PWM switching frequency

[thinker]

We are in the process of selection a PWM drive system with space vector control for highly dynamic application (torque response time < 2 msec). Is there a relationship between PWM switching frequency and a system dynamic behavior? We were told that the most high end drives use higher switching frequencies (8-10 kHz) versus 3-5 kHz for non-demanding applications.

 

[Skogsgurra]

Yes, there is a relationship. But 2 ms is a rather moderate torque response time. I think that you would be OK with 5 kHz. That will give you ten samples in 2 ms and that should be enough to get a good torque response.

On the other hand, if higher carrier frequenies are available, all you loose if you go to, say 16 kHz, is some extra power loss and more EMI plus bearing current issues becoming more prominent. So, I would keep switching as low as possible.

Gunnar Englund

http://www.gke.org

 

[ozmosis]

Sometimes, depending on the drive, you don't have any control over the switching frequency. The DTC(Direct Torque Control) technology currently used by ABB in a number of their drives will automatically alter the switch.freq subject to load. Interesting to see a harmonic spectrum at a site with these installed and you see a prominent 17th order harmonic, then a 19th, then 23rd...and back to the 17th, all in the space of a few minutes.

 

[cswilson]

sed:

The ABB DTC drives do not use PWM at all, and there is no fixed frequency. Rather, they drive the power stage much like a single-bit DAC, switching only as necessary to drive the current towards the desired value. They make this decision every 25 usec. (Their marketing claims there is no modulation at all, but that's bunk. It's just a different modulation scheme.)

skogs:

Remember that you can sample and control twice per PWM cycle (but not more often). Our default for our digital PWM servo/space-vector drives is to sample at 10 kHz for 5 kHz PWM.

Curt Wilson
Delta Tau Data Systems

 

[thinker]

Thanks to everybody for good advices. We will stick with 5 kHz.