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Hz Frequency directly represents no of rotation of rotor for that machine, the higher the frequency the more the rotation of the rotor, i.e. the rpm of the motor there by result in bearing wear and tear, if the motor run at no load, voltage will be very high than the rated value which may result in insulation failure for the motor winding.

The rated operating voltage must be the same, the motor power will reduced by 50/60 Hz. so if the motor working at the full load, load must be reduced by the same ratio, to keep the motor temperature within the rated limit.

Torque/speed curve is on blower flux. You must reduce load or increase voltage, being careful of insulation voltage level. If motor is fed by variable frequency drive, speed reference must be changed to ensure correct V/Hz control (open loop control).
You can even get low cost variable frequency drives with low commutation resolution (8 bit and less) that WILL cause more audible noise in EITHER type motor irregardless of PWM frequency! The variables are infinite. The fact remains BLDC does not have to have different noise than an equivalent induction motor.
Efficiency is built into a motor on design and on the quality of the steel that makes up the laminated core. The fan and bearings are also a factor. "T" frame motors have quite a heavy slot fill so there is not much you can do with conductor size. There is not a lot you can do with bearings because the housings and journals are manufactured for a specific bearing series and size. You could put a smaller fan but the motor would overheat. In a rewind situation you have to be extremely careful to maintain the efficiency.
The Tan-Delta test is an off-line AC test, usually sinusoidal. The Tan Delta is the ratio of the real (resistive) current to the capacitive current. This ratio should be relatively constant despite test voltage making the Tan Delta value a pass/fail. The resistive current is typically much smaller than the capacitive current on a good insulation system.
Surge and hipot testing can be potentially destructive. But, so can a Megger test on a motor which is in very poor condition. When testing a motor, tests are performed starting with the low voltage tests and moving on to the higher voltage tests. When a motor fails any test, the more rigorous tests are not performed, so no further damage is done. If a motor fails a resistance test or a Megger test, you do not perform the hipot and surge tests.
Motor Circuit Analysis (MCA) is a de-energized test method and the test can be initiated from the Motor Control Center (MCC) or directly at the motor. The advantage to testing from the MCC is the entire motor system, including the connections and cables between the test point and the motor, is evaluated.
Noise and sound are a bit tricky to deal with. Generally the noise from the motor fan is "white noise" and not objectionable to the ear, except for large machines that were mentioned or as mentioned below. The objectionable noise normally comes from the slot combination used in the design. The slot combination generates a variety of frequencies, many of which can be easily calculated based on the number of slots, various combinations, slip, speed, frequency of operation, etc.
I have been in the electric motor repair industry all my working life, (50 years) and I have watched this industry progress. During my apprenticeship in the UK we were taught all about the damages that can be done to a stator core if proper procedures were not followed and in all honesty when I first arrived on Canadian shores I was horrified at some of the methods.
INVERTER-READY means that some of the extras required to protect the machine from the damaging effects of the drive output waveform are included.
INVERTER-DUTY (also sometimes listed as "inverter ready") means even more extras are built in. All the inverter-ready items are there.
Case: The motors from approx 22kw - 250kw and are double star-wound to facilitate a wide speed range. The motors and frequency inverters are matched. The bearings are not showing the usual ESD damage indications but we are experiencing short life (less than 8000 hours in some cases). The bearings on some units are insulated (80kw plus). Is it the bearings that re undersized or if there may be another issue?
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