- 19th January 2005, 18:25 #1

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## magnetization characteristics of dc generator

I was going through an article about DC Shunt Motors when i came up with some unknown concepts... I searched some books but none explained the concepts i needed... can someone please help me with the following:

1. What are the magnetisation characteristics of a DC Shunt Motor?

2. What is its Significance?

3. How to draw the Magnetisation Characteristics given some data about its Open Circuit Voltage and Field Armature Resistance - TABLE

4. How to calculate the various parameters like Critical Resistance, Critical Speed from the graph and from the data given...

5. What is Critical Resistance, Critical Speed?

- 19th January 2005, 18:25

- 19th January 2005, 19:56 #2
## critical resistance dc generator

Are you refering to DC shunt motor or DC shunt generator?

- 19th January 2005, 20:04 #3

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## characteristics of dc motors

"Critical Resistance" is found to related to DC shunt generator as follows:

*The value of resistance of shunt field winding beyond which the shunt generator fails to build up its voltage is known as***critical resistance**

- 19th January 2005, 20:04

- 20th January 2005, 03:27 #4

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## characteristics of dc motor

Sorry...all this magnetisation characteristics are given under the topic DC Shunt Generator...

- 20th January 2005, 03:37 #5

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## critical resistance of dc generator

Magnetisation characteristics of a DC motor is the plot of variation of torque with speed. it's signoficance lies in the designing of a motor.Of course,there are more factors to be consoidered for a faithful construction of a DC motor.

Regards

drdolittle :)

- 20th January 2005, 13:58 #6
## magnetisation characteristics of dc shunt motor

Originally Posted by**purifier**

Now say E is the generated armature EMF.

E=KΦω, where Φ is field flux and ω is generator speed or turbine speed and K is a proportionality constant.

Now Φ is dependent on I which is the field current.

Therefore E = GIω where G is a proportrionality factor which varies as the current varies. It depends on the µ of the core material.

Also I ≈E/Ra where Ra is the armature resistance.

Now you can generate two graphs for E vs I. One will look similar the B-H curve since B is proportional to Φ and hence depends on E while H is proportional to I.

The other is a straight line graph with a slope of Ra. The point of intersection of the two graphs gives us E and I.

Now if Ra is too high the straight line graph will coincide with the linear portion of the B-H curve so the resulting intersection will be at a low E and low I. So to avoid this situation Ra should be lower than a particular critical value. If you draw the two graphs you will understand it better.

Also in a DC generator some residual magnetism should be present or else when the generator is started Φ = 0 and hence E=0 irrespective of speed and therefore I is always 0. Another important point is that the field magnetism should aid rather than oppose the residual magnetism direction or else once again we'll end up with a Φ = 0 situation when the DC generator starts.

- 2nd November 2006, 19:26 #7

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## critical speed of dc generator

critical resistance is tangent to the o.c.c (open circuit curve) curve. beyond critical point machine dosnot perform voltage buldup and prior to critical resistance the machine cannot build up voltage.

the speed at which critical resistance exist is critical speed of the machine

both these quantity can be calculated from graph i dont know if they have theoritical formula.

magnetization charactristics curve are different for generator and motor you can

find diagrams in any books of electrical machine..........

there is only one characteristics curve for generator but two in case of motor

for generator it's voltage-rated current curve

for motor it's torque-speed curve and torque-armature current characteristics curve...

- 17th April 2011, 05:01 #8

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