The determination of which motors are worthy of an inspection and which aren't cannot be made by any one standard rule. It is instead pretty specific to a facility based on type of industry, size of the facility, number and size of motors in use, operational philosophy, etc. Even within similar facilities a wide variation may exist. As an example, there are three plants with identical processes and similar size that are clients of my company. One performs no inspections on it's motors that I know of. One inspects only 3 of it's motors (complete inspection of 3000 hp, 180 rpm, 4160V synchronous motors...about $100,000 U.S. and two weeks of intense work to to perform a stator rewind on one of those babies if it fails). The last plant inspects all motors over 100hp to some degree and performs complete inspections on the big ones.
Another example to think about...I have seen an entire plant shut down by loss of a 2hp motor. The motor was part of an electrostatic precipitator at a manufacturing plant and environmental regulations prevented this facility from running without the precipitator in service. They had not identified it as critical , so no checks were performed on the motor and they had no spare. It get's worse... the motor was an unusual type so the only one available was across the country. We flew the motor in (freight cost was more than the motor cost) and they started the plant back up after a loss of about 12-16 hours of production.
Anyway, with respect to your question, I recommend the following guidelines as a start:
1 - Identify any motors which are CRITICAL to your client. Any motor that would cause a major loss of production or would create a hazard if it failed should be considered critical. These motors should be inspected at a frequency based on age, operating environment, and how critical they are. A spare should be acquired and you may want to recondition or replace these motors at regular intervals (5-20 years) based on the application and the environment regardless of the condition as assessed by maintenance inspections.
2 - Identify motors which would be DIFFICULT OR IMPOSSIBLE TO REPLACE if they were critically damaged. Many facilities operate obsolete motors or specialty motors which cannot be purchased or, if available, have a very long delivery lead time (months). These motors should be inspected at a frequency based on age and operating environment and should definitely be reconditioned at regular intervals (5-20 years) depending on the motor, the application, and the environment regardless of the condition as assessed by maintenance inspections.
3 - Identify motors which would be considered EXPENSIVE by your client to rewind or replace. It is much cheaper to recondition a motor with weak insulation than to rewind or replace it. These motors should be inspected at a frequency based on age and operating environment and should definitely be reconditioned at regular intervals (5-20 years)depending on the motor, the application, and the environment regardless of the condition as assessed by maintenance inspections.
Inspection workscope should include at a minimum an insulation resistance check. A PI check, a surge test, and a bearing inspection is recommended for a complete inspection workscope. For wound rotor and synchronous motors additional checks would be required. Obviously, DC inspections are completely different.
