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I_am_learning

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I see some trouble with how symmetrical components are treated. In lots of texts, I have heard something along these lines

When the current in a 3 phase motor is unbalanced, we can resolve it into three sets of balanced currents.

1. Positive sequence currents.

These produce rotating magnetic fields in usual direction and produces +ve torque and power

2. Negative sequence currents

There produce rotating magnetic fields in opposite direction and produce -ve torque

3. Zero sequence currents

These produce stationary and pulsating magnetic fields and produce no torque.

Fine upto now. the net torque is sum of all torques.

I fully agree that mathematic ally, 3 phase unbalanced set of currents (or any phasors) can be thought off as sum of 3 sequence components.

What I disagree is when people talk about the effects of these currents.

For example I don't think we can say the losses in stator winding = (I_positive^2*r + I_negative^2*r + I_zero^2*r).

Mostly, I find texts talking about negative sequence currents.

"Since negative sequence currents produce rotating magnetic fields in opposite direction, it will have large relative velocity with rotor (nearly 2*Ns , Ns is synchronous speed). It will create large induced currents in rotor and huge eddy losses."

But I don't think we are allowed to find out the induced currents and eddy losses due to the 3 sequence currents individually, just like we can't find the I^2*R losses individually.

For example, consider that a DC current of 2A is flowing in one of the winding of a motor. Or even better, let's consider a motor with no current in any winding.

We can think of 0 current as 0 = 100*Sin(wt) + (-100*Sin(wt)) , as composed of two AC currents. Now Clearly, we can't individually calculate the losses (eddy and I^2*R) due to each current and Add.

Please help me learn. :)

When the current in a 3 phase motor is unbalanced, we can resolve it into three sets of balanced currents.

1. Positive sequence currents.

These produce rotating magnetic fields in usual direction and produces +ve torque and power

2. Negative sequence currents

There produce rotating magnetic fields in opposite direction and produce -ve torque

3. Zero sequence currents

These produce stationary and pulsating magnetic fields and produce no torque.

Fine upto now. the net torque is sum of all torques.

I fully agree that mathematic ally, 3 phase unbalanced set of currents (or any phasors) can be thought off as sum of 3 sequence components.

What I disagree is when people talk about the effects of these currents.

For example I don't think we can say the losses in stator winding = (I_positive^2*r + I_negative^2*r + I_zero^2*r).

Mostly, I find texts talking about negative sequence currents.

"Since negative sequence currents produce rotating magnetic fields in opposite direction, it will have large relative velocity with rotor (nearly 2*Ns , Ns is synchronous speed). It will create large induced currents in rotor and huge eddy losses."

But I don't think we are allowed to find out the induced currents and eddy losses due to the 3 sequence currents individually, just like we can't find the I^2*R losses individually.

For example, consider that a DC current of 2A is flowing in one of the winding of a motor. Or even better, let's consider a motor with no current in any winding.

We can think of 0 current as 0 = 100*Sin(wt) + (-100*Sin(wt)) , as composed of two AC currents. Now Clearly, we can't individually calculate the losses (eddy and I^2*R) due to each current and Add.

Please help me learn. :)

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