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Reference Number: AA-01933// Views: AA-01933// Created: 2020-07-23 13:39:17// Last Updated: 2020-07-23 19:57:26
eFPGASIM
How to calculate RC Snubber values for coupling inverter and PMSM


The PMSM machine in the PMSM example with eFPGASIM is fed by a two level inverter, as visible in the FPGA circuit (PMSM_eHS_UB.mdl) of the model.



We use RC snubbers which allow for proper coupling of the inverter to the motor.

The computation of the R_Snubber and C_Snubber values can be done as follows:


1. Assume R_Snubber = 100 ohms and choose C_Snubber such that  R_Snubber*C_Snubber = 5*(Time Step of the FPGA model/eHS)
For example if Ts_FPGA equals 250ns, then C_Snubber is 12.5nF


2. Run the simulation to check whether it is stable without PWM gating signals.
Calculate the losses using 3*0.5*C_Snubber*(Vdc^2)*(PWM_frequency)

Vdc is the dc link voltage for the inverter.
In the formula above, we use the multiplication with 3 since there are three RC snubbers.


3.If the losses are too high, you can double the R_Snubber value and reduce the C_Snubber value by half.
Repeat this step until the value of losses is acceptable.


4.If the losses are acceptable but the simulation is not stable, reduce the R_Snubber value by half and double the C_Snubber value unitl the simulation is stable.

(Please note the exact connection of the RC Snubbers will have an impact on the calculations, for example, if there is a star connection to a floating node, the snubbers will charge only to 0.5*Vdc)