Synchronous motors 1

[Jk1996]

With synchronous motors when loaded current increases just as induction motors. I know this is due to back emf getting smaller as it’s loaded. How is back emf less when loaded? I know it’s something to do with the rotor magnetic flux?

 

[xnuke]

The increase in armature current when load increases in a synchronous motor is not due to back emf magnitude getting smaller as occurs in an induction motor. It is due to an increase in the angular difference between the stator terminal voltage applied and the internal stator armature voltage at a given excitation. This angular difference in these two voltages, δ, known as the torque angle or power angle, goes up as the rotor briefly slows during the transient due to the additional mechanical load. Once the transient finishes, the rotor field lags the applied stator field by a larger angular amount but resumes rotation at synchronous speed. This increase in the voltage differential angle allows more stator current to flow per Ohm's law.

If you want to see the math and some phasor diagrams, look here.

xnuke
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[Jk1996]

Thank you xnuke I’m just trying to visualise this how does the Torque angle increase the current drawn from the supply? And also is the greater the load the greater the shaft torque until the magnetic linkage breaks?

 

[xnuke]

I_A = (V_T - E_A)/X. As the phase angle of the back emf in the armature E_A lags the phase angle of V_T by a larger angle δ when the rotor slows due to a mechanical load increase, the numerator of the equation goes up, and therefore so does the armature current. Since torque is proportional to armature current, it also increases as the angle increases (which is why I mentioned the angle is called the torque angle) unless maximum torque is exceeded and pole slip occurs when the magnetic linkage breaks as you state. Note that the magnitude of the armature voltage E_A only changes if the excitation changes, so unless that occurs, only the torque angle increasing is responsible for the increase in armature current when the load increases.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Jk1996]

So what causes the back emf on these types of motor is it the strength of the DC field used in the rotor?

 

[xnuke]

Yes. The stator conductors, the rotor field, and the relative motion of the stator conductors to the rotor field means the requirements for generator action according to Faraday's law are met while the rotor spins, therefore an induced voltage is created in the stator windings that opposes the applied terminal voltage per Lenz's law.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Jk1996]

So when a mechanical load is applied does this effect the back emf just as in an induction motor?

 

[RRaghunath]

Yes and the increased back emf tries to weaken the air gap flux which is countered by increasing the field current in rotor winding.