# DC Shunt Motor

1. Dec 4, 2016

### Reefy

1. The problem statement, all variables and given/known data
A 250V shunt dc motor runs at 1000rpm while taking a current of 25 A. The motor has armature and field resistance as 0.2W and 250W respectively. Calculate

(a) The speed when the load current is 50A.

(b) Armature torque developed at both speeds.

2. Relevant equations
IL = Ia + Ish

Vt = IshRsh = Ea + IaRa

Poutput = Tω = T(2πN)

3. The attempt at a solution

I already solved part a and got N2 = 979.6 rpm

I actually just have a question of solving part B. How exactly do I compute the output without the torque and angular speed.

Is Poutput = IaRa

or Poutput = IaRa - (Iron + Friction) losses

2. Dec 4, 2016

### cnh1995

Is it the armature current or the total current supplied by the supply?
This means you need to find the mechanical power developed in the motor. Do you know the relation between back emf and mechanical power developed?

3. Dec 4, 2016

### Reefy

I dont know why it says W. It is ohms.

The load current is the current supplied by Vt.

4. Dec 4, 2016

### Reefy

Pinput(1) = VtIL1 = (250 V)(25 A) = 6250 A at the first speed

and it is Pinput(2) = VtIL2 = (250 V)(50 A) = 12500 A at the 2nd speed

5. Dec 4, 2016

### cnh1995

Ok.
You need to find the armature current first. You know the total current and field resistance. How would you calculate the armature current?

6. Dec 4, 2016

### Reefy

Ia1 = IL1 - Ish = 25A - 1A = 24A

Ia2 = IL2 - Ish = 50A - 1A = 49A

7. Dec 4, 2016

### cnh1995

Correct.
You can calculate back emf from this. What is the equation describing the relation between back emf and mechanical power developed? Back emf is the electrical feedback of mechanical energy developed.

8. Dec 4, 2016

### Reefy

That's what I'm unsure of. I know that Pinput - Poutput = the total losses (Copper, iron, friction).
And Copper Loss is I2aRa + I2shRsh

9. Dec 4, 2016

### cnh1995

Those are all electrical powers. The relation between back emf and mechanical power is Pm=EbIarmature. Can you proceed from here?

10. Dec 4, 2016

### Reefy

Ooohh. Yeah, its the conceptual idea I don't get. I don't get why we have to use mechanical power instead of electrical

11. Dec 4, 2016

### cnh1995

You are asked to find the torque and speed. These are associated with mechanical power. Now, how do you write mechanical power in terms of torque and speed?

12. Dec 4, 2016

### Reefy

T = PM/ω = EaIa/(2πN)

13. Dec 5, 2016

### cnh1995

Right.

14. Dec 5, 2016

### Reefy

Thanks, I see the mistake I was making! It was pretty simple, I must be tired. Appreciate the help again