# How does rated horsepower compare with input power?

If I look at the motor specs, it gives a rated HP:

1 HP @ 2740 RPM & 42.6 Amp, 1.91 ft-lb.

My end goal is to get torque as a function of time, but I cannot remove it from the machine. I can only record things such as current, voltage, etc. vs time as I give various inputs. I also have a plot of Amps vs torque and rpm vs torque for the motor from the manufacturer. However, if I use the rated values, the chart doesn't match up. If I measure input power, using current clamp and voltmeter, and log these over time, I'm wondering how that value would compare to the output power of the motor over time? I know input will be more than output because of numerous losses, but I was wondering why the chart and rated power values don't coincide.

• What kind of motor is it? Is there an electronic speed control? – Charles Cowie Feb 3 '17 at 16:07
• The motor is a PM DC motor. – mrkevelev Feb 3 '17 at 17:03
• I think torque is linearly related to current over a wide range. If you measure current vs time, and use the torque constant to convert to torque, this will give you an approximation of torque. – mkeith Feb 3 '17 at 17:46
• What is the voltage for rated Hp? What is the voltage for the plot? If they are different, the rating specs will not match the plot. – Charles Cowie Feb 3 '17 at 17:59
• Voltage for rated HP is not listed. Voltage of the plot is 30 V. – mrkevelev Feb 6 '17 at 13:20

## 1 Answer

Without being able to see the charts you're looking at, it's impossible to give anything other than a very generic answer.

The ratio of output power to input power is called the efficiency of the motor. The efficiency of any given motor varies widely over its operating range, and different types of motors have very different characteristics. Unless you match all of the conditions specified in the charts exactly — including the measurement methods they used — you won't get the same numbers.

• What I mean is this: Lets say I take the rated power given, which also comes with an rpm, amperage, and torque value, and I plug in those values. Lets say I take 1.91 ft-lb of torque given. I look on the chart with an x-value of 1.91, go up and find the lines, and see that the RPM is 2250, and the current is 30 Amp. These values are not what are given for the rated power. Why is that? – mrkevelev Feb 3 '17 at 17:00
• Like I said, unless you show us the charts, it's impossible to say. But in general, the peak torque, the peak current, and the peak horsepower will not all occur at the same speed. – Dave Tweed Feb 3 '17 at 17:10
• The amps vs torque and rpm vs torque are both linear. I can give to values which can form those lines. – mrkevelev Feb 6 '17 at 13:22
• Torque is on the x-axis and current and speed are on opposing y-axes. Torque 1 is 0 ft-lb, which has current at 5 A and speed at 2600 RPM, torque 2 is 8 ft-lb which has current at 105 A and speed at 1400 RPM. The plot says everything is at 30 V. – mrkevelev Feb 6 '17 at 13:30