# HF 8x14 DC Motor Conversion



## briankenyn (Dec 4, 2010)

Hi all,

I have had the HF 8x14 lathe for about 2 1/2 years and its a great machine. Recently I have been having motor problems which could be the motor itself, the connectors and terminal strip, or both. I'll probably have to change the terminal strip and crimp on new terminals to identify the problem. But since I change spindle speeds fairly frequently and use the lathe for threading, I am thinking about converting to a DC Treadway Motor, but am uncertain about the best choice of motors, in terms of voltage, HP, frame, etc for the conversion. Has anyone had any experience with a similar conversion and recommendations?


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## John Rudd (Dec 5, 2010)

Brian,

Steve Bedair did a dc motor conversion of his 9*20, google steve's site for details..

I converted my 9*20 to a vfd rather than messing trying to obtain a suitable motor, best wad of money I spent ( around £150 not sure where you are geographically..)


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## Noitoen (Dec 5, 2010)

I would think that VFD with a 3 phase motor is a better choice. The motor is cheaper and no fuss with brushes and the speed control is much better under load.


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## Kermit (Dec 5, 2010)

If you care to take the risk of opening the motor. a cleaning and brush replacement might be all that's needed.

may be best to do this to an old motor AFTER you have installed the new motor in the lathe.


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## briankenyn (Dec 5, 2010)

Thanks to all,

I am not familiar with the term VFD, but I suppose that it is some from fractional wave control to be used with a standard AC motor. Am I close?


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## RonGinger (Dec 5, 2010)

I have the HF 8x14 lathe, which I converted to CNC. I think it is a nice machine. I changed the motor to a 1/3hp 3 phase motor I found in a surplus shop for $50, then bought a VFD for under $100 for it.

I had to make a new mounting plate to fit it.

There are photos of my CNC conversion at http://plsntcov.8m.com/CNClathe/CNClathe1.html


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## AllThumbs (Dec 5, 2010)

I have the same lathe and also put a VFD and 3 phase 1/2 HP motor on it. I love it. The DC motor conversion is not as good. No torque at low speeds and poor speed control at all speeds (slows down a lot under load compared to a AC motor and VFD)

VFD stands for Variable Frequency Drive

The other cool thing about a VFD is you can over speed the motor. This allows a low gear ratio and still get decent spindle speeds. I run my 1750 RPM motor up to 3500 RPM with the VFD no problems so far.

I have done this conversion to both my Milling machine and Lathe.

Eric


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## AllThumbs (Dec 5, 2010)

[ame]http://www.youtube.com/watch?v=gPA6kHWjUlw[/ame]


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## AllThumbs (Dec 5, 2010)

Pat J  said:
			
		

> I have seen a lot of large 3-phase VFD's on pump motors, used for energy savings.
> These were 3-phase input, 3-phase output arrangements.
> 
> Can you put a 3-phase motor on a lathe, and add a VFD that has a 220V single-phase input and a 3-phase output? Is there such an animal?
> ...



Almost all VFD's accept both single and 3 phase input, and all are 3 phase output. If you connect a 3 phase (input) VFD to single phase, you need to derate it by 30%. In other words, a 3 HP VFD can only run a 2 HP motor if it's connected to single phase input.


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## Stan (Dec 5, 2010)

> The DC motor conversion is not as good. No torque at low speeds and poor speed control at all speeds (slows down a lot under load compared to a AC motor and VFD)



A DC motor is a constant torque motor at all speeds. Like all motors, HP drops with speed unless you overcurrent the motor, whether AC or DC. The purpose of a DC motor controller is to supply constant current at all voltage levels so that speed is controlled while torque is held constant.

I have been using a matched pair (motor and control) of industrial quality for more than fifteen years on two different Logan lathes. The only advantage of the VFD system is cost and I believe even that is marginal if you get the same features such as speed and torque feedback and dynamic braking.


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## briankenyn (Dec 5, 2010)

Many thanks to all,

The VFD is far and away from what I was thinking, and a three phase solution was the furthest thing from my mind. On further inspection, it seems that the terminal block burnt up, opening up the circuit. 
I am sold on the VFD approach, particularly since it seems to deal with so many of the problems that normally crop up with the DC motor. I think I'll start hunting for the motor and VFD for Christmas!

Many thanks again.


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## AllThumbs (Dec 5, 2010)

Stan  said:
			
		

> A DC motor is a constant torque motor at all speeds. Like all motors, HP drops with speed unless you overcurrent the motor, whether AC or DC. The purpose of a DC motor controller is to supply constant current at all voltage levels so that speed is controlled while torque is held constant.
> 
> I have been using a matched pair (motor and control) of industrial quality for more than fifteen years on two different Logan lathes. The only advantage of the VFD system is cost and I believe even that is marginal if you get the same features such as speed and torque feedback and dynamic braking.



All I am saying is an AC motor will have better speed regulation than a DC motor, assuming neither has feedback to the drive.


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## AllThumbs (Dec 5, 2010)

A DC motor will produce much more torque at low speed than an AC motor. In order to do it at a predictable speed you need feedback from the motor to the drive. Few low end DC drives or motors have provisions for this. A DC motor with feedback and a matching drive would be the cats meow for a spindle drive.

IMHO the best low cost compromise is a 3 phase AC motor on a VFD with no feedback. Set the lathe motor belts (or gearbox) on a "medium" speed gear ratio, then set the VFD to overspeed the motor to 120 Hz. This will permit spindle speeds from zero to double the selected belt gear ratio. In other words if the belt ratio you picked is a spindle speed of 1500 RPM, the VFD will allow for 0-3000 rpm spindle speed without changing belts. You still have the option to change belt or gearbox ratio if you need very slow spindle speeds with high torque.

As a bonus, reversing the spindle just required a push of a button on the drive.

Some things you need to consider are:

1) You are over speeding the motor. I have not had any trouble with this with standard (non VFD rated) 3 phase motors but you do so at your own risk. Most VFD's will run your motor up to 400 hz (over 6 times rated name plate speed) but I would not dare exceed 120 hz (double rated speed). It is possible to buy VFD rated 3 phase motors but they are expensive. These are usually rated for higher RPM and will run cooler at low RPM as well. 

2) One of the issues with non VFD rated motors is the fan at the back can't cool the motor very well when turning slowly. In a home shop this may or may not be a big deal. If you run a very long and heavy job at low spindle speeds you may want to change gears on the lathe to allow the motor to run faster and cooler.

3) It's possible to over speed the spindle. If you select a spindle gear ratio for max speed, then over speed the motor, you will over speed the spindle. Your spindle bearings may not like this.

The above are just some things I have found to work well. Your mileage may vary.


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## Stan (Dec 5, 2010)

> A DC motor will produce much more torque at low speed than an AC motor. In order to do it at a predictable speed you need feedback from the motor to the drive. Few low end DC drives or motors have provisions for this. A DC motor with feedback and a matching drive would be the cats meow for a spindle drive.



Without getting into a lot f technical details, a DC motor provides it's own feedback by the back EMF from the armature. This is picked up by a third stator winding and fed to the controller. Industrial type DC drives bear no resemblance to the treadmill motor or similar. They are shunt wound with the field drawing about one amp so that is all the current you have to switch to reverse the motor. The smallest of the two or three printed circuit boards is bigger than the entire cheap control.
That is why I said to get that feature on an AC drive would eliminate some of the price differential on an industrial type CD drive. The dynamic brake feature on the DC motor is is another feature that is superior to a foot operated mechanical brake on a lathe.

Having said all that, unless you can source an industrial DC motor and control at low cost, a cheap VFD and cheap used old style three phase motor is a good compromise.


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## pipcount (Jan 7, 2014)

Fellows, my first post for help.  I have a lathemaster version of the 8x14 lathe, and have been very happy so far, a great little lathe.

Recently I have experienced motor issues and wish to go the VFD/3 phase path.  I picked up two industrial VFD off Craigslist (list $800-$900!) for $150 each, NEMA rated for exterior installation, heavy duty, seem like GREAT units.  If anyone would like one, let me know.

Where I have problems so far: the 3/4 HP motor I have is simply too large to fit behind my lathe.  I was wondering if anyone could send me the model number of the motor they used that fit.  Ideally I would like a "drop in"... I enjoying working with the lathe, not on the lathe


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