Model Engineers’ Workshop – August 2019

58 http://www.model-engineer.co.uk Model Engineers’ Workshop

Start L (Z1) 1

Single Phase Machines

Forward

Reverse

3

7

11

15

5

9

13

2

4

8

112

16

6

10

14

Start N (Z2)

L In

N In

Run L (A1)

Run N (A2)

Start N (Z1) 1

3

7

11

15

5

9

13

2

4

8

112

16

6

10

14

Start L (Z2)

L In

N In

Run L (A1)

Run N (A2)

Dewhurst Drum

Switch

Repair

T

he contacts of the Dewhurst Type

A Reversing Drum Switch, photo 1,

that was fi tted to my 1960s Myford

Super 7 lathe burnt out, and as new

replacements are no longer available, I cast

around for a solution. The lathe is fi tted

with an MEM No Volt Release contactor,

so I only used the drum switch to set the

motor to run in either forward or reverse

and its failure was somewhat surprising.

Anyway, frazzled as it was, I needed a

new reversing switch. A trawl of the Farnell

catalogue revealed a Moeller T0− 4 −8213/

EZ three position switch at about £27 as an

aff ordable possibility, so I bought one.

Single phase induction motors have

two windings – a start and a run. Both

are connected to the mains supply to

start the motor, but a centrifugal switch

in the motor then cuts power to the

start winding when the motor runs up

Glyn Davies off ers

some advice on fi xing

these switch units that

were fi tted to many

postwar lathes.

Dewhurst Switch

1