What happens if...

[Dieselman 63]

... I fit a motor to a machine that has much more hp than the original but the same rpm?

I'm tossing up between vfd and motor swap for my grinder but I can't find any motors 3/4hp which run at 2850rpm. I've thought about 'swapping' the sizes of pulley and using a 1425rpm motor (much easier to find) but I've been wondering what would the outcome be if I fitted a 2hp motor working at 2850rpm?

Can anyone enlighten me?

 

[Dieselman 63]

On your grinder it'll be fine.

The only 'danger' is if the machine is overloaded - instead of stalling the motor the extra power could cause damage (say on a lathe, a big cut that'd normally stall the motor could strip out the gearbox instead).

So, unless you stand on something on the wheel......


Edit: if you swapped to a 1425 motor you'd need at least double the power to equal the original in use.

Thanks for that. That is what I thought, seeing as it's belt drive any serious overloading would surely slip the vbelt. Of course not having much electrical experience it always pays to confirm your theory with others!

 

[rtbcomp]

...

Edit: if you swapped to a 1425 motor you'd need at least double the power to equal the original in use.

Don't think so, if a machine requires x Kw it doesn't matter what rpm the motor runs at, you just use gearing to attain the correct rpm (& torque) for the machine.

Power = rpm x torque, so a 2hp motor running at 1400 rpm will develop twice the torque of a 2hp motor running at 2800 rpm.

 

[Maker]

Of course not having much electrical experience it always pays to confirm your theory with others!

Is it the electrics you're worried about? As long as the switch you're using is rated high enough (It almost certainly will be) you shouldn't have a problem.

 

[Dieselman 63]

Is it the electrics you're worried about? As long as the switch you're using is rated high enough (It almost certainly will be) you shouldn't have a problem.

To be honest it's all a big learning curve as of about two years ago - the living generation of my family don't do anything like this, though there is a history of it in people who have sadly passed away.

It's not so much the electrics, it's just a worry about whether it could cause damage to the machine (in this case the spindle) if I used a motor with more grunt but as @pdg has already said, with there being no gearbox it shouldn't be an issue. I'd really prefer a 1hp motor, 3/4 is just lacking a bit for 10" wheels I feel.

 

[RichardM]

check the physical size and the motor mounts as well.

 

[Dieselman 63]

@Maker did remind me of something...

What type of switch is fitted?

No switch, but I'll sort all this out afterwards. I've got spare switchgear.

 

[Shox Dr]

Motors fitted to grinders surface/tool/die are usually very well balanced, So are princely price.

I priced up a 3/4hp motor for my Ward Tool grinder, and it was far cheaper to get it changed from star to delta (or tuther way) and fit a vfd, than to buy a 1ph motor.

Also possible to get rid of any harmonics, by fine tuning the motor speed

 

[Dieselman 63]

Motors fitted to grinders surface/tool/die are usually very well balanced, So are princely price.

I priced up a 3/4hp motor for my Ward Tool grinder, and it was far cheaper to get it changed from star to delta (or tuther way) and fit a vfd, than to buy a 1ph motor.

Also possible to get rid of any harmonics, by fine tuning the motor speed

Did you change it yourself or get someone to do it for you?

 

[Shox Dr]

Did you change it yourself or get someone to do it for you?

Work of the devil, paid someone else.

 

[Pete.]

... I fit a motor to a machine that has much more hp than the original but the same rpm?

I'm tossing up between vfd and motor swap for my grinder but I can't find any motors 3/4hp which run at 2850rpm. I've thought about 'swapping' the sizes of pulley and using a 1425rpm motor (much easier to find) but I've been wondering what would the outcome be if I fitted a 2hp motor working at 2850rpm?

Can anyone enlighten me?

If you fit a 1425 motor and use pulleys to bring the spindle speed up to where the 2800 motor had it then you'll notice no difference except the extra power will prevent it from slowing down when loaded. Take care not to go beyond the original rpm though.

If you just fit the slower motor without using pulleys to adjust the speed of the spindle back up you'll change the characteristics of the grinding wheel. It will behave like a 'softer' compound wheel to what it is.

 

[Dieselman 63]

So internally it looked like this

But a clean up has it looking much better

Now to decide whether to dive into the windings...

 

[Seadog]

It's not too clear, but looking at the left-most piece of insulation, just below the black wires, it seems like there may be three wires running into it. If so, that's your star point.

 

[Pete.]

Be aware that if you cut those binders to find the star point you have to make sure that you re-tie the wires back down securely after you've finished. The coil wires will move and rub in operation if you don't until it wears through the varnish insulation and causes a short. It's very important.

 

[Dieselman 63]

Okay, I know you're probably hoping I do try...

...the string is going to be cut, I will begin to enter and see what I can find

 

[Dieselman 63]

Be aware that if you cut those binders to find the star point you have to make sure that you re-tie the wires back down securely after you've finished. The coil wires will move and rub in operation if you don't until it wears through the varnish insulation and causes a short. It's very important.

Thanks, I have read that elsewhere. Is it normal string or is it something special which I'll need to buy?

 

[Pete.]

When I did some time winding prototype motors it was some kind of waxy coated tape, though I have used normal heavy string since. If you use normal string I would suggest putting something like thin mylar under it where it pulls across the windings so that it doesn't abrade through the insulating varnish.

 

[brightspark]

normal string and varnish it

 

[Dieselman 63]

I have successfully found the star point.

What do I do now? Will I need to buy a connection box to take the six wires so that I can add linkages so that it is in delta?

 

[Seadog]

You could extend the wires outside and use a connector strip. Solder wires on and use heatshrink, if you have it. Make sure you identify the pairs of wires and then wire the star end of one to the start of another. The sequence of connections is not important but you must maintain the 'polarity.'