Is there a such thing as Asymetric Current?

[Eduardo Maun]

I took an exam unfortunately I'm not able to answer the question with Asymetric Current. The question is what is the effect of an asymetric current in a motor? Is this the same thing as In Rush Current?During starting of the motor?I checked on the net but I found no result(s)

Thank you.

God Bless...

 

[mivey]

They were probably talking about asymmetric currents that might result from the voltages being unbalanced.


add: It can cause the motor to overheat.

 

[Hameedulla-Ekhlas]

There are two type of Short Circuit,

1. Asymmetrical Faults which contains
line to ground fault
line to line fault
two line to ground

2. Symmetrical Faults

Three line to ground

 

[mivey]

From the wording of the question, I don't thiink they were talking about fault current.

 

[gar]

100211-2332 EST

From dictionary.com
a⋅sym⋅met⋅ric 

?adjective 1. not identical on both sides of a central line; unsymmetrical; lacking symmetry: Most faces are asymmetric.
2. (of a logical or mathematical relation) holding true of members of a class in one order but not in the opposite order, as in the relation ?being an ancestor of.?
3. Chemistry. a. having an unsymmetrical arrangement of atoms in a molecule.
b. noting a carbon atom bonded to four different atoms or groups.
c. (of a polymer) noting an atom or group that is within a polymer chain and is bonded to two different atoms or groups that are external to the chain.

If I put a diode in series with one lead to a motor the result is an asymmetric current. Probably big problems unless it is a DC motor.

.

 

[ohmhead]

Well Symmetric is normal current flow in a motor perfect balance operation under load running and asymmetric current would be motor under odd load swings or other issues like unbalanced phase to voltage ground faults or short circuit .

Normal three phase ac power thur zero a perfect wave symmetric!

 

[Hameedulla-Ekhlas]

Asymmetrical and Symmetrical currents are nothing except fault currents which are balance and unbalance during any short circuit calculation.

 

[mivey]

If I put a diode in series with one lead to a motor the result is an asymmetric current. Probably big problems unless it is a DC motor.

.

or a big enough resistor.

 

[mivey]

Asymmetrical and Symmetrical currents are nothing except fault currents which are balance and unbalance during any short circuit calculation.

If you are ok with calling loads faults then I guess you might see it that way.

 

[hurk27]

What about the effect of the capacitors in single phase motors, they will shift the phase angles making the currents lead and lag depending upon the rotor position?

 

[Hameedulla-Ekhlas]

2-4.2.1.2 Generators. Short circuit current flow from a generator is limited by thegenerator impedance and the circuit impedance between the generato r and the fault.The magnitude of the generator fault current depends on the armature and fieldcharacteristics, the time duration of the fault, and the load on the generator


.2-4.2.1.3 Motors. Synchronous motors appear as a source of generation during ashort circuit, with the current magnitude limited by the motor impedance and the circuitimpedance between the motor and the fault. Induction motors also provide short circuitcurrent, but the current supplied decays faster than with a synchronous motor


.2-4.2.1.4 Transformers. Supply transformers are not sources of short circuit current,but they have a significant impact on the magnitude of short circuit current supplied to afault location. The transformer impedance will function to limit the short circuit current,but the short circuit current available at the transformer will be magnified or reduced inaccordance with the turns ratio as it passes through the transformer.


2-4.2.2 Short circuit current from the above sources varies with time; not a ll sourcescan sustain the peak short circuit current. Instead, the short circuit current available inthe system reaches some peak value and decays over the next few cycles to a smallersteady-state value. With time, the short circuit current provided b y rotating machinesfalls to zero as they brake to a complete stop. Short circuit current is modeled as afunction of time by three distinct impedances:?Subtransient reactance (Xd'')-- the effective reactance defining the short circuitcurrent during the first few cycles after a fault occurs. Use this value in all shortcircuit studies.?Transient reactance (Xd')-- the effective reactance during the period after the firstfew cycles up to about 30 cycles after a fault occurs. Use this value in voltageregulation and stability studies.?Synchronous reactance (Xd)--the effective reactance after a steady-state conditionhas been reached. In terms of short circuits, steady-state conditions occur severalseconds after the fault occurs


.2-4.3Symmetrical Versus Asymmetrical Current.2-4.3.1 Include the effect of asymmetric current in all short circuit studies. Normalcurrent is symmetric about the zero axis. S hort circuit current tends to have symmetricand asymmetric behavior. The degree to which the current waveform is asymmetricaldepends on when the fault occurs in relation to the voltage waveform peak or zero, andthe proportions of resistance and reactance in the circuit. If a short circuit occurs in aninductive reactive circuit at the peak of the volta ge waveform, the resulting short circuitcurrent is completely symmetrical as shown in Figure 2 -2. If a short circuit occurs in thesame circuit but at the zero of the waveform, the resulting short circuit current will becompletely asymmetrica

 

[hurk27]

100211-2332 EST

From dictionary.com
a⋅sym⋅met⋅ric 


If I put a diode in series with one lead to a motor the result is an asymmetric current. Probably big problems unless it is a DC motor.

.

But put in a capacitor and you get rotation from the asymmetric current? in a ac motor

 

[hurk27]

2-4.2.1.2 Generators.

.2-4.2.1.3 Motors. Synchronous motors appear as a source of generation during ashort circuit, with the current magnitude limited by the motor impedance and the circuitimpedance between the motor and the fault. Induction motors also provide short circuitcurrent, but the current supplied decays faster than with a synchronous motor

This could be described as dynamic braking, and could very well be the results the OPs question is asking, but with out more info on the question?

 

[Hameedulla-Ekhlas]

This could be described as dynamic braking, and could very well be the results the OPs question is asking, but with out more info on the question?

I just wanted to add some information about question.

Yes, the last paragraph is explains all, which is below

2-4.3Symmetrical Versus Asymmetrical Current.2-4.3.1 Include the effect of asymmetric current in all short circuit studies. Normalcurrent is symmetric about the zero axis. S hort circuit current tends to have symmetricand asymmetric behavior. The degree to which the current waveform is asymmetricaldepends on when the fault occurs in relation to the voltage waveform peak or zero, andthe proportions of resistance and reactance in the circuit. If a short circuit occurs in aninductive reactive circuit at the peak of the volta ge waveform, the resulting short circuitcurrent is completely symmetrical as shown in Figure 2 -2. If a short circuit occurs in thesame circuit but at the zero of the waveform, the resulting short circuit current will becompletely asymmetrica

 

[Hameedulla-Ekhlas]

This could be described as dynamic braking, and could very well be the results the OPs question is asking, but with out more info on the question?

I just wanted to add some information about question.

Yes, the last paragraph explains all, which is below

2-4.3Symmetrical Versus Asymmetrical Current.2-4.3.1 Include the effect of asymmetric current in all short circuit studies. Normalcurrent is symmetric about the zero axis. S hort circuit current tends to have symmetricand asymmetric behavior. The degree to which the current waveform is asymmetricaldepends on when the fault occurs in relation to the voltage waveform peak or zero, andthe proportions of resistance and reactance in the circuit. If a short circuit occurs in aninductive reactive circuit at the peak of the volta ge waveform, the resulting short circuitcurrent is completely symmetrical as shown in Figure 2 -2. If a short circuit occurs in thesame circuit but at the zero of the waveform, the resulting short circuit current will becompletely asymmetrica

 

[K8MHZ]

I just wanted to add some information about question.

Yes, the last paragraph explains all, which is below

2-4.3Symmetrical Versus Asymmetrical Current.2-4.3.1 Include the effect of asymmetric current in all short circuit studies. Normalcurrent is symmetric about the zero axis. S hort circuit current tends to have symmetricand asymmetric behavior. The degree to which the current waveform is asymmetricaldepends on when the fault occurs in relation to the voltage waveform peak or zero, andthe proportions of resistance and reactance in the circuit. If a short circuit occurs in an inductive reactive circuit at the peak of the voltage waveform, the resulting short circuitcurrent is completely symmetrical as shown in Figure 2 -2. If a short circuit occurs in thesame circuit but at the zero of the waveform, the resulting short circuit current will becompletely asymmetrica

I am confused about the use of the word 'occurs'.

Does that mean the fault starts at a particular time or only happens at a particular time?

An diagram of the difference would help me understand this better.

I was under the impression of an example of an asymmetrical current would be a rectified half wave.

 

[mivey]

But the question seemed to be asking about the effect of an asymmetric current in a motor, not the motor's contribution to a fault. The phrase asymmetric current does not have to mean fault current.

IMO, they wanted to know the effect of asymmetric currents on the motor. These currents can be caused by asymmetric voltages and the effect is an increase in the winding temperature.

 

[mivey]

I was under the impression of an example of an asymmetrical current would be a rectified half wave.

That would be an example of an asymmetrical current.

 

[weressl]

If you are ok with calling loads faults then I guess you might see it that way.

Listen to the kid, he knows what he is talking about.

 

[weressl]

That would be an example of an asymmetrical current.

Nope. Asymetrical to what? Symetry is formed when comparing two or more elements.