Why is maximum output power constant in field resistance control method?

In summary, there are three methods for controlling the speed of a shunt DC motor: field resistance control, terminal voltage control, and armature resistance control. In the field/flux control method, the induced torque decreases and speed increases due to a decrease in flux, resulting in the power remaining constant. This is explained by the fact that the rotor must spin faster in a weaker field to generate the same back emf, but the force causing this rotation (repulsion between armature and field) is reduced. This can also be seen mathematically, as the maximum torque is proportional to maximum armature current, which is inversely proportional to rotational speed, resulting in a constant power.
  • #1
nazia_f
20
0
There are three speed control methods in a shunt DC motor -
Field resistance control
Terminal voltage control
Armature resistance control

In the field/flux control method induced torque decreases and speed increases due to decrease in flux, that is why power remains constant. This is what I got from the book.

I want to know if there is any other explanation about why the output power remains constant or is there any mathematical proof?
 
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  • #2
nazia_f said:
In the field/flux control method induced torque decreases and speed increases due to decrease in flux, that is why power remains constant. This is what I got from the book.

I want to know if there is any other explanation about why the output power remains constant or is there any mathematical proof?
It's easy to imagine that starting torque would be reduced when field current is less, there now being reduced repulsion between the armature and weaker field flux.

If we overlook losses, then the rotor of a DC motor will speed up until the rotational speed generates an emf that exactly opposes that of the power supply. If the field's magnetic strength is reduced by reducing the field current, then the rotor must spin faster in that weaker field to generate the same back emf. So it's rotating faster, but the force that is causing this rotation (repulsion between armature and field) is less because you have reduced the field's strength.

I think that is the qualitative question you are asking. The answer suddenly came to me as I was walking to the beach yesterday. It must have been playing on my subconscious mind. :smile:
 
  • #3
Maximum torque is proportional to maximum armature current but armature current in inversely proportional to rotational speed. So we can write
T = k/ω
Again P = Tω = (k/ω)*ω = k
So the power remains constant. This answer is what our teacher was expecting from us.
Thanks a lot for your answer but there's something that I didn't clearly get. Could you please explain "less repulsion between armature and field due to reduced field strength" in details to me? How are armature and field repulsing each other?
And I'm really sorry for replying this late.
 
  • #4
nazia_f said:
Could you please explain "less repulsion between armature and field due to reduced field strength" in details to me? How are armature and field repulsing each other?
I used that phrase instead of saying "motor action". :smile:

After all, isn't that what causes the rotor to spin--repulsion between the rotor (i.e., armature) and field? The strength of this repulsive force being proportional to the current in each.
 
  • #5
Ah I get it now. :)
Thank you for trying to help me out. ^_^
 

Related to Why is maximum output power constant in field resistance control method?

1. Why is maximum output power constant in field resistance control method?

Maximum output power is constant in the field resistance control method because the field resistance is adjusted in such a way that it keeps the output power at a constant level. This is achieved by varying the amount of current flowing through the field winding, which in turn affects the magnetic field strength and controls the generator's output voltage. As long as the load remains within the generator's capacity, the output power will remain constant.

2. How does field resistance control method work?

Field resistance control method works by adjusting the amount of current flowing through the field winding of the generator. This is done by varying the resistance in the field circuit. Changing the resistance affects the magnetic field strength, which in turn controls the output voltage of the generator. The output power is kept constant by adjusting the field resistance to match the load demand.

3. What are the advantages of using field resistance control method?

One advantage of using field resistance control method is that it provides a simple and efficient way to regulate the output power of a generator. It also allows for a constant output power to be maintained, regardless of changing load demands. Additionally, this method does not require complex control systems or feedback mechanisms, making it cost-effective and easy to implement.

4. Are there any limitations to field resistance control method?

Field resistance control method may not be suitable for all types of generators. It is primarily used in DC generators and some AC generators that have separately excited fields. Additionally, the output power can only be controlled within a certain range, and exceeding this range may lead to instability in the system. Furthermore, this method may not be suitable for applications with highly variable load demands.

5. How does field resistance control method differ from other methods of controlling generator output power?

The field resistance control method differs from other methods of controlling generator output power in that it directly regulates the amount of current flowing through the field winding. In comparison, other methods, such as voltage control or speed control, indirectly affect the field current by controlling other parameters such as the excitation voltage or the speed of the generator. Additionally, the field resistance control method is simpler and more cost-effective, making it a popular choice for smaller generators.

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