I am looking for the formula, not a rule of thumb. Is there a way to calculate the no-load current?
Current Measurement – Formula for Calculating No Load Current of a 3-Phase Motor
currentcurrent measurementmotorthree phase
Related Solutions
No guarantees as to correctness, but this shows the general method.
- Find the real and reactive power flowing in each phase.
- Determine the current flowing in each phase, including the angle between phase current and phase voltage.
- Add all currents vectorially - real and imaginary components. A diagram helps.
By convention, phase rotation is anti-clockwise. An inductive load will make the current lag behind the voltage. (Remember, inductors resist changes in voltage dv/dt.)
Diagrams below are not to scale.
I don't see any way to do this. You need the power factor of the individual phases and that's not available. The equation is incorrect.
\$P_{actL1} = P_{appL1}\cdot\cos\varphi_1\$
\$P_{act,tot}/P_{app,tot}= \$
\$= (U\cdot I_{l1}\cdot \cos\varphi_1 + U\cdot I_{l2}\cdot \cos\varphi_2 + U\cdot I_{l3}\cdot \cos\varphi_3)/(I_{l1}\cdot U + I_{l2}\cdot U + I_{l3}\cdot U)\$
\$= ( I_{l1}\cdot \cos\varphi_1 + I_{l2}\cdot \cos\varphi_2 + I_{l3}\cdot \cos\varphi_3)/(I_{l1} + I_{l2} + I_{l3})\$
and that is not equal to \$\cos\varphi_1\$.
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Best Answer
As both Andy aka and Bimpelrekkie have commented, there is no useful formula because there are far too many variables both known and unknown. It depends on the design and type (not all motors are induction) of the motor and how well it has been maintained. By far the easiest way of finding out is to get yourself a clamp on ammeter (readily available from multiple sources) and actually measure it on the particular motor in which you are interested.