P-Power hacksaw

Hi
Planning to build a printed power hacksaw.
I would use a windscreen wiper motor to drive this hacksaw. Just bought this motor in UK but problem is after after some time this motor gets very hot, and I mean HOT..! Only way to keep it cool during a long (free)run is to install a 80 or 120 mm fan against it. It gets warm still but not as hot as before.
Is there a reason these motors getting so hot. I have an older windscreen wiper motor and it's just the same, it gets very hot to.
Idea's...?

Thanks

Chris

Windscreen wiper motors are continuously rated but the question is what is the rated power for wiping windows. If you slow it down with a higher than expected load the current will be higher than it is designed for. You probably need something with a higher rated power for a hacksaw unless you reduce the load by gearing or changing pulley sizes..

Martin C

Martin,

There was no load during testing and it gets very hot anyway.

Chris

All motors are different, which might sound obvious, but a windscreen wiper motor has been designed to wipe windscreens on cars.

So, they have to deliver high torque, and have to be compact, so as not to impact on the "look" of the car. On the up side, it's pretty easy to have them sited in a high flow air-stream.

A compact high power motor is always going to get hotter than a large heavy one. For the same number of wire turns, the wire can be thicker and the magnets stronger. Weaker magnets means more current required, and thinner wire means more heat for that current.

One way to fix it, is to attach your new saw to the front of your car, and drive around whilst sawing. That way you would get the required airflow.

A more practical solution would be a bigger motor, or different/better gearing.

I suspect you won't achieve cooling equivalent to the automotive environment with a practical fan setup.

Edited By Andy Ash on 16/04/2017 17:49:16

.....

Hi Chris

I take it from the size of the motor that you will be building a working model Hacksaw and not for serious work.

Up size as Andy Ash suggests if you want the job in cut the same day.

Target 60 to 100 strokes per minute @ 100 mm stroke with a frame weight of around 5 kgs with a 300 mm blade.

Most of all enjoy building something you have designed and built yourself It's a good feeling "I Made That".

Eric

Eric,

Busy designing this hacksaw this very moment. It will be capable sawing rather thick diameters. I was thinking about 100mm(4" max. It"ll take some time to work through 100mm steel or brass with this hacksaw but I'm not in a hurry at all....

Chris

The limitation is generally how much temperature the materials can stand. If you make a nice motor that barely gets hot, somebody else will make a motor with the same performance that is a fraction of the size and costs a fraction of the price. Then you won't make that mistake again because you won't be in business. That's how it works - and we have been making electric motors for well over a century now, so there is a lot of knowledge and experience out there.

Generally it's the "magnet wire" used to wind the rotor that is the limitation. Again, there is a tradeoff between how hot the wire can be and its cost (the varnish and ease of termination). In the end, the sweet spot is typically wire rated in the 200-220C region.

The pain threshold for most people is in the region of 50C or so. So when you say "really hot", that could mean anything from 50C to the best part of 150C. Engineers generally like to put numbers on things - does it boil droplets of water?

I can't imagine a Corsa window winder cutting much metal. Mind you, I'm also wondering what a "printed" hacksaw is. Any chance of a picture or a link to something that explains what you mean?

Murray

Haha - 100mm diameter? The blade will become blunt and polished in no time if you don't take a sensible depth of cut (= power). Take a cue from similar machines and fit a motor with some hundreds of watts rather than 10s.

Edited By Muzzer on 16/04/2017 19:57:43

Murray,

I can't post a picture yet because I'm busy desinging/drawing it. In a few days maybe I'll post some drawings.
When I say really hot, I mean I can hold my hand on it for just a few seconds.

Chris

Thats nae proper hot. 

Edited By George Clarihew on 16/04/2017 20:32:10

A class B motor is rated to run with the windings at 130 centigrade.

Neil.

I use a wiper motor for a mill table x-axis feed. It's for VW Golf, made by FEBI. Cost about the same as yours.

I think they do run hot. Off load @12.8V the casing on mine tops out at some 53oC according to my IR Thermometer. after some 30 mins. or so.

Does not get so hot in service despite being loaded as it's PWM

(Sorry, can't see any way of doing superscript for 'degree' ).

D.

This one, I think.

http://www.mister-auto.co.uk/en/wiper-motor/febi-bilstein-17092_g295_a10117092.html

Edited By Dave Daniels on 16/04/2017 21:39:26

It might be worth having a quick read of the Lathes.co.uk article on the Kennedy power hacksaw

Tony mentions that the smaller one was fitted with a "Feeble" 1/6 hp motor and the larger one with 1/4 hp motor, so I'd suggest that is probably what you should be aiming for.

The internet suggests a 20A fuse for a Corsa wiper motor, so let's say it runs at 12 to 15A under load, i.e. 144 to 180 watts @ 12v

Now at 750 watt to the hp, it looks like the wiper motor is in the region of 0.19 to 0.24 hp input, so maybe in the right ball park in theory. I have to say that this seems rather on the high side for something that size, and the one that Dave mentioned in the post above claims to have a rated capacity of only 40w

Are you able to measure the current under load?

I may of corsa be a talking a complete balderdash, but I'm sure I'll be corrected.

Edited By peak4 on 16/04/2017 22:08:38

Strangly enough I measured (at 12V) 0.5A free running. Seems a rather low value.

I do have a 12v 1/4hp motor kicking around somewhere off an old lawnmower, but from what I remember it's a lot larger than a wiper motor, and is also fan cooled.

Have you tried running it in the opposite direction as a wiper motor may have the brush position and mechanical parts Optimised to run in one direction.

Mike

Yep, did that to, same result.

btw, there are two leads comming from that wiper motor, white and yellow, what is the correct procedure.
In electronics red=+ and black=- but with cars I do not know.

I cannot imaging a car windscreen wiper motor being capable (eventually) cutting through a 100mm diameter metal bar but there is something not quite right about it getting very hot when it is unloaded. 12Volts as 0.5A is only about 6 Watt so its not a very powerful heater.

Do the yellow and white wires go direct to the brushes?

Most wiper motors I have seen incorporate internal switches as part of the self-parking mechanism. Also is it a two speed motor?

Ian P

Same with cars, except Vauxhall battery connections which are black + red -.

Don't ask how I know, but it's not good watching think copper jump leads get so hot they start to droop...

Neil