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A 200 V DC series motor, when operating from rated voltage while driving a certain load, draws 10 A current and runs at 1000 r.p.m. The total series resistance is 1 Ω. The magnetic circuit is assumed to be linear. At the same supply voltage, the load torque is increased by 44%. The speed of the motor in r.p.m. (rounded to the nearest integer) is ________ .
Correct answer is '823 to 827'. Can you explain this answer?
Most Upvoted Answer
A 200 V DC series motor, when operating from rated voltage while drivi...
To determine the new speed of the motor when the load torque is increased by 44%, we need to analyze the motor's characteristics and use the torque-speed relationship.

1. Initial Parameters:
- Supply voltage (V) = 200 V
- Current (I) = 10 A
- Speed (N) = 1000 RPM
- Total series resistance (R) = 1 Ω

2. Calculating Initial Torque:
The torque (T) of a DC series motor is given by the equation:
T = K * Φ * I
where K is a constant, Φ is the flux, and I is the current.

Since the magnetic circuit is assumed to be linear, the flux Φ is directly proportional to the supply voltage V:
Φ ∝ V

Therefore, the torque T is directly proportional to the square of the supply voltage:
T ∝ V^2

We can calculate the initial torque by substituting the values:
T_initial = K * V^2 * I

3. Calculating New Torque:
When the load torque is increased by 44%, the new torque can be expressed as:
T_new = 1.44 * T_initial

4. Calculating New Speed:
The torque-speed relationship of a DC series motor is given by the equation:
T = K * (N - No)
where No is the no-load speed.

From the torque equation, we can rearrange it to solve for the speed:
N = T/K + No

Substituting the values, we have:
N_new = (1.44 * T_initial) / K + No

5. Finding the No-Load Speed:
To find the no-load speed, we need the torque at no-load conditions. At no-load, the torque is zero, so we can rewrite the torque equation as:
0 = K * (N_no-load - No)
N_no-load = No

Therefore, the no-load speed is equal to the initial speed:
N_no-load = N_initial = 1000 RPM

6. Final Calculation:
Substituting the values into the new speed equation:
N_new = (1.44 * T_initial) / K + N_no-load

Since N_no-load = N_initial, we have:
N_new = (1.44 * T_initial) / K + N_initial

Now, we know that the motor is operating from the same supply voltage, so the constant K remains the same. Therefore, the new speed can be calculated as:
N_new = (1.44 * T_initial) / K + N_initial

Substituting the values, we get:
N_new = (1.44 * (K * V^2 * I)) / K + N_initial

Simplifying the equation:
N_new = 1.44 * V^2 * I + N_initial

Plugging in the given values:
N_new = 1.44 * 200^2 * 10 + 1000
N_new ≈ 823 to 827 RPM

Hence, the speed of the motor, when the load torque is increased by 44%, is approximately 823 to 827 RPM.
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A 200 V DC series motor, when operating from rated voltage while driving a certain load, draws 10 A current and runs at 1000 r.p.m. The total series resistance is 1 . The magnetic circuit is assumed to be linear. At the same supply voltage, the load torque is increased by 44%. The speed of the motor in r.p.m. (rounded to the nearest integer) is ________ .Correct answer is '823 to 827'. Can you explain this answer?
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