View Full Version : Repowering an M head...

01-18-2009, 09:39 AM
Hey guys!
I've been searching for a new motor for my M head.
I'm a little disappointed...
i can't find a 1200 rpm C face motor.
I can't find a low profile pancake style motor like the original (I just like the lower motor better...)
A 1750 rpm motor is going to be waay too fast and there isn't enough room to do meaningfull pulley changes.
So...not a big deal...buy a 3ph motor...and buy a VFD. Still not a pancake motor but it will work. But then...I found this...it'll work out cheaper than a 3ph with VFD and has a lot of cool features.
What do you lectrical geniuses think of this idea?

01-18-2009, 10:06 AM
Russ, if you have an M-Head without a motor, I'd highly recommend doing a vari-speed conversion on it -- you'll almost never have to change belts.

So get a 6-pole, 1125 RPM, C-face motor with 50 - 100% more horsepower than the stock motor, and put a single sheave on the drive side that mates with the middle RPM, so when you over-clock or underclock the VFD, you'll get the full range of spindle speeds.

This approach also has the huge advantage that you don't have to extend the motor shaft (since you don't need the entire cone sheave). The stock Bridgeport motors are custom pancake motors with extra long shafts.

01-18-2009, 10:27 AM
Robert..that motor I provided the link to is a variable speed AC motor. After we where discussing the possibility of upping the rpm on that M spindle...I've wanted to do something like you suggest. This motor here (the link)is already "overclocked"...@2400 rpm....twice the R's of the original. Like you said...a single pulley...
Why is this motor not a good idea? there's just no way I can buy a new 3ph motor and VFD for anywhere close to that price.
and this one is just plug and play!!!

01-18-2009, 10:45 AM
Robert..that motor I provided the link to is a variable speed AC motor.

It's Plug-N-Play, but you're paying for it: $330.

But the bigger issue is that when you vary the RPM of an induction motor with a VFD, the power is proportional to the V/Hz ratio. So if you take that 1/2 HP, 1125 RPM motor that you linked, and run it at half RPM, you're getting 1/4 HP at 562 RPM, 1/8 HP at 281 RPM, ...

That's why people usually double the horsepower of the motor on a VFD varispeed conversion.

You can get a good quality 1 HP motor for around $75 - $150, and the 1 HP VFD will run you about $150, so you'd still end up ahead of the SurplusCenter motor.

01-18-2009, 10:49 AM
Robert...YOU can get that stuff that cheap...I can't. I pay almost double for all that stuff.
BTW..the motor I linked is 250/2400rpm.
I'll have to check and see if the face on a 1hp motor will adapt into the BP plate.

Just Bob Again
01-18-2009, 10:51 AM
there's just no way I can buy a new 3ph motor and VFD for anywhere close to that price.

A 1-horse VFD is $200. A 1-horse 3-phase motor is $100. That's $300 total. A 1/2 horse VFD is $150 and a 1/2 horse 3-phase motor is about $75. Total $225. The surpluscenter motor is $330 and it's only 1/2 HP and the controller cannot be separated. You really want to reach on top in the back every time you want to change something? Plug and play? A line cord and 6 feet of wire is all it takes.

01-18-2009, 10:51 AM
Robert..I have a near new 1hp 3ph motor here(not a c face tho).
It is HUGE compared to the lil pancake on the M head now.
OK..but what you are saying then..the motor I'm looking at won't have the power down low...and it prolly won't run as smooth...correctumundo?

01-18-2009, 10:52 AM
BTW..the motor I linked is 250/2400rpm.

I'm guessing that's a stock 6-pole, 1200 RPM motor, and they're quoting the VFD overspeed/underspeed limits.

Edit: Russ, I just sent you a link for a brand-new Baldor 1 HP motor for $100 with free shipping.

01-18-2009, 11:51 AM
Dont forget the extra cost of the remote that motor will need also some means of quick stop for those times....?

Personally I would look to the 2 separate items as I do not like all the eggs in the one basket!


01-18-2009, 12:20 PM
the controller cannot be separated. You really want to reach on top in the back every time you want to change something
That's exactly how the original motor is set up...the forward reverse/stop switch is at the top on the side of the motor.

01-18-2009, 12:23 PM
And don't forget guys...VERY few US Ebay sellers will ship to Canada.
I've been thru that a hundred times. Believe me..
And UPS...they will make your good deal a nightmare...

01-18-2009, 12:38 PM
I agree with the others to avoid that motor. The M head originally was underpowered, or at least, marginal. A 1/2HP with VFD will be even worse.

I would consider a 1HP as a minimum. The C flange is available on many different motors, it is the Frame size that determines the physical size of the motor. Here is a 56C 1HP motor that is the same size and weight as the 1/2HP.


01-18-2009, 01:10 PM
Ok...That's what I really wanted to know.
The extra few bucks isn't the issue.
I was hoping I could just buy that motor and bolt it on and go.
I REALLY don't want to get into another VFD manual that's going to take me days to figure out.
THAT is money down the drain...while i'm sitting on my butt trying to figure this stuff out...I'm losing $75 an hour.
I'll phone Surplus on Monday about the 1hp. I just bought a DC motor from them for my positioner.
They are really great people to deal with!
And they ship USPS!!!