1 Name three types of de motors and Make a sketch of the connections.

2 Explain what is meant by the generator ef-fect in a motor.

3 What determines the magnitude and polarity of the counter-emf in a dc motor?

4 The counter-emf of a motor is always slightly less than the applied armature voltage. Explain.

5 Name two methods that are used to vary the speed of a dc motor.

6 Explain why the armature current of a shunt motor decreases as the motor accelerates.

7 Why is a starting resistor needed to bring a motor up to speed?

8 Show one way to reverse the direction of rotation of a compound motor.

9 A 230 V shunt motor has a nominal arma-ture current of 60 A. If the armature resist-ance is 0.15 Ω, calculate the following:

a. The counter-emf [V]

b. The power supplied to the armature [W]

c. The mechanical power developed by the motor, [kW] and [hp]

10 a. In Problem 9 calculate the initial start¬ing current if the motor is directly connected across the 230 V line.

b. Calculate the value of the starting resistor. needed to limit the initial current to 115 A series winding, per pole. The shunt field has a total resistance of 115 Ω, and the nominal ar-mature current is 23 A. If the motor is con-nected to a 230 V line, calculate the following:

a. The mmf per pole at full-load

b. The mmf at no-load

12 A separately excited de motor turns at 1200 drain when the armature is connected to a 115 V source. Calculate the armature voltage required so that the motor runs at 1,500 r/min. At 100 r/rnin.

13 The following details are known about a 250 hp, 230 V, 435 r/min de 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.

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

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

14 We wish to stop a 120 hp, 240 V, 400 r/min motor by using the dynamic braking circuit shown in Fig. 5.17. If the nominal armature current is 400 A, calculate the following:

a. The value of the braking resistor R if we want to limit the maximum braking current to 125 percent of its nominal value

b. The braking power [kW] when the motor has decelerated to 200 r/min, 50 r/min, p r/min

15 a. The motor in Problem  14 is now stopped by using the plugging circuit of Fig. 5.19. Calculate the new braking resistor 1? so that the Maximum braking current is 500 A.

b. Calculate the bralcing power [kW] when the motor has decelerated to 200 r/min, 50 r/min, 0 r/min.

c. Compare the braking power developed at 200 r/min to the instantaneous power dissipated in resistor R.

Advanced level

16 The armature of a 225 kW, 1200 r/min mo¬tor has a diameter of 559 mm and an axial length of 235 mm. Calculate the following:

a. The approximate moment of inertia, knowing that iron has a density of 7900 kg/m3

b. The kinetic energy of the armature alone when it turns at 1200 r/min

c. The total kinetic energy of the revolving parts at a speed of 600 r/min, if the J of the windings and commutator is equal to the J calculated in (a)

17 If we reduce the normal exciting current of a practical shunt motor by 50 percent, the speed increases, but it never doubles.

Explain why, bearing in mind the saturation of the iron under normal excitation.

18 The speed of a series motor drops with ris¬ing temperature, while that of a shunt motor increases. Explain.

Industrial Application

19 A permanent magnet motor equipped with cobalt-samarium magnets loses 3% of its magnetism per 100°C increase in tempera-ture. The motor runs at a no-load speed of 2500 r/rnin when connected to a 150 V source in an ambient temperature of 22"C. Estimate the speed if the motor is placed in a room where the ambient temperature is 40°C.

20 Referring to Fig. 5.30, calculate the following:

a. The number of conductors on the armature

b. The value of the counter-end at full load

c. The flux per pole, in milliwebers [mWb]

21 A standard 20 hp, 240 V, 1500 r/min self-cooled dc motor has an efficiency of 88%. A requirement has arisen whereby the motor should run at speeds ranging from 200 r/min to 1500 r/min without overheating. It is de¬cided to cool the machine by installing an external blower and channeling the air by means of an air duct. The highest expected ambient temperature is 30°C and the temperature of the air that exits the motor should not exceed 35nC. Calculate the capacity of the blower required, in cubic feet per minute.

22 A 250 hp, 500 V dc shunt motor draws a nominal field current of 5 A under rated load. The field resistance is 90 U. Calculate the ohmic value and power of the series re¬sistor needed so that the field current drops to 4.5 A when the shunt field and resistor are connected to the 500 V source.

23 A 5 hp dc motor draws a field current of 0.68 A when the field is connected to a 150 V source. On the other hand, a 500 hp motor draws a field current of 4.3 A when the field is connected to a 300 V do source.

In each case, calculate the power required Othe field as a percentage of the rated powel of the motor. What conclusions can you draw from these results?