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mecheng46

Single Phase motor wiring question

Question

mecheng46

I recently purchased a very old and heavy duty single phase bench grinder and I am trying to put some life in it. The motor manufacturer label can be seen here: 

s-l1600.jpg

 

The motor is a single phase 240V, 1HP, 50Hz, 6.1A, 1500 RPM.

 

At the moment there is DPDT switch wired on the motor and no capacitor. The capacitor possibly has been removed at somepoint.

https://ibb.co/rKhQmSg

 

The 15A/250V toggle switch wiring as it stands is:

DPDT-toggle-switch-diagram.png 

1 - Mains power blue - Neutral

2 - Motor Cables 2 & 3

3 - Motor Cable 1

4 - Mains power brown - Live

5 - Nothing

6 - Nothing

 

Although the toggle switch has three positions only two actually do work. The middle which is the Off position and one of the side positions. When I flick the switch to the On position the motor is buzzing and with a bit of push it eventually goes up to speed. This motor should only run in 1 direction but something must be wrong with the wiring because the direction of it when on the On position is 50/50. Sometimes it runs clockwise and sometime anti-clockwise.

 

It sounds like I need to start from scratch which is 3 wires on the motor (not colour coded) and 3 wires on the mains plug, a toggle switch and possibly a capacitor.

 

I like the idea of having a DPDT switch with reverse polarity if possible so i can use the grinder in either direction but I am a bit worried that I might flick the switch to the opposite direction by mistake. I am not sure if DPDT ''safety'' switches exist.

 

To start with I am looking to get the grinder back in working order with 1 direction and without me having to push it to start.

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Andy™

sounds like the start cap is faulty (or missing, as you said it may have been removed) & needs replace or the start winding is open circuit

Edited by Andy™

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mecheng46

Thanks for your reply Andy, yes the start cap is definitely missing as I have to push the motor to get it going. Also it is possibly the problem with the wrong motor direction.

 

I really need to start from scratch with this as the wiring as it stands now it has no capacitor involved in the wiring. All I have for now is 3 wires from the motor, 3 wires on the mains and a switch which it might not be the right one. The motor needs a capacitor somewhere

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ProDave

You don't need to start from scratch. You need to get a capacitor and connect it and all will be well.

 

Are there any signs of cut off bits of cable on any of the motor terminals to give you a clue where it might have been connected?

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mecheng46

The wiring is messed up by the previous owner and it's impossible for me to say what the original wiring supposed to be.

 

No signs of where the capacitor was originally. Also the switch that is currently on the machine doesn't look to be the original, it's an On-Off-On switch and has 6 terminals which only 4 of the terminals are used. The original switch would be an On-Off with the motor rotating in one direction only.

 

Another question is what size and what type capacitor do i need for this motor?

 

 

Edited by mecheng46

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Andy™

the fact there are spare terminals on the switch is irrelevnt. usually the switch would be a 2 position but yours is 3 so you effetively have on/off/off instead of on/off. there are also plenty switch with the same terminals that are just on/off

 

you need to find where the capacitor goes and fit it. chances are, thats all it needs. there are plenty disgrams on google for single phase motor wiring / windings. you need to find the start winding and fit it to that. if you have 3 cables sticking out of the motor, where are they conencted to? there is usually 4 (2 per winding) but it could have 2 connected inside the motor making only 3 on the outside

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ProDave

Can you take the cover off the motor terminal box and photograph that as close and clear as you can get?

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mecheng46

Taking the bench grinder motor apart would be my last resort as it not as simple as it sounds. I am hoping that somebody came across this type of motors in the past and might be able to help. Google and all the wiring diagrams are not helpful due to the age of the motor, I can't find a wiring diagram of a single phase motor with 3 wires coming out of it.

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Andy™

looks like youre taking it apart then

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ProDave

Okay, so you have 3 wires coming out of the motor.  TWO are presently connected, to L and N

 

So the third is the starter winding.  We just need to know if the other end of the starter winding is internally connected to L or N.  Do you have a multi meter to take some ohms readings?

 

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mecheng46
48 minutes ago, ProDave said:

Okay, so you have 3 wires coming out of the motor.  TWO are presently connected, to L and N

 

So the third is the starter winding.  We just need to know if the other end of the starter winding is internally connected to L or N.  Do you have a multi meter to take some ohms readings?

 

 

Much appreciated for your reply Dave.

 

Two of the motor cables are connected together at the moment.

 

Yes I have a multi-meter, what readings do i need to get?

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Andy™
Just now, mecheng46 said:

 

Much appreciated for your reply Dave.

 

Two of the motor cables are connected together at the moment.

 

Yes I have a multi-meter, what readings do i need to get?

 

ohms between all 3 cables

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ProDave
52 minutes ago, Andy™ said:

 

ohms between all 3 cables

but MAKE SURE you note first which cable goes where so you can put them back exactly as they were when you have made your measurement.

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Evans Electric

On a H&Safety issue  ,   you don't need a bench grinder that turns either way  , its an accident waiting to happen .   In my experience they spin down towards the tool rest or bench ,  spinning the other way could throw a piece of work up, unexpectedly   at the operator.     

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Tony S

Wise advice.

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kerching
3 hours ago, Evans Electric said:

On a H&Safety issue  ,   you don't need a bench grinder that turns either way  , its an accident waiting to happen .   In my experience they spin down towards the tool rest or bench ,  spinning the other way could throw a piece of work up, unexpectedly   at the operator.     

Never mind it unscrewing the nut and the disc coming off!

tool,jams

nut undoes

disc goes all moody

tears all,around

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mecheng46
4 hours ago, Evans Electric said:

On a H&Safety issue  ,   you don't need a bench grinder that turns either way  , its an accident waiting to happen .   In my experience they spin down towards the tool rest or bench ,  spinning the other way could throw a piece of work up, unexpectedly   at the operator.     

 

I will get some readings tomorrow from the motor windings :)

 

As I mentioned on my first post, I am not confident having two directions on my bench grinder without some sort of a safety device. On a side note, Tormek market their grinders turning clockwise. 

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Geoff1946

I also can't think of a good reason to have a bench grinder run backwards. Apart from anything else it would direct the detritus into your face rather than down the guard .

Presumably you are electrically knowledgeable enough to know the importance of checking the insulation of this old machine.

With a machine of that power and age there could be a centrifugal start switch. I would be inclined to be dismantling it to establish just what I'm dealing with.

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ProDave

My guess would be L to one thick cable, N to the other thick cable Start capacitor to the thin cable.  Then you will have to try other side of start capacitor to N and if that does not work other side of start capacitor to L

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mecheng46

i will try you suggestions Dave, thanks.

 

What size capacitor shall i try with this motor?

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roys

Wild guess, try a 10uf 450V 

try tryping cbb60 as well when you do a search, think I got my last one from Amazon, cheap as chips.

Edited by roys
Additional

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ProDave

I would have said about 4 or 5 uF  make sure it IS a motor start capacitor usually rated at 400V ac.

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mecheng46

I went through the pictures I took yesterday and I just noticed that the resistance readings I got from the starting windings yesterday are not healthy, unless I am missing something?

 

My only hope is if there is a centrifugal switch in the motor somewhere that might be the culprit. Otherwise i might have to ignore the starting widnings and wire only the run windings directly on an SDST switch and push start it every time i want to use it.

 

Shame as is not an easy task finding a brand new single phase 240v 1HP bench grinder, for some reason manufacturers don't make heavy duty single phase grinders anymore. Heavy duty usually goes with 3 phase. 

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Tony S

A centrifugal switch is of no use without a capacitor to give a starting phase shift.

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