TERM TEST

SOLVE THE FOLLOWING:

1. What is the watts output of a 25hp motor at full load?

2. A 150 kW generator has a efficiency of 91% at full load. Calculate the input in kilowatts and the power loss.

3. A 15hp motor operates at an efficiency of 87.5% at full load. If the stray power loss is approximately one-fourth of the total loss, calculate the copper loss.

4. The rotational loss in a generator was found to be 780 watts when the generated emf was 132 volts. Determine the rotational loss for generated voltages of 138 and 126 volts.

5. The output torque of a motor is 69.2 lb-ft when it operates at 950 rpm. Calculate the losses in the machine and its efficiency if, under this condition, the power input is 10900 watts.

6. A 10 kW, 220 V compound generator is operated at no load at the proper armature voltage and speed, from which the stray power loss calculations are determined to be 705 watts. The shunt-field resistance is 11o ohms, the armature resistance is 0.265 ohm, and the series-field resistance is 0.035 ohm. Assume a 2V brush drop and calculate the full load efficiency.

7. A 250V shunt generator has a rated armature current of 40A and the following losses at full load: friction and windage = 200 watts; core loss = 260 watts; shunt field loss = 100 watts; brush contact loss = 80 watts; armature copper loss = 400 watts. Assuming that the maximum efficiency occurs when the approximated constant losses are equal to those losses that vary as the square of the load, calculate the armature current for the condition of maximum efficiency.

8. A 25 kW series generator has a efficiency of 85% when operating at rated load. If the stray power loss is 20% of the full load loss, calculate the efficiency of the generator when it is delivering a load of 15 kW, assuming that the stay power loss is substantially constant and the other losses vary as the square of the load.

9. A 250V shunt generator has a full load armature current of 40A, under which condition the losses are: friction+windage = 200 watts; shunt field = 100 watts; core = 260 watts; brush contact = 120 watts; armature copper = 400 watts. For the operation at maximum efficiency, when the constant losses are equal to those losses that vary as the square of the load, calculate: a. tha armature and line currents, b. the load power, c. the maximum efficiency.

10. A 20 hp motor has an efficiency of 88.5% and operates continuously to drive a ventillator at full load. Calculate: a. the power loss, b. the energy loss per month, assuming operation for 200 hours during that period, c. the cost of the energy loss at 1 1/2 cents per kilowatthour.

11. An enclosed motor has a rating of 50 hp. The cover plates are removed and the machine is located where it is capable of cooling itself extremely well. If tests show that it can carry 28% more load without excessive heating, what rating should be given the motor?

12. A dynamotor has an output rating of 0.16A at 500V, under which condition it takes 11A from a 12V storage battery. Calculate the efficiency.

13. When a motor is operating under load, the armature takes 8280 watts and its current is 36A. If the armature circuit resistance, including brushes, is 0.4 ohm, what horsepower is developed by the motor?

14. A 5 hp, 230V shunt motor takes 18A when operating at full load. The shunt field resistance is 115 ohms and the armature resistance is 0.25 ohm. Calculate the value of the starter resistor if the armature current is limited to 1.5 times the rated value at the starting instant.

15. A 220V shunt motor has an armature resistance of 0.32 ohm and a field resistance of 110 ohms. At no load the armature current is 6A and the speed is 1800 rpm. Assume that the flux does not vary with load and calculate: a. the speed of the motor when the rated line current is 62A (assume 2V brush drop), b. the speed regulation of the motor.