As I understand it, it is current flow that creates magnetic lines of force. In reading a textbook on DC electric motors, I come across this statement Re. speed control: "DC motors can be operated below base speed by reducing the amount of voltage applied to the armature and above base speed by reducing the field current". Why would not current be the determining factor in both cases? Is there some way that voltage itself impacts the magnetic field that I am missing here? I know P=IE. Isn't magnetic flux always be in direct proportion to my Power?
Voltage, Current and Magnetic Flux
- Thread starter Wes G
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gar
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e = K*N*d phi/dt
The induced voltage in a coil is some constant times the number of turns times the rate of change of magnetic flux.
When the motor rotor rotates a voltage is induced into the rotor coil from the coil rotating in the magnetic field of the stator. This induced voltage is proportional to the magnitude of the magnetic flux, the number of turns, and the speed of rotation. This induced voltage is called the counter-EMF and when at speed equals the external voltage applied to the armature.
I have left out some minor details, but this is the basic theory of the motor speed. If you lower the applied armature voltage, then the armature must slow down so the voltages are equal.
The above was assuming a constant stator magnetic intensity. Now consider the armature voltage a constant. Lower the current to the stator field and that lowers the magnetic field intensity, then what happens?
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e = K*N*d phi/dt
The induced voltage in a coil is some constant times the number of turns times the rate of change of magnetic flux.
When the motor rotor rotates a voltage is induced into the rotor coil from the coil rotating in the magnetic field of the stator. This induced voltage is proportional to the magnitude of the magnetic flux, the number of turns, and the speed of rotation. This induced voltage is called the counter-EMF and when at speed equals the external voltage applied to the armature.
I have left out some minor details, but this is the basic theory of the motor speed. If you lower the applied armature voltage, then the armature must slow down so the voltages are equal.
The above was assuming a constant stator magnetic intensity. Now consider the armature voltage a constant. Lower the current to the stator field and that lowers the magnetic field intensity, then what happens?
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cadpoint
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Its the sum of various quantites that make magnetic lines of force. It will always be ratio problem in A DC system its more dramatic result of this propotional math equation.
If a reduction of voltage in DC then what happen to the current number in the eqaution. Yes there less over all push as that voltage is applied.
Well probably the biggest things that you might be missing is how the two types of motors are wound!
In general: AC motors are mainly wound to the Outside, and DC on the armature (the Shaft). So where the real work is accomplished from the rotation, DC generally give better torque output from their own design,
the voltage is applied to the shaft. In AC it has to work the field that is on outside to make the inside shaft rotate.
You have to remember that DC sum of power is at some Line be it 12 V or 480v while ohm's law is still appliable, comparing that DC Flat line to 480v 3ph AC
its the sum of the AC power pulsing that creats the RMS the useable/work power. I can't recall seeing any graphs on DC line summary with voltage or current detracting from a DC line graph of its power.
You could think of it this way, you know that the power is based on the Sum
of voltage and current in either cause, something has to give. With AC you'd probably burn up the motor with higher current their not built for that excess
application where DC motors are. DC are just more bigger in all respects.
Last edited:
Mayimbe
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From what I remember for DC motors
in the field
If=Vf/Rf
where
If= field current
Vf= Voltage applied in the field
Rf= Field Resistance
in the armature
Ia=Va/Ra
where
If= armature current
Vf= Voltage applied in the armature
Rf= armature Resistance
G= inductive constant [Henrys] (usually you can get it from the power factor of the machine, and the power in the nameplate)
then the voltage induced is
e = G*w*If
w = motor velocity in radians/seg
and the torque is
T=G*If*Ia [Newton.meters]
playing with those equations you can find your answers, hope it helps you
gar
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Mayimbe:
I believe in the paragraph
Ia = armature current
Va = Voltage applied in the armature
Ra = armature Resistance
However, if Va is the voltage applied to the brushes, then the equation Ia = Va/Ra is incorrect. If Va is the difference in the applied voltage to the brushes minus the counter-EMF, then the equation is correct ignoring minor factors.
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Mayimbe:
I believe in the paragraph
You meant to write
Ia = armature current
Va = Voltage applied in the armature
Ra = armature Resistance
However, if Va is the voltage applied to the brushes, then the equation Ia = Va/Ra is incorrect. If Va is the difference in the applied voltage to the brushes minus the counter-EMF, then the equation is correct ignoring minor factors.
,
Mayimbe
Senior Member
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- Horsham, UK
DC motor speed control
DC motor speed control
In DC motors the armature is a bar, basically a dead short between the brushes. As the armature bar rotates it cuts the flux of the field coils and this induces a back EMF on the armature bar, an additional voltage drop across the armature bar, and effectively, less current through the armature bar.
DC motors have very high armature bar currents and very low field coil currents. This is why DC motors have very high starting torque, they have very high armature current at starting.
Most all power tool motors are series connected universal AC - DC motors. They work great on 120 vDC and have good low speed torque.
To answer your question why "DC motors can be operated below base speed by reducing the amount of voltage applied to the armature and above base speed by reducing the field current".
Reducing armature voltage reduces armature current flow and thus motor torque output. Assuming constant load, the motor slows down.
Here's the interesting part. Reducing field coil current reduces the back EMF on the armature bar and increases armature current and thus motor torque output. Assuming constant load, the motor speeds up for reduced coil current because the armature current goes up a lot.
For a running DC motor that loses all field coil current, this results in a speed runaway condition where the armature current is again similar to a short between the brushes and the high armature current, high motor torque output, causes runaway high speed.
DC motor speed control
In DC motors the armature is a bar, basically a dead short between the brushes. As the armature bar rotates it cuts the flux of the field coils and this induces a back EMF on the armature bar, an additional voltage drop across the armature bar, and effectively, less current through the armature bar.
DC motors have very high armature bar currents and very low field coil currents. This is why DC motors have very high starting torque, they have very high armature current at starting.
Most all power tool motors are series connected universal AC - DC motors. They work great on 120 vDC and have good low speed torque.
To answer your question why "DC motors can be operated below base speed by reducing the amount of voltage applied to the armature and above base speed by reducing the field current".
Reducing armature voltage reduces armature current flow and thus motor torque output. Assuming constant load, the motor slows down.
Here's the interesting part. Reducing field coil current reduces the back EMF on the armature bar and increases armature current and thus motor torque output. Assuming constant load, the motor speeds up for reduced coil current because the armature current goes up a lot.
For a running DC motor that loses all field coil current, this results in a speed runaway condition where the armature current is again similar to a short between the brushes and the high armature current, high motor torque output, causes runaway high speed.
Thanks to all for your input. The question I wrestle with has to do with the forces involved with magnetic flux.
In my understanding, reducing the voltage to the fields also reduces the field current. Reducing the field current reduces the field magnetic flux which in turn reduces the CEMF which allows more effective EMFin the armature and hense greater current flow in the armature, thus greater speed and torque.
On the other hand reducing the current flow by lowering the voltage applied to the armature will result in a lower speed.
It seems to me that it is ultimately the current flow that we are seeking to regulate in both cases. What am I missing? Does voltage somehow of itself affect lines of flux or does it do this only indirectly by affecting current flow?
In my understanding, reducing the voltage to the fields also reduces the field current. Reducing the field current reduces the field magnetic flux which in turn reduces the CEMF which allows more effective EMFin the armature and hense greater current flow in the armature, thus greater speed and torque.
On the other hand reducing the current flow by lowering the voltage applied to the armature will result in a lower speed.
It seems to me that it is ultimately the current flow that we are seeking to regulate in both cases. What am I missing? Does voltage somehow of itself affect lines of flux or does it do this only indirectly by affecting current flow?
gar
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Wes G:
Theoretically there is no current in the armature and therefore no flux generated from the armature (rotor) when there is no mechanical load on the armature. Yet because there is an armature coil moving in in a magnetic field from the stator there is a voltage induced in the rotor coil proportional to the speed of rotation of the rotor. It is this voltage that must equal to the external voltage applied to the armature for the rotor to be at an equilibrium speed.
If this were not so then there would be a current flow in the armature that would accelerate or decelerate the rotor unless there was a mechanical load on the rotor. See if this makes sense.
On speed of the rotor vs field current. If the field current is reduced, then the field flux is reduced and the rotor has to rotate faster to produce the same counter-EMF as before field reduction.
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Wes G:
Theoretically there is no current in the armature and therefore no flux generated from the armature (rotor) when there is no mechanical load on the armature. Yet because there is an armature coil moving in in a magnetic field from the stator there is a voltage induced in the rotor coil proportional to the speed of rotation of the rotor. It is this voltage that must equal to the external voltage applied to the armature for the rotor to be at an equilibrium speed.
If this were not so then there would be a current flow in the armature that would accelerate or decelerate the rotor unless there was a mechanical load on the rotor. See if this makes sense.
On speed of the rotor vs field current. If the field current is reduced, then the field flux is reduced and the rotor has to rotate faster to produce the same counter-EMF as before field reduction.
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Mayimbe
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Agree, since If=Vf/Rf
Vf proportional to If
lower If increases Ia, agree with that, since
e= G*w*If (1)
and Ia = (Vsource - e)/Ra
greater speed of course, because from (1) we see that w=e/G*If, the lower the If the greater the w. Inverse proportional.
greater torque?? dont know that because
T=G*If*Ia, you have to put values on those variables to see how the motors behaves.
what do you mean by that?
there are three ways of reducting the current flow in the armature
Ia= (Vsource - e)/Ra
1) decrease Vsource
2)Increase e (increase the Vf or If)
3)Increase Ra
only the number 2) causes a change on the velocity, see (1). The others dont cause that.
agree with that, we are regulating the current flow.
Dont see the point of those questions.
gar
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Wes G:
There has to be current in the field coil to produce the stator magnetic field, or use a permanent magnet for a fixed magnetic field intensity.
Armature current has nothing to do with base motor speed. It does modify it because of armature resistance. My use of base speed here is the theoretical speed with no armature current.
Armature current is required to produce power output, or when the motor is run as a generator to produce output power. As a generator the current flows in the opposite direction compared to motoring.
If you have a DC motor with a wound coil and no permanent magnet for the field, then what happens if you remove current from the field coil?
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Wes G:
There has to be current in the field coil to produce the stator magnetic field, or use a permanent magnet for a fixed magnetic field intensity.
Armature current has nothing to do with base motor speed. It does modify it because of armature resistance. My use of base speed here is the theoretical speed with no armature current.
Armature current is required to produce power output, or when the motor is run as a generator to produce output power. As a generator the current flows in the opposite direction compared to motoring.
If you have a DC motor with a wound coil and no permanent magnet for the field, then what happens if you remove current from the field coil?
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gar
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- Ann Arbor, Michigan
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- EE
090921-1724 EST
A correction to previous post.
Armature current has nothing to do with base motor speed. It does modify the motor speed from base speed because of armature resistance. My use of base speed here is the theoretical speed with no armature current.
New comment.
Armature current is not used to control speed. Armature current in a motor is dependent upon the motor load, and the field magnetic intensity.
Output torque is mechanical load dependent. The independent variable is not generally armature current. However, it would be if I wanted the motor to be a constant torque device.
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A correction to previous post.
This should read:
Armature current has nothing to do with base motor speed. It does modify the motor speed from base speed because of armature resistance. My use of base speed here is the theoretical speed with no armature current.
New comment.
Armature current is not used to control speed. Armature current in a motor is dependent upon the motor load, and the field magnetic intensity.
Output torque is mechanical load dependent. The independent variable is not generally armature current. However, it would be if I wanted the motor to be a constant torque device.
.
A DC machine doesn't have a power factor.
Mayimbe
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LarryFine
Master Electrician Electric Contractor Richmond VA
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Surrrrre, you did.I said that on purpose to see if somebody was paying attention
gar
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090922-1955 EST
Wes G:
Do you comprehend what I have said? As a first order approximation RPM is a function of stator magnetic field intensity (inverse relationship), and applied voltage to the armature (direct relationship). Nothing to do with armature current in this first order approximation. Until you really understand this relationship you will not understand how a DC motor works.
You can run the experiment. Get a shunt wound DC motor with the field coil brought out to separate terminals from the brushes. A 6 to 24 V motor is probably a good voltage range. You need variable power supplies, volt and amp meters, and a means of measuring speed.
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Wes G:
Do you comprehend what I have said? As a first order approximation RPM is a function of stator magnetic field intensity (inverse relationship), and applied voltage to the armature (direct relationship). Nothing to do with armature current in this first order approximation. Until you really understand this relationship you will not understand how a DC motor works.
You can run the experiment. Get a shunt wound DC motor with the field coil brought out to separate terminals from the brushes. A 6 to 24 V motor is probably a good voltage range. You need variable power supplies, volt and amp meters, and a means of measuring speed.
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