An important part of the Electrical Red Seal IP exam is understanding the use of transformers and how to interpret diagrams. Transformers are used to transfer electrical voltage from one circuit to another. This common use case today are step-down transformers where higher voltage is transformed into lower voltage, for example, 600 V to 120/240 V for residential.
Within the Electrical Red Seal IP Exam, the following topics will be tested on:
1) Transformer Turns & Taps
2) Types of Transformers (Single Phase, Delta, Wye and More)
3) Transformer Calculations (Power, Phase and Line Voltage & Current)
4) Dual Voltage Transformers
5) Transformer Polarity
6) Transformer Short Circuit & Impedance
7) Transformer Grounding
For part 1 of our transformers breakdown, we’ll talk about how a transformer works and how turns and taps function within transformers.
Firstly, a transformer consists of a primary and secondary circuit. Primary is the input into the transformer and secondary is the output of the transformer. So, for your typical 600 V to 120 V transformer, the 600V is the primary and 120 V is the secondary.
Turns
The transformation from primary to secondary is based on the number of turns. Turns is the number of times that the wire is wound around the primary or secondary. Primary and secondary turns identified here in this diagram:
If primary turns is more than secondary turns, the voltage decreases from primary to secondary and defined as step-down transformer. If primary turns is less than secondary turns, the voltage increases from primary to secondary and defined as step-up transformer.
Primary turns over secondary turns is defined as the turns ratio. Alongside this ratio, it is important to memorize and understand these formulas in order to pass the exam.
Turns ratio is equal to primary voltage over secondary voltage and equal to the secondary current over primary current.
Np/Ns = Vp/Vs = Is/Ip
For example, if the turns ratio is 5:1, the primary voltage is 5 times the secondary voltage, and the primary current is 5 times less than the secondary current.
NOTE: Although the voltage and current may change between primary and secondary, the power stays the same.
KVAp = KVAs
Taps
Taps will add or subtract a number of turns to change the turns ratio. Usually, taps would be supplied to the primary windings in 2.5% and 5% increments. For example, let’s say that we have 480 V : 120 V transformer. With the taps, we would be able to intake the following voltages and still output 120V:
504 V -> 480V + 5%
492 V -> 480V + 2.5%
480 V -> no taps
468 V -> 480V - 2.5%
456 V -> 480V - 5%
Autotransformers
An autotransformer share the same winding for both primary and secondary. However, the transformer is provided with taps to determine the turns ratio between primary and secondary.
Red Seal IP Practice Exam Questions
There are 480 turns on the primary of an auto-transformer having a source voltage of 240 V. To obtain a 24 V secondary voltage, how many tap turns are required on the secondary winding?
Step 1 - Identify variables in questions
Primary voltage: 240 V
Secondary voltage: 24 V
Primary Turns: 480 turns
Step 2 - Find turns ratio
Turns ratio = Np/Ns = Vp/Vs
Turns ratio = 240 V / 24 V
Turns ratio = 10 : 1
Step 3 - Find tap turns for autotransformer
10 / 1 = Np / Ns
10 / 1 = 480 / Ns
Ns = 480 / 10
Ns = 48 turns
Stay tuned with Part 2, where we dive into topics around the different types of single-phase transformers.