Dewhurst Type A Specifications

The Dewhurst Type A switch manual suggests a maximum 0.75hp at 240V, no amp rating. Does this mean it's essentially a "no go" for my motor?

The motor someone installed on my lathe (before I purchased) is single phase 240V/50Hz, 5.2A, 1hp.

The thing to remember with the Dewhurst switch is that it is designed for REVERSING the direction of the motor NOT for stopping/starting which should be via a proper NVR starter switch. If operated this way the circuit is made/ broken by the NVR switch whose contacts are designed to cope with the arcing which evevitably occurs when the circuit is made/ broken - it will also ensure that the motor will not restart automatically following restoration of power in the event of cut. The Dewhurst should be set to the required direction before the NVR switch is operated hence there will be no arcing at its contacts. The question I would have is why do you need to reverse the direction of your lathe? I know there are some occasions but I've never found it necessary and, in consequence, I don' t have one fitted to my S7 !

Norman

Have you ever had the need to cut a metric thread on and imperial lathe or an imperial thread on a metric lathe? If so you surely would know why a lathe has a need for spindle reversing.   Short threads could be wound back by hand but a bit tiresome on long lengths!

Edited By D Hanna on 21/10/2016 05:16:50

Norman,

I recently did a job for a friend that required cutting the thread all the way down on some 110mm cap screws, 10 x M10 and 10 x M12 (he had trouble getting all thread cap screws in the size needed). Since my tapping box is not auto reverse I used spindle reverse to remove the die from the threaded cap screws after threading. Without reverse this would have taken a lot longer than it did.

Martin

While the digression is interesting Does anyone know the answer to my question? My understanding is the HP rating of the switch doesn't matter if it is not being used to control the motor i.e. on/off but what max. amperage is the Dewhurst rated for? Interesting that it is not in the manual but understandably it's "old school".

Edited By mrbuilder on 21/10/2016 09:13:05

Re reversing a lathe - the manufacturers would not have fitted a reversing switch unless deemed an advantage in use. Even fitted on lathes with threaded chucks (these should not be operated in reverse under load because the chuck can unscrew!). Some cheaper lathes require either reversal of direction, for long travel return after a cut, or laborious winding back manually. As above, non standard (for the lathe) thread cutting necessitates keeping the lead screw engaged at the same setting for the full operation.

I've seen videos of people turning cutters upside down to direct dusty cast iron swarf to a more collectible area (Keith Fenner, for one) in order to keep the mess away from the ways - perhaps not the best practice as loads on the saddle and ways is opposite than designed.

It is a shame, and probably illegal, to not display the electrical ratings. If not in the manual, they shoud still be on the device. A decision of maximum power transferrable can then be made. A soft start motor will draw less peak current at start up, a parameter required to detetmine the excess capacity of the product, compared to normal running. So HP rating is not necessarily the best 'yard stick' for determining the switch's capability. There is a difference between switches and isolators - they are not the same and these reversers should be treated as isolators, something many do not comprehend.

1 hp is equivalent to 746 watts. 0.75 hp is about equal to 500 watts which at 230 volts gives a current of just under 2.2 amps.

Apparently not within this community

But here's a few clues:

1. This page illustrates not only the switch, but the cover of what looks like an instruction leaflet.
2. The company still exists: **LINK**
3. The terminals and contacts are very substantial, and, for non-switching applications should carry far more than a couple of amps.
4. Schneider makes something broadly similar: **LINK**

MichaelG.

In regard to that, for example I've seen 10A, 240V, 0.75hp rated switches. So I was under the impression that there is a bit more to it than just that calculation - I think I remember something along the lines that the "hp rating" is the amount of current the switch can handle at the time the motor (or other device) is turned on.

However, I'm certainly not an expert, I'm constantly trying to get it clear in my mind how _everything_ I use/come in contact with works and to increase my understanding!

Thanks for the links. I might contact them directly or investigate a modern (possibly better) equivalent.

mrbuilder

The contacts IMO in a good condition Dewhurst are of adequate size when closed to carry the starting and running current of your 1HP motor, these devices are old design when contact size/surface area was far greater than modern devices have.

The 5.2A is full load current, you will be not always be running at full load. DOL starting current can be 3 or more times FLC for a brief period, depending on load, this is the cause of burnt contacts when used as a starting switch. Before using check all cable terminations on the switch are secure.

You are already using a NVR starter and state the Dewhurst is not used for starting/stopping but only when reverse running is called for so you are aware of the possibility of burning contacts if used as the starting/stopping device.

Check after extended periods of use the metal cover of the Dewhurst switch remains at ambient temperature.

Emgee

I have a Dewhurst drum switch on a single phase 3/4hp Myford.

It does O.K., but it is important to realise that the handle can get out of sync with the contacts. The handle detent looks right from the outside of the switch but the actual contacts are not properly aligned. If this happens the contacts can deteriorate fairly quickly. I spotted mine fairly swiftly by the smell.

If I was wiring mine up again, I'd use low power mains rated push button switches, and have a box with a forward and reverse contactor.

The thing with the contactor is that you can always get a bigger one, and you can put them in parallel if you want.

The best is that if it burns out you can just buy a new one and swap it out. Cheap and easy.

Edited By Andy Ash on 21/10/2016 21:47:00