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Tesla car - mathematical calculations
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nForce:
I was wondering if we measure DC current which goes to the inverter of Tesla's car. And we know the magnetic flux, can we then calculate the torque on the crankshaft of the induction motor? By T = psi*I?
 
Nitrousoxide:
Not a motor expert. But surely you would have to determine some proportional constants to account for losses?

IIRC there's Kv, Km, Kt?
MagicSmoker:

--- Quote from: nForce on July 12, 2019, 03:41:57 pm ---I was wondering if we measure DC current which goes to the inverter of Tesla's car. And we know the magnetic flux, can we then calculate the torque on the crankshaft of the induction motor? By T = psi*I?

--- End quote ---

Nope. While DC input current to the inverter will go up as torque from the motor goes up, this assumes the speed is constant; DC input current will also go up at a constant torque output if speed is increasing.

This is because the inverter effectively acts as a buck converter when the motor is operating as a motor (and a boost converter when the motor is regenerating). Current Torque is most directly correlated with phase current, but even then you need to know the slip percentage and phase angle displacement with the integrated voltage waveform to calculate it.

Determining torque of a PM or other kind of synchronous AC motor is far easier - at least when operated below field-weakening speed - as then it is directly proportional to phase current (neglecting some effects from saturation, etc.).

EDIT - corrected striked out word
max_torque:
If you measure the DC voltage and DC current that flows into (and out of) the inverters DClink, you know the power that is being used by the inverter and motor.

If you know the speed of the electric motor, then you can estimate the torque of that motor, because power = speed x torque.

I say "estimate" because you don't know either the inverter or motor efficiencies, however if you estimated a 5% loss for the inverter and a 5% loss for the motor you wouldn't be a mile off........
nForce:
Thanks guys.

I have one question if we measure the DC current to the inverter, should then the 3-phase currents if we sum them up be this DC current, minus the losses in the inverter?
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