Dewhurst Switch 3 phase wiring

[GrottoParticipant @grotto]

[GrottoParticipant @grotto]

hoping someone can help me with wiring up a Dewhurst switch.

I have 4 wires from socket;

L1 – Red

L2 – Blue

L3 – White

Earth – Black

Dewhurst switch has 8 terminals, 1-4 down one side, 4-8 on the other.

I did have photos of how it was wired (replaced motor with single phase) but can’t find photos anywhere! Now I’ve put the 3 phase motor back on but can’t work out wiring. I’ve had a hunt on the web but can’t find any diagrams which seem to match for 3 phase

[EmgeeParticipant @emgee]

Incoming phases to 1, 3, 4 (Red-1, W-3, B-4)
Motor lines out from 2, 6, 8

If you need to change direction to match the switch indication change over any 2 wires going to the motor.

From the colour description sounds like an old cable, earth needs connecting to the metal switch casing and the motor frame.

Emgee

[GrottoParticipant @grotto]

Thanks!

One section of cable (motor to switch) is older but looks to be in good condition & is in a metal sheath, so I’ll reuse it. Rest is more recent with a green earth wire.

I reckon I should have paid a bit more attention at school when we did electricity.

[Phil WhitleyParticipant @philwhitley94135]

Have you checked that the switch is 415V rated (if you are using normal mains 3 phase) It doesnt look like it is!! flashover danger !!

Phil.

Edited By Phil Whitley on 05/10/2018 20:05:39

[EmgeeParticipant @emgee]

Dewhurst switches were designed for use on 3 phase supplies and from memory were rated to 500v, perhaps Grotto can confirm if there is any marking.

Emgee

[GrottoParticipant @grotto]

Switch is rated 500v 3 phase.

I’ve wired it up and it works, but sometimes it tries to start the motor rotating the wrong way. Motor won’t start if belt on Myford is tight (need to have belt slack to start). Once motor is running it’s fine.

I’m wondering if I’ve got the R, W & B wires going to the motor around the wrong way?

[EmgeeParticipant @emgee]

Hi Grotto

Sounds as if some switch contacts have been damaged by long term use for switching on/off, this causes high resistance and hence the problems encountered.

If you have 3 phases on the supply side and there are no problems with burnt contacts switching on should give instant starting on a good motor.

You need to visually check (and use a resistance meter) the condition of the switch contacts replacing as required.
Also check the winding resistance of the 3 motor coils, they should all be the same resistance.
If available use an insulation resistance meter (Megger) at 500v to check for leakage live to earth.

If you are using the switch as the only device controlling the motor you need to consider fitting a DOLtype starter and overload with on/off pushbuttons for starting, only using the Dewhurst as an off load device for changing direction.

Emgee

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