Machines; Direct Current Machines Objective 2 Flashcards
What is the difference between a DC generator and a DC motor?
The way they are used
Torque in a motor rotor opposes _______________ applied by the load
Counter-rotational force (counter torque)
What two magnetic fields interact to produce the force that causes a motor’s armature to rotate?
Magnetic field created by current in the:
1) armature winding and
2) field windings.
Describe the Lorentz force
Pushes current-carrying conductor at right angles to a magnetic field.
The right hand rule uses (conventional/ electron) flow to define current direction.
Electron
Explain the different components of the right hand rule for motors
First finger = main field flux (points south)
Middle finger = direction of polarity of emf induced into the armature conductor
Thumb= direction of Lorentz force
Describe how the concentration of flux contributes to Lorentz Force
Feild flux attempts to straighten out after being forced to bend around armature conductor magnetic field. This makes field flux concentrated on the side the armature flux is also concentrated.
True or false Flux produced by the armature is perfectly symmetrical around the conductor
False. Armature field is compressed on one side. Opposite side of direction of force (see figure 21)
What is the quantity of motor action compared to rotational force applied by the prime mover (in a generator)? Why?
Motor action < rotational force.
Because the armature current is due to a voltage induced by the rotation of the prime mover.
What is the quantity of motor action compared to counter-rotational force applied by a mechanical load (in a motor)? Why?
Motor action = counter-rotational force
Because current is due to a voltage applied from an outside source.
What is another name for induced voltage in a conductor?
Counter emf
What is flemmings left hand rule for generators used to find?
Polarity of cemf
What is the right-hand rule for motors used for?
Determine lorentz force on the conductor that causes a motor rotor to turn.
What is the magnitude of output voltage dependent on in a DC generator?
1) speed of rotation
2) intensity of magnetic field.
What is the equation used to find generated voltage of a generator?
E(g) = K × flux per pole × n
K - constant
n - r/min
What is the equation used to find cemf of a motor?
Cemf = K × flux per pole × n
Describe the state of the following at the instant a DC motor is started:
1) armature motion
2) generated cemf
3) properties limiting armature current
1) not turning
2) no Cemf generated
3) only resistance of armature winding (usually less than 1ohm)
What is the formula for the fundamental motor equation?
V(A) = cemf + (I(arm) × R(arm))
What happens to Cemf as a motor accelerates from ‘start’?
Cemf increases
What is the main role of cemf in a motor?
Limits armature current by opposing applied voltage
When does cemf stop increasing within an accelerating motor?
When applied voltage - cemf = voltage just sufficient to produce enough armature current to meet the torque requirements of the load.
Electrical balance
What is the starting torque of a motor?
Compared to cemf
Compared to starting current
High compared to cemf
Proportional to starting current (both high at start).
What happens to motor cemf when load increases? Why?
Cemf decreases because speed decreased
(Cemf = K × flux per pole × rotational frequency)
What happens to armature current when load on a motor increases? Why?
Armature and torque both increase
(I(arm) = (V(arm) - V(cemf))÷ R(arm) (ohms law)
Since cemf decreases with increased load, this will effect current according to ohms law.
Torque and current are proportional.
Having added load to a motor, it will stabilize at ___________(higher/lower) speed, with a(n) ______________(increase/decrease) in torque and a(n) _____________ (increase/decrease) in armature current.
Lower; higher; higher
How can we reduce the starting current of a DC motor?
1) Placing a resistance in series with the armature at start. (Decrease resistance as speed /cemf increase)
2) apply less than rated voltage to the motor at the start. (Increase applied voltage as frequency increases)
What component in a DC motor allows current to reverse at 180° in order to maintain rotation in the same direction (positive rotation)
Commutator
Why are brushes located in the neutral plane?
At this location, they short out the commutator bars connected to a coil at a point where there is no or very low generated emf.
What is armature reaction?
Distortion of the main feild flux pattern that causes brush sparking
What is the result of main feild distortion due to armature reaction?
Causes neutral plane to shift
What are two methods used to oppose the magnetic field produced by armature current (armature reaction)?
1) Interpoles wound with commutating windings
2) Compensating windings placed in main pole faces
Which particular distorting effects do the compensating windings aim to oppose?
Distorting effects of armature current at THE POLES
Which distorting effects do the commutating windings aim to oppose?
Distorting effects of armature current in the INTERPOLE area.
What are three factors that affect the speed of a DC motor?
1) amount of load on the motor
2) intensity of the magnetic field
3) amount of armature voltage
In order to regain equilibrium between torque and counter-torque, what happens to speed of motor when
1) generated torque is greater than counter-torque?
2)When counter-torque is greater than generated torque?
1) Motor accelerates until counter torque equals torque.
2) motor slows until developed torque equals counter torque.
What happens to the following when load is added to a DC motor?
1) motor speed
2) cemf
3) armature current
4) torque
1) motor speed = decrease
2) cemf = increase
3) armature current = increase
4) torque = increase to equal counter torque
Cemf = V(arm) - (I(arm) × R(arm)) electrical balance
What is a common method of motor speed control?
Adjusting strength of field current
What happens to motor speed when field intensity is decreased? Why? Describe the process
Motor speeds up.
1) Armature cuts fewer lines of flux and therefore generates less cemf.
2) Less cemf means increased armature current
I(arm) = (V(arm) - cemf) ÷ R(arm)
3) increased I = increased torque (proportional)
4) motors developed torque is now greater than counter-torque of the load, so motor speeds up.
According to ohms law, what is armature current proportional to?
Voltage
According to ohms law, what is armature current inversely proportional to?
Resistance
What is the process that occurs when we apply lower voltage to a DC motor?
1) armature current decreases proportionally
2) developed torque decreases due to lower I
3) speed decreases because of reduced torque
4) cemf decreases because of less lines of flux being cut
5) armature current stabilizes when electrical balance is achieved at lower speed.
V(arm) = cemf + (I(arm) × R(arm))
