# Electrical Machines DC Shunt Motor

• Engineering
• Edges_sharp
In summary, an 8.25 kW shunt DC motor is supplied with a terminal DC voltage of 300 V. The armature resistance of the motor Ra is 0.25 , and the field resistance Rf is 40 . The field winding consists of 1500 turns. An adjustable resistance Radj is connected in series to the field winding. Radj may be varied over the range from 0 to 80  and is currently set at 20 . Armature reaction may be ignored in this machine. The magnetization curve for this motor, taken at a speed of 1200 rpm, is shown in Figure Q1.
Edges_sharp
New poster has been reminded to use the Homework Help Template in schoowork posts here
An 8.25 kW shunt DC motor is supplied with a terminal DC voltage of 300 V. The armature resistance of the motor Ra is 0.25 , and the field resistance Rf is 40 . The field winding consists of 1500 turns. An adjustable resistance Radj is connected in series to the field winding. Radj may be varied over the range from 0 to 80  and is currently set at 20 . Armature reaction may be ignored in this machine. The magnetization curve for this motor, taken at a speed of 1200 rpm, is shown in Figure Q1.
(i) What is the speed of the motor when operating at rated load with an armature current of 30 A?
(ii) If the motor is now unloaded with no changes in terminal voltage or Radj, what is the no-load speed of the motor?
(iii) Iron core losses are 250 W. What are the copper losses and mechanical losses in the motor at rated load? (ignore brush and stray losses)
(iv) What is the efficiency of the motor at rated load?
(v) If the terminal voltage is reduced to 260 V and the armature current stays at 30 A, calculate the required Radj in order to restore the speed of the motor to the value in (i).

I am having difficulty with Question 1(I) of this example. I have tried finding the internal voltage, then finding "If" to then work out however many Ampere Turns that will produce and read the Internal Generated voltage from the graph. If provided with the no load speed I would not find this an overly difficult question however due to the omission of this information I am struggling quite a bit. I am not asking for the full solution to all of the questions. However I would really appreciate a nudge in the right direction with 1(I).

Many Thanks

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Hi Edges_sharp.

Have you calculated the field current at the settings in use?

Edges_sharp
Yes I am getting it to be 4.875A

Edges_sharp said:
Yes I am getting it to be 4.875A
How did you calculate that?

Not sure if my method is right but using Ea=VT-IaRa
Ea=300-30*0.25=292.5

Then to calculate field current I did If=Ea/Rf=292.6/60=4.875

Edges_sharp said:
the field resistance Rf is 40 .
I'm puzzled by your symbol after the 40? Does it look like Ω on your screen? It doesn't on mine. Maybe it's a windows character you copied?

Yes it is copied and should be ohms

Edges_sharp said:
Not sure if my method is right but using Ea=VT-IaRa
Ea=300-30*0.25=292.5

Then to calculate field current I did If=Ea/Rf=292.6/60=4.875
Rf of 60 Ω is right. But in a shunt machine the field current is determined by the terminal voltage.

You should always have a model circuit at hand when attempting problems on motors.

Edges_sharp
Thanks very much :)

Edges_sharp said:
Yes it is copied and should be ohms
It looks like a tiny subscript 2 to me.

If you click on the ##\Sigma## symbol at the top of the editor window in your browser then a line of Greek letters will appear lower down from where you can click on one to have it included in what you're typing. (If you are using the mobile app...then I have no idea what it does.)

Edges_sharp said:
An 8.25 kW shunt DC motor
Edges_sharp said:
What is the speed of the motor when operating at rated load with an armature current of 30 A?
I believe mechanical output of the motor is 8.25 kW when it is drawing 30 A armature current. This is not possible with a supply voltage of 300V.

Last edited:
I am only giving the information present in the question

cnh1995 said:
I believe mechanical output of the motor is 8.25 kW when it is drawing 30 A armature current. This is not possible with a supply voltage of 300V.
Power engineers are known for their healthy disdain of exactitude.

Edges_sharp said:
I am only giving the information present in the question
Well, in my opinion, you can solve the question only by "assuming" the magnetization curve to be linear, while still using the values on the non-linear graph.

Can someone please come back to this example.
I'm working on a similar problem and would appreciate the help. Thanks

Last edited:
SK97 said:
Can someone please come back to this example.
I'm working on a similar problem and would appreciate the help. Thanks
Edit: I see you already have.

cnh1995 said:
My problem is actually the same.

I have found I(f) to be 5A
therefore the mmf is 7500A.turns

and reading from the graph I saw the internal voltage at that point is around 275 V.

Now I'm not sure where to go from there, any help would be appreciated.

## 1. What is a DC shunt motor?

A DC shunt motor is a type of electrical machine used to convert electrical energy into mechanical energy. It consists of a field winding connected in parallel with the armature winding, giving it its characteristic "shunt" configuration.

## 2. How does a DC shunt motor work?

A DC shunt motor works by using the interaction between the magnetic field produced by the field winding and the current flowing through the armature winding. This creates a force that causes the armature to rotate, converting electrical energy into mechanical energy.

## 3. What are the advantages of using a DC shunt motor?

DC shunt motors have several advantages, including high starting torque, simple and reliable construction, and the ability to control speed by adjusting the field current. They are also relatively inexpensive and easy to maintain.

## 4. What are the applications of DC shunt motors?

DC shunt motors are commonly used in applications that require constant speed, such as conveyors, elevators, and machine tools. They are also used in industries such as automotive, aerospace, and manufacturing.

## 5. How do you troubleshoot issues with a DC shunt motor?

The most common issues with a DC shunt motor include low speed, overheating, and sparking. To troubleshoot these issues, one can check for loose connections, clean the motor and brushes, and adjust the field current. It is also important to regularly inspect and maintain the motor to prevent potential problems.

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