# Controlling the reactive current of an PMSM in an open loop setting

for my master thesis I'm investigating the EMI of a 3-phase inverter, wich drives a PMSM. The inverter's structure is the common 3-Phase DC/AC Voltage Source Inverter and it uses EPC2020 GaN transistors at 100 kHz switching frequency. For my investigations the system needs to be in a stationary state with exactly periodic currents and signals, hence no closed loop control is used.

Instead I only set the desired stator voltage and frequency and let the motor do its thing. For startup I slowly increase the stator frequency and at the same time increase the voltage my inverter produces to compensate for the higher back EMF at higher frequencies.

So far so good, although this method is not practical, it lets the motor run at my desired electrical frequency of 50Hz in an idle state, but the phase currents are concerningly high.

The system currently runs with a DC rail voltage of 20V. The DC current going in the inverter is peaking at about 5A. The phase current of the PMSM on the other hand is peaking at nearly 20A. Hence my conclusion the majority of this current is reactive current between the motor and my DC link cap. So now my real question: How do I reduce that reactive current? From my understanding of synchronous machines, the reactive current is not needed for torque built up and should be controlable through the ratio of stator voltage and induced back EMF. So if I adjust the absolute value of my stator voltage it should be possible to reduce the reactive current to a minimum, while still drawing the real current needed to drive the motor.

But that's unfortunately not what I experienced until now. If I increase the stator voltage, the reactive current gets bigger. If I reduce it, the current does reduce a bit, but before it gets reduced significantly, the motor stops, so somehow also the real current got reduced, what I find counterintuitive.

I plotted one phase current and the DC current for one whole period, you can take a look. They also don't look like I expected, but I have no explanation on why that is...

So I know this may be an unusual use case, but maybe there are some machine experts out there, who may guide me in the right direction ;)

• Have you ever heard about FOC algorithm? Also, for torque control a current control is nedded, voltage control is not so important for motor control. Why to reinvent warm water? Jun 26, 2019 at 7:33
• Thanks for your answer. Yes I know about FOC, but for my case I don't need a closed loop algorithm, but a deterministic control signal. I just want to put a voltage on it and want it to rotate, even though this is not the way you would normally do it. Jun 26, 2019 at 7:38
• You will get better performance under load disturbances with feedback. infineon.com/cms/en/product/power/motor-control-ics/… VFD is good but with FOC even better with PFC and current feedback to reduce reactive current. Hall and Angle sensors are used for most efficient sine drive. Jun 26, 2019 at 7:46
• Yeah the performance would be better with FOC, no doubt. But as I tried to explain, closed loop control is not an option in my case. Jun 26, 2019 at 9:58
• By "stationary state," do you mean the shaft is prevented from turning or do you mean "steady state" - constant speed, neither accelerating nor decelerating? Is the shaft free or connected to a load? What kind of load? What is the motor rating plate information, rated voltage, frequency, current, speed or number of poles, mechanical output power? Are the voltages and currents balanced among the three phases? Is the inverter a commercial product?
– user80875
Jun 26, 2019 at 10:24