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Consider running a 3 phase BLDC motor controller having 20KW inverter fed by 100V whence designed for high amperage. What happens if we turn it on and start gradually increasing duty cycle of the MOSFET gates PWM without connecting motor leads? Or a somewhat similar situation but more complex - connecting 3 phase motor but one phase wire is broken ... What can we observe on the phase output leads if we connect oscilloscope probe A-B, or B-C etc...? What happens to the inverter bridge MOSFETs ? Can this (no load) case be used to test at least some of the inverter specs? I think the conducting MOSFETs will not have a half bridge closed circuit for the current to run but, if so, I am confused about what happens if 2 phases are conducting but one is broken (unbalanced inverter's load)? Thanks in advance for any meaningful ideas as this question tests our understanding of the nature of the power inverters.

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  • \$\begingroup\$ I doubt that you'd get much information from doing that: The drive would notice very quickly that it has no load (either from the lack of phase current, the lack of back-EMF or the lack of hall effect sensor/optical encoder feedback) and shut down. What are you trying to do, to troubleshoot a hybrid electric vehicle? \$\endgroup\$ – jms Feb 12 '18 at 17:46
  • \$\begingroup\$ Having a new PCB motor controller to debug and waiting for test motors to arrive , I don;t want to start by hooking my expensive motors intended for test flight (piloted aircraft). Another chance is to use motor which has one burned wire... \$\endgroup\$ – VladBlanshey Feb 12 '18 at 19:06
  • \$\begingroup\$ Hall sensors neither back EMF not needed for just open loop run at fixed frequency. \$\endgroup\$ – VladBlanshey Feb 12 '18 at 19:08
  • \$\begingroup\$ Not really. The controller of a sensorless motor does spin up the motor open loop, but if it can't detect zero crossings of the back-EMF (required for switching to closed loop mode), it very quickly stops and tries again, eventually giving up. If the driver is designed for sensored (encoder/hall) motors, it won't operate open loop at all. \$\endgroup\$ – jms Feb 12 '18 at 19:27
  • \$\begingroup\$ my system is designed for sensored operation but, for testing and debugging, when running open loop software drive, this software needs neither sensor nor back EMF - that's why it is called "open loop" \$\endgroup\$ – VladBlanshey Feb 13 '18 at 19:54

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