Basics of Low Voltage Distribution Transformers A quickSTEP Online Course © Siemens industry, Inc. www.usa.siemens.com/step Trademarks Siemens is a trademark of Siemens AG. Product names mentioned may be trademarks or registered trademarks of their respective companies. National Electrical Code® and NEC® and NFPA 70® are registered trademarks of the National Fire Protection Association. NEMA® is a registered trademark and service mark of the National Electrical Manufacturers Association. UL® is a registered trademark of UL, LLC. Other trademarks are the property of their respective owners. © Siemens Industry, Inc. 2017 Page 1-2 Course Topics Welcome to Panelboards. This course covers the following topics: Chapter 1 - Introduction •Overview • Transformer Basics Chapter 2 – Transformer Information • Specifications and Ratings Chapter 3 – Siemens Transformers • Transformer Types Final Exam If you do not have an understanding of basic electrical concepts, you should complete Basics of Electricity before attempting this course. © Siemens Industry, Inc. 2017 Page 1-3 Course Objectives Upon completion of this course you will be able to… • Describe the role of low voltage distribution transformers in an electrical distribution system. • Describe how transformers work and the theories and concepts behind their design. • Identify important standards for low voltage distribution transformers • Describe the typical data found on a low voltage distribution transformer nameplate. • Describe various types of Siemens low voltage distribution transformers and the appropriate applications for each type. © Siemens Industry, Inc. 2017 Page 1-4 SITRAIN® Training for Industry Online Self-paced Learning – Programs with maximum flexibility so students can easily fit courses into their busy schedules Virtual Instructor-led Learning - Classroom lectures delivered in the convenience of your home or office Classroom Learning - Expert and professional instructors, proven courseware, and quality workstations combine for the most effective classroom experience possible at your facility or ours How-to Video Library - Quick, affordable, task-based learning options for a broad range of automation topics for training or purchase Simulators - World-class simulation systems available for training or purchase This course also describes learning options available from the Siemens SITRAIN USA organization and our global SITRAIN partners. For additional information: www.usa.siemens.com/sitrain © Siemens Industry, Inc. 2017 Page 1-5 Utility Power Distribution Power, originating at a power generating plant, is distributed to residential, commercial, and industrial customers through various transmission lines and substations. The most efficient way to do this is to increase the voltage while at the same time reducing the current. This is necessary to minimize the energy lost in heat on the transmission lines. These losses are referred to as I2R (I-squared-R) losses because they are equal to the square of the current times the resistance of the power lines. Once the electrical energy gets near the end user, the utility steps down the voltage to the level needed by the user. The device that utilities use to step up the voltage at the generator end and step down the voltage at the user end is called a transformer. Transformers are also used inside facilities to further reduce or increase the voltage to the level required by systems and devices. This course is intended to cover the low voltage distribution transformers used by commercial and industrial facilities as part of their power distribution systems. Siemens also sells equipment to electrical utilities; however, utility transformers are not covered in this course. © Siemens Industry, Inc. 2017 Page 1-6 Residential Power Distribution Power distribution systems are used in every residential, commercial, and industrial building to safely control the distribution of electrical power throughout the facility. Most of us are familiar with the power distribution system found in the average home. Power purchased from a utility company enters the house through a metering device. The power is then distributed from a load center to various branch circuits for lighting, appliances, and electrical outlets. Because the voltage applied to the utility meter in a typical residential application has already been stepped down to 120/240 VAC, a level usable by many electrical devices, the roll that transformers play in residential applications is not obvious. However, many devices have transformers built-in to further step down the voltage where necessary. This course does not cover these small, embedded transformers, but the principles of operation are the same as those that are covered. © Siemens Industry, Inc. 2017 Page 1-7 Commercial and Industrial Distribution Power distribution systems used in commercial and industrial locations are often complex. They include metering devices to measure power consumption, main and branch disconnects, protective devices, switching devices to start and stop power flow, conductors, and transformers. Power may be distributed through various switchboards, transformers, and panelboards. The primary role that low voltage distribution transformers play in commercial and industrial applications is essentially the same as in other applications, to transform voltage and current to the levels needed by the equipment employed. For example, in the accompanying simplified illustration, transformers are used to reduce the voltage from 480 VAC to 120 VAC for lighting and office equipment use. © Siemens Industry, Inc. 2017 Page 1-8 Low Voltage Distribution Transformers Transformers come in a variety of sizes and ratings. Most of transformers covered in this course are referred to as low voltage distribution transformers. Most of the transformers covered in this course are also referred to as dry-type transformers because they are not filled with any fluid. These transformers are rated for 600 VAC and below and intended for supplying appliance, lighting, and power loads. Common supply voltages include: 600, 480, 277, 240, and 208 VAC. © Siemens Industry, Inc. 2017 Page 1-9 Online Self-paced Learning With Siemens online self-paced learning, you select the topics and set your own pace for completing chosen courses. All course material can be accessed online. Instruction starts upon completing the purchase of a subscription. You can choose from over 500 courses consisting of high- quality graphics, on-screen text, supporting voiceover narration, and interactive exercises. Features include printable course content for reference and underlined key vocabulary terms with definitions displayed with a simple mouse-over action. Depending on the subscription purchased, you can choose any 10 or 25 courses or select the entire online self-paced course catalog. These courses are offered 24/7/365, so you can begin your subscription at any time. From the date of registration, you have one year to complete your course selections. For additional information: www.usa.siemens.com/sitrain © Siemens Industry, Inc. 2017 Page 1-10 Chapter 1 – Introduction This chapter covers the following topics: • Overview • Transformer Basics © Siemens Industry, Inc. 2017 Page 1-11 What is a Transformer? Transformers are electromagnetic devices that transfer electrical energy from one circuit to another by mutual induction. As previously mentioned, they are frequently used to step a voltage or current up to a higher level or down to a lower level. They can also be used to limit electrical noise transfer or to match circuit impedances. A single-phase transformer has two coils, a primary coil and a secondary coil. Energy is transferred from the primary to the secondary via mutual induction. The accompanying illustration shows an AC source connected to the primary coil. The magnetic field produced by the primary induces a voltage in the secondary coil, which supplies power to a load. Mutual inductance between two coils depends on their flux linkage. Maximum coupling occurs when all the lines of flux from the primary coil cut through the secondary coil. To maximize the amount of coupling, both coils are often wound on the same iron core, which provides a good path for the lines of flux. Keep in mind that the coils of wire are insulated from each other and from the core. The following discussions of transformers apply to transformers with a core made from iron or steel. © Siemens Industry, Inc. 2017 Page 1-12 Basic Principles Transformers operate on alternating current, rather than direct current. This is because it takes constantly varying current to allow the transformer to operate. The alternating current applied to the transformer’s primary coil causes magnetic flux to flow in the core. This flux, in turn, induces an alternating current in the secondary coil. Because this type of transformer has a core of iron, steel, or similar material, the magnetic flux is concentrated in the core. This allows virtually all the energy from the primary to be transferred to the secondary. Some energy is lost in heat in the conductors or core, but transformers are designed to minimize these losses. © Siemens Industry, Inc. 2017 Page 1-13 Transformer Turns Ratio There is a relationship between primary and secondary voltage, current, and impedance and the ratio of transformer primary turns to secondary turns. When the primary has fewer turns than the secondary, voltage is stepped up from primary to secondary. For the circuit on the left, the transformer secondary has twice as many turns as the primary, and voltage is stepped up from 120 VAC to 240 VAC. Because the impedance of the load is also higher than the impedance of the primary, current is stepped down from 10 amps to 5 amps. When the primary has more turns than the secondary, voltage is stepped down from primary to secondary. For the circuit on the right, the primary coil has twice as many turns as the secondary coil, and voltage is stepped down from 240 VAC to 120 VAC. Because the impedance of the load is lower than the impedance of the primary, the secondary current is stepped up from 5 amps to 10 amps. Transformers are rated for the amount of apparent power they can provide.