Dc Motor Vs Ac?

great white

great white

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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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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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101_0578.jpg 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
 
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.
 
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.
 
Hmmm, I've been thinking of trying the 1.5 HP continuous duty treadmill motor I have sitting on the shelf....
 
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.

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.
Click to expand...
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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