PWM servo amp switching frequency
PWM servo amp switching frequency
(OP)
Hi all,
We have been currently driving a brushless DC servo with an amplifier that was recently obsoleted. This amp had a PWM switching freq. of 40 Khz. We MAY still be able to get a similar model that switches at 20 Khz. What I would like to know is 1) How critical is this switching frequency as far as performance goes? 2) What can I expect to see by using a substitue that operates at a different switching frequency?
This servo is used in an EDM head application. The overall mass of the moving components is quite small and response of the servo is critical to optimal performance.
Thank you in advance for any assistance.
Scott
We have been currently driving a brushless DC servo with an amplifier that was recently obsoleted. This amp had a PWM switching freq. of 40 Khz. We MAY still be able to get a similar model that switches at 20 Khz. What I would like to know is 1) How critical is this switching frequency as far as performance goes? 2) What can I expect to see by using a substitue that operates at a different switching frequency?
This servo is used in an EDM head application. The overall mass of the moving components is quite small and response of the servo is critical to optimal performance.
Thank you in advance for any assistance.
Scott
In a hundred years, it isn't going to matter anyway.





RE: PWM servo amp switching frequency
The servo amp current loop bandwidth is limited to about 1/10th the switching frequency but the actual bandwidth depends on the motor inductance and currnt loop compensation values. For high performance, this is more important (usually) than the switching frequency.
RE: PWM servo amp switching frequency
But very important issue that must be checked - current ripple defined by motor inductance. The some PWM amp manufacturers limit it - see Elmo (www.elmomc.com) for example.
RE: PWM servo amp switching frequency
I have found that the higher switching freq. gives a better efficiency for the amp. However, one bit I read stated that power output ratings were reduced when using the models with 40 or 60Khz switching.Hmmm..have to look into this. Thanks again.
Scott
In a hundred years, it isn't going to matter anyway.
RE: PWM servo amp switching frequency
I have seen this myself on very high speed PM motors, the higher the PWM frequency the greater the eddy current losses in the rotor magnets. Any PWM-induced harmonics in the stator currents will set up a rotating field at that frequency, rotating faster than the rotor. The higher the frequency the greater the induced eddy currents in the rotor as per Faraday's law. In addition, the iron losses in the stator may be higher too.
Rotor eddy-current loss not an issue of course if the rotor can and magnets are non conducting.
RE: PWM servo amp switching frequency
More details regarding motor power losses I found at: http://www.drbrushless.com/articles.html#A2.