# Synchronous Machine Voltage Curves & Torque Relation to Armature Current

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1. Mar 31, 2017

### Alesa

Hello everyone

Im new on the forum and I just discovered what are sync machines...

I have really basic question what I still cannot figure out.. I did some experiments with motor for different field currents and torques. In the result I measured armature currents and did V curves. But next question asks to do find theoretical solution for same problem. As I guess, torque related to the power through sync speed and power related to I armature as I=P/(Va*cos(theta)). However it doesnt work for me and Im pretty lost.

Maybe someone can give me at least direction where and what I should read about.. I have air gap voltages, open and short circuit charachteristics and different input torques and calculated different sync inductance values for open loop voltages. Machine is 400v 0.8kW cos=0.75 50Hz 4 poles 1500rpm.

2. Mar 31, 2017

### Dr.D

Get a good book on electrical machine theory. I recommend the ones authored by Krause (Purdue U).

3. Apr 1, 2017

### jim hardy

The way to think about synchronous motors is an electromagnet in motion being dragged along by the magnetic field of stator ..
A generator is identical except torque is reversed, rotor is held back by stator.

From that mental picture it's a small step to phasor representation of what's going on. Brush up your grade school geometry and dust off your old drafting compass...

see if this one is any help.

4. Apr 1, 2017

### Dr.D

The one thing that I would add to Jim's answer is that the magnetic pole on the rotor is always locked to the corresponding pole on the stator; it is not allowed to slip. If the torque is so great that it does slip, the machine falls out of synchronism.

5. Apr 3, 2017