# Calculating Armature Torque for 250V DC Motor

• Reefy
In summary, at 1000rpm, the motor takes a current of 25 A and develops armature torque of 0.2W. At 50A load current, the armature torque is increased to 1.2W.
Reefy

## Homework Statement

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.

## Homework Equations

IL = Ia + Ish

Vt = IshRsh = Ea + IaRa

Poutput = Tω = T(2πN)

## 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

Reefy said:
field resistance as 0.2W**Ω** and 250W**Ω**
Reefy said:
The speed when the load current is 50A.
Is it the armature current or the total current supplied by the supply?
Reefy said:
Armature torque developed at both speeds.
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?

I don't know why it says W. It is ohms.

The load current is the current supplied by Vt.

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

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

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

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

Reefy said:
Ia1 = IL1 - Ish = 25A - 1A = 24A

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

cnh1995 said:
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.

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

Reefy said:
That's what I'm unsure of. I know that Pinput - Poutput = the total losses (Copper, iron, friction).
And Copper Loss is I2aRa + I2shRsh
Those are all electrical powers. The relation between back emf and mechanical power is Pm=EbIarmature. Can you proceed from here?

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

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

cnh1995 said:
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?

T = PM/ω = EaIa/(2πN)

Reefy said:
T = PM/ω = EaIa/(2πN)
Right.

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

## 1. What is armature torque?

Armature torque is the measure of the rotational force produced by the armature of a motor. It is responsible for driving the rotation of the motor's shaft.

## 2. How is armature torque calculated for a 250V DC motor?

The formula for calculating armature torque is T = Kt * Ia, where T is the torque in Newton-meters, Kt is the torque constant of the motor, and Ia is the armature current. To calculate for a 250V DC motor, you will need to know the torque constant and armature current specifications of the specific motor model.

## 3. What factors affect armature torque for a 250V DC motor?

The main factors that affect armature torque for a 250V DC motor include the strength of the magnetic field, the number of windings in the armature, and the amount of current flowing through the armature. Additionally, the design and condition of the motor's brushes and commutator can also impact armature torque.

## 4. How does armature torque affect the performance of a 250V DC motor?

Armature torque is directly proportional to the motor's rotational speed and power output. This means that a higher armature torque will result in a faster and more powerful rotation of the motor's shaft. However, if the armature torque is too high, it can cause the motor to overheat and potentially damage its components.

## 5. How can armature torque be increased for a 250V DC motor?

There are a few ways to increase armature torque for a 250V DC motor, such as increasing the strength of the magnetic field by using stronger permanent magnets or increasing the number of windings in the armature. Additionally, using a larger armature or increasing the current flowing through the armature can also help to increase armature torque.

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