Synchronous motor field application timing
Synchronous motor field application timing
(OP)
I have been going through old product data on a 5,500hp synchronous motor with a brushless exciter as part of an upcoming modernization project. The manual states that the optimum angle for field application occurs when the current induced in the field discharge resistor moves through a positive going zero crossing because it is at this time that maximum induced flux, and hence maximum pull-in torque, is available. This indicates that the flux is 90 degrees out of phase with the induced current, and I do not understand why.
My thinking was that the induced voltage in the field winding would be Vfld = -N*d(phi)/dt, where phi is the flux from the stator winding. Subsequently, the current in the field winding would be Vfld/Zfld. I struggle to get from this point to a current-flux relationship with a 90 degree difference between the two. Any thoughts on what I am missing?
My thinking was that the induced voltage in the field winding would be Vfld = -N*d(phi)/dt, where phi is the flux from the stator winding. Subsequently, the current in the field winding would be Vfld/Zfld. I struggle to get from this point to a current-flux relationship with a 90 degree difference between the two. Any thoughts on what I am missing?





RE: Synchronous motor field application timing
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Synchronous motor field application timing
I'm not following you. I realize that the time derivative of a sinusoidal flux is cosine and thus 90 degrees out of phase, but that describes the induced voltage. The exciter manual states that it tracks zero crossings of induced current in the discharge resistor. Where does the 90 degree shift between induced current and induced flux come from?
RE: Synchronous motor field application timing
Referencing the induced field current, you want to fully establish the applied field current in the quarter between the positive going zero crossing and the peak positive induced current. You want to get the applied field established by the time the peak positive current would occur because that is the point when the rotor and stator magnetic fields are aligned the same way as they will be when the motor is synchronized.
The flux isn't at a maximum when the induced field current is crossing zero.
RE: Synchronous motor field application timing
There is no phase shift in the resistor, so when current in the resistor is zero, the flux is at maximum.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Synchronous motor field application timing
RE: Synchronous motor field application timing
No, I will try once more. Then I will give up.
The situation is as follows:
1. There is NO external current in the field winding
2. OP shall apply current.
3. He reads in the manual that "the optimum angle for field application occurs when the current induced in the field discharge resistor moves through a positive going zero crossing"
4. First Conclusion: The flux is at its maximum and no voltage is induced in the field winding (minimum di/dt).
5. Second conclusion/observation: If you want the excitation current to flow immediatley, you shall apply it when there is already a current flowing (Lenz' Law and all that).
6. Third conclusion: The manual is right and it seems that most people cannot read.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Synchronous motor field application timing
Checking back through some old textbooks, I came to a similar conclusion. I haven't been able to find anything that leads me to believe there is a shift between current and flux due to said current. The timing explanation you provided above makes sense.
Thinking about it a bit more, when the slip is under 5%, the reactance of the field winding would be fairly low (X = 2*pi*f*L, f = 3Hz or less). It seems reasonable to conclude that the circuit composed of the field winding and discharge resistor is mostly resistive at this point, thus induced voltage, current and flux would be more or less in phase with one another.
RE: Synchronous motor field application timing
Trying to consider the circuit and a series resistor and inductor is wrong.
RE: Synchronous motor field application timing
old field guy
RE: Synchronous motor field application timing
Why do you say it is wrong to consider the combination field winding / discharge resistor as a series RL circuit? I envision it as a voltage source (the result of -N*d(phi)/dt from the stator currents) in series with R and L of the field winding as well as the discharge resistor. When the current in the discharge resistor is zero, which flux are you saying is maximum -- flux from the stator currents or that resulting from the current in the field circuit?
Brushless exciter.