E1 Flashcards
What principles of the D’Arsonval, or moving coil meter make the meter work?
A coil of wire becomes an electromagnet when an electric current flows through it.
Like magnetic poles repel and unlike magnetic poles attract
List the basic parts of a D’Arsonval meter
Moving coil wound on an aluminum frame
Moving coil is mounted with jewel bearings
Jeweled bearings allow meter to move freely between a permanent magnet
Pointer is attached to coil
List the basic parts of a D’Arsonval meter
current being measured is conducted through the spiral springs
Mirror to assist in reading accurately
Zero-adjust screw is used to set the pointer to zero
When current flows through the coil or the D’Arsonval meter what happens
magnetic field interacts with permanent mag
Produces clockwise torque on the coil
The torque, or turning force, is directly related to the amount of current flowing in the coil
What is the D’Arsonval meter coil can handle without wrecking
50 mA
Define deadbeat as it pertains to D’Arsonval meters
when the pointer move across the scale smoothly and reaching it steady position without oscillation due to effective damping
What are the two types of damping used on D’Arsonval
Eddy-current damping
Air damping
Greatest accuracy on a meter
select the range so that the needle will be a near as possible to FSD
What are the three names for a meter
D’Arsonval
Moving coil
Galvanometer
Accuracy of a meter is
ohms per volt or 1/amps or the current though the coil
To get an accurate reading on a voltmeter
the total resistance of the meter should be about 10 times that of the load under test
Why must an amp meter have short switch on it amperage range selection switch
if it did not have the make-before-break switch the coil would get full amperage for a split second. Can only handle up to 50 mA
Amp meter sensitivity is defined as
the amount of current necessary to cause FSD. The lower the current to cause FSD the better sensitivity the meter has.
Define meter loading
when the meter being used changes the values of the current or voltage being measured due to adding or removing resistance.
External shunts are used
to measure large DC currents
What two materials are shunts made of?
Copper and manganin
What temperature coefficient does manganin have?
negative
what resistance would a shunt have? high or low
low. must be able to conduct large amounts of current
List the fundamental parts of an AC clamp meter
Iron Core, clamp, secondary coil
AC clamp ammeter operates on the principle of an
transformer
If the current in the conduct is too small for your AC clamp ammeter what can be done?
Loop the wire around the clamp adding amperage for each turn.
What is the movable coil in a watt meter called?
voltage or potential coil
What two types of coils are in a watt meter
current carrying coil
voltage coil or potential coil
what must be done when hooking up a watt meter to insure that the pointer defects the correct way
make sure that the terminals are connected to the one marked with an asterix or plus sign
what keeps the pointer at zero when the meter is not registering any power
a spiral spring.
List basic functions of Solenoid-and-Plunger voltage tester
when not in use the spring holds plastic disk t 0V
Higher the voltage the further the disk goes from 0V
On DC the tester doesn’t vibrate
On AC voltage the tester vibrate
Only use for 15-20 seconds
A megger is used to
test the megaohm resistance of wire insulation
Typical meggar output voltage would be 100 to 1000V
What should be done before and after meggering a conductor?
Short it to ground
Make sure that the conductor to be tested is not hooked to any electronic curcuits
What does DMM stand for?
Digital Multi-Meter
The impedance value of most digital voltmeters is
10 Megaohms
List three causes of insulation deterioration
Oil
Moisture
Excessive heat
Mechanical damage
what is the minimum value that is expected when meggering a motor conducto
1 Megaohm
DMM have a
3.5 digit display
For DMM accuracy
+ and - (%stated accuracy X reading) +
change in the number of least significant digit
