Transformer Questions and Answers 3 (3)

Question and Answer Electrical Power Transformers

1. What is Secondary Voltage Rating?
Designates the no-load circuit voltage for which the secondary winding (winding on the output side) is designed.


2. What is a Secondary Winding?
The transformer winding connected to the load or output side.


3. Define Series/Multiple?
A winding consisting of two or more sections which can be connected for series operation or multiple (parallel) operation. Also referred to as dual voltage or series-parallel.


4. What is a Short Circuit?
A short circuit condition occurs when an abnormal connection or relatively low impedance, whether made accidentally or intentionally, occurs between to points of different potential in a circuit.


5. What is a Solid State Device?
It is a device that contains components that do not depend on electronic conduction in a vacuum or gas. Semiconductors or the use of otherwise completely static components such as resistors or capacitors performs the electrical function of a solid-state device.


6. What is a Step-Down Transformer?
It is a transformer where the high voltage winding (primary) is connected to the input or power source and the low voltage winding (secondary) to the output or load.


7. What is a Step-Up Transformer?
A transformer in which the low voltage winding (secondary) is connected to the input or power source and the high voltage winding (primary) is connected to the output or load.


8. What is a Tap?
A tap is a connection brought out of a winding at some point between its extremities, usually to permit changing the voltage or current ratio.


9. What is a T-Connection?
A Scott connected three-phase transformer utilizing two primary and two secondary coils called the main and the teaser.


10. What is Temperature Class?
It is the maximum temperature that the insulation can continuously withstand. The classes of insulation systems in a transformer are rated as follows:
Class 105°C
Class 150°C
Class 180°C
Class 220°C


11. What is Temperature Rise?
It is the increase over ambient temperature of the winding due to energizing and loading the transformer.


12. How do you calculate the Total Losses in a transformer?
It is the transformer electrical losses, which include no-load losses (core losses) and load losses (winding losses).


13. What is a Transformer?
It is a static electrical device, which, by electromagnetic induction transforms energy at one voltage and current to another voltage and current at the same frequency.


14. Describe Transient?
It is a temporary or brief change in a given parameter. This is typically associated with input voltage or output load parameters.


15. What are some of the Tests performed on transformers?
Normal, routine production tests include: (1) core loss; (2) load loss - winding or copper loss;
(3) Impedance; (4) hi-pot - high voltage between windings and ground; (5) induced - double induced two times voltage. Optional special tests include: (a) heat run - temperature testing;
(b) Noise tests - sound level measurement; (c) impulse tests - BIL tests: (d) partial discharge.


16. What is Transverse Mode?
It is electrical noise or voltage disturbance that occurs between phase and neutral (between lines), or from spurious signals across the metallic hot line and the neutral conductor.


17. What is the abbreviation UL?
Underwriters Laboratories


18. Define VPI Impregnation?
A vacuum and pressure impregnation process using a resin which, is then oven cured to completely seal and protect the surface of a transformer and provides a strong mechanical bond. This process is standard on all Hammond transformer products.


19. What is Voltage Regulation?
The change in secondary voltage that occurs when the load is reduced from rated value to zero, with the value of all other quantities remaining unchanged. Regulation may be expressed in percent (per unit) on the basis of rated secondary voltage at full load.


20. What is Volt-Amperes (VA)?
It is the current flowing in a circuit multiplied by the voltage of the circuit. It is an expression of the output rating of a transformer.


21. What is a Wye Connection?
A standard 3-wire transformer connection with similar ends of the single-phase coils connected. This common point forms the electrical neutral point and may be grounded.


22. What is a Zig Zag Connection?
It is a special transformer connection commonly used in grounded transformers. See also grounding transformers.


23. What is a Control Transformer?
A control transformer is an isolation transformer designed to provide a high degree of secondary voltage stability (regulation) during a brief period of overload condition (also referred to as "inrush current"). Control transformers are also known as Machine Tool Transformers, Industrial Control Transformers or Control Power Transformers.


24. When you calculate the VA requirement of a Transformer, do you use the Primary or the Secondary Voltage?
When selecting the VA requirement, you use the Secondary Voltage.


25. Can you use a Control Transformer connected in reverse?
Yes, a control transformer can be connected in reverse. However, keep in mind the output voltage will be less than it's rating, due to the compensation factor of the windings.


26. Can a Control Transformer regulate the output voltage?
A control transformer will not regulate the voltage. Output voltage is a function of the coil's turn ratio only, times the input voltage.


27. What is the benefit of "Vacuum Impregnation" on a Transformer?
All Hammond Control Transformers are vacuum impregnated with "VT Polyester Resin" and oven cured which seals the surface and eliminates moisture. Impregnating the entire unit provides a strong mechanical bond and offers protection from environmental conditions.


28. Explain the "VA" or "Volt Ampere Output" Rating?
The VA or volt-ampere output rating designates the output that a transformer can deliver for a specified time at its rated secondary voltage and rated frequency, without exceeding its specified temperature rise.


29. Insulating Materials, what are they made of and what is their purpose?
Hammond Power Solutions utilizes Mylar, Nomex and other high quality insulating materials. Insulation is used to electrically insulate turn-to-turn windings, layer-to-layer windings, primary to secondary windings and ground.


30. What is the effect of "Overload"?
When a transformer is continually overloaded, excessive heat develops and the insulation system will begin to breakdown. As a result, the life expectancy of the transformer is shortened due to the heat exceeding the rating of the insulation system.


31. What is a buck-boost transformer?
Buck-boost transformers are small single-phase transformers designed to lower (buck) or raise (boost) line voltage from 5-20%. The most common applications for buck-boost transformers
Include boosting 208 volts to 230 or 240 volts for air conditioning systems, boosting 110 to 120 volts and 240 to 277 volts for lighting applications, heating systems and induction motors of all types. Many applications exist where supply voltages are frequently above or below nominal.
Buck-boost transformers are conventional low voltage, single-phase distribution transformer, with standard primary voltages of 120, 240 or 480 volts, and secondary voltages of 12, 16, 24, 32 or 48 volts. They are available in sizes ranging from 50 VA to 10,000 VA. The primary and secondary are wired together to form a single-winding autotransformer. Utilizing the additive and subtractive polarity, small amounts of voltage are either added or subtracted from a distribution circuit.


32. How does a buck-boost transformer differ from an isolating transformer?
A Buck-Boost transformer is manufactured as an isolating transformer, with separable primary and secondary, and is shipped from the factory in that configuration. When the end user at site connects it, the primary is connected to the secondary changing the transformer's electrical characteristics to those of an autotransformer. This provides the smaller voltage correction that is typical of Buck-Boost. The primary and secondary windings are no longer isolated as they are connected together.


33. What is the difference between a buck-boost transformer and an autotransformer?
As noted above, when the primary and secondary are connected together to buck or boost voltage, the transformer becomes an autotransformer. If the connection between the primary and secondary winding is not made, then the unit remains as an isolation transformer.


34. Why are they used?
A Buck-Boost transformer is a simple and effective way of correcting off-standard voltages. Electrical and electronic equipment is designed to operate within a standard tolerance of nominal supply voltages. When the supply voltage is consistently too high or low - typically more than 10%, the equipment will operate below peak efficiency.


35. Can Buck-Boost transformers be used to power low voltage circuits?
Installed as two-winding, isolation transformers, these units can be used to power low voltage circuits including control, lighting circuits, or other low voltage applications that require 12, 16, 24, 32 or 48 volts output, consistent with the secondary of these designs. The unit is connected as an isolating transformer and the nameplate kVA rating is the transformer's capacity.


36. Why do Buck-Boost transformers have 4 windings?
A four winding buck-boost transformer with 2 primary and 2 secondary windings can be connected eight different ways to provide a multitude of voltages and kVA's. This provides the flexibility necessary for the broad variety of applications. A two-winding transformer can only be connected in two different ways.


37. Will a Buck-Boost transformer stabilize voltage?
Autotransformers will not stabilize supply line voltage. The output voltage of an autotransformer is a function of the input voltage. If the input voltage varies, then the output voltage will also vary by the same percentage.


38. Are there any restrictions on the type of load that can be operated from a Buck-Boost transformer?
There are no restrictions as to application for Buck-Boost, including single or three-phase motor loads.


39. As an Autotransformer, how can a Buck-Boost transformer supply kVA power?
This is a function of adding voltage - a small amount of voltage is added and a small amount of corresponding power capacity is added as well. For example, if the transformer is connected in such a way that 22 volts is added to a 208 volt primary, a 230-volt output will result.
Using this example, the calculation for autotransformer kVA is as follows:
KVA = (Output Volts x Secondary Amps)/1000
KVA = (230V x 41.67 Amps)/1000 = 9.58 KVA

40. How are single phase and three phase load amps and load kVA calculated?
Single phase Amps = (kVA x 1000)/Volts
Three phase Amps = (kVA x 1000)/Volts x 1.73
Single phase KVA = (Volts x Amps)/1000
Three phase KVA = (Volts x Amps x 1.73)/1000