# Reconciling DC motor volt, amp, watts

#### mvoltin

Joined Dec 18, 2015
4
I always found DC motor ratings to be very straightforward (in terms of A V W relationship) but currently on the market for the geared DC motors from Groschopp and totally lost:

For example: PM6015 motor shows 24VDC, 13.25amps and 110 watts. The manufacturer website lists HP for this at 0.08.

So, If it was 13.25 amp motor at 24 volts, it should be 318 watt. Also, it should be 38 horsepower at 90% efficiency. I just cannot reconcile any of these numbers.

They also have another motor (PM6025) that has same V and A but higher (178) watts. So, somehow they are rating these motors differently?

#### BobTPH

Joined Jun 5, 2013
3,006
The 13.25 A is probably stall current, which is a lot higher than running current.

Bob

#### wayneh

Joined Sep 9, 2010
17,059
... it should be 38 horsepower at 90% efficiency.
0.38

The cited 0.08HP is 60 watts. That is a little hard to reconcile with usage of 110W but the devil is in the details of exactly how each rating is defined and measured. Loaded versus unloaded, stall, at peak, etc.

Last edited:

Joined Jul 18, 2013
22,672
Some of the figures quote V & A RATINGS, IOW this is the maximum that the motor should be operated at.
Generally motors have two maximum ratings, one is for a continuous rating and one for peak operating point, the latter should only be operated for a very short period, otherwise the motor can be damaged.
The maximum continous torque current value for e.g. is around zero RPM.
Max.

#### Papabravo

Joined Feb 24, 2006
15,518
Be sure brain is in gear before engaging clutch to fingers.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,565
I would list nominal voltage and max current. In reality, when the motor first starts, the winding resistance prevails which is easier to understand with an AC motor.

The number of poles probably has a lot to do with the discrepancies with brushed DC motors.

AC motors typically have an LRA value or Locked-Rotor-Amps.

#### Tonyr1084

Joined Sep 24, 2015
5,820
I thought 745.4 watts equaled one horse power.

Joined Jul 18, 2013
22,672
when the motor first starts, the winding resistance prevails which is easier to understand with an AC motor.
Why is that?
With a DC brushed motor the current is maximum at zero RPM and is controlled purely by the resistance of the winding.
This is why maximum torque occurs at zero RPM.
Max.

#### mvoltin

Joined Dec 18, 2015
4
Thank you all! it seems more complicated that my supra-tentorial space can handle. I will just order one and see if it works for my application.

P.S. I called the company (super nice people) and they seem to be making these custom for every client - thus, their website data vs. what is actually out there don't match. The 13.25 seems to be a stalling current and he quoted me around 3amps for this particular items (special order) that still doesn't reconcile with the 110amps.

Interestingly, this item with 110 watts has higher torque than another with 177 watts even though they use the same gear reduction. Again, they must be measured differently...