90kW motor protection

[St_Isakovic]

Hello Colleagues,

I have a project and part of it was driving a 90kW motor for a hammer mill where I specified star-delta starter protected with NH2 fuses and bimetallic relay in the branch.
Contractor built the cabinet and changed protection from fuses-bimetall combo to single circuit breaker, let's not go into who approved the change and why. Single circuit breaker (motor protection) is legitimate, but I had bad experience with them for this kind of motor drive, I've seen two 90kW and one 110kW circuit breaker fail where contact of one phase would burn out and wouldn't conduct.

All equipment is Schneider, circuit breaker that is installed is CVS250F TM200D.

https://www.se.com/id/en/product/LV525342/easypact-cvs-cvs250f-tm200d-circuit-breaker-4p-3d/

Datasheet says application for this breaker is distribution, not motor protection. Suitable motor circuit breaker would be:

https://www.se.com/id/en/product/GV...-tesys-gv5-3p-220a-icu-36ka-thermal-magnetic/

They are rated for the same current, have same breaking capacity, same thermal protection adjustment range. Installed circuit breaker doesn't have tripping class selection.
Should I expect problems with this setup?
Is there any other difference between these types of breakers in general besides tripping class selection?
Motor has additional protection with PTC probe relay.

 

[SilverfoxUK]

Maybe a daft question but are you starting the hammer by simply closing the breaker or do you have a contactor in line and a proper control circuit?

 

[St_Isakovic]

There are 3 contactors and a proper control circuit for star-delta starting.

 

[panter]

You should look at the expected starting current of the motor, bearing in mind that newer motors with improved efficiency have a higher starting current of 10 IN.
Then this four-pole distribution switch could be tripped by its characteristic .

 

[che12345]

I have the following opinion for your consideration
"... Single circuit breaker (motor protection) is legitimate, but I had bad experience with them for this kind of motor drive, I've seen two 90kW and one 110kW circuit breaker fail where contact of one phase would burn out and wouldn't conduct... ".
1. Both types of CBs are suitable. CB with burn out contacts are extremely rare.
1.1 Strongly suggested a) check the conductor size and the recommended tightening torque, both data are given in their data sheet.
b) Carryout thermal scan every six monthly with full load running for say 3-4 hours.
c) Do NOT use the CB for starting and stopping, which should be by the Contactors in the starter.
2. CB without protection Class should be installed with an external motor thermal overload protection relay of the appropriate Class in the motor starter circuit.
2.1 For hammer mill application, the thermal overload may? required Class 30.
3. Suggestion: a) with SD starting, the main CB should be rate > 1.3 * motor rated current.
b) the thermal overload which is usually connected in the phase conductor, set at 0.85 * motor rated current.
c) if all these precautions failed, select the next higher current CB, but set the Ir down in order to protect the conductor. Note: Motor overload protection is usually by the thermal overload relay installed in the starter.
Che Kuan Yau (Singapore)

 

[St_Isakovic]

Thank you for replies,

I looked at the graphs for CB and motor data (motor is ELVEM 7T2 280M2 90kW 2970rpm IE2). Motor starting current is 5.9In, so I should expect 2In starting current through CB, reaction time for this current is about 50 seconds.
Hammer mill does have a difficult start and there are many factors so I guess we'll wait and see what happens when we do the testing. I'll submit results if it trips or not in couple of months if anyone stumbles upon this post.

You should look at the expected starting current of the motor, bearing in mind that newer motors with improved efficiency have a higher starting current of 10 IN.
I didn't know this about starting currents of more efficient motors. Motor in question is grade 2, grade 4 being the highest.

1. Both types of CBs are suitable. CB with burn out contacts are extremely rare.
1.1 Strongly suggested a) check the conductor size and the recommended tightening torque, both data are given in their data sheet.
b) Carryout thermal scan every six monthly with full load running for say 3-4 hours.
c) Do NOT use the CB for starting and stopping, which should be by the Contactors in the starter.

Faulty CB were NSX250 type, I've seen 3 failed ones on the same kind of drives, mills and presses, that's why I'm suspicious. Can't say if it was maintained properly or not, it was another company I was called for consulting. Those instalations are now on NH2 fuses.

2. CB without protection Class should be installed with an external motor thermal overload protection relay of the appropriate Class in the motor starter circuit.
2.1 For hammer mill application, the thermal overload may? required Class 30.

I also think there should be thermal overload relay for proper motor protection in this setup.