Machines; Direct Current Machines Objective 3 Flashcards
What does “shunt” wound mean?
Windings connected in parallel
What does “compound” wound mean?
Series and parallel
True or false: basic field and armature winding connections are the same between dc motors and dc generators
True
What are the physical characteristics of a shunt field winding?
Many turns of fine wire
What is a shunt field winding resistance compared to resistance of the armature conductors?
Much higher.
Where is the largest current in a shunt motor? Why?
Through the armature. Because parallel circuit, current splits based on resistance (same voltage in each branch). Resistance lowest in armature therefore current highest.
How does shunt current compare to armature current in a shunt motor?
Shunt current can be as low as 1% of the armature current.
What is the relationship between speed and torque in a shunt motor?
Small decrease in speed = substantial increase in torque
What is the relationship between armature current and torque?
Proportional
What is the relationship between speed and armature current?
Decreased speed = increased armature current (due to decrease in counter emf)
What can be done to reduce high starting current of a shunt motor?
Add resistor in series with armature circuit at start.
How do we control the speed of a shunt motor?
Vary shunt field resistance
What is motor “runaway”?
High armature current due to not enough cemf
Weak cemf can be due to high field resistance (low field current; weak magnetic shunt-field)
What can be done to prevent motor runaway in a shunt motor?
Add resistor in series with armature during starting
What are the physical characteristics of a series field winding? Why is it constructed as such?
Few turns of heavy wire. Being in series with armature, it needs to carry the same current
What is the relationship between torque and current in a series motor?
Torque is proportional to I²
How does increasing armature current affect both series motors and shunt motors’ magnetic field intensity?
??? Pg.32
Series= current increases in both windings
Shunt = current only increases magnetic field intensity around armature winding conductors.
Which aspects of the fundamental motor equation remain constant?
V(arm) = cemf + (I(arm) × R(arm))
V(arm) remains constant
R(arm) remains constant
How do certain aspects of the fundamental motor equation change with a decreased load?
V(arm) = cemf + (I(arm) × R(arm))
I(arm) decreases with less load
Cemf increases to maintain electrical balance
What precaution must be taken with a series motor?
Dangerously high speeds under light load
How does starting torque compare to full load torque on a series motor?
Starting torque can be as much as 500% higher than full load torque.
How do series and shunt windings compare? Physical characteristics and location?
Series - few turns, heavy wire, on top
Shunt - many turns, fine wire, on bottom
What are two ways the field windings of a compound motor may be connected?
1) cumulative-compounded
2) differentially compounded
How would you wire field windings on a compound motor if you wanted the magnetic fields to aid each other? Oppose?
Aid = cumulative compounded Oppose= differentially compounded
What is the difference between a long shunt and a short shunt?
Little difference in actual operation
Long shunt in parallel with series AND armature windings
Short shunt in parallel with armature winding only.
True or false: when load is added to cumulative compounded motor, shunt field strength remains constant.
True
What would happen to a compound motor if the shunt winding disconnected and light load was connected?
Likely runaway, as it is now a series motor.
Which type of motor is considered stable? What does this mean?
Cumulative-compound motor. It can handle a load that suddenly increases or decreases.
When is a differentially compounded motor used?
When a nearly constant speed is required.
How does a differentially compounded motor differ from the cumulative-compounded motors?
Current in series field winding flows in opposite direction to that of shunt field.
What is the purpose of having a series field winding with opposite current flow?
To oppose shunt field flux to weaken total field flux so motor can speed up.
What happens to a differentially compounded motor as load increases? What is an inherent risk with this?
Subtracting effect of series field winding lowers cemf thereby increasing series and armature currents.
If this series current raises above shunt current the motor can reverse with damaging effects.
What is the degree of compounding of a given machine dependent upon?
Strength of flux produced by series winding compared to that produced by shunt winding.
Increasing series turns (compared to shunt turns) increases compounding
Describe the relationship between speed and torque for:
1) shunt motor
2) series motor
3) cumulative-compound motor
Speed is inversely proportional to torque.
1) small drop in speed with increased torque
2) large drop in speed with increased torque
3) combined chanarcyeristics of above
What does motor speed regulation compare?
Motor speed and load
Which type of motor has the highest starting torque?
Series
Why is the torque vs. Current curve a straight line for a shunt motor?
Torque produced varies only with changing armature current
Why is the torque vs. Current curve curved for a series motor?
Torque produced with armature current flux AND series field flux
What is the formula for speed regulation?
%n(r) = ((no load) - (full load) ÷ full load) × 100
Which type of motor has poor speed control?
Series
