Synchronous motor field application timing

[AZElectrical]

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?

 

[Skogsgurra]

It is correct. And your Vfld = -N*d(phi)/dt is correct. But you seem to forget that d(sin(t))/dt is the same as cos(t) and hence 90 degrees out of phase.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[AZElectrical]

It is correct. And your Vfld = -N*d(phi)/dt is correct. But you seem to forget that d(sin(t))/dt is the same as cos(t) and hence 90 degrees out of phase.

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?

 

[LionelHutz]

It's done at the zero crossing because the current and magnetic fields in the rotor poles are moving towards the same state as they'd be with the field applied. So, the motor "assists" in building the fixed rotor field and it also synchronizes smoothly when you apply the field current at that time.

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.

 

[Skogsgurra]

The important words are "zero crossings of induced current in the discharge resistor".

There is no phase shift in the resistor, so when current in the resistor is zero, the flux is at maximum.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

I know flux = Li and since the resistor and field are in series the current in both must be equal which means when the resistor current is 0 that the winding current is also 0 and the flux is also 0.

 

[Skogsgurra]

I give it up!

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[AZElectrical]

I know flux = Li and since the resistor and field are in series the current in both must be equal which means when the resistor current is 0 that the winding current is also 0 and the flux is also 0.

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.

 

[LionelHutz]

I thought about this more and had to remember that the field is acting as an AC alternator winding during the motor start. Starting with the field applying it's output voltage to the resistor, you should be able to understand why the flux is at a peak when the resistor voltage and current are zero.

Trying to consider the circuit and a series resistor and inductor is wrong.

 

[oldfieldguy]

Brushless exciter? Or brushless synchronous motor?

old field guy

 

[AZElectrical]

I thought about this more and had to remember that the field is acting as an AC alternator winding during the motor start. Starting with the field applying it's output voltage to the resistor, you should be able to understand why the flux is at a peak when the resistor voltage and current are zero.

Trying to consider the circuit and a series resistor and inductor is wrong.

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? Or brushless synchronous motor?

Brushless exciter.