Originally posted by David_G:
How in a 'Y' wound stator circuit, a zero sequenced 3rd current harmonic could exist all together?
Saying that the 3rd harmonic can flow in Y or delta, I was reacting to the above. And indeed, any time harmonic can flow in delta or star. Nonlinearity of the circuit is enough to deform the current causing harmonics.
But inducing the current into Y is a different story. You certainly cannot induce 3rd harmonic from rotor into the Y connected stator. You can induce it to the Delta and the current can circulate in the Delta. The question is how much? I did not have a clue what is the zero sequence impedance. Luckily I have a 20 hp Delta connected motor in my garage. So I broke the Delta connection and measured the zero impedance. To my surprise it was 10 Ohms at 60 Hz, it means it is 30 Ohms at 180 Hz (3rd harmonic). The locked rotor impedance is about 1.45 Ohm, so the zero sequence is much, much larger. In order to push any significant current (such as 24% FL) through this impedance would require the 3rd harmonic voltage to be huge. Does not seem realistic to me.
The Delta connection is routinely used even on medium voltage machines (2300/ 4160 Volts). Never have I heard about necessity to de-rate the motor when ran in Delta. Yet the flattening of the spatial flux density is an absolutely normal and very well known fact and it is present in every motor.
The paper is an interesting new look at the torque due to 3rd harmonic. As I have indicated above, the deformation of the spatial flux density has been known and accounted for differently than thinking about 3rd harmonic. One way (I am familiar with) increases the calculated flux by an empirical formula. The increase is larger for small motors and smaller for large motors.
The fact is that most of the large motors are connected in Y. As I have said I am not aware of necessity to de-rate due to the 3rd harmonic (due to saturation). I do know that the delta connection may support “secondary armature reaction”. It is a very complicated thing. The rotor harmonics can induce frequencies other than the 60 Hz in the stator. Those currents can freely circulate in Delta and create parasitic torques during the acceleration. On the top of it the parallel branches in the stator can contribute to the problem. But I have to admit, I have never looked into the details.
I have just realized that some time ago I have taken current signature on the Delta connected motor. I took the signature inside the delta and on the input line. I am attaching the spectra and waveforms. It was taken on 150 hp motor, 575 Volts. The nameplate current is roughly 150 Amps. You can see that the third harmonics inside Delta is indeed larger (2.89 Amps) compared to the 3rd harmonic in the input line (0.89 Amps). The increase seems to be significant until you take into account the full load current (the signature was taken at no load). Hence, the 3rd harmonics will drown in the full load current once the motor is loaded.