Problems

Author: E.E. Kimberly

	Problem	Answer
1	Calculate the flux density in part a of the magnetic path in Fig. 7-6 (a) when the total ampere-turns are 15,000.
2	Why is the toy motor frame of Fig. 7-11 open at the top?
3	A coil of 1360 turns of No. 18 copper wire (see Appendix) is wound in one layer on a wooden ring whose circular cross-section is 4 sq in. and whose mean radius is 10 in. What will be the current when the coil is connected to a source of 20 volts and the current is direct? What will be the total flux in the core?	1045 lines
4	If the wooden ring in Problem 3 is replaced by a cast-steel ring, how many volts would be needed at the terminals of the coil to produce 200,000 lines of flux? See page 389 in the Appendix for the magnetization curve.
5	What will be the flux density in lines per square centimeter in Problem 4, if the voltage is halved?
6	A change of 4 amp in a certain 1000-turn coil causes a flux change of 1,000,000 flux lines. Calculate the inductance of the coil.
7	A direct current of 4 amp flowing through 500 turns of wire is required to maintain a flux of 4 x 106 lines in an air-core reactor. What is the inductance of the reactor?
8	An air-core reactor of negligible resistance has an inductance of 0.2 henry. What current will flow when a 60-cycle voltage of 230 volts is applied?
9	In Problem 8, what is the maximum instantaneous rate at which energy is stored in the magnetic field?
10	A magnetic circuit of cast steel is magnetized to a flux density of 45,000 lines per square inch. What percentage change in current will be necessary to double this flux density? See the magnetization curve on page 389.
11	What percentage change would be required in Problem 10 if the circuit were of 1% silicon steel instead of cast steel?
12	When the armature iron of a d-c motor is magnetized to a density of 95,000 lines per square inch and the speed is 1800 rpm, the hysteresis loss is 420 watts. What would be the hysteresis loss if the flux density were reduced to 90,000 lines and the speed were increased to 2500 rpm? Assume that all parts of the armature have the same flux density.
13	When the speed and flux density of the motor of Problem 12 are changed as specified, what will be the per cent of change in the armature eddy-current loss?
14	A magnetic contactor, when closed, requires a sustained current to hold it so. What becomes of the power used?
15	A direct-current motor running at 2000 rpm is found to have a hysteresis loss of 200 watts and an eddy-current loss of 300 watts in the armature iron. What will be the total power loss in the armature iron if the speed is increased to 2500 rpm while the field flux remains constant? The eddy-current loss is proportional to the square of the frequency of reversal of magnetic flux.	718,7 watts