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Power hack saw motor switch

Power hack saw electrics

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Sam Slater30/11/2017 15:12:56
8 forum posts
2 photos


I obtained a Qualters & Smith Power Hacksaw.

The motor switch was not working. It was the original switch made by MEM. One of those older electro-mechanical switches, were a transformer breaks/connects the circuit.This MEM switch also had thermal protection.

The motor is 3 phase. I decided to replace the MEM switch with a modern motor switch with thermal protection, as I could not find a replacement on ebay with the right amperage.

I purchased a Schneider motor breaker with thermal protection

I connected it up fine.

Now on this power hacksaw there is a limit switch actuated by a cam so that the saw cuts off once the saw reaches the bottom or top. The plan was to connect the cables of the limit switch to one of the 3 phase cables so that when the limit switch is pressed down one of the 3 phases cuts off. So that is what I did and Yes one of the 3 phases I selected to connect the wires of the limit switch does cut off when the limit switch in pressed down.

Well the problem is the motor will still run with only 2 of the phases connected to it. So my original thinking that I could shut down the motor by switching one of the phases off with the limit switch is completely wrong.

I need to break the circuit completely, all 3 phases, onece the limit sitch is actuated.That is what the original MEM switch was doing.

So how do I actually do this?

Brian Sweeting30/11/2017 15:37:15
437 forum posts
1 photos

Sadly that push button contactor is not designed to have remote stop/start control.

If you can find a way to get at the wiring to the buttons you may have a chance.

You shouldn't really wire one phase through the limit switch as the switch probably is not designed to take the current through its contacts.

larry Phelan30/11/2017 16:44:59
544 forum posts
17 photos

Just had a look at my spindle moulder,there is a mass of wires in the start switch,but it looks as if two wires come from the foot switch,one being connected to the incoming side and the other to the out going.

Just keep in mind that this switch is intended to accept a foot or remote switch.

Oldiron30/11/2017 16:54:45
481 forum posts
22 photos

Its usually only the coil that fails on the original switch. You should be able to get a coil for the MEM switch with a little searching around . Try contacting a local electrical wholesalers like City Electric Factors or similar.. Even a 240v coil will do the trick if you have, or can run a neutral up to the box.

BTW.  MEM is now owned by EATON all spare parts are supplied by them now.

Edited By Oldiron on 30/11/2017 17:09:49

Martin Cargill30/11/2017 17:48:38
140 forum posts

What you have is a circuit breaker that protects the motor and can switch it on and off. To control your saw in the manner that you describe you probably need to run it in series with a contactor.

Phil Whitley30/11/2017 18:11:36
1250 forum posts
147 photos

The limit switch should be wired to the motor starter to break the holding in circuit, it is acting as a remote stop button. It needs to turn off the coil that holds the starter in the on position. TBH as has already been said, you would be far better off using the original MEM starter, I will look at the schematic for the new one and get back to you in a minute.


Phil Whitley30/11/2017 18:34:50
1250 forum posts
147 photos

Right, as Brian Sweeting says above, that is a mechanical starter so cant be tripped remotely (Brooks used to make a similar mechanical starter with thermal overloads) , I thought this type of starter had been outlawed as it appears to have no no-volt release, which means that that the motor will restart by itself after a power failure, Odd. See if you can get the parts for the MEM starter. On the MEM there is a link to remove and the remote stop is connected to the terminals that were linked. There is also a wiring diagram inside the starter cover. In the old days, things were done properly, and old MEM starter is far better than anything you will buy today. The last starter I fitted a few days ago was a 3 phase on a compressor, it was supplied by the customer, made in India, and cost about £45.00. If you buy 500 from India they cost $4.50 each! sorry I can't be more help.


Sam Slater30/11/2017 19:29:40
8 forum posts
2 photos

With the MEM switch some of the bakelite was missing at the area were the push button is. So you couldn't push it on. Also it wasn't tripping when the limit switch is pressed. I spent a long time trying to wire it so that it trips but no luck.

Les Jones 130/11/2017 20:09:30
2152 forum posts
147 photos

If you post some pictures of the inside of the starter (And any documentation that came with it.) we may be able to help. The cut out switch will need to be connected in series with the stop button.


David George 101/12/2017 08:17:43
1303 forum posts
447 photos

Hi Sam It appears that you have the wrong switch and if it was me I would bite the bullet and get another one which has a coil control with a contactor and then you can wire the down switch and a emergency switch as necessary. Where are you based perhaps someone with electrical expertise near you can help.


peak401/12/2017 14:12:25
1154 forum posts
135 photos

According to RS, that particular switch is designed to be used in conjunction with a contactor;

See Here


What did the ebay seller's supplied wiring diagram show?



Edited By peak4 on 01/12/2017 14:13:34

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