Term Final Practice

Term Final Practice

Midterm Practice Units 1-3

1-82. 0.3 pounds per cubic inch to kilograms per cubic meter =

2-14. The current in a 60 Hz single-phase motor lags 36 degrees behind the voltage. Calculate the time interval between the positive peaks of voltage and current.

T = 1/ƒ = 1/60 = 1.67 ms

3-17. A motor drives a flywheel having a moment of inertia of 5 kg·m2. The speed increases from 1600 r/min to 1800 r/min in 8 s. Calculate

a. the torque developed by the motor [N·m]

b. the energy in the flywheel at 1800 r/min [kJ]

c. the motor power [W] at 1600 r/min

d. the power input [W] to the flywheel at 1750 r/min

4-24. A 240 kW, 500 V 1750 r/min separately excited dc generator has an overall efficiency of 94%. The shunt field resistance is 60 ohms and the rated current is 5 A. The I2R loss in the armature is 0.023 pu. Calculate

a. the rated armature current

b. the total losses in the machine

c. the I2R losses in the armature

5-13. The following details are known about a 250 hp, 230 V, 435 r/min dc shunt motor: nominal full-load current: 862 A insulation class: H weight: 3400 kg external diameter of the frame: 915 mm length of frame: 1260 mm

a. Calculate the total losses and efficiency at full-load.

P_o= 250 *746 =186.5 kW; P_i = VI = 230 *862 =198.26 kW

P_loss = P_i – P_o = 198.26 kW - 186.5 kW = 11.76 kW

? =P_o / P_i = 186.5 kW / 198.26 kW = 94%

b. Calculate the approximate shunt field exciting current if the shunt field causes 20 percent of the total losses.

P = (11.76 kW / 100) 20 = 2.35 kW

I = P/V = 2.35 kW / 230 V = 10.23 A

c. Calculate the value of the armature resistance as well as the counter-emf, knowing that 50 percent of the total losses at full-load are due to armature resistance.

d. If we wish to attain a speed of 1100 r/min, what should be the approximate exciting current?

6-15. An electric motor driving a skip hoist withdraws 1.5 metric tons of minerals from a trench 20 m deep every 30 seconds. If the hoist has an overall efficiency of 94 percent, calculate the power output of the motor in horsepower and in kilowatts.

7-21. A coil having a reactance of 10 ? and a resistance of 2 ? is connected in parallel with a capacitive reactance of 10 ?. If the supply voltage is 200 V, calculate

a. The reactive power absorbed by the coil

b. The reactive power generated by the capacitor

c. The active power dissipated by the coil

d. The apparent power of the circuit

8-11. A 3-phase heater dissipates 15 kW when connected to a 208 V, 3-phase line.

a. What is the line current if the resistors are connected in wye?

b. What is the line current if the resistors are connected in delta?

c. If the resistors are known to be connected in wye, calculate the resistance of each.

13-15.

a. Calculate the synchronous speed of a 3-phase, 12-pole induction motor that is excited by a 60 Hz source.

b. What is the nominal speed if the slip at full-load is 6 percent?

13-17.

a. Calculate the approximate values of full-load current, starting current, and no-load current of a 75 kW, 4000 V, 3-phase, 900 r/min, 60 Hz induction motor.

b. Calculate the nominal full-load speed and torque knowing that the slip is 2 percent.

14-15. We wish to make an asynchronous generator using a standard squirrel-cage induction motor rated at 40 hp, 208 V, 870 r/min, 60 Hz (Fig. 14.14). The generator is driven at 2100 r/min by a gasoline engine, and the load consists of three 5 ? resistors connected in wye. The generator voltage builds up when three 100 ?F capacitors are connected in wye across the terminals. If the line voltage is 520 V, calculate the following:

a. The approximate frequency generated

b. The active power supplied to the load

c. The reactive power supplied by the capacitor bank

d. The stator current

e. If the following gasoline engines are available—30 hp, 100 hp, and 150 hp—which one is best suited to drive the generator?

14-17 The motor in Problem 14-16 is cooled by circulating 350 gallons (U.S.) of water through the heat exchanger per minute. Calculate the increase in water temperature as the water flows through the heat exchanger.

revolving flux. Does the machine operate as a generator? Calculate the torque.

15-4. A 550 V, 1780 r/min, 3-phase, 60 Hz, squirrel-cage induction motor running at no-load draws a current of 12 A and a total power of 1500 W. Calculate the value of Xm and Rm per phase (see Fig. 15.2).

15-7. A 440 V, 3-phase, 1800 r/min, squirrel-cage motor has the following characteristics If the magnetizing branch can be neglected, calculate the value of the starting torque and the breakdown torque if a 4.5 ? resistor is connected in series with each line.

15-10. The 5 hp motor represented by the equivalent circuit of Fig. 15.12 is connected to a 503 V (line-to-line), 3-phase, 80 Hz source. The stator and rotor resistances are assumed to remain the same.

a. Determine the equivalent circuit when the motor runs at 2340 r/min.

b. Calculate the value of the torque [N-m] and power [hp] developed by the motor