Electrician Apprenticeship Year 3

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  • What is the main difference in the construction of a DC generator and a DC motor?

    The type of enclosure.
    generator (open frame) motor (fully enclosed).

    The rotating part of a DC generator is called the ___________?

    Armature.

    What are two basic types of armature windings?

    Lap and Wave.

    Which armature winding is used in high current, low voltage applications?

    Lap windings are used in high current, low voltage applications.
    Wave windings are used for low current, high voltage applications.

    What is the electrical function of the commutator?

    The commutator maintains the current in one direction to the external circuit.

    What are the common materials used in making brushes?

    Carbon, graphite, electro-graphite, and copper graphite.

    Suitable brush pressure is usually between _________ and _________ psi.

    One and two psi.

    What is the function of the field poles?

    Field poles provide the necessary magnetic field.

    DC machines use _________ or _________ bearings.

    Sleeve or ball bearings.

    Grease nipples in the end shields indicate that the machine contains which type of bearings?

    Open faced ball bearings.

    What are small generators that use permanent magnets for field excitation are called?

    Magnetos.

    What is the difference between separately excited and self-excited fields?

    The current for separately excited fields is obtained from a source external to the machine.

    the two ways in which field coils are connected in self-excited generators.

    Series and shunt (parallel).

    Which field windings are made of a low number of turns of large gauge wire?

    Series windings.

    What device in the shunt field circuit is used to control generator voltage?

    Field rheostat.

    The build-up of voltage in a self-excited generator requires?

    Residual magnetism.

    What are the 5 key factors that influence generated voltage?

    Number of poles, flux per pole, rotational speed, number of armature paths, number of active armature conductors.

    What factors that influence generated voltage can be varied in an operating generator?

    Flux per pole ( excitation and rotational speed.

    What is the effect on generated voltage if the speed of the prime mover is decreased by one third?

    The generated voltage is reduced by one third.

    In a self-excited generator, voltage build-up requires the presence of ___________?

    Residual magnetism.

    Voltage build-up occurs until the generator reaches its ___________?

    No-load terminal voltage.

    Appropriate residual magnetism can be established on the field poles by a process commonly called __________ the _____?

    Flashing the field.

    The value of field resistance that will limit field current to a value insufficient for voltage build-up is called _______________?

    Critical field resistance.

    What should be done at generator start-up to avoid the problem associated with critical field resistance?

    Field rheostats should be set to their minimum ohmic value.

    What is armature reaction?

    Armature reaction is a distortion of the main field flux caused by current flowing in armature conductors.

    What are two effects of armature reaction on the operation of a generator?

    A reduction in terminal voltage with an increasing load. Impaired commutation.

    Interpole coils consist of?

    A small number of turns of large gauge wire.

    How are interpole coils connected?

    Interpole coils are connected in series with the armature.

    The effects of armature reaction can also be minimized by placing windings called _____________ in the main pole faces.

    Compensating windings.

    What are three types of power loss in a generator?

    Mechanical, electrical, magnetic.

    Rotational losses include ___________ and ____________.

    Bearing friction and windage.

    How is the effect of eddy currents minimized in the construction of DC machines?

    By laminating the core materials.

    Heat produced by current flowing through the armature and field coils is called?

    Copper losses.

    Define efficiency.

    Efficiency is the ratio of power out to power in.

    A generator characteristic curve shows the effect of _______________ on _______________.

    Connected load on terminal voltage.

    Describe the conditions that must be satisfied to obtain the desired performance when connecting a generator.

    The field coils must be properly connected with respect to the armature and to each other.

    the letters that are used in the NEMA system to identify the armature, shunt and series fields.

    Armature = A; Shunt field = F; Series field = S

    The terminal voltage of a shunt generator is controlled by?

    A rheostat in the field circuit. (field rheostat)

    What determines the amount of current that flows through an armature?

    The impedance of the connected load.

    the factors that cause a reduction in terminal voltage with increasing load.

    IR drop of the armature, armature reaction and loss of field current due to reduction in generated voltage.

    Is the series generator a practical machine?

    No.

    Why would it not be practical to control the output of a series generator with a field rheostat?

    The rheostat would have to carry the high armature current resulting in high wattage losses.

    Why is the no-load voltage of a series generator extremely low?

    Without a load the circuit is an open circuit so no current flows through the series field coils.

    In a series generator, why does the terminal voltage rise with increasing load?

    As armature current increase, the strength of the series field increase.

    Describe how the voltage characteristic of series field coils could be useful.

    The rising voltage characteristic is used to offset the decrease in terminal voltage associated with shunt field excitation in a compound generator.

    A compound generator contains what type of field?

    Both series and shunt.

    The condition under which the series field adds to the shunt field is called.

    Cumulative compound.

    The condition under which the magnetic flux of the series and shunt fields oppose each other is called.

    Differential compound.

    Describe the difference between long shun and short shunt connections.

    In long shunt, the shunt field is connected across the line. In short shunt, the shunt field is connected across the armature.

    A compound generator that has a relatively constant terminal voltage characteristic is referred to as __________.

    Flat-compound.

    Line drop can be compensated for by using a generator that is __________.

    Over-compound.

    Which generator has the most severe drop in terminal voltage with load?

    Differential compound.

    Which generator would be most suitable for arc welding?

    Differential compound.

    With respect to long vs. short shunt connections, what is a general rule?

    Connect the shunt field across the source of emf.

    What is torque?

    Torque is a twisting or turning force that is capable of producing rotation about an axis.

    What is the motor effect principle of magnetism?

    Whenever a conductor carries current in the presence of a magnetic field, a force will act on the conductor at right angles to the field.

    What is Fleming's right-hand motor rule used to determine?

    The direction of force acting on a conductor.

    How does the operation of the armature differ between a DC generator and a DC motor?

    In a generator, the armature is a source of emf. In a motor, the armature is a load.

    What are two ways that the direction of rotation of a DC motor can be reversed?

    Reverse the connection of the armature leads. Reverse the connection of both the series and shunt field leads.

    Torque is directly proportional to the product of ________ and _______________.

    Flux, armature current.

    The motion of armature conductors cutting a field, results in an induced emf that _____________ the applied emf.

    Opposes.

    The emf induced in armature conductors as they cut the magnetic field is called _____________.

    Counter emf.

    The counter emf of a motor is proportional to ______________ and ______________.

    Flux, speed.

    Effective armature voltage is the product of __________ and __________.

    Armature current, armature resistance.

    The difference between applied voltage and counter emf is called _____________?

    Effective voltage.

    At standstill, the counter emf of a motor is __?

    0

    At start-up, the current drawn by a DC motor is limited only by __________________.

    Armature resistance.

    How is the starting current limited to a safe value in large DC motors?

    Resistance is placed in series with the armature.

    A locked rotor usually means that the motor will ________________.

    Fail to start.

    Why is a locked rotor condition undesirable?

    The persistence of high starting current may damage the motor.

    An increase in mechanical load on the shaft of a motor causes motor speed to?

    Decrease.

    When the speed of a motor decreases due to an increase in mechanical load, the counter emf will _________.

    Decrease.

    A reduction in counter emf causes armature current to _____________.

    Increase.

    What would happen if an increase in mechanical load exceeded the ability of the motor to develop additional torque?

    The motor would stall.

    In a motor, power is proportional to ______________ and ____________.

    Toque and speed.

    The variation in motor speed with load is called _____________________?

    Speed regulation.

    The speed at which a DC motor operates without speed control devices is called _____________?

    Base speed.

    Why does increasing the resistance in the field circuit of a DC motor cause the speed to increase?

    The increase in armature current )due to a decrease in counter emf) is greater than the decrease in excitation.

    To operate a motor below base speed, it is necessary to decrease _____________.

    Armature current.

    A practical method of operating below base speed involves controlling ____________.

    The voltage applied to the armature.

    What are two adverse effects of armature reaction if uncorrected?

    Weakening of the main field, Exvessive brush sparking.

    The most common solution for minimizing the effects of armature reaction involves the use of _________?

    Interpoles.

    Why are interpoles connected in series with the armature?

    So that the strength of the interpole matches the strength of the armature field.

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