Calculating Max DC Bus Voltage from PMSM Line-to-Line Voltage Using 6-FET Inverter

Hi everyone,

We want to use a Permanent Magnet Synchronous Motor as a generator as well as a motor.

We'd like to use a classical 6-FETs inverter to run it and then use it as a rectifier to provide DC voltage.

But what would be the VDC bus vs. line-to-line voltage?

I read in this pdf (p;13) : http://projekter.aau.dk/projekter/files/14498144/s08_stefan_partyka.pdf
that inverters can give higher voltage than classical rectifier diode bridges. But by how much? How can I calculate the max /nominal DC voltage coming out of our inverter from the line-to-line voltages?

 

Thanks for your input!

 

A.

A mosfet (or "ideal diode") bridge will create an output voltage that is higher by two times the diode forward voltage drop over a traditional diode based bridge rectifier.

The forward voltage drop of a traditional diode rectifier will vary by the diodes that make up the bridge and also the current flowing through the bridge. Values between 0.5V and 1.5V are typical for the forward voltage drop making the difference between traditional diode bridge output and mosfet ideal diode bridge output between 1 and 3V (typically).

The paper you cited references using PWM boost controllers on the output of the motor. The output voltage of these circuits is dependent on the duty cycle of the PWM signal driving the mosfet switch in the boost regulator. This is typically generated using a closed-loop PWM controller that uses a reference signal generated from the output voltage through a resistive divifer. In this case, the output voltage is dependent on the values of the resistors in the divider used to generate this reference voltage. The paper references a C2000 DSP used to generate the PWM signals, so you would have to know what divider resistors are used and what software was used on the DSP in order to determine the output voltage.

As for what is possible, large multipliers in the boost circuit

As for what is possible, large mutipliers in the boost circuit require short duty cycles on the PWM signal and mosfet switch. Short duty cycles mean increased current at the switch and filtering requirements (capacitors) at both the input and output of the boost converter. These all come at additional parts cost and in the case of the switch usually reduced efficiency. 2:1 and 3:1 ratios are achievable, but determining the maximum boost ratio is dependent on many variables and would require detailed analysis.

Designing your system to have an AC output voltage higher than your desired DC output voltage and using a Buck or buck-boost switching regulator may create higher system efficiency (at a given cost) than using a boost converter. A 3-phase transformer could also be used to increase the output voltage before an ideal-diode bridge or mutli-phase buck converter

Hi Dean,

Thanks for the answer,

In our case, we are not looking for an increased voltage (apart from 0,7V gained vs. a diode rectifier) so the "boost" effect is not necessarily welcome.
So, the question is basically : if we take a 100% modulation (100%PWM), will we see the "rated" voltage of the motor at this speed or will we see more because of some "resonance" between the motor and the inverter filtering cap?

Also, I can't seem to calculate a (average? nominal?) VDC Bus voltage value from a line-to-line voltage, all the formulas I find give different values... Would you have a reference regarding that?

Thanks again,

A

Because this is a 3 phase rectifier, I'm pretty sure that the DC bus voltage will be Vpeak of the AC input voltage. There is no ripple as in single phase rectifier circuits and the capacitors will only filter high frequency noise caused by load transients (or the PWM noise if the PWM duty cycle is not 100%).

If you are driving a resistive load, then the filter caps can be eliminated. If you are driving an active load you may or may not need the filter caps.

Linear makes a reference design for a 3 phase ideal diode bridge rectifier, the support documents may have information you can use. http://www.linear.com/solutions/7232