Transformers

An Electronic Device, and also a Action Hero…

Transformers

•Transfers electrical energy from one circuit to another.

•No physical connection between the circuits.

•May involve changes in voltage and current.

•Frequency does not change.

Construction

•Primary coil or winding.

•Secondary coil or winding.

•Neither are connected to each other.

3 Types of Transformers•Step-Up Transformer

▫It delivers higher voltage than the source, aka, it steps the voltage up. The voltage on the secondary side is higher than the primary side.

•Step-Down Transformer▫It delivers lower voltage than the source,

aka, it steps the voltage down. The voltage on the secondary side is lower than the primary side.

•One-to-One Isolation Transformer▫The voltage on the primary is the same as

the secondary. There is no increase or decrease in voltage.

RVOTD: Danger, Will Robinson!

How Transformers Function

•The primary winding receives the source AC and creates a changing magnetic flux.

•The secondary winding receives it’s energy from the magnetic flux created by the primary winding and sends it to the load.

•The core material provides a path for the magnetic lines of flux.

Explaining how the magnetic field “cuts” the windings in a transformer. (again)

The Core of a Transformer

•The material of the core can be made from a variety of materials. It can also be made out of air.

•Typically and air core is useful for high frequency applications while iron cores are used for low frequency applications.

If the core is made out of iron, it is usually in the form of a shell core.•Laminations are insulated from each

other.•This makes shells electrically insulated

from each other (good for less eddy currents) but not magnetically isolated from each other.

•Take 5 minutes and read section 19.2 to yourselves.▫No load conditions and load conditions on a

transformer.

What happens to the fuse on the primary side of a transformer if there is a short in the secondary?The fuse blows even though they aren’t connected!!!!!!!!!

Transformer Ratios

NS VS

VPNP

Where:

Np = number of turns in the primaryNs = number of turns in the secondaryVp = voltage applied to the primaryVs = voltage induced in the secondary

•You can calculate if a transformer is a step-up, step-down, or one-to-one by simply looking at how many turns there are of wire on the primary and secondary sides.

•It can also be given by the equation below

Useful Ratio Form

•Sometimes you may be given a “turns” ratio. This may be one number that represents:

NSVS VP

NP

NP

NS

such as 10.0, or a ratio like 10:1

Example #1:

200

50

120V

Vs?

50 VS

120200

NS VS

NP VP

VS 30V

The Turns Ratio

•Common ways to express the ratio▫10:1▫ or 1:10▫Which of these steps-up voltage?

•Ratio is always quoted “Primary to Secondary”

•Ratio does not indicate the number of turns

NP

NS

Example #2:• Step-down Transformer• Aka “Buck” Transformer

• Find:

5V

7:1

VP

VSVP

NP

NS

VP 5V

7

1VP

35V

Vp?

Example #3:• “Boost” Transformer

• Find:

1:5

Transformer labeled:

120V

Vs

?

VS

VSVP

NP

NS

VPVS

NS

NP

VS600V

•Voltage does not just magically increase out of thin air. Something must be lost in the process, or else we would be making energy from nothing.

•What do we lose in the process?▫Current!

•If we have a perfectly efficient transformer, the power transferred from one side to the other must be kept the same.

•Since P = VI, the Voltage multiplied by the current on the primary must be equal to the voltage multiplied by the current on the secondary.Pp = VpIp = VSIS = PS

•Therefore, if the voltage increases then the current must decrease to keep the power the same.

•So current can also be calculated using the number of turns on the primary and secondary side.

•IpNp = ISNS

Pp = VpIp = VSIS = PS

Example

•A transformer has 100 turns in the primary and 600 in the secondary. There is 3A of current in the primary. What is the value of current in the secondary?

IpNp = ISNS

Example

•A transformer with a turns ratio of 1:8 has 2A of current in the primary. What is the value of secondary current?

IpNp = ISNS

Example – try these on your own

Transformer Schematic Symbol

Sometimes there are taps off the secondary.Sometimes there are multiple secondary windings.

Phase Dots•Phase Dots refer to whether the voltage

on the primary side is in phase with voltage on the secondary side. There are either in phase or 180˚ out of phase.

In Phase Out of phase

Maximum Power Rating for a Transformer•Transformers have a maximum power

rating.•This means the wires can only handle so

much combined voltage and current before they break down.

•Transformers usually have their power rating printed right on them, such as 24VAC (Take a look in lab.)

•The power rating is given in Volt-Amp rather than watts because transformers are providing power, not using it like resistors.

Measuring for transformer type.•You can measure the actual physical

resistance on the primary winding and the secondary winding.

•This will tell you if you have a step-up, step-down, or one to one transformer.

•Do you think the windings in the secondary side of a transformer has more or less resistance than the primary?

• Why – because more windings for higher voltage and those windings are usually thinner because they don’t have to handle as much current, which causes the resistance to go up even more.

Multiple Winding Transformers used in power supplies

Variac or Auto-Transformers

Review – for team points. Please raise your hand!

•What type of transformer has a larger voltage on the secondary than the primary?

•In a step down transformer, what has more resistance, the primary or the secondary?

•Suppose you overpowered your transformer, what do you think will happen?

•If the voltage is increased in the secondary side, what is decreased?

•What are the phase dots used for?•What causes an induced voltage on the

secondary?•What 3 things determine the amount of

voltage induced on the secondary?