Test1

Question 2: In a DC machine, the armature current has no effect on the field circuit
a) it’s always true.
b) this is true only in steady state.
c) it’s always wrong.
d) it can be true or not.

Question 3: In a DC motor, permanent magnet or independent excitation, the electromagnetic torque supplied to the mechanical system (consisting of the rotor of the machine and the mechanical load that is coupled to it) is proportional to the armature current.
a) it’s true.
b) this is true only in steady state.
c) it’s wrong.
d) this is true only in transient state..

Question 4: The electromotive force that appears in the armature circuit is proportional to the speed of rotation.
a) it’s true.
b) this is true only in steady state.
c) it’s wrong.
d) this is true only in transient state..

Question 5: If the armature current is expressed in amperes, the electromotive force in volts, the torque in newton.meter and the speed of rotation in radians per second, the constants of proportionality between current and torque on the one hand and between speed and force electromotive on the other hand are the same..
a) it’s true.
b) this is true only in steady state.
c) it’s wrong.
d) this is true only in transient state..

Question 6: The speed of a permanent-magnet or independently excited DC machine can be easily adjusted by modulating the supply voltage of its armature circuit.
a) it’s true.
b) this is true only in steady state.
c) it’s wrong.
d) this is true only in transient state..

Question 7: When the inductive circuit of a separately excited direct current machine (that is to say when the inductor current if is canceled), the electromagnetic torque produced by the machine is canceled and, therefore, if the torque of load is mainly a friction torque, the machine stops spinning.
a) it’s true.
b) this is true only in steady state.
c) it’s wrong.
d) this is true only in transient state.

Question 8: Select the correct item:
a) Magnetic power losses correspond to the total flux that circulates in the air.
b) Magnetic power losses are, in general, 40-50% of the nominal power of an electric machine.
c) Magnetic power losses are associated with the leakage flux in a magnetic circuit.
d) Magnetic power losses can increase with the squared of applied voltage.

Question 9: Select the correct item:
a) If the flux in a machine does not vary with time, the core of that machine must be laminated.
b) The inductance of a magnetic circuit decreases when an air-gap is cut in it.
c) The induced emf in a coil is maximum when the flux linking it is maximum.
d) Reducing laminations thickness can reduce core loss because reduce the Eddy-currents in a machine.

Question 10: The following data were obtained on a thin sheet of silicon steel. Compute the hysteresis loss and the eddy-current loss and after select the correct affirmation.

a) Calculations show that the eddy-current loss is smaller than the hysteresis one.
b) Calculations show that the eddy-current loss is greater than the hysteresis one.
c) Calculations show that the hysteresis loss at 25Hz could be neglected.
d) Calculations show that the eddy-current loss at 60Hz almost doubled the hysteresis one.