Induction and Inductive Reactance

Question 1

What is an inductor?

Answer 1

Insulated wire wound around a core.

Question 2

Air, ceramic, plastic or iron are all considered acceptable core materials.

Answer 2

Yes, however they will all differ in magnetic strength.

Question 3

What kind of electrical equipment utilize coils?

Answer 3

Motors,
transformers,
solenoid valves,
relays and contactors,
electromagnetic lighting ballasts,
and line reactors for VFDs.

Question 4

When a symbol for a core is drawn with two parallel lines next to it, whet is signified by this addition?

Answer 4

The core is a magnetic material like iron.

Question 5

What factors will affect the inductance of a coil?

Answer 5

The number of wraps of wire,
permeability of the core,
cross-sectional area of the core,
and the length of the magnetic path.

Question 6

The symbol for inductance is ______, it is measured in ______.

Answer 6

L,

Henrys (H).

Question 7

What is inductance?

Answer 7

The property of a circuit to oppose a change in current.

Question 8

What does Faraday’s law state?

Answer 8

The magnitude of induced emf in a conductor is directly proportional to the rate of change of flux cutting the conductor.

Question 9

What is mutual induction?

Answer 9

When the changing magnetic field around one conductor causes current to flow in a nearby, electrically separate conductor.

Question 10

What is self induction?

Answer 10

The generation of an emf in a conductor by the change in flux density produced by that same conductor.
This is prevalent in coils and also skin effect.

Question 11

What does Lenz’s law state?

Answer 11

An induced emf opposes the change that created it.

Question 12

What factors will affect counter emf?

Answer 12

The inductance of the coil and the rate of change of current.

Question 13

When an inductor is placed in a DC circuit, how will the current rise to it’s maximum value as the circuit is closed?

Answer 13

The current will rise quickly at first and slow down gradually. It may never reach 100% of it’s steady value.

Question 14

How can we define the time it takes for current to reach it’s steady value?

Answer 14

We can define it as five time constants, which are defined as the ratio of Inductance to resistance.

Question 15

How much will the current change in each time constant?

Answer 15

Current will increase by 63.2% of the remaining value to reach it's maximum, after every time constant. 
Tc1 = 63.2%
Tc2 = 86.4%
Tc3 = 95%
Tc4 = 98.1%
Tc5 = 99.4%, Which is essentially 100.

Question 16

What is an inductive kick? How do we mitigate the damage of this reaction?

Answer 16

When a DC circuit containing an inductor is opened by a near instantaneous switch, the counter emf produced is enormous and may easily arc across the switch.
We can mitigate damage by adding a discharge resistor in parallel with the inductor, which allows a path for the voltage to “discharge”.

Question 17

What is the effect of an inductor in an AC circuit?

Answer 17

In AC the current continually changes, so we can reason that there will be a continual opposition to current flow. We define this effect as inductive reactance.

Question 18

How is inductive reactance represented? What is it measured in?

Answer 18

Inductive reactance is X_L, and is measured in Ohms.

Question 19

What factors will affect inductive reactance?

Answer 19

The frequency of the supply, and the inductance present in the circuit.

Question 20

How is an inductive circuit classed, regarding time relationships?

Answer 20

We call this type of circuit a LAGGING circuit.

Question 21

When Reactive power due to induction is positive, how is energy being used?

Answer 21

It is taken by the coil to establish the magnetic field.

Question 22

When Reactive power due to induction is negative, how is energy being used?

Answer 22

It is delivered back into the circuit by the collapsing magnetic field.