# Motor power and current relationship

### Calculations | Simple Electric Motors

In a DC motor, the output torque is directly proportional to the current through the windings, and is given as: For most DC motor cases, we can assume the flux, φ, is constant, making torque directly proportional to the current: When examining the torque-current curve for a DC. The relationship between a motor's electrical characteristics and .. Voltage and current are the essential components of power a.k.a. the ability. Thus, a horsepower (hp) motor has an acceptable load range of. 5 to 10 hp . Figure 3 Relationships Between Power, Current, Power Factor and Motor Load.

Motor torque changes with the speed. At no load you have maximum speed and zero torque. Load adds mechanical resistance. The motor starts to consume more current to overcome this resistance and the speed decreases. If you increase the load at some point motor stops this is called stall.

When it occurs the torque is at maximum and it is called stall torque. While it is hard to measure stall torque without special tools you can find this value by plotting speed-torque graph. You need to take at least two measurements with different loads to find the stall torque.

## Calculations

How accurate is the torque calculation? While voltage, current and speed could be accurately measured, efficiency of the motor may not be correct. It depends on the accuracy of your assembly, sensor position, friction, alignment of the motor and generator axles etc. If you want to get meaningful numbers you might use a second generator kit as explained in Torque and Efficiency Calculation section.

## DC motor electrical and mechanical power relationship

Speed, torque, power and efficiency of the motors are not constant values. Usually the manufacturer provides the following data in a table like this one sample data from one of the motors used in generator kit: Also the manufacturers usually provide power curves for the motor at nominal voltage: These curves are generated by plotting motor speed, consumed current, and efficiency as functions of the motor torque.

So all things considered, how does the motor voltage stack up? For the same motor, ideally if you apply double the voltage you'll double the no-load speed, double the torque, and quadruple the power. This is assuming of course the DC motor doesn't burn up, reach a state which violates this simplistic ideal motor model, etc.

### DC motor electrical and mechanical power relationship | Physics Forums

However, between different motors it's impossible to tell how two motors will perform compared to each other based only on the voltage rating. So what do you need to compare two different motors? Ideally you'd want to know the voltage rating and stall current so you can design your electronics appropriately and you'd want to know the no-load speed and stall torque so you can calculate the mechanical performance of your motor.

You may also want to see the current rating of the motor some motors can be damaged if you stall them for too long! This analysis also somewhat neglects the efficiency aspect of the motor. This would cause the power calculations using the two equations to be equal i. However, real motors aren't perfectly efficient.

Some are close, some aren't.