Motor Testing & Inspections for shutdown 1

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.

Suggestion: Generically, it is good idea to evaluate each motor function in terms of safety, production cost of downtime if it fails, damages, etc. according to understanding of the plant processes. Some motors will probably have to be tested thoroughly, some replaced, some maintained, some will probably need very little of testing perhaps insulation test only and routine maintenance check. There are industry standards, e.g. IEEE Stds that cover induction motor testing, and dc motor testing. Check
for the current names and revisions.
There are also specialized electric motor services available good to contact for potential subcontracting.

rhatcher and jbartos, thanks for your suggestions which i will certainly incorporate.

jbartos, i have already completed a criticality analysis on the plant in terms of impact to production, process, breakdown frequency, etc. im now using this and kW rating of the motor as a starting point in defining what tests/mtce checks to be done on which motors.
if you have any further suggestions or handy tips, please let me know. thanks for your input

All very good points that the level of maintenance is evaluated based on criticality of equipment, history (historically-unreliable recieves more), cost and effort for repair, etc.

At many facilities in addition to the above there is a push towards condition-based maintenance, rather than time-based maintenance.

Your decision to schedule a motor for off-line cleaning is more likely if on-line monitoring has detected increasing trend in temperature, or perhaps if vibration indicates imbalance (sometimes dirt on rotor).

Likewise at out plant we have on-line partial discharge monitoring for 13.8kv motors. If an anomaly is indicated here we schedule it for further off-line diagnostic tests, possibly including dc hi-pot, and doble power factor tests, and a special off-line partial discharge.

A motor which has seen constant abuse from dirty environment and/or persistent overload should perhaps be cleaned/inspected more often.

etc etc.

That's the in-vogue philosophy of condition-based maintenance.

In terms of a standard, I think that NFPA 70E has some guidance on recommended test frequencies, as does NETA.

IR/PI is your bread and butter test. You also have to decide if you want to disconnect leads or not for the test... adds a lot of time if these are high voltage leads which need to be taped with raychem.

We don't do winding resistance testing any more.... seems like not a very useful test. (unless you have a multi-tester like PDMA which picks it up without any additional effort).

Greasing of lub'd bearings is sometimes performed off-line, although much preferred on-line.

You may want to make sure your space heaters are working on critical motors. That's a pretty good investment in time. Either measure the current with a clamp-on or feel the motor case.