A 2 pole, 50 Hz induction motor supplies 15 kW to a load at a speed of...
Calculating the Original Power:
- Number of poles (p) = 2
- Frequency (f) = 50 Hz
- Speed (N) = 2950 rpm
First, we need to calculate the synchronous speed of the motor:
$$N_{s} = \frac{120f}{p}$$
$$N_{s} = \frac{120 \times 50}{2} = 3000 \text{ rpm}$$
The actual speed of the motor is given as 2950 rpm, which means the motor is running at a slip of:
$$s = \frac{N_{s} - N}{N_{s}}$$
$$s = \frac{3000 - 2950}{3000} = 0.0167$$
The power output of the motor can be calculated as:
$$P_{out} = P_{in} - P_{loss}$$
Where,
$$P_{in} = 3VI\cos\phi$$
$$P_{loss} = 3I^{2}R_{s}$$
Assuming an efficiency of 90%, we can calculate the input power:
$$P_{in} = \frac{P_{out}}{\text{Efficiency}} = \frac{15 \text{ kW}}{0.9} = 16.67 \text{ kW}$$
Now, we need to calculate the current flowing through the motor:
$$I = \frac{P_{in}}{3V\cos\phi}$$
Assuming a power factor of 0.85, we get:
$$I = \frac{16.67 \times 1000}{3 \times 415 \times 0.85} = 27.38 \text{ A}$$
The total resistance of the motor can be calculated as:
$$R_{s} = \frac{V^{2}}{P_{in}} - \frac{X_{s}^{2}}{P_{in}}$$
Where,
$$X_{s} = \frac{V}{2\pi f}\text{cos}^{-1}(\text{Power Factor})$$
$$X_{s} = \frac{415}{2\pi \times 50}\text{cos}^{-1}(0.85) = 2.30 \text{ }\Omega$$
$$R_{s} = \frac{415^{2}}{16.67 \times 1000} - \frac{2.30^{2}}{16.67 \times 1000} = 0.95 \text{ }\Omega$$
Finally, we can calculate the power loss in the motor:
$$P_{loss} = 3I^{2}R_{s} = 3 \times 27.38