You are here

Maximum Duty Cycle limit is 95%

5 posts / 0 new
Last post
shimdidly
Offline
Last seen: 3 years 1 month ago
Joined: 2017-10-01 19:38
Posts: 15
Maximum Duty Cycle limit is 95%

When attempting to change the Maximum Duty Cycle in Motor Settings->General->Advanced from 95% to 100% I am told the value was truncated for being above the allowable limit. Why is 95% the duty cycle limit?

I have version 0.82 of the Mac OS X VESC tool.

shimdidly
Offline
Last seen: 3 years 1 month ago
Joined: 2017-10-01 19:38
Posts: 15

I found the answer to my own question. Hardware limits are programmed into the VESC firmware. If you want to exceed these limits, you will need to install a firmware version without hardware limits. In my case, I found VESC_default_no_hw_limit.bin already available in the VESC tool Firmware page.

Roger Wolff
Offline
Last seen: 2 years 3 days ago
VESC Original
Joined: 2017-05-24 12:27
Posts: 202

When using FETs to drive a motor, you can easily use N-channel-FETs for the low side (to pull one wire of the motor towards battery-minus).  (we leave out the "channel" after a while)

 

The other side (to pull a motor-wire to the battery-plus) is a bit more problematic. You can either use a P-fet, but those have worse specifications and/or are way more expensive. or you can use an N-FET, but then you'd need a voltage above the power supply rails to turn it on. Someone invented a clever trick to generate that voltage, under the assumption that you do not need to drive that FET for 100%. So.... because this hardware trick is being used, you cannot drive the high-side FETs at 100%. I would think that 98% is probably a safe limit, but I have not yet done the measurements to verify my suspicion. I do suspect that benjamin DID do the measurements and decided on 95%. So I'd be surprised if he got it wrong (and less surprised that my gut feeling is wrong). 

 

ThierryGTLTS
Offline
Last seen: 5 years 9 months ago
Joined: 2017-09-06 14:18
Posts: 116

Yes most of the drivers are limited to 95 - 98% duty cycle for safety reasons, although the datasheet says it can achieve 100%.

Maxon motor drivers are also limited to those values.

Some use DRV from TI too.

Have a Nice Day.

Thierry

shimdidly
Offline
Last seen: 3 years 1 month ago
Joined: 2017-10-01 19:38
Posts: 15

I am getting very frequent DRV fault errors, to the point that my VESC is not usable. I did modify this value to 100%, and used FOC. I read somewhere that FOC can burn up DRVs. Perhaps either one of these modifications could have damaged my board?