Variable Speed Concerns

Whenever load speeds are varied, there are many considerations which must be taken into account. These concerns are both electrical and mechanical in nature.

The electrical problems associated with electronic drives generally concern the insulation. Because of the type of output generated by the inverter, there is great stress placed upon the insulation and the temperature rise of the windings may increase. In other cases, the motor may be run below its minimum self-cooling speed. The main trouble is that for every 10 degrees C, the insulation life of the windings are reduced by half. If the temperature rise is allowed to climb too high, the motor will overload and burn-up in a very short time. An additional problem, which is rare, is inverter resonance.
These difficulties can be avoided through the following means:
· Rewind or replace the motor – Rewind the motor to a higher insulation class, or replace it with a new motor. the new motor may be of the energy efficient or inverter duty type.
· Provide external cooling – This is especially important in cases where the self-cooling ability of the motor is compromised.
· Re-set the parameters – inverter resonance is found in cases where the drive parameters are not properly set. If this is not the case, the drive should be programmed to by-pass those frequencies where the problems are found.

Mechanical considerations include mechanical resonance and driven load incompatibility. Mechanical resonance can be defined as the speed of the driven load that matches its natural frequency. If this speed is found and maintained, the equipment will develop extremely high levels of vibration and may shake itself apart. Load incompatibility can be defined as loads which may not be operated at speeds lower than their design speed. For instance, many gearboxes have a minimum speed at which the lubricating oil may not be properly moved over the contacting parts.
Mechanical resonance can be avoided by programming the drive to avoid the appropriate frequency(s). The resonance levels may be determined by using a vibration analyzer and operating the machine through the entire speed range. Another way is by performing a “ring-test” using vibration analysis equipment. Load incompatibility can only be avoided by not allowing the drive to operate below a minimum speed.

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