There is no recuperation (regenerative) braking in the inverter. Would you please explain why without recuperation braking, the expected cause that kills the FETs is Vgs overvoltage and not drain-to-source (Vdss) overvoltage?If the motor is actually sourcing enough power to raise the bus voltage to 115 V, then it would be surely sufficient to blow the FETs. Could happen if you switch the control circuit from full speed forward to recuperation brake without connecting a braking resistor. Without recuperation, I would rather expect Vgs overvoltage to damage the FETs, e.g. due to a bad gate driver circuit layout.
In my application, the maximum DC-link voltage supplied to the HMP-Inverter is 70V. Let's the DC-link voltage is to be clamped at 80V. Do you think a LittelFuse 30KPA (https://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_TVS-Diodes_30KPA.pdf), which is rated for 30kW, is enough to protect the FETs from overvoltage?Another question is if you provide sufficient supply voltage margin. I would expect 60, maximum 80 V regular bus voltage with 100 V FETs. Raising the bus voltage from 80 to 100 V implies an energy of 36 Ws stored into the 20000 uF capacitors. That's quite a lot and rarely handled by a small TVS diode.
There is regenerative action in the inverter. It is automatic.There is no recuperation (regenerative) braking in the inverter.
Yes, of course. I presumed, powersys knows about.If you try to decelerate your motor quickly (means faster than natural deceleration of the motor), then stored mechanical energy in the rotor will go to DC bus automatically.
Some tests were carried out yesterday... The Xantrex power supply output voltage was set as 70Vdc, while its OVP limit was lowered to 80V. The motor was operated at 2000rpm, then the speed command was reduced to zero instantly. When the motor was slowing down, the voltage reading on the Xantrex power supply display panel increased and sometimes it triggered the OVP of the power supply. Thus, it looks to me that the motor, while slowing down, did "generate" energy back to the inverter. However, the HMP-Inverter does not have a mechanisme/circuit (e.g. braking resistor) to recover the energy.without regenerative braking, I don't see how the bus voltage should be raised more than a few volts. In this case, overvoltages can be expected rater low energy surges, that should be absorbed by the FETs itself through avalanche breakdown with damage. TVS aren't bad as additional protection.
The 30kW TVS may be able to absorb the transient energy if the DC-link capacitance is reduced to half (e.g. 10000uF). I agree with you that it still difficult to find a TVS with suitable "Reverse Stand Off Voltage (Vr)" and "Max. Clamping Voltage (Vc)" for my application. According to this article (**broken link removed**I would translate the 30000 W TVS absorption capability specification very roughly to 30 Ws energy handling. That's more than a small TVS. It can be still difficult to select a voltage that guarantees TVS clamping before FET breakdown.
True... I noted such phenomenon in some tests carried recently...Yes, of course. I presumed, powersys knows about.
Thanks. I think this exactly what happened to the system described above.There is regenerative action in the inverter. It is automatic.
If you try to decelerate your motor quickly (means faster than natural deceleration of the motor), then stored mechanical energy in the rotor will go to DC bus automatically.
Great explanation! Thank you. It seems that TVS is not an easy solution for this problem. Do you have other better option?So it turns out that you do use regenerative braking. To decide if the TVS diode can manage the energy, you can estimate the energy stored in motor's and possibly connected system's moment of inertia.
It doesn't help to reduce the bus capacitance, because the energy is supplied by the motor, not the capacitors. In contrast, increasing the capacitors can possibly reduce the overvoltage. If it's true, that the bus voltage has been raised from 70 to 115 V OVP trip in your previous test, the motor energy is at least 83 Ws. So you would in fact need several paralleled TVS diodes, and they must be allowed to cool down before next braking action.
Thank you. I will implement the brake_resistance + FET + voltage_comparator.Break resistance (switched by a FET) is the best option. It can be automatically operated by a voltage comparator.
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