# Motor is consuming less than rated current and more than rated power

I have a brushless DC motor of following specs.

I am using a roboteq controller to operated the motor. While running the motor i have collected the few params and here are the plots.

power curve - expressed in watt. Current curve - expressed in amps.

Its running at 42 rad/sec (400 rpm). But i am not sure why the power is so high and current lower than rated. Can some one give a possible explaination also how can i increase current and reduce the power.

I am using a motor controller in a closed loop (small PID gains). As far as i understood, I only command the motor controller with the desired rmp. Based on the varying load ,i think the motor controller will vary the voltage(not constant) to stay close to the command rpm. On top of the roboteq controller , I wrote a small piece of code to capture the current and power (can't capture how roboteq controller is varying the voltage). Correct me if I am wrong.

I am new to the motor control, if you need more information kindly let me know.

• These graphs are identical - not likely in the real world. – Cecil - W5DXP Apr 5 at 11:28
• What is the Voltage? – StainlessSteelRat Apr 5 at 11:31
• If you don't apply the rated load, it won't consume the rated current at the rated voltage. – Huisman Apr 5 at 11:32
• @Cecil-W5DXP , I am sorry, I attached wrong image. Thanks for point it out. – BhanuKiran Apr 5 at 12:03

The rated speed is where the black vertical line (rated operating point) crosses the blue line (speed). This graph only applies when working at the nominal voltage.

So, when you applied 24Vdc and the motor speed is 400 RPM, while the rated speed is 3000 RPM, you are quite likely close to stalling the motor, so operating in the right lower corner, at, let's say, torque Q.
Looking at the yellow/orange line (output power) at this torque Q, you'll find your output power is very low.
Looking at the red line (current) at this torque Q, you'll find the corresponding current. It should be higher than the rated current. Note that the input power is the(fixed) 24V times this current.

So, at 400 RPM your measured current should be higher than the rated currrent. Your findings seem to contradict each other. Please double-check your the RPM, the current.

Some numbers:
Using on purpose $$\w\$$ instead of $$\ \omega\$$ as the first is in RPM, the latter in rad/sec, but they are proportional

$$\ \tau_{rated} = 0.32 \text{ Nm}\$$
$$\ w_{no load} = 4000 \text{ RPM}\$$
$$\ w_{rated} = 3000 \text{ RPM}\$$

Extrapolation the (blue) speed line:
$$\ k_{speed} = \frac {w_{rated} -w_{no load} }{\tau_{rated} - 0} \$$

$$\ \tau(w) = \big( w_{no load} - w \big) \frac{1}{k_{speed}} \$$

So, stall torque (@ RPM = 0) = $$\ \tau(0) = 1.28 \$$ Nm
And at 400 RPM, the torque = $$\ \tau(400) = 1.15 \$$ Nm

Extrapolation the current (red) line:
You can do the same calculations regarding the current and find that the current at 1.15 Nm equals to 24.4 A.

If your current curve is expressed in amps, then your findings and above calculations are close.

With DC motors, current is proportional with torque, voltage is proportional to speed. So, when controlling the speed, the voltage is being regulated. As torque is related to current and not (directly) to voltage, voltage control hardly limits the torque.

It is likely your motor is not driven on 24V but a slightly lower voltage. At a lower voltage, the (blue) speed line drops (towards the x-axis), so the torque corresponding to the 400 RPM is slightly lower. And that torque will calculate to the correct current.

• I measures the output shaft speed. Now i corrected it in the question. Its 42 rad/sec , which is 400 rmp. Thanks for pointing it out. – BhanuKiran Apr 5 at 12:21
• @BhanuKiran Ill update my answer. Please update the question as well how the current and power in the graphs are expressed (in mA or A? and in W?). And provide the input voltage (or is it being PWM'ed?). – Huisman Apr 5 at 14:36
• Please note that the graph you provided is for a brushed DC motor. The question is about a brushless DC motor. – JG97 Apr 5 at 15:10
• @Huisman ,@ JG97 . I am using a motor controller in a closed loop. As far as i understood, I only command the motor controller with the desired rmp. Based on the varying load ,i think the motor controller will vary the voltage(not constant) to stay close to the command rpm. On top of the roboteq controller , I wrote a small piece of code to capture the current and power (can't capture how roboteq controller is varying the voltage). Correct me if I am wrong. – BhanuKiran Apr 5 at 16:05
• @BhanuKiran Please find updated answer. I think your current measurements are good (provided those are in Amps). – Huisman Apr 5 at 17:20

If you calculate (rated torque)x(rated speed) where the rated speed is said as radians/second, you get the rated power. The rated power seemingly is the available mechanical output power and the motor takes in electrical power substantially more than it outputs as rotating force. The difference is wasted as heat.