DickDV
Member
I've studied the white paper by MJ Melfi of Reliance Electric and am persuaded that the equivalent circuit he offers as Figure 3 is valid for a loaded induction motor.
It is clear from his Figure 3 that there is nothing in the equivalent circuit but inductance and resistance. There is no provision for any kind of internal generator in this circuit.
In equation #2, Melfi labels Eg as "internally generated voltage due to motor rotation". At the end of that same page and the beginning of the next page, he goes on to define Eg as Vt minus the voltage across the stator leakage resistance and the leakage reactance, none of which have anything to do with motor rotation. I believe that this is a labelling error of Eg and is probably due to Melfi being a former DC motor specialist (almost all Reliance motor engineers came up thru the DC ranks).
Further along in his paper under the heading "Field Oriented Control", Melfi defines motor lead current as the vector sum of Flux Producing Current and Torque Producing Current. This is a well-known definition and has been thoroughly tested in the field and found completely reliable. I have personally used this relationship numerous times to find flux amps. The procedure involves energizing the motor to rated volts and frequency and measuring current with no load on the shaft. Or, as I have often done, half the rated voltage and half the rated frequency can be applied, again with no load and the rotor turning at half the rated speed, with the same measured current. There is no provision in the definition and no findings in the measured current that would indicate an internally-generated voltage from rotation or any other source.
I conclude from the above that Melfi's equivalent circuit is valid and that there is no internal generator present in a loaded induction motor.
Having said all this, I realize that the above clearly does not explain, under overhauling conditions, how an induction motor regenerates power back to the supply, whether the AC line or an inverter.
I cannot claim to understand this mode of motor behavior and have never had it adequately explained either. As with all AC generation in a network, phasing of the AC wave is a critical indicator of energy flow. How this is brought to bear in an overhauled induction motor is not clear to me either.
I conclude that Melfi's equivalent circuit is valid only for a loaded motor and that there must be some internal generating element that appears only when the motor is overhauled or, expressed another way, when the motor is driven into positive slip.
I would like a more complete explanation with supporting documentation of the overhauling motor condition and accompanying regeneration.
Looking forward to further discussion on this, DickDV
It is clear from his Figure 3 that there is nothing in the equivalent circuit but inductance and resistance. There is no provision for any kind of internal generator in this circuit.
In equation #2, Melfi labels Eg as "internally generated voltage due to motor rotation". At the end of that same page and the beginning of the next page, he goes on to define Eg as Vt minus the voltage across the stator leakage resistance and the leakage reactance, none of which have anything to do with motor rotation. I believe that this is a labelling error of Eg and is probably due to Melfi being a former DC motor specialist (almost all Reliance motor engineers came up thru the DC ranks).
Further along in his paper under the heading "Field Oriented Control", Melfi defines motor lead current as the vector sum of Flux Producing Current and Torque Producing Current. This is a well-known definition and has been thoroughly tested in the field and found completely reliable. I have personally used this relationship numerous times to find flux amps. The procedure involves energizing the motor to rated volts and frequency and measuring current with no load on the shaft. Or, as I have often done, half the rated voltage and half the rated frequency can be applied, again with no load and the rotor turning at half the rated speed, with the same measured current. There is no provision in the definition and no findings in the measured current that would indicate an internally-generated voltage from rotation or any other source.
I conclude from the above that Melfi's equivalent circuit is valid and that there is no internal generator present in a loaded induction motor.
Having said all this, I realize that the above clearly does not explain, under overhauling conditions, how an induction motor regenerates power back to the supply, whether the AC line or an inverter.
I cannot claim to understand this mode of motor behavior and have never had it adequately explained either. As with all AC generation in a network, phasing of the AC wave is a critical indicator of energy flow. How this is brought to bear in an overhauled induction motor is not clear to me either.
I conclude that Melfi's equivalent circuit is valid only for a loaded motor and that there must be some internal generating element that appears only when the motor is overhauled or, expressed another way, when the motor is driven into positive slip.
I would like a more complete explanation with supporting documentation of the overhauling motor condition and accompanying regeneration.
Looking forward to further discussion on this, DickDV