I recently have been presented with a few cases where it was questioned weather or not a motors overload protection value could be increased to allow the motor to run into its service factor. For the sake of this thread lets just say we are talking about a motor with a FLA of 10A and a S.F of 1.15.
Let say this motor is in operation and it is seen that due to the load the motor typically runs at around 11A, and is tripping the motor. We then ask the question weather or not the motor overload protection can be increased to 11.5A to account for the service factor. Now my typical answer would be that yes you can increase this value however you are shortening the life of the motor if it is run continuously. But what if the load is not a continuous load such as one that is cyclical?
Of course my example above is typically referring to an electronic overload where an exact FLA value can be entered. But one thing I am constantly struggling with is the application of bi-metal relays. Typically these bi-metal relays are listed in ranges. So for the sake of example what if the motor is operaing with a bi-metal overload with a high range of 10.5A and we look at the next range up to possibly allow operation into the service factor. Lets say this next range goes between 11-14A or so. If we increase to this next level to allow operation into the service factor, are we properly protecting the motor considering the high limit of this range?
I also notice that most bi-metal relay charts have a fine print note stating that that a particular bi-metal realy range accounts for 125% of the motors FLA for 40deg ambient motors and if the motor is not a 40deg ambient motor then the next range down should be selected. Does this 125% account for the same 125% that the NEC allows for 40deg ambient motors or motors with 1.15 SF's? Does the 40deg statement listed in this note represent the S.F. of a motor? If my motor does not have a 1.15 S.F. should I select the next range down? How does all of this change with electronic overloads.
I recently read an article that stated a motor that is protected with RTD's could be operated higher than its FLA rating as long at the RTD's do not increase over the insulations max temperature rating. So using my example above what if we were able to operate this motor up to a load of 15A as long as we stayed under the insulation class max rating determined by RTD's?
Looking at Class F insulation it has a max temperature of 155deg. For a 1.0 S.F motor it appears that a temp rise of 105 degrees is allowed bringing the max temp to 145 deg and allowing 10deg for hot spots. For a 1.15 S.F. motor a 115 deg rise is allowed bringing the total temp rise to the max insulation temp rating. Is this why S.F. operation is dangerous due to the fact that if there are hot spots when operating at 1.15SF then we are risking exceeding the insulations max temp rating?
Let say this motor is in operation and it is seen that due to the load the motor typically runs at around 11A, and is tripping the motor. We then ask the question weather or not the motor overload protection can be increased to 11.5A to account for the service factor. Now my typical answer would be that yes you can increase this value however you are shortening the life of the motor if it is run continuously. But what if the load is not a continuous load such as one that is cyclical?
Of course my example above is typically referring to an electronic overload where an exact FLA value can be entered. But one thing I am constantly struggling with is the application of bi-metal relays. Typically these bi-metal relays are listed in ranges. So for the sake of example what if the motor is operaing with a bi-metal overload with a high range of 10.5A and we look at the next range up to possibly allow operation into the service factor. Lets say this next range goes between 11-14A or so. If we increase to this next level to allow operation into the service factor, are we properly protecting the motor considering the high limit of this range?
I also notice that most bi-metal relay charts have a fine print note stating that that a particular bi-metal realy range accounts for 125% of the motors FLA for 40deg ambient motors and if the motor is not a 40deg ambient motor then the next range down should be selected. Does this 125% account for the same 125% that the NEC allows for 40deg ambient motors or motors with 1.15 SF's? Does the 40deg statement listed in this note represent the S.F. of a motor? If my motor does not have a 1.15 S.F. should I select the next range down? How does all of this change with electronic overloads.
I recently read an article that stated a motor that is protected with RTD's could be operated higher than its FLA rating as long at the RTD's do not increase over the insulations max temperature rating. So using my example above what if we were able to operate this motor up to a load of 15A as long as we stayed under the insulation class max rating determined by RTD's?
Looking at Class F insulation it has a max temperature of 155deg. For a 1.0 S.F motor it appears that a temp rise of 105 degrees is allowed bringing the max temp to 145 deg and allowing 10deg for hot spots. For a 1.15 S.F. motor a 115 deg rise is allowed bringing the total temp rise to the max insulation temp rating. Is this why S.F. operation is dangerous due to the fact that if there are hot spots when operating at 1.15SF then we are risking exceeding the insulations max temp rating?
