Dewhurst reversing switch

Hello all,

Finally have a moment to post a head scratcher I'm having just now. The motor that came with my lathe would run, i tested it on the bench, but using my spider senses it had that gut wrenching hot motor smell. I used my FLIR camera and found after around a minute it was in excess of 120°c!!

So with that pronounced dead at the scene, I bought another. I took the 4 post terminal from the old one and set about bringing 4 wires out of the replacement as per the original motor. You can guess where I'm going. I've seen the period schematic publications and after applying that to the motor it just trips the garage. There is a reset on it which hasn't need to be reset any time I've tried.

I did run the replacement motor before hand and it was spot on, it was sold as old stock with new bearings fitted at overhaul.

Some what out of my grasp at this stage of the week, I expect this is covered regularly and to more capable minds I'm sure the following images will be useful.

Apologies big thumbs a constant problem on a mobile phone! Any advice gratefully received, thank you

You do not make the significance of the two pictures of the conection boxes clear clear . As the middle picture shows the start capacitor is mounted on the side of the motor I would expect the wires from it to come out to the connection box. I would also expect the wires from the centrifugal switch to come out to the connection box, So I would expect a total of 8 wires to come to the connection box. Show us pictures of before and after your modification.

Les.

Good morning Les, thank you for the reply. I have a few more on my phone I have uploaded and hopefully the 2 more relevant ones to show the motor before the 4 wires were taken outside.

New old stock but needed new bearings ? strange ! I take it from the title that this is all about fitting a dewhurst drum reverser ? So the square terminal block on the outside is not original but from the old motor ? These motors should have a circuit diagram on the inside of the cover for the terminal block. Noel

Where did you connect the power to?

The 2 yellowish wires to A2 and T3 are the thermal overload switch. Yellow and blue could be the start windings and red and black the run windings.Cap and cerntrifugal switch in series with the start windings. To change rotation swap yellow and blue. Dewhurst left hand terminals numbered 1 (top) 2,3,4, and right 5 (top)6,7,8. Live to 1, Neutral 3. Run to 5 & 7, start to 6 & 2 (4 & 8 not connected) Your motor MAY have different colours of wire. It is VITAL that both the motor and switch are earthed ! How is the terminal box on the outside fixed to the motor ? Noel.

Good evening,

Thank you for the reply and the information, I have followed it as you outlined. Yes the power was correctly wired to the left bank of the switch, live 1 neutral 3. The motor runs in forward rotation but when stopped and reverse selected and powered up, click! So a bit of progress however still an issue as tripped upon reverse selection. Earth of course a no brainer, you can see the 5 core armoured cable leading to the motor, the supply to the Dewhurst is 3 core armoured cable from a NVR switch.

The terminal box is fixed through the motor case into the void ahead of the field windings, 2 flush cut m5 bolts in half nuts protruding approx 4mm into the case about 15mm from the windings themselves.

What is tripping an MCB, short circuit or an RCD, earth fault ? Do you have start and run windings mixed up ? Does it only have one Cap, or 2. Noel

Can you draw a diagram of how you have connected the start winding, the run winding, the capacitor, the centrifugal switch and the thermal cut out switch with letters or numbers on the connection that we can refer to when we reply.
To use the thermal cut out switch you will need an extra 2 conductors in the cable to the Dewhurst switch. (So that the thermal switch isolates power from both the run and start windings.
It would be a good idea to see if the motor runs in both directions without the Dewhurst switch just by reversing the phase relationship between the main and start circuits.

Les