A&B: Wiring Diagrams Up to 400MBH

What is a transformer and what are its key features?

AC device using mutual induction from two coils on the same core

Magnetic circuit made of low reluctance material with no air gaps

Two main purposes:

Change circuit voltage

Isolate circuits

Built from laminated silicon steel to reduce eddy currents

High voltage side usually marked "H", low voltage side "X"

Multi-winding transformers allow for flexible voltage stepping

What are the main transformer construction types and their characteristics?

Core Type: most flux leakage

Shell Type: moderate flux leakage

H Type: least amount of flux leakage

What are typical efficiency levels of transformers and why?

95–98% efficient

No moving parts

Low reluctance path

Small flux leakage through the iron core may cause minor inefficiencies

What are common voltage applications for ignition and control transformers?

Step-up: 120V to 6000V (ignition transformers)

Step-down: 120VAC to 24VAC (control transformers)

How does an ideal transformer behave?

Voltage ratio equals turn ratio

No impedance or voltage drops

Example: 100 turns on primary with 100VAC = 1 VAC/turn

10-turn secondary = 10VAC output

How does a practical transformer differ from an ideal one?

Primary and secondary have resistance and reactance

Voltage drop exists due to excitation current or winding losses

Drops are minor but real in application

What is exciting (magnetizing) current and what does it do?

Flows continuously at 2–10% of rated current

Establishes magnetic flux for operation

Manages hysteresis and eddy current losses in iron core

What does Lenz’s Law state about transformer voltage?

Induced voltage opposes the change that caused it

Supply voltage and induced voltage are always in opposition

What happens in a transformer under no load vs. full load?

No Load: secondary voltage equals output primary flux cuts secondary winding

With Load: secondary current generates opposing flux
primary adjusts flux to maintain balance

What is transformer capacity rated in and what does it represent?

Measured in VA or kVA (apparent power)

Reflects full output power (not input)

1 VA = 1 Watt

Rating must account for power factor
Example: 20W load at 0.8 PF = 25 VA required

How do you calculate transformer voltage per turn and turn ratios?

Formula: a = Vprimary / Vsecondary = TurnsPrimary / TurnsSecondary

Example: 120VAC across 240 turns = 0.5V/turn

What happens when a transformer reaches saturation?

Flux no longer increases with current

Saturation typically occurs around 110% of rated voltage

Beyond this point, primary current rises rapidly and may cause burnout

Why is voltage rating dependent on iron content in the core?

More iron allows for more flux capacity

Helps avoid early saturation and maintain output under load

What are current limiting transformers and where are they used?

Small transformers with high internal impedance

Even a shorted secondary produces low current

CEC limits to 100VA (Class 2)

Common in gas control applications

How does the air gap in current limiting transformers affect performance?

Limits secondary current delivery by saturating core

Prevents increase in magnetic flux

Eliminates need for overcurrent protection

What information is required to size a control transformer?

Load voltage

Load amperage

Available line voltage

Likely power factor of the load

What are the installation styles of transformers?

Open type or totally enclosed

Enclosed transformers can be used in outdoor elements if safe

What are the three main types of wiring diagrams and what does each show?

Pictorial: shows actual images of components
Connection (line diagram): shows wire terminals and physical locations best for tracing or locating wires
Ladder: shows logic and symbols, but not physical locations of components

What does an arrow through a wiring diagram symbol typically indicate?

It means the component is adjustable (e.g., adjustable setpoint)

What does a typical wire diagram represent in its initial state?

It shows the unit in standby and powered, but with no call for heat

What are common thermostat wire functions in a heating/cooling system?

R: power feed to thermostat (usually 24VAC)
W: initiates heating (R to W)
Y: initiates cooling (R to Y)
G: activates blower
(R to G) may also trigger higher blower speed during cooling

What is the first step in a sequence of operation for a heating system?

A call for heat at the thermostat

What are common causes of transformer overheating in circuit troubleshooting?

Heavy-duty transformers are oil-filled and may have fins to dissipate heat

Residential units often aren’t and may overheat

Varnish that holds core laminations can fail, leading to short circuits

Always ensure proper airflow around the transformer

What can cause a transformer short circuit?

Secondary windings overloaded beyond capacity

Causes excess current draw

May damage primary wires

What is core insulation breakdown in a transformer and its effect?

Eddy current losses result in heat inside the core Single solid cores have higher losses, laminations reduce this If laminations separate, eddy losses increase and heat rises

How is heat dissipation capacity identified in transformers?

Via letter designation for temperature rise:

A = 55°C

B = 80°C

F = 115°C

H = 150°C

Based on a 40°C starting internal temperature

What is the rule of thumb for transformer loading to prevent failure?

Do not exceed 80% of VA rating under continuous load

Why are transformers rated in VA and not watts?

Because transformer loads may vary in power factor, VA accounts for both real and reactive components