Wiring A Dewhurst Switch-Single Phase

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Wiring A Dewhurst Switch-Single Phase

This topic has 12 replies, 4 voices, and was last updated 31 August 2015 at 09:25 by Les Jones 1.

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24 August 2015 at 05:18 #7769
Grotto
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@grotto
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24 August 2015 at 05:18 #201632
Grotto
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@grotto
Further to my previous post (year or so ago)…

I've had a good trawl on the net, and have found numerous pages showing how to do this, but I think I must lack the knowledge to understand.

I'm replacing my 3 phase motor with a single phase (looked at getting 3 phase but way too expensive, 3 phase motor is not the correct type to run single phase).

I have a single phase motor (0.6 hp, type BSP) which only has two terminals.

All the diagrams I've seen for wiring motor to Dewhurst have more terminals.

Should I be separating the wires?

Any help would be appreciated, as I even tried getting an electrician to sort it but he said he didn't know what to do.

Thanks
24 August 2015 at 07:01 #201633
Grotto
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@grotto
I'm wondering if it would be best to use a different (single phase) motor.

I have a Gryphon on my bench saw which is 3/4 h.p. and has 4 terminals (A1, A2, Z1, Z2)

It's capcitor start, but still not sure how to wire it as at present live and neutral (form power source) only go to 2 terminals
24 August 2015 at 08:48 #201641
Les Jones 1
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@lesjones1
Hi Grotto,
It will probably be possible to use a reversing switch with either of the motors. On the 0.6 HP motor two of the wires are probably the main winding and two the auxiliary winding. (Possibly via a centrifugal switch and start capacitor.) You would need to do some tests with a multimeter to work out which wires are the main winding. Does this motor have a start capacitor ? You would need to add a connector block so that the four wires (Plus the earth wire.) could be brought out to the reversing switch.
on the 0.75 HP motor you would need to take a photograph of the connection box on the motor so that the labels on the terminals could be read. (Or add labels to the picture.) I am assuming this is a capacitor start motor and the cable from the bottom right hand side of the connector box goes to the capacitor.

Les.
24 August 2015 at 10:36 #201657
Robbo
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@robbo
It will be easier to use the Gryphon with the 4 terminals.

This diagram may then help.
25 August 2015 at 00:45 #201806
Grotto
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@grotto
Thanks

The Gryphon motor is 1.25 hp which seems a little grunty for an ML7, it 's also 2850 rpm which may (?) cause issues (not sure about this.

The other motor (Gill & Son) is 1,450rpm which is what my 3 phase one was. I don't think it's capacitor start (no visible capcitor).

I've undone the wires on the Gill motor and it looks like there is (just) enough length to attach them to a connector block.

I have a multi meter, but am not sure how to test which are main winding & which are auxiliary. At present, one black and one green attach to each of the terminals (there are 2 green wires & 2 black).

On the Gryphon motor the capcitor wires (bottom right of photo) attach Black to A1, and Red to Z1. The power input wires attach Red to A1, and Black to A2.
25 August 2015 at 09:09 #201832
Les Jones 1
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@lesjones1
Hi Grotto,
I agree that the 1400 rpm motor is more suitable. The green wires will probably be one winding and the black the other winding. I would expect the resistance of the main winding will be between 5 and 10 ohms and the start winding between 10 and 20 ohms. (For connecting it up to the reversing switch it does not matter which is which.)

On the Dewhurst switch connect live and neutral VIA AN NVR SWITCH to terminals 1 and 3 (It does matter which way round.)
Connect the main winding to terminals 5 and 7
Connect the start winding to terminals 2 and 6

If forward and reverse are the wrong way round then swap over the wires on either terminals 5 and 7 OR 2 and 6 (BUT NOT BOTH)

Les.

Edited By Les Jones 1 on 25/08/2015 09:10:28

Edited By Les Jones 1 on 25/08/2015 09:15:27
25 August 2015 at 11:10 #201849
Grotto
Participant
@grotto
Thanks Les, really appreciate the advice

the resistance of the black wires is around 3.8 ohms.

the green wires don't appear to form a circuit(?) as resistance doesn't read.

I'll have a better look in the morning
25 August 2015 at 13:00 #201862
Les Jones 1
Participant
@lesjones1
Hi Grotto,
The black wires will be the main winding. The start winding is probably in series with the centrifugal switch (Which should be closed when the motor is stopped.) If the motor had a start capacitor that would also be in series with the start winding. So if there is a start capacitor you would not get a resistance reading between the green wires as a capacitor does not pass DC. If you are sure that the motor does not have a start capacitor then the centrifugal switch is faulty. (Or the start winding open circuit but this is unlikely.) You will need to dismantle the motor to get to the centrifugal switch to confirm that is the fault and if so repair it.

Les.
31 August 2015 at 05:23 #202575
Grotto
Participant
@grotto
Had a go at pulling the 4 wires out, and wiring up.

worked forwards but looked like there was a fireworks display going on inside the motor.

reverse blew the trip switch.

I've put it back to standard and will just use the lathe without reverse until I get a more suitable motor.

thanks for the help, I'll be back!
31 August 2015 at 08:44 #202587
J Hancock
Participant
@jhancock95746
You can do no better than buying the Model Engineer of 2nd November 1979.

The article REVERSING FRACTIONAL HORSE POWER MOTORS by R Jeffrey covers all of this in

'super simple ' detail.
31 August 2015 at 08:49 #202589
J Hancock
Participant
@jhancock95746
You will also need to buy the Model Engineer of 7th March 1980 in order to see the three corrected

diagrams for the above !!!
31 August 2015 at 09:25 #202594
Les Jones 1
Participant
@lesjones1
Hi Grotto,
If the motor works correctly when connected directly then it should work at least in the forward direction via the Dewhurst switch as in one direction it is connected exactly the same way. You have either wired it wrong or the Dewhurst switch is faulty.

Test the switch as follows. (With no other wires connected to the switch.)

With the switch in the off position.
Test for continuity between terminal 1 and the following terminals. 2, 3, 5, 6 , 7 There should be no continuity.
Test for continuity between terminal 3 and the following terminals. 2, 5, 6 , 7 There should be no continuity.

With the switch in the forward position.
Test for continuity between terminal 1 and the following terminals. 2, 3, 5, 6 , 7
You should get continuity to terminals 5 and 6 but NOT terminals 2, 3, 7
Test for continuity between terminal 3 and the following terminals. 2, 1, 5, 6 , 7
You should get continuity to terminals 2 and 7 but NOT terminals 1, 5, 6

With the switch in the reverse position.
Test for continuity between terminal 1 and the following terminals. 2, 3, 5, 6 , 7
You should get continuity to terminals 5 and 2 but NOT terminals 3, 6, 7
Test for continuity between terminal 3 and the following terminals. 2, 1, 5, 6 , 7
You should get continuity to terminals 6 and 7 but NOT terminals 1, 2, 5

(It is possible that the forward and reverse positions may the the wrong way round.)

Did you take the motor apart to check the centrifugal switch ? (To understand why you did not get ant continuity between the green wires.)

Les.
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