Help with DC motor thermal protection

[Dee]

Hi, I'm retrofitting an RF45 mill with DC servo motors and Uhu drives.
The motors are nice Minertia units. The data sheet for the motors specifies the maximum continuous current at around 6 amps, with peaks of 15 amps allowed for up to 12 seconds.
I was thinking of using regular 3 phase motor circuit breakers, but am unsure if they will be suitable for DC motors. I realise one contact will be wasted.
Will phase loss protection prevent me using mcb's with DC motors?
Will DC change the rating of the mcb's as opposed to the AC they are rated for?
Thanks for your help.

[Al_The_Man]

Breakers traditionally have time delay built in. Don't the drives have current limit protection?
Most modern drives have this feature, and in most cases the drive current limit together with fast acting fusing in the motor lead would be sufficient, if you think it is needed.
AC breakers are not really designed for breaking DC, if you did use one, loop one of the conductors back through the 3rd terminal.
Also if the actuating mechanism is by current transformer method, they may not function correctly on DC.
Al.

[Dee]

Thanks Al,
Looping back through the contact is a really simple solution I hadn't thought of, and so obvious now that you suggest it.
The Uhu drives do have current limiting, but it's absolute limiting only, equivalent to a fuse.
So I could set that for say 15 amps, but this wouldn't protect the motors from overheating; I think it would be more for usefuel for protecting couplings or ballscrews with very large motors.
Or, I could set the limit at 6 amps, but would then not have the overload torque available, and would waste alot of the motors' capability. They are excellent motors, but not huge, so I want to use them to near their limits, without damaging them.
I'm not sure how the magnetic side of the breaker will work on DC.
Big advantages of the MCB are ease of use, resettable, adjustable, trip/no-trip state is visible at a glance, thermal and short protection in one package, and auxilliary contacts available for reporting the state of the MCB to software or panel lights.
I don't have my heart set on using MCBs, so if someone is using another method for reliable DC motor thermal overload protection, I'm all ears!
Thanks again.

[Al_The_Man]

Search in the marine/RV supplies, they use AC/DC single pole breakers, which is all you need with a DC motor.
Al.