A3 LT4 Flashcards
Where is the movable electric contact mounted?
On the armature
What happens when current flows through the coil?
Armature attracted to coil core
What is the basic meter movement used in most commercial analog meters?
D’Arsonval meter
What does current flow in the coil of an analog meter produce?
Clockwise torque
What is the basis for meters measuring AC and DC currents and voltages, as well as resistance?
D’Arsonval movement
Where is the stationary contact mounted?
On the same base as the coil core
What happens when the current stops flowing?
Spring causes armature to move away
What is the basic construction of the meter movement called?
D’Arsonval meter
What is the torque in the coil of an analog meter directly proportional to?
The amount of current flowing in the coil
What causes the pointer in an analog meter to oscillate?
Combined action of moving coil and restraining springs
What is the purpose of contacts?
Isolate the coil from the current.
What is the moving coil in an analog meter made of?
Fine copper wire
Why is the instrument movement in a D’Arsonval meter extremely delicate?
Can only handle milliamps or microamps
Why is damping necessary in an analog meter?
To suppress oscillation
What must be done to release the material attracted to the lifting magnet?
Open the switch to stop current flow
What does the current passing through the coils produce?
Magnetic field
What is the aluminum frame in an analog meter mounted on?
Shaft pivoted between two jewelled bearings
What is a relay?
Electromagnetic switch
How does the current flow in the system when the bell button is pressed?
Current starts to flow again
What happens as the cycle repeats itself with the bell system?
Armature repeatedly strikes the bell
What are motors and generators commonly used for?
Produce mechanical energy from electrical energy
How are field poles created in these machines?
Passing a current through coils wound around a magnetic core called the stator
What enables the coil in an anoalog meter to move freely?
Jewelled bearings
What happens when the current through an undamped analog meter is increased suddenly from 0 to some value A amps?
Pointer oscillates about the value A before coming to rest.
What happens when current is passed through a coil?
Attracts a spring-loaded armature
What causes the contacts to come together and close the switch?
Current flowing through the coil
What controls the magnitude of the current through the field coils?
Field rheostat
What is attached to the coil in an analog meter?
Pointer
Why is it difficult to read the analog meter quickly and accurately?
Pointer oscillates with every fluctuation of current.
How are magnetic relays used?
Small current controls large current
Why is the flux density of the field varied in these machines?
To control speed, torque, and voltage
How is the current conducted to and from the coil in an analog meter?
Spiral springs
How does the deadbeat instrument behave?
pointer moves across smoothly without oscillation
What is similar about the operation of a bell and a buzzer?
Same basic operation.
What keeps the coil in the rest position when no current flows in an analog meter?
Spiral springs
What happens when the armature moves toward the coils?
Strikes the bell
How is current indicated in the coil of an analog meter?
By the attached pointer on a calibrated scale
What is the function of the two springs in the coil of an analog meter?
They conduct current to the coil and restrain its movement
What does the D’Arsonval movement require to measure AC values?
Built-in rectifier
What happens to the core when a current is passed through the coil?
Becomes magnetized and poles are formed
What happens when the current stops flowing in a core with low retentivity?
Field collapses and material is released
What pulls the armature away from the coils?
Spring
What two principles does the operation of an analog meter depend on?
Coil becoming an electromagnet; Like poles repel, unlike attract
What does the D’Arsonval movement require to measure resistance?
Internal battery
What is the difference between a buzzer and a bell in terms of the armature?
No bell to strike for the buzzer
What do generators use to produce electrical energy from mechanical energy?
Magnetoelectric effect
What are the magnetic poles required for generators and motors called?
Field poles
When does the pointer stop moving in the coil in an analog meter?
When the torques balance each other out
What is the maximum current the coils in a D’Arsonval meter movement can handle?
About 50 mA
