Calculating the Resistance and Capacitor for a Snubber Circuit with 3HP DC Motor

[bencyp]

Hi,

I have build a PWM to control a 3HP DC motor. The problem is that the MOSFET is getting damaged because of high return current/voltage. I used a Diode at the motor end but that diode is also getting burnt.

Main DC 380 Volt
PWM used to control a 220 DC 3HP motor

I found out that a snubber circuit is need to take up the spike generated by the motor when the MOSFET is switched off. Can anyone help me with the exact calculation of Capacitor and Resistance needed for Snubber?

Regards,
Bency

 

[berkeman]

Generally with the snubber circuit, you will size the capacitor to resonate with the power inductance at some frequency, and will size the resistor to damp that resonance rather strongly. You want to use the cap to temporarily absorb the transient energy, and the resistor to dissipate it over about a half cycle of the LC resonance.

I googled snubber circuit tutorial, and got lots of good hits. Here's an app note from Maxim, for example:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/848

 

[ravenprp]

Hi Bency,

It seems like you are experiencing some issues with your PWM circuit and MOSFET due to high return current/voltage from your 3HP DC motor. This is a common problem in motor control circuits and using a snubber circuit is a good solution to protect your components from damage.

To calculate the resistance and capacitor values for your snubber circuit, you will need to know the switching frequency of your PWM and the inductance of the motor. The formula for calculating the resistor value is R = (Vd * T)/Ipk, where Vd is the voltage rating of your diode, T is the switching period (1/frequency), and Ipk is the peak current through the diode. The capacitor value can be calculated using the formula C = (Ipk * T)/Vspike, where Ipk is the same as before and Vspike is the maximum spike voltage.

It is important to note that these calculations are just estimates and you may need to experiment with different values to find the optimal snubber circuit for your specific setup. Additionally, you may need to consider the power rating of your resistor to ensure it can handle the current and the voltage rating of your capacitor to prevent it from breaking down.

I hope this helps and good luck with your snubber circuit! Remember to always take necessary precautions when working with high voltages.