Inductance and Inductive Reactance Flashcards
The symbol for inductance is:
L
Inductance is measured in:
henrys
The inductance of an ac circuit is measured in ohms
True or False?
False
The number of time constants required for the current of a dc operated coil to reach its steady value is:
A) 1
B) 3
C) 5
D) 7
C) 5
After one time constant has elapsed, the percentage of the steady state current flow in a dc operated coil will be approximately:
A) 20%
B) 37%
C) 63%
D) 100%
C) 63%
The purpose of a discharge resistor across a dc relay coil is to:
A) reduce high voltage spikes due to coil flux collapse
B) allow the relay to pick up faster
C) provide a short delay in the pick-up
D) offset the inductance of the coil
E) take care of more than one of the above
A) reduce high voltage spikes due to coil flux collapse
A relay is operated on an ac supply. When the armature of the relay pulls in, the inductive reactance of the coil will:
A) increase
B) decrease
C) stay the same
A) increase
The two factors that determine the inductive reactance of a coil are:
A) frequency and voltage
B) inductance and frequency
C) inductance and voltage
D) resistance and inductance
B) inductance and frequency
Which of the following frequencies will give the lowest inductive reactance value if the same coil is used in all cases?
A) 50Hz
B) 60Hz
C) 100Hz
D) 400Hz
A) 50Hz
One of the properties of a series ac circuit is inductance. In a series ac inductive circuit, the current will always lag the reactive component of voltage by 90 degrees.
True or False?
True
A simple inductor is made up of _____ ______ wound around a ________
insulated wire, core
Name three types of equipment where inductors are used.
- Motors
- Solenoids
- Relay or magnetic starter coils
- Electromagnetic lighting ballasts
- VFD line reactors
What are the four factors that affect the inductance of a coil and the symbol for each.
- Permeability of core material (µ)
- Length of magnetic path (ℓ)
- Area of core (a)
- Number of turns (N)
State Faraday’s Law.
The amount of induced emf is directly proportional to the rate of change of flux
What is mutual induction?
The generation of an emf in one conductor or circuit by a change of flux around another separate conductor or circuit.
What is self-induction?
The generation of an emf in a conductor or circuit by a change of flux in that conductor or circuit
State Lenz’s Law.
The polarity of any inducted voltage must be such that any current resulting from it develops a flux that tends to oppose any change in the original flux.
What are the two factors that affect the cemf of a coil?
1) inductance of the coil
2) rate of change of current (which determines the rate of change of flux)
Define time constant for an RL circuit.
One time constant of an inductive circuit is the ration of inductance (L) of the circuit divided by the resistance (R)
T=L/R
During the time span of one time constant after closing an inductive circuit, the current rises to ____% of its steady value.
63.2%
What is the equation for the time constant of a resistive-inductive circuit?
T=L/R
What is the unit of measurement for a time constant?
Seconds
If the iron core of a coil is removed, the time taken for the current to reach its steady value:
A) increases
B) decreases
C) stays the same
B) decreases
A circuit has a coil with 1000 turns of No. 3 AWG copper wire. If this coil is replaced with one that has 1000 turns of No. 20 AWG copper wire, the time constant for this coil as compared to the first coil would be:
A) greater
B) less
C) the same
A) greater
What happens when an inductive circuit is opened?
The collapsing magnetic field can induce a very high voltage into the circuit
What is the purpose of a discharge resistor?
Reduces arcing at the switch when an inductive circuit is opened
Inductive reactance is the ____________ created by inductance in an ac circuit
opposition
What are the two factors affecting the inductive reactance?
- Inductance of the coil
- Frequency of the supply voltage
