# Dc Motor Vs Ac?



## great white (Apr 5, 2015)

Is there any real functional difference between a 3/4 hp DC motor and a 3/4 HP AC motor?

I know the arguments for and against each (speed control, cost, vfd, maintenance, etc).

What I am wondering is if a lathe powered by a 3/4 hp DC motor will function any differently than one powered by a 3/4 HP AC motor.

For the sake of argument, lets say the DC motor is controlled by the latest up to date speed controller and the AC motor by the latest VFD.

Reason I ask is I've been reading conflicting info on DC motors being harder on lathes (IE: higher startup torque) than a comparable AC motor.

I'm going DC on my Atlas TH42 (probably with a Baldor 3440 3/4 HP run by my Cycletrol 150), I just don't want to be unpleasantly surprised by "breakage".

I'm thinking it shouldn't be a problem since the Atlas is factory rated (max, IIRC) for a 1/2 HP AC motor.

Besides the fact that I have somewhat of a preference for DC, the controller was inexpensive (40 bucks for the cycletrol and I have another in the drawer for spares) and the Baldor motor looks like it will be around 100 bucks plus 50 odd dollars to ship it.

So, around 200-250 bucks and I'll have a variable speed 3/4 HP drive with Jog/Stop/Brake/Rev vice my current single phase 1/8 HP AC drive.

So, any difference in 3/4 HP DC or 3/4 HP AC I should be concerned about?


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## JimDawson (Apr 5, 2015)

I don't see a problem with either motor.  A modern controller should give you a soft-start in either case so there won't be an instantaneous spike in torque.  Much more gentle than just turning on a switch.


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## Kernbigo (Apr 5, 2015)

i had a 1 hp dc on my atlas 10" for years worked fine. Now i have a south bend 9" with a 1 hp dc on it also works great


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## Kernbigo (Apr 5, 2015)




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## great white (Apr 5, 2015)

Kernbigo said:


> View attachment 98983
> View attachment 98983
> View attachment 98983


Is that a treadmill motor?


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## stevez (Apr 5, 2015)

This is intended to  be a respectful response - I have no idea what you know or don't know. I'm not an expert but have some information that might be of use.

HP is the product of torque and RPM. Two 3/4 hp motors will produce the same amount of torque at full load if the full load RPM of each motor is the same - and important point is "full load" . If the RPMs are different you have to account for that difference unless you made pulley or gear changes to address that.

How each motor behaves as it's speed is reduced is dependent on the motor and the means to reduce the speed. For some motor/speed controller combinations the torque drops off rapidly - in others it does not drop off so rapidly. As a mechanical engineer I have to be careful with fans and pumps that the motor torque equals or exceeds the load as the RPM is reduced. I'd like to think that for most conditions the 3/4 hp motor would outperform your 1/8 HP motor.

I have a nice 1/2 hp Baldor DC motor that I am tempted to put on my Craftsman/Atlas lathe for the same reasons.


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## Kernbigo (Apr 5, 2015)

yes it is i used the same thing on my 10" atlas, except for the speed control, i used the thread mill board, both worked good.


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## great white (Apr 5, 2015)

Hmmm, I've been thinking of trying the 1.5 HP continuous duty treadmill motor I have sitting on the shelf....


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## great white (Apr 5, 2015)

stevez said:


> This is intended to  be a respectful response - I have no idea what you know or don't know. I'm not an expert but have some information that might be of use.
> 
> HP is the product of torque and RPM. Two 3/4 hp motors will produce the same amount of torque at full load if the full load RPM of each motor is the same - and important point is "full load" . If the RPMs are different you have to account for that difference unless you made pulley or gear changes to address that.
> 
> ...


Not taken as disrespectful at all.

I was assuming 1750 rpm for an ac or dc motor in order to eliminate that variable. I should have specified.


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## Kernbigo (Apr 5, 2015)

here is the wiring
	

		
			
		

		
	



	

		
			
		

		
	
 for a mc60 board


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## great white (Apr 5, 2015)

I'm not understanding why you have a 120 feed into the DC motor in the diagram?

Nor do I understand why the cb is in the return line instead of the "hot" line.

I have some knowledge of wiring, but not so much house services....I'm all aircraft and automotive wiring knowledge.



Edit- oh wait. Black is the hot and white is the neutral in house wiring, isn't it? That would make more sense with the diagram...


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## John Hasler (Apr 5, 2015)

great white said:


> I'm not understanding why you have a 120 feed into the DC motor in the diagram?
> 
> Nor do I understand why the cb is in the return line instead of the "hot" line.
> 
> ...


The AC is probably routed through a thermal overload in the motor.


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## Kernbigo (Apr 5, 2015)

use the drawing where it shows the mc 60 contol, the second one it makes more cents , and put the motor stop and start switch in the pot middle wire the (wiper) that way you don't have to change the pot setting you can start the speed setting where you set it from before


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## great white (Apr 5, 2015)

Kernbigo said:


> use the drawing where it shows the mc 60 contol, the second one it makes more cents , and put the motor stop and start switch in the pot middle wire the (wiper) that way you don't have to change the pot setting you can start the speed setting where you set it from before


No worries, I'm going to be using a cycletrol 150. Different beast than a treadmill controller.


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## Kernbigo (Apr 6, 2015)

I'am using a cycletrol 2000 now but the mc60 worked just as well


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## British Steel (Apr 6, 2015)

I'm going to chip in here in the AC/DC/variable speed discussion...

If you're running either at low speeds and heavy loads, add an external fan to cool the motor, at low RPM the motor's fan won't push much cooling air so it's easy to overheat the motor to the point where the insulation starts breaking down, result: loss of magic smoke, dead motor which shorts and blows the controller...

I worked at an importer of Chinese, DC-motor powered lathes and mills, the most common warranty claim was exactly that scenario - the Chinese don't bother putting thermal overload switches in their motors, they cost money... The importer would refuse warranty claims if he could get away with it, calling it "incorrect operation" although there were no warnings about it in the appalling "handbook".

The larger the fan the better, although even a PC cooling fan (5"?) is better than nothing - I have a Papst 10" equipment fan on my lathe's (dustbin sized) motor that cuts in below 30Hz on the VFD using one of its normally-closed relay outputs ("above supervisory frequency" is the menu choice). plus a 125 degreeC thermal switch in the VFD fwd/rev control circuit, no problems so far, even down to 5Hz, and the noise of the fan running reminds me to power everything off when I'm finished!

Dave H.
(the other one)


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## Kernbigo (Apr 6, 2015)

yes i agree make sure your dc motor has a fan on the end of it


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## great white (Apr 6, 2015)

Went with a Baldor CDP3440 TEFC, class F insulation. Got a good price, couldn't pass it up.


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## great white (Apr 6, 2015)

Kernbigo said:


> I'am using a cycletrol 2000 now but the mc60 worked just as well


Interesting.

I have second cycletrol 150.

I am back and forth on whether or not to use it on my small drill press when I install the treadmill motor in place of the single phase ac motor it currently has.

I was planning to use the treadmill controller due to its smaller size (better suited to my small bench top drill press).


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