Can you Ladies and Gents help me out here? As I understand it, self-induced voltage (CEMF) is directly proportional to current flow. Yet I read that inductive reactance remains constant as long as the frequency is constant and that inductive reactance is not affected by current flow. Are CEMF and inductive reactance not the same?
Inductive Reactance
- Thread starter TJK
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That is not correct. The voltage created by an inductive load is not proportional to the amount of current flow, but rather to the rate at which current is changing. In a DC circuit, current is constant, so the amount of counter electromagnetic force (CEMF, for those who did not know what the initials meant) is zero. In an AC circuit, the current rises and falls every cycle. The amount of CEMF depends on the rate of those rises and falls, or simply is dependent on the frequency. That is why with a constant frequency, the inductive reactance is also constant.
mivey
Senior Member
Charlie,
There is a CEMF in a DC circuit.
Think of the DC circuit which has a DC motor. As the motor starts, there is a high current and no CEMF. As the motor gets up to speed, the CEMF increases until it is slightly less than the applied voltage and the current to the motor drops. The difference in the applied voltage and the CEMF maintains the motor at a constant speed.
When a load is applied, the speed of the motor and the value of the CEMF drop. This drop in the difference between the applied voltage and the CEMF causes an increase in the current to the motor.
mivey
Senior Member
For those that want to play along, first consider this statement:
gar
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101218-1931 EST
Following is one discussion:
http://www.answers.com/topic/counter-electromotive-force
The following is interesting in that names and standards for electrical units were just getting started in the late 1800s. In a different thread I was trying to determine when the word WATT was first used as a unit of measure of either electric or mechanical power. So far I have not found a source for this information. The following does not help me on that subject, but is interesting.
http://www.ieeeghn.org/wiki/index.p...cal_and_Electronic_Engineers_(IEEE)_Standards
The following reference provides an official time for definition of the WATT, but it does not say whether watt was used as a measure of power prior to this. Horsepower had been used to measure power long before this 1889 time frame.
http://www.ieeeghn.org/wiki/index.php/System_of_Measurement_Units
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.
Following is one discussion:
http://www.answers.com/topic/counter-electromotive-force
The following is interesting in that names and standards for electrical units were just getting started in the late 1800s. In a different thread I was trying to determine when the word WATT was first used as a unit of measure of either electric or mechanical power. So far I have not found a source for this information. The following does not help me on that subject, but is interesting.
http://www.ieeeghn.org/wiki/index.p...cal_and_Electronic_Engineers_(IEEE)_Standards
The following reference provides an official time for definition of the WATT, but it does not say whether watt was used as a measure of power prior to this. Horsepower had been used to measure power long before this 1889 time frame.
http://www.ieeeghn.org/wiki/index.php/System_of_Measurement_Units
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tallgirl
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How is there CEMF? V = dI/dT and dI is zero since I = V / R and V is the DC voltage (constant) and R is the resistance (not reactance) of the motor windings. For AC motors, the windings are inductors and R becomes X.
What am I missing ("Everything" could be an answer ...) here?
(And please be gentle -- too much digital electronics the past 30 years and not enough anything else ...)
gar
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tallgirl:
Counter-emf probably dates to the mid 1800s or maybe the 1870 to 1880 time frame. This refers to the induced voltage in the motor armature resulting from the rotation of the armature in a a fixed field. The generated voltage is an ac output but is demodulated by the commutator on the rotor. This counter-emf is equal to a constant times armature speed for a fixed field excitation. In a PM dc motor you can get a fairly accurate measure of RPM by pulsing current to the armature, such as a half wave SCR control, and measure the armature voltage between current pulses.
For a fixed field excitation the motor RPM = K*(Vsupply - I*R) where R is the armature resistance. Vsupply - I*R = the counter-EMF of the motor. With J. G. Tarboux as a teacher of DC Machinery these basic concepts were emphasized. He was an outstanding teacher in both DC and AC Machinery and used basic concepts and intuition as his basic teaching tools. Absolute constants were of no great importance, but rather the relationships of one factor to another were the critical items taught. Tarboux was both a PhD and a full professor. I was lucky to have had a number of very good professors. and these were in relatively small classes. Anywhere from 5 to maybe 35 students in most cases.
In 1879 Edison and his workers developed a means on his dynamo to get constant output voltage under varying load conditions by adding a winding to the field excitation that had load current thru it. Thus, as the IR drop from load on the generator reduced the output voltage the load current increased the field excitation to compensate for this voltage change. They determined the proper relationship to get good voltage regulation. Note: that they also discovered the characteristic of saturation of iron core magnetic circuits. Therefore, they did not over drive the magnetic circuit and waste excitation power, as competitors did.
This same concept of a compound field was later used in dc motors to maintain a more constant output RPM.
I generally described induced voltage by e = K*N*dp/dt where p is phi, magnetic flux or flux density, because it more directly indicates what is happening. In most ferromagnetic circuits L is constantly changing and thus harder to conceptually visualize what is happening.
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tallgirl:
Counter-emf probably dates to the mid 1800s or maybe the 1870 to 1880 time frame. This refers to the induced voltage in the motor armature resulting from the rotation of the armature in a a fixed field. The generated voltage is an ac output but is demodulated by the commutator on the rotor. This counter-emf is equal to a constant times armature speed for a fixed field excitation. In a PM dc motor you can get a fairly accurate measure of RPM by pulsing current to the armature, such as a half wave SCR control, and measure the armature voltage between current pulses.
For a fixed field excitation the motor RPM = K*(Vsupply - I*R) where R is the armature resistance. Vsupply - I*R = the counter-EMF of the motor. With J. G. Tarboux as a teacher of DC Machinery these basic concepts were emphasized. He was an outstanding teacher in both DC and AC Machinery and used basic concepts and intuition as his basic teaching tools. Absolute constants were of no great importance, but rather the relationships of one factor to another were the critical items taught. Tarboux was both a PhD and a full professor. I was lucky to have had a number of very good professors. and these were in relatively small classes. Anywhere from 5 to maybe 35 students in most cases.
In 1879 Edison and his workers developed a means on his dynamo to get constant output voltage under varying load conditions by adding a winding to the field excitation that had load current thru it. Thus, as the IR drop from load on the generator reduced the output voltage the load current increased the field excitation to compensate for this voltage change. They determined the proper relationship to get good voltage regulation. Note: that they also discovered the characteristic of saturation of iron core magnetic circuits. Therefore, they did not over drive the magnetic circuit and waste excitation power, as competitors did.
This same concept of a compound field was later used in dc motors to maintain a more constant output RPM.
I generally described induced voltage by e = K*N*dp/dt where p is phi, magnetic flux or flux density, because it more directly indicates what is happening. In most ferromagnetic circuits L is constantly changing and thus harder to conceptually visualize what is happening.
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Consider whether there would be CEMF inputting a square-wave positive-only voltage. Now consider the difference between the preceding and flipping a DC-voltage supply switch on and off. Any "light bulbs" yet?
mivey
Senior Member
The voltage may be constant enough, but the coil current is not. With enough coils, it looks smoother and smoother. Here is a nice picture:
http://hades.mech.northwestern.edu/index.php/Brushed_DC_Motor_Theory
add:
With AC we say the waves change direction.
With DC, we normally think of things as being constant but they can also oscillate from high to low without changing direction.
Also, there is more than just a fixed R in the DC motor or the current would remain at the starting current.
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