electrical boiiii
A four-pole induction motor supplied from a 50 hertz supply will have synchronous speed of:
1500 ppm.
The speed of a four pole squirrel cage motor in a 60 Hz power system is ______ rpm.
1750
A four pole induction motor has a synchronous speed of 1800 rpm. The actual rotor speed when measured with a tachometer is 1764 rpm. What percentage slip is there?
2
How many poles does a 60 cycle alternator running at 3600 rpm have?
2
The modern AC generator us almost always of the:
2 - pole design.
Two alternators are operated in parallel to supply power to a load having a unity power factor. The load requires 48 kW at 240 volts. Both alternators operate at unity power factor. Output of one = 30 kW and the output of number two = 18 kW. They are single-phase alternators each rated 30 kW, 240 volts and 60 HZ. What is the total current supplied to the load?
200A
A lagging power factor load, such as induction motors, fluorescent lighting, and electric welders will cause the alternator terminal voltage to drop as much as ____ % to _____ % between no-load and full-load.
25, 50.
Speed regulation in a squirrel cage motor is in the range of:
3-5%
Hydrogen and air form an explosive mixture between the limits of ____ % and ______ % hydrogen in air by volume.
4, 75%
In general, the larger the motor, the higher the full-load efficiency. Average values of full-load efficiencies of squirrel-cage motors would be about:
5 hp 83% and 100 hp 90%
The mass of a large two pole generator design with a solid steel forging rotor may reach masses of:
50 to 60 tonnes.
If the starting voltage is reduced to 80% for an induction motor, the starting torque will be _______ percent.
64
The power factor of an induction motor, even at full load, will not exceed ________ percent.
90
During normal running it is not difficult to maintain the air purity of the hydrogen in the stator at least ______ % or above.
95.
Circuit-protective and switching devices in general are rated by:
A continuous rating and an interrupting rating.
Eddy currents in the stator core of AC generators, due to the flux of the revolving field can be reduced by the use of:
A laminated core.
The wound rotor construction of a wire wound induction motor:
Allows for control of the motor speed and provides a high starting torque.
The main limitation on AC generator output from any given turbo-alternator frame is:
Amount of heat that can be dissipated from the rotor without having a temperature rise in the windings exceed permissible limits.
A delta connection used for connection in three phase AC generators to connect the leads to the load provides:
An increase in current but no increase in voltage.
A wye connection used to connect the leads to the load in three phase AC generators provides:
An increase in voltage but no increase in current.
In a synchronous motor at no load the torque __________ is zero as the mechanical load increases the torque __________ increases.
Angle, angle.
For an alternator to obtain maximum efficiency, it should be operated:
At or near the rated full load output.
The power factor of a lightly loaded induction motor will:
Be very low.
Why is induction motor torque low at no load?
Because there is only a slight difference between the rotor speed and the synchronous speed of the stator flux.
The point of maximum torque for an induction motor is called:
Breakdown point.
The starting torque of a synchronous motor is almost nil, since the stationary poles are alternately attracted and repelled by the revolving stator field. How is this counteracted in most cases?
By having a squirrel-cage winding built into the faces of the rotor poles and during starting the field winding on the rotor is short-circuited through a resistance and start-up uses the squirrel cage windings.
How can the effectiveness of hydrogen cooling of alternators be increased?
By increasing hydrogen pressure and allowing the gas direct access to the copper conductors on the rotor windings.
What is done to avoid having an explosive mixture of air and hydrogen in the stator of a hydrogen-cooled alternator at times of charging or purging?
Carbon dioxide is used as a purging medium.
In order to avoid having an explosive mixture of air and hydrogen in the stator at times of charging or purging, ______ is used as a buffer gas.
Carbon dioxide.
The condition of hunting when two or more alternators are operating in parallel:
Causes a torque that may cause the rotor of the alternator to be pulled ahead of and then behind its normal running position and is a pulsing or oscillating effect.
An AC generator voltage regulator is a device that:
Causes the excitation of the generator to be automatically raised or lowered.
Reversing two of the three incoming supply leads to a three-phase squirrel-cage induction motor will cause the motor to:
Change direction of rotation.
How is the power factor altered in a synchronous motor?
Changing DC field excitation.
The supply frequency is fixed the speed of a squirrel-cage induction motor can only be changed by:
Changing the number of poles.
Percentage voltage regulation is calculated at:
Constant speed and a fixed field excitation.
The standard squirrel-cage induction motor has excellent speed regulation from no load to full load, and because of its good speed regulation it is considered to be a :
Constant speed motor.
The losses that vary the most in a given induction motor would be:
Copper losses.
Intermittent or sudden heavy loads on an AC motor can slow it down excessively because motor speed is what produces cemf (back emf). What does this limit through the windings?
Current flow.
A capacitor-start single phase motor uses a capacitor to achieve:
Current phase displacement.
When a squirrel cage induction motor is started with full line voltage the starting __________ may be 3 to 6 times the rated __________ of the motor.
Current, current.
In an alternator the magnitude of the generated voltage depends upon:
DC field excitation.
This type of AC generator voltage regulator gives quick response and close regulation. It adjusts the field strength directly.
Direct acting rheostatic regulator.
An AC generator cannot supply its own field current. The field excitation must be:
Direct current.
The rotation speed of the field of an induction motor is:
Directly proportional to the frequency but inversely proportional to the number of pairs of poles.
The output from a generator is _____ dependent upon the _____ power.
Directly, excitation.
In AC motors what causes winding damage or excessive heat?
Excessive current.
For two alternators operating in parallel, the load division between the machines must be obtained without causing a change in:
Frequency
The speed of a squirrel cage induction motor can be changed by changing the:
Frequency and/or number of poles.
Load sharing between alternators is accomplished by adjusting the __________ of the prime mover.
Governor.
There are many types of overload protection of AC motors. Some use a bimetallic strip that bends with __________.
Heat.
The efficiency of synchronous motors is, in general, is _________ induction motors of the same horsepower and speed ratings.
Higher than.
The efficiency of wound rotor motors is decreased because of _____ losses.
I^2R
When may a synchronous motor apply full-line voltage for starting?
If the supply line disturbance isn't objectionable and the load can stand the shock of starting under full voltage.
When may an induction motor apply full-line voltage for starting?
If the supply line disturbance isn't objectionable and the load can stand the shock of starting under full voltage.
This type of AC generator voltage regulator controls the movement of a motor operated rheostat in the generator field-circuit using an auxiliary apparatus. It is used in large plants.
Indirect acting rheostatic regulator.
This type of AC motor has no electrical connection from the rotor winding to any source of electricity.
Induction
The three single-phase windings of an alternator may be connected in either delta or wye. The wye connection is more commonly used because:
It gives higher terminal voltage.
When two or more alternators are operating in parallel, the load is shared between them and the load taken by each machine is proportional to what?
KVA rating.
Why must AC generators be driven at a definite constant speed?
Keeps frequency correct.
What type of load on the alternator will cause the terminal voltage to drop 25% to 50% between no-load and full load?
Lagging load (ie. induction motors, fluorescent lighting, welders)
f a power system has __________ power factor reactive KVA is supplied by the equipment if a system has __________ power factor the equipment requires reactive KVA.
Lagging, lagging.
What are the advantages of AC generators with revolving fields and stationary armature conductors?
Lower speeds can be used.
In an induction motor, to obtain a greater torque the rotor is given:
More resistance and less reactance.
The most common type of AC motor is the squirrel cage induction motor. It requires:
No slip rings.
A synchroscope gives an accurate indication of:
Paralleling alternators' phase relationship between the two voltages and frequency.
By reversing the connections of any two leads of a three phase motor a reverse torque is set up which stops the motor quickly. Before the motor starts to turn in reverse the circuit can be interrupted. This method of motor breaking or stopping is called:
Plugging.
Power consumed in an AC circuit depends on the circuit __________. Therefore an AC generator could supply a moderate amount of true power for a load, but if the __________ of the load was low, the total, or apparent, power actually delivered by the generator could be very large, which could result in the generator burning out.
Power factor, power factor.
The maximum torque value that can be developed by a synchronous motor without dropping out of synchronism, and is usually 150% to 200% of the rated torque output is called:
Pullout torque.
The amount of any generated emf depends upon the:
Rate at which magnetic lines of flux are cut.
Poor starting torque of the standard squirrel cage motor comes from its relatively high inductive __________ at rest compared to its __________
Reactance, resistance.
In AC generators the effective armature resistance, the armature leakage reactance, and the armature reaction effect:
Regulation
What controls the exciter current to the field winding to compensate for changes in output voltage in AC generators?
Regulators.
This type of AC motor operates at constant speed and its power factor is under the control of the operator.
Synchronous.
What type of motor can be operated as capacitors to improve a plant's power factor?
Synchronous.
Devices called current, thermal or combination cutouts are built in protection for larger AC motors which will:
Temporarily shuts off the motor if it gets too hot or draws too much current.
Leading power factor loads will:
Tend to raise the alternator voltage.
What determines the power factor of an AC generator system?
The characteristics of the load connected.
A change in rotor frequency in an induction motor will cause:
The inductive reactance and impedance to increase for increase in frequency.
What problems does the basic AC motor have?
The initial direction of rotation is unpredictable.
For AC generators to be synchronized, what conditions have to be met?
The output voltages must be equal and in phase and the frequencies must be the same.
To insure that the strongest possible electromagnetic induction action takes place in a squirrel-cage motor:
The periphery of the rotor is separated from the stator by a very small air-gap.
In a synchronous motor as the field current increases the stator supplies a smaller component of current. How will this affect the power factor?
The power factor increases.
Most large AC generators are of the stationary armature type. What is the main reason behind this?
The slip rings do not have to carry the generator power output.
Why does the output voltage of a simple AC generator constantly change direction?
There is no rectification of the output by the slip rings.
A three phase alternator has three separate windings which are arranged so that:
Three voltages are produced and are 120 electrical degrees apart.
A simple AC motor has a centrifugal switch built into it. What is it for?
To operate start circuits.
The __________ output of an induction motor varies as the square of the terminal voltage for a given percent slip.
Torque.
Alternators for hydro-electric or diesel engine generating stations are of the:
Two.
The modern turbo-alternator is almost without exception of the _____ pole design.
Two.
These motors are fractional horsepower AC motors specially designed for use with DC or AC power. If they're operating on AC the current reversals affect both field and armature so that the motor direction isn't changed. They'll run at 4000-8000 rpm and have high starting torque.
Universal motors.
A DC motor that can operate with an AC power supply is a ______ motor.
Universal.
A quick acting positive control of alternator __________ is essential as large __________.
Voltage, voltage.
Alternators that have salient field poles are driven by:
Water turbines and diesel engines.
When may AC generators be rated in watts or kilowatts?
When they're designed for use with loads having a constant power factor.
An AC motor which has a polyphase winding of the same number of poles as the stator, and the terminals are brought out to slip rings so that external resistance may be introduced is a:
Would rotor.
Energy efficient motors having higher than standard efficiencies obtain this additional efficiency through the use of:
low-loss lamination steel, longer stators and longer rotors.
Alternators that have cylindrical type rotors are driven by:
steam and gas turbines.
The wye connection windings in a three phase alternator allows the:
terminal voltage to be 1.73 times the phase winding voltage.
If an alternators' load consists of pure resistance, such as lighting and heating units, then the power factor will be unity and the terminal voltage drop will be about _____ % to ______ % between no-load and full-load.
1, 10.
An AC generator field voltage is usually between:
100V and 250V.
One volt is produced when _____ line(s) of magnetic flux are cut per second.
10^8
These single-phase motors have a rotor that is constructed like a DC motor armature with a centrifugally controlled mechanism to short-circuit to commutator when the motor reaches 75% of rated speed. They are suitable for applications requiring a strong starting torque and fairly constant speed from no load to full load and are used in commercial refrigerators, compressors and pumps.
Repulsion, start induction motor.
A capacitor start motor is very similar to a:
Resistance start motor.
The __________ of an AC generator carries the field windings, supplied through brush gear and slip-rings.
Rotor.
A shaded pole motor:
Runs at 10% to 25% below synchronous speed, built in sizes from about 3 to 75 watts, have low starting torque and cannot be reversed unless specially constructed.
The ratio of the field current required to produce rated voltage at rated speed and no load to the field current required to circulate rated stator current when operating at rated speed under sustained short-circuit conditions is called:
Short circuit ratio.
The basic solid state power device used in motor starters to produce smooth, stepless motor acceleration is a type of thyristor called a:
Silicon controlled rectifier.
In AC generators with revolving armatures, what is used to connect the rotating coils to the brushes and thus to an external circuit?
Slip rings.
A synchronous motor source of the magnetic field of the rotor is DC supplied from an external force therefore __________ which is so essential in an induction motor is absent in this motor.
Slip.
The difference in synchronous speed of the motor and the actual speed of the rotor is called:
Slip.
What determines the frequency that is generated from an AC generator?
Speed and number of poles.
The induced field voltage of an AC generator is large enough to damage equipment. Therefor to eliminate this danger the following is used:
Speed field discharge switch, field contactor or circuit breaker.
The kilowatt division of load between synchronous generators in parallel is determined by:
Speed load characteristics of the prime movers only.
A _____-_____ AC motor resembles a two-phase induction motor because the currents of the windings are approximately 90 electrical degrees out of phase with each other. A single phase source supplies the motor however.
Split phase.
The most common type of AC motor is the:
Squirrel cage induction motor.
The rotor of this AC motor consists of a cylindrical core made from steel laminations. Copper bars are mounted near the surface and are connected at the ends to copper end rings.
Squirrel-cage.
This type of AC motor can be used in locations where there is a possibility of an explosion due to arcing, such as chemical plants, flour or lumber mills because brushes and slip rings are not required.
Squirrel-cage.
The __________ is the stationary part of an induction motor and consists of a cylindrical laminated core that is pressed in a frame.
Stator
Torque is produced in an induction motor by the interaction of:
Stator and rotor magnetic fluxes.
A three phase alternator has three separate windings which are placed in the slots of the __________ core.
Stator.
The __________ of an AC generator consists of a magnetic steel core, built up in laminated sheets.
Stator.
