ASD DC bus over voltage decreased by changing base frequency

Thread Starter


Joined Feb 2, 2020
Hi guys!

This is my first post and I am not really sure if this is the appropriate category to post it so if it isn't I apologize for it.

I'm new working with ASDs in an oil field and I really understand pretty much how it works but there's one thing I don't understand so let me tell you so you can help me to understand why it happens:

Most ASDs use a dynamic braking resistance that prevents over voltage on the DC bus during the regeneration caused by the motor surpassing the synchronous speed but sometimes when those resistances are broken and there's no stock to replace them what they do is to set the motors base frequency on the ASD a little lower (i.e. If the nameplate says it is 60 Hz they set it at 50 Hz).

This causes the motor to draw more current but also decreases the over voltage on the DC bus when the motor is regenerating.

I'd like to understand why this happens. They told me something related to the V/f ratio but I'd really like to hear what you have to say of it.