TRANSFORMERS

Guide Card

At the end of the discussion, you should be able to:

•      Define a transformer and how it works, and

•                      Solve a problem that involves transformers.

Assessment Card

A transformer is a device used to raise or lower the voltage of an AC source. A transformer consists of two coils called primary and secondary coils placed side by side having a common iron core. The primary coil is the one connected to the source; the secondary coil to the load. There are two types of transformer: a step down transformer and a step up transformer. A step down transformer lowers the voltage, while a step up transformer raises the voltage. A step up transformer has more turns in the secondary than in the primary. Assuming a 100 percent efficient transformer, the following relations hold:

I_pV_p = I_sV_s

V_s = N_s

V_p = N_p

where I_p and I_s refer to the current in the primary and secondary, respectively; N_p and N_s to the number of turns in the primary and secondary coils, respectively, and V_s and V_p to the voltage in the secondary and primary. The ratio N_p / N_s us called the turn ratio of the transformer.

            A transformer works on the principle “a changing flux in the primary coil induces an emf (electromotive force) on the secondary coil.” Hence, it cannot be used with steady direct current in as much as a steady direct current in the primary coil produces a constant flux.

            Transformers play a major role in power transmission. Whenever electricity is transmitted, there is always loss of power because of the resistance of the transmission wires or cables. To compensate for this loss, power companies use a step up transformer to increase the voltage, while reducing the current. Consider a power plant with 2200 V. From the power plant, a step up transformer is used to raise the voltage to around 220 000 V. This high voltage power is sent over long transmission lines to a substation. At the substation, a step down transform reduces the voltage to its original value, 2200 V. This is further reduced to 220 V or 110 V by a step down transformer mounted on electric posts before any domestic use.

Sample Problem

            A transformer operates on a 2500 V line and delivers a current of 80 A. If its turn ratio N_p:N_s is 20:1, determine the (a) secondary voltage, (b) the primary current, and (c) the power output. Assume 100 percent efficiency.

Solution:

            Given: Vp = 2500 V

                        Is = 80 A

                        N_p = 20

                        N_s      1

            a. To solve for the secondary voltage,

                        N_p = V_p

                        N_s    V_s

                                                20 = 2500 V

                         1         V_s

                        V_s = 125 V

            b. Using the formula I_pV_p = I_sV_s and substituting values,

                        I_p(2500 V) = (80 A)(125 V)

                        I_p = 4 A

            c. Power output = V_sI_s

                                                P = (125 V)(80 A)

                        P = 10000 watts or 10 kilowatts

Evaluation:

                
            1. The primary of a transformer has 100 turns and the secondary 20000 turns.

                        (a) What kind of transformer is this?

                        (b) If the primary voltage is 6 V, what is the secondary voltage?

Activity Card

             1. A step-up transformer has 400 turns on its primary coil and 600 turns on its secondary coil. 

                If the primary coil is supplied with 120 V of alternating current, what will be the secondary              
(               (output) voltage?

              2. Continuing the problem on number 2, suppose the same transformer produces a secondary
                 (output) current of 8 A. What was the primary (input) current?

Enrichment Card

            Briefly explain how a transformer works.

Reference Card

Books:

            Silverio, Angelina A. (2007). Exploring Life Through Science: Physics. 927 Quezon Avenue, Quezon City: Phoenix Publishing House, Inc.

            Physics: Science and Technology Textbook for Fourth Year. Quezon City: Book Media Press, Inc.

Website Link:

            http://www.buzzle.com/articles/how-does-a-transformer-work.html