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[Tech] Motor Inductance, Resistance and Maximum RPM: Estimated vs Actual

Discussion in 'Technical' started by kennybobby, Apr 23, 2015.

  1. kennybobby

    kennybobby Member

    Sep 14, 2014
    Heart o' Dixie
    #1 kennybobby, Apr 23, 2015
    Last edited: Apr 24, 2015
    It will likely be a long time before anyone can actually measure these parameters to verify, but i'll take a shot at calculating them.

    Why is this important or of any interest? Because these electrical characteristics are used in the calculations made by the inverter to drive the motor using a Direct Torque Control (DTC) scheme, described in patents and discussed in another thread.

    For a 3-phase, 4-pole wye motor with the poles wired 2s2p, and with 4 Turns per pole using 16 AWG copper wire 12-in-hand:

    phase inductance ~ 493 nH
    phase resistance ~ 5.3 mR

    This gives an L/R time constant of 93 usec. If we use 3 tc to reach full current, then the motor max speed ~ 26,858 rpm. With a 9.7:1 gearbox the theoretical max speed would be 228 mph.

    For a P85D to reach 155 mph the motor(s) will need to spin at 18,200 rpm and make at least 50 kW each. The inverter would need to be generating a current waveform of 606 Hz, and the per phase L/R time constant would have to be less than 138 usec.

    Send data when you get some...

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