I know both of these values are used for safety/protection in the NEC but are they adequate for protecting the life of the motor in most cases>
Following is personal opinion and design philosophy. My comments are limited to normal industrial, low voltage, 300hp or less
Overloads don't protect the long term life of a motor - unless you are planning on having a big enough motor to be able to set the overloads at 100% FLA or less. And if you are planning on this, the accountants should be beating you with a nerf bat.:roll:
There are only three things I know of that protect the long term life of a motor.
1. System design to limit motor power required to that available from the motor, keeping the motor inside of mechanical stress/thermal stress parameters. System design includes a stable power source within the motor design parameters.
2. Operational parameters/practices set to keep the motor within mechanical stress/thermal stress parameters. This follows the design theory that a ham-fisted operator can tear up an anvil with a rubber mallet.
3. Keep up the maintenance - bearings, alignment, moisture, dirt, connections, contacts.
Setting the overloads down won't do anything to save the motor. It is going to draw the current it needs, and run as hot as required to dump the heat from the current, to produce the power required by the mechanical design. There is nothing you can do to the overloads to change that.
In fact, setting the overloads down may well cause the motor to fail sooner - I'll go over this one if it causes anyone to say, "Huh - where is that screwball coming from?"
Electrical testing: Megging, PI, DAR, negative sequence monitoring (voltage and current balance) won't make the motor last any longer. It might save a few bucks in the cost of a rewind. It can save money in allowing for a scheduled replacement instead of running to failure.
Vibration monitoring can help in scheduling bearing replacements/alignments prior to motor catastrophic failure.
Alarming/Shutdown protective relaying on voltage/current imbalance can definitely help in larger end of sizes. This is perhaps more true above 200hp (and MV motors)
Some electronic OLs incorporate a motor thermal model that accounts for overload, ambient temp, as well as multiple starts. They are really good units
For SF 1.15, unless there is a reason not to, I tend to set overloads at 140%. Rarely is there a reason not to. I've never seen any evidence that setting the OLs up to the max, shortens the life of a motor.
cf
