# Thread: Shunt DC motor question

1. gar
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Besoeker3:

There is not no flux.

Have you studied ferromagnetic magnetic circuits? Look at hysteresis curves.

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2. gar
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Besoeker3:

Also ask yourself how a self-excited DC generator starts to produce any output, if there was not some residual flux. For example an automotive generator.

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3. gar
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A basic equation for a DC motor is:

Vsupply = ( Iarmature * Rarmature ) + Vcounteremf

Vcounteremf = Constant * Fieldintensity * RPM

In an ideal motor, no losses, then with no mechanical load the Iarmature is 0.

Thus, ideally at no load we get:

Vsupply = Constant * Fieldintensity * RPM
and
RPM must increase if Fieldintensity decreases for a constant supply voltage.

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Originally Posted by gar
190116-2358 EST

A basic equation for a DC motor is:

Vsupply = ( Iarmature * Rarmature ) + Vcounteremf

Vcounteremf = Constant * Fieldintensity * RPM

In an ideal motor, no losses, then with no mechanical load the Iarmature is 0.

Thus, ideally at no load we get:

Vsupply = Constant * Fieldintensity * RPM
and
RPM must increase if Fieldintensity decreases for a constant supply voltage.

.
In the real world, a shunt motor losing its field will overspeed. If you have changed the scenario into a motor with no supply to the field winding, Besoeker3 is right that there is no torque, or to put it mildly, the motor won't run.

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Originally Posted by topgone
In the real world, a shunt motor losing its field will overspeed. If you have changed the scenario into a motor with no supply to the field winding, Besoeker3 is right that there is no torque, or to put it mildly, the motor won't run.
That overspeed can be catastrophic simply because DC motors are designed so that at its rated speed the resistance of the rotor winding is small, and the resistive voltage drop is small compared to the counter EMF.
At some stator field strength the rotor current will be limited by the DC resistance rather than the counter EMF, so the idealized speed with no mechanical losses does not approach infinity as the field strength decreases. It does usually exceed the limits of the rotor.

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Originally Posted by GoldDigger
That overspeed can be catastrophic simply because DC motors are designed so that at its rated speed the resistance of the rotor winding is small, and the resistive voltage drop is small compared to the counter EMF.
At some stator field strength the rotor current will be limited by the DC resistance rather than the counter EMF, so the idealized speed with no mechanical losses does not approach infinity as the field strength decreases. It does usually exceed the limits of the rotor.
However from gar's equations below, it is to be inferred that speed tends to infinity as field intensity is reduced towards zero.

Originally Posted by gar
190116-2358 EST

A basic equation for a DC motor is:

Vsupply = ( Iarmature * Rarmature ) + Vcounteremf

Vcounteremf = Constant * Fieldintensity * RPM

In an ideal motor, no losses, then with no mechanical load the Iarmature is 0.

Thus, ideally at no load we get:

Vsupply = Constant * Fieldintensity * RPM
and
RPM must increase if Fieldintensity decreases for a constant supply voltage.

.
So where is the error in gar's equation, if there is any ? i.e how to introduce any correction in his equation?
Last edited by Sahib; Yesterday at 06:03 AM.

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Originally Posted by gar
190116-1915 EST

Besoeker3:

There is not no flux.

Have you studied ferromagnetic magnetic circuits? Look at hysteresis curves.

.

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Originally Posted by Sahib
However from gar's equations below, it is to be inferred that speed tends to infinity as field intensity is reduced towards zero.

So where is the error in gar's equation, if there is any ? i.e how to introduce any correction in his equation?
It is not an error, it is an idealization. The condition that Iarmature is zero is necessary for there to be no torque, since there is no mechanical load. But even with no external load, there will be some friction (bearing, commutator, air resistance) which will require some minimal torque and that minimal load will in fact also increase as motor speed increases. The lower the stator field strength gets, the higher this minimal Iarmature will be.
So in a physical motor with no external load there will still be some speed for which Iarmature times Rarmature will in fact equal the applied voltage. But all it has to do is equal the applied voltage times whatever EMF is created by the reduced field. Note that the counter EMF is not dependent on the armature current, just the magnetic characteristics of the residual field, the rotor winding, and the speed.

The speed runaway in an unloaded series wound motor, on the other hand, can happen even in the absence of residual field because the field current does not initially go all the way to zero. A series wound motor with no iron in it at all will still run out of control as long as you assume that even the internal mechanical load can be treated as zero.

I once did some playing with a military surplus series wound motor, and it had a centrifugal governor switch which opened the entire circuit at the maximum allowable motor speed.

9. gar
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When one analyzes a circuit there are simplifications, and assumptions made in order estimate what may happen in the real world.

Most of the time when you talk about a resistor the assumption is that it is a pure resistance, but it really isn't. It is all of resistance, inductance, and capacitance, and not even lumped as single items of these. A 10 ohm wire wound resistor at 1 MHz may be far from a simple resistor in its real world operation.

In post #1 I started with a running motor, so stated. Thus, what happens when starting from 0 speed was was not part of the question. Unstated, but assumed was a motor with a ferromagnetic core. Since in the question it was obvious that current was flowing in the field at the start this meant there was an initial magnetic field. You can't loose current if there was none to start with. Also assumed was that the reader has knowledge of magnetization curves.

An assumption of no mechanical load means just that. Not true in the real world, and not a constant with speed. But to get a basic understanding of how the circuit works it is a useful assumption.

There is a lot that you can study about a DC motor by making some assumptions that simplify the view of the device.

For those that don't want to read and understand statements of a problem, then you are going to get lost in getting a solution. Turned around another way. You are confronted with a problem. Asking the correct questions about the problem becomes critical to solving the problem. But you don't know what are the correct question or questions to ask. So you probe all around the tree with experiments and probing questions to try to point in the correct direction.

Without a lot of erroneous details about what happens when you loose field excitation in a running motor a basic understanding of a DC shunt motor tells you it is likely to mechanically destroy itself.

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10. gar
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dpeter:

In your post #3 I think you knew what would happen, but you did not provide a why. I really did not want to comment (respond) to avoid influencing other responses. So this response is to finish my statement that I was not ignoring you.

A major problem with multiple choice questions on tests is that the why part is not present.

Some motor controls for DC shunt motors made some use of reduced field excitation to obtain greater output speed, but at reduced continuous torque.

Note: torque load on a DC motor is a thermal problem. So maximum continuous load current is a function of internal temperature rise, ambient temperature, and whatever criteria has been set for maximum hot spot temperature.

I think a number of responses indicated a lack of a basic understanding of a DC shunt motor. Should an ordinary electrician know how a motor works? I don't know. If one never works with a motor, then probably no need. Otherwise I would say yes.

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