Jump to content

Overload Setting


Recommended Posts

 

Hello, I am new in this site. Just want to know if its allowed to use one overload for two motors, they are the motors in each side of a Overhead crane. If so, how do I set the overload? Should I add the FLC of each motor? thank you

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Hello Froilan

 

There is a 15% safety margin in the ratings of the motors to allow for tolerances in the calibration of the protection devices.

Theoretically, a protection device must not trip at less than rated current and must trip at 115% rated current.

Typically, with good protection relays, the average trip point is in the order of 105% rating, but commonly there are 3% - 5% tolerances in CTs etc.

If you set the trip point at 115%, you can expect the motors to trip at 120% - 125% on average. This is enough to shorten the life of the motor.

If the protection operates late, the motor can operate at a higher temperatures and not necessarily fail immediately. Every ten degrees C hotter you run the motor, you halve it's life, so you may not see the effects of a higher overload setting for some time.

 

Best regards,

Mark.

 

 

 

 

 

Link to comment
Share on other sites

So, put it the value what Mark had mentioned (if 10 A each, then 20 A + 15 %). Also, usually, all motors' currents slightly (but in some cases considerably) differ between phases. Such as one I saw recently working with 120/90/140 A. Then, it would be better if there were 2 contactors, and you rearanged phase sequencies for each motor so, that the common currents differ the least (preserving the same rotation direction of the motors, of course).

 

However, of course, no need for two contactors.

Link to comment
Share on other sites

  • 4 weeks later...

I would not recommend the situation you are suggesting if you are planning on running the motors anywhere near full load current. If you set the overload current relay to 215 % then you could be running 50 % load on one motor and 150 % load on the other motor and not trip. Depending on how the motors are coupled it may be possible to have a bearing failure on one motor, and not catch the problem because your overload is set so high, or if the track the crane is riding on is jamming on one side, but not the other, you could see significantly more load in one motor than the other. Thermal overload relays are relatively inexpensive, and I would recommend using one on the leads to each motor. If you have higher currents than the standard overload relays handle you can run both of the contacts into the same trip circuit.

 

Hope this opinion is helpful

 

Thanks

 

 

Dave

Link to comment
Share on other sites

Of course you are right Dave. Saved a hundred or so on a proper safety device today may and wiil lead to lost thousands tomorrow. I do not reccomend it also. However, economy on one contactor, one relay....and given the application: if motor is jammed in one shoulder of the cran the motor in another shoulder unable to cope with the load would also be.
Link to comment
Share on other sites

I would only consider this as a last resort, if and only if the motors were identical and were driving the same shaft which they probably will not be. Just a small difference in speed can yield a big difference in load.

 

Best regards,

Mark.

Link to comment
Share on other sites

Actually can. But we must sure that the 02 motors normally run together to ensure of protection. For the small motor, just adjust the setting range slightly above the trip point. It means that you run these 02 motors, then reduce the setting until the overload relay trips. Then move one step beyond this point.

 

Regards,

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...