Motor controls

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The voltage transient which occurs whenever there is a sudden change in current in an inductive device. Inductors resist a sudden current change.
V=L di/dt
In electric motors this occurs at start up when the contactors close and shut down when the contactors open. Soft starts reduce the start up transient, but not the shutdown transient.
This also occurs with variable frequency drives which switch the current rapidly and repeatedly.
In speaking with many engineers over several decades, a large manufacture of large electric motors, have decided once a rotor is identified as the problem, rebarring, or any single repair is usually unsuccessful, and their procedure is to scrap the rotor completely.
The condition of the rotor bars will determine how much torque your motors will deliver. As a person who has been in the electric motor repair business all my life it is something I constantly check. When a motor is developing open rotor bars it will become noisy on start up. Noisier with each bar that becomes open. It can sound like a cement mixer or as if there is no lubrication in the bearings.
The noise level created by the motor at any speed is in a fixed environment, take two motors same HP, Speed, Enclosure, and the applied voltage could be a factor of the noise, the installed conditions of a 1000 motors could vary from alignment to load, to piping connected to load, to actual load.
Babbitt Bearings, properly cared for and maintained will last much longer than anti-friction bearings. Properly set up and aligned there is no contact between the babbitt and the journal other that a scuff mark at the bottom that is caused by minimal contact at starting. As long as the motor is properly aligned to the load, the oil is kept clean and continually fed to the bearing the bearing will last a very long time.
Some motor manufacturers go from CU to Al because they try to reduce costs.
Then it just works the other way. Al wire needs to have a larger diameter than Cu if you want the same motor performance.
Then you may face problems with the slot opening and the slot fill, there may not even be enough space at all for the Al wire. You may need to change your lamination.
For a DC Motor Armature, There is a simple method of determining the condition of the Armature. Drop Test Method: Give a DC Voltage across the commutator Segments for one pole pitch area from a Power supply or Battery. Connect Positive end of the DC power supply at one end and the Negative end at the opposite end.
I am currently investigating the design of a three phase axial flux PM motor, but replacing conventional materials with high temperature superconductors. I'm interested to know the thoughts of group members regarding design rules/rules of thumb relating to the number of stator coils and rotor poles. Many in the amateur wind turbine community seem to use a 4:3 ratio (magnets:coils), but I can't seem to find anything 'official' on the topic.
AC Motors - Variable torque: AC motors have a speed torque characteristic that varies as the square of the speed.
AC Motors - Constant torque: These ac motors can develop the same torque at each speed, thus power output varies directly with speed.
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