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61 Cards in this Set
- Front
- Back
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Define Torque |
Twisting or turning force that is capable of producing rotation about the axis |
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Define the Motor Effect principle of Magnetism |
Whenever a conductor Carrie's current in the presence of a magnetic, a force will act on the conductor at a right angle to the field |
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Fleming's right hand rule is used to determine... |
Direction of force acting on the conductor |
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How does the operation of the armature differ between a DC Motor and DC Generator? |
In a Generator, armature is source of emf In a Motor, armature is the load |
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State 2 ways the direction of rotation of a DC motor can be reversed. |
-Reverse armature leads -Reverse the connection of the shunt and series field leads |
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Torque is directly proportional to the product of ______ and _______ |
Flux, Armature Current |
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The motion of armature conductors cutting a field results in an induced emf that _____ the applied emf |
Opposes |
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The emf induced in the armature conductors as they cut the magnetic field is called.... |
Counter emf |
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The counter emf of a motor is proportional to ______ and _______ |
Flux, Speed |
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Effective armature voltage is the product of _______ and _______ |
Armature Current, Armature Resistance |
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The difference between applied voltage and counter emf is called... |
Effective Voltage |
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At standstill, the counter emf of a motor is... |
Zero |
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At start-up, the current drawn by a DC motor is limited only by... |
Armature Resistance |
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How is the starting current limited to a safe value in large DC motors? |
Resistance is placed in series with the armature |
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A locked rotor usually means that the motor will... |
Fail to start when voltage is applied |
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Why is a locked rotor condition undesirable? |
Persistance of High Starting current may damage the motor |
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An increase in mechanical load on the shaft of the motor causes motor speed to.... |
Decrease |
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When the speed of a motor decreases due to an increase in mechanical load, the counter emf.... |
Decreases |
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A reduction in counter emf causes armature current to... |
Increase |
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What would happen if an increase in mechanical load exceeded the ability of the motor to develop additional torque? |
The motor would stall |
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In a motor, power is proportional to ______ and _______ |
Torque, Speed |
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The variation in motor speed with load is called.... |
Speed Regulation |
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The speed at which a DC motor operates without speed control devices is called... |
Base Speed |
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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 |
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To operate a motor below base speed, it is necessary to decrease... |
Armature Current |
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A practical method of operating below base speed involves controlling... |
The voltage applied to the armature |
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List 2 adverse effects of armature reaction if it remains uncorrected |
-Weakening of the main field -Excessive brush sparking |
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The most common solution for minimizing the effects of armature reaction involves the use of.... |
Interpoles |
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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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Excitation of a shunt motor is provided by field coils connected in series with the armature (T/F) |
False |
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When using NEMA subscripts, the convention of A1 positive indicates (CCW or CW) rotation |
Counter Clockwise (Viewed from opposite drive end) |
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When a DC generator and a DC motor are operated with the same line voltage, polarity and direction of rotation, the respective armature currents will be in opposite directions (T/F) |
True |
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2 ways to reverse the direction of the force acting on the armature conductors of a shunt motor? |
-Reverse direction of Armature current Or -Reverse direction of Shunt current |
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What is the most practical method of reversing the direction of rotation of a shunt motor? |
Reverse the armature leads |
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What happens to the direction of rotation if the polarity of the line terminals is reversed? |
It stays the same |
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A shunt motor is commonly called a _______ motor |
Constant Speed Motor |
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Why must the field circuit of a shunt motor never be allowed to become open while the motor is in operation? |
Motor can accelerate to unsafe speeds |
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What is the main difference between series and shunt excitation? |
Magnetic Strength of series field coils varies with load current |
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State 2 ways to reverse the direction of rotation of a series motor |
-Reverse leads of the armature -Reverse leads of the SERIES field |
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Will changing the polarity of the line leads change the direction of rotation of a series motor? |
No. Reversing line polarity reverses both armature current and field polarity, maintaining torque in the same direction as before |
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Why is the series motor unsuitable for constant speed applications? |
Since torque varies with the square of armature current, changes in mechanical load are accompanied by large changes in operating speed |
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In practice, speed control of series motors is accomplished using... |
Solid State Drives |
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The main advantage of the series motor is it's very high starting torque (T/F) |
True |
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Why should a series motor always be mechanically coupled to the driven load? |
When a load is suddenly removed, the motor can operate to unsafe speeds |
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The main characteristics of a series motor are _____ and ______ |
High starting Torque, Poor Speed Regulation |
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3 common uses for series motors? |
Cranes, Hoists, Locomotives, Mine Haulage trucks, Automobile Starters |
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In most compound motors, the motor characteristics are predominantly like the ______ motor |
Shunt |
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When the flux of the series field aids the flux of the shunt, the connection is called a ______ compound |
Cumulative Compound |
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When flux of the series field opposes the shunt, the connection is called a _____ compound |
Differential Compound |
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Difference between long and short shunt connections? |
Long shunt is across the line Short shunt is across the armature |
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State 2 ways of reversing the direction of rotation of a compound motor |
-Change leads of armature -Change leads of series and shunt coils |
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Which of the 2 methods of reversing rotation is preferred? |
Changing leads @ armature |
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What determines the position of the compound characteristic curve with respect to series and shunt curves? |
The degree of compounding |
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The speed of a compound motor can be increased by increasing resistance in the shunt field circuit (T/F) |
True |
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Is it necessary to take precautions to prevent runaway of a compound motor? |
Yes. Degree of compounding may not be sufficient to prevent dangerous 9ver-speed in the event of an open in the shunt field circuit |
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Is it acceptable to have belt-driven mechanical loads with a compound motor? |
Yes. No-load speed hazard of the series motor is not present |
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When selecting a compound motor, the _____ and ______ characteristics must be matched to the specific application. |
Torque, Speed |
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Compound motors are more suitable than shunt motors for handling.... |
Sudden increases in mechanical load |
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List 3 applications for compound motors |
Presses, Shears, Compressors, Elevators |
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To prevent runaway in a shunt motor, an _______ is used |
Overspeed sensor or loss of field relays |
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3 typical uses for a shunt motor? |
-Drives for paper machines -Printing Presses -Drill Presses -Lathes -Blower -Motor Generation Sets |
