# dc rpm

Discussion in 'General Electronics Chat' started by bladerunner, Nov 18, 2012.

Apr 15, 2012
83
4
SOme questions about rpm and Torque(I think?)

Since in most AC and DC are similar in current needs, is the torque equal.
I may have the wrong term, but will a DC motor of the same amperage pull the same apparatus as an AC motor will? If not, how can I determine an equal sized DC motor?

If I have an DC motor that runs 3000 RPM at 7 amps; I turn the power down to the point where the motor is running at 1750 RPM ; Will it take less amps to run or simply use the same at a lower RPM.

If you could please include any formula's so I may work my way thru this. I understand Ohm's Law to a point, but not sure if it applies here and if it does, then where does everything fit to the formula?

Many thanks for any info here.

When events look their worst, there is always a light at the end!

2. ### SPQR Member

Nov 4, 2011
379
49
HERE's a nice little paper on torque calculations for DC and AC motors.

It seems that the generated torque is a function of maximum horsepower rating and RPM.

Lower down on the page is a section on selecting AC and DC motors that might directly answer your question.

There are tons of formulas on the page.

3. ### crutschow Expert

Mar 14, 2008
20,067
5,652
If the motors have similar efficiency then they will use similar power with the same load torque and RPM. Starting torque is a different story as that's highly dependent on the type of motor.

Apr 15, 2012
83
4
OK, by the formulas:

I have a 1/3 Hp 115VAC, 1750RPM (base Rate) , rated at 7A and should produce a torgue of 1 Lb-ft.

If I use a 1/2 HP 24VDC, 3000 RPM, rated at 7A and its torque is .87 lb-ft.,
Then I only need to slow it down to 1500 RPM to reach the 1.0 lb-ft torque.

Would this DC motor use less current (A) by reducing its speed to the 1500 RPM?

5. ### BillB3857 AAC Fanatic!

Feb 28, 2009
2,456
372
In a DC motor, voltage is speed (rpm) and current is torque.