose
some safety problems with suitable belts, bearings, pullies etc.

I did suggest that not over egging it on the belt drive might be a good
plan so as to maintain a intentional weak link. I also set the current
limit on the starter to prevent the motor being able to ever develop
full output power.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/