A basic understanding of transformers is an essential skill for every electrical worker; familiarity with and mastery of fundamental transformer knowledge is crucial. The following will detail 16 key points about transformers.
1. What is a transformer?
In an AC circuit, a device that raises or lowers voltage is called a transformer. A transformer can transform any voltage value into the voltage value we need with the same frequency to meet the requirements of electrical energy transmission, distribution and use.
For example, the electricity generated by a power plant has a low voltage level, which must be increased before it can be transmitted to a distant power consumption area. The power consumption area then needs to step down the voltage to a suitable level to supply power equipment and daily electrical equipment.
2. How does a transformer change voltage?
A transformer is made based on electromagnetic induction. It consists of an iron core made of stacked silicon steel sheets (or silicon steel sheets) and two sets of coils wound around the iron core. The iron core and the coils are insulated from each other and have no electrical connection.
The coil connected to the power source side of the transformer is called the primary coil (or primary side), and the coil connected to the electrical equipment is called the secondary coil (or secondary side). When the primary coil of the transformer is connected to an AC power source, changing magnetic lines of force are generated in the iron core.
Since the secondary coils are wound on the same iron core, the magnetic field lines cut through the secondary coils, inevitably inducing an electromotive force (EMF) and causing a voltage across the coils. Because the magnetic field lines alternate, the voltage across the secondary coils also alternates, and its frequency is exactly the same as the power supply frequency.
Theoretical analysis has confirmed that the voltage ratio between the primary and secondary coils of a transformer is related to the turns ratio between the primary and secondary coils, which can be expressed by the following formula: Primary coil voltage / Secondary coil voltage = Primary coil turns / Secondary coil turns. This means that the more turns, the higher the voltage. Therefore, it can be seen that a transformer with fewer turns in the secondary coil than in the primary coil is a step-down transformer, while the opposite indicates a step-up transformer.
3. What are the different types of transformer designs?
Transformers can be classified into single-phase and three-phase transformers based on the number of phases.
According to their uses, transformers can be classified into power transformers, special power transformers, voltage regulating transformers, measuring transformers (voltage transformers, current transformers), small power transformers (used for low-power equipment), and safety transformers.
Based on structure, there are two types: core-type and shell-type. The coils can be double-winding or multi-winding, and there are autotransformers.
According to the cooling method, there are oil-immersed type and air-cooled type.
4. What are the components of a transformer?
Transformer components mainly consist of iron core and coils, as well as oil tank, oil conservator, insulating bushings, and tap changers.
5. What are the uses of transformer oil?
The function of transformer oil is:
(1) Insulation function
(2) Heat dissipation function
(3) Eliminating the arc effect
6. What is an autotransformer?
An autotransformer has only one set of coils. The secondary coil is tapped from the primary coil. Its electrical energy transfer involves both electromagnetic induction and electrical transmission. This type of transformer uses fewer silicon steel sheets and copper wires than a regular transformer and is commonly used for voltage regulation.
7. How does a voltage regulator adjust the voltage?
The voltage regulator is constructed in the same way as an autotransformer, except that the iron core is made into a toroidal coil which is wound around the toroidal iron core.
The secondary coil tap uses a sliding brush contact, which slides in a ring along the coil surface to achieve smooth voltage regulation.
8. What is the relationship between the currents in the primary and secondary coils of a transformer?
When a transformer is operating under load, changes in the secondary coil current will cause corresponding changes in the primary coil current. Based on the principle of magnetomotive force balance, the current in the primary and secondary coils is inversely proportional to the number of turns; the side with more turns has a smaller current, and the side with fewer turns has a larger current.
It can be expressed by the following formula: Primary coil current / Secondary coil current = Number of turns of secondary coil / Number of turns of primary coil.
9. What is the voltage change rate of a transformer?
The voltage change rate of a voltage regulator is one of the main performance indicators of a transformer. When a transformer supplies power to a load, the voltage at the load terminal of the transformer will inevitably drop. The percentage of this voltage drop compared to the rated voltage is the voltage change rate.
This can be expressed by the formula: Voltage change rate = [(Secondary rated voltage - Load terminal voltage) / Secondary rated voltage] × 100%. For a typical power transformer connected to its rated load, the voltage change rate is 4–6%.
10. How to ensure that the transformer has a rated voltage output?
Both excessively high and low voltage can affect the normal operation and lifespan of a transformer, so voltage regulation is necessary.
Voltage adjustment is achieved by drawing several taps from the primary coil and connecting them to a tap changer. Rotating the tap changer alters the number of turns in the coil. Simply changing the tap changer position yields the desired rated voltage. It is important to note that voltage adjustment should typically be performed after disconnecting the load from the transformer.
11. What are the common types of small transformers? In what situations are they used?
Small transformers refer to single-phase transformers with a capacity of less than 1 kVA. They are mostly used as power transformers for electrical equipment control, power transformers for electronic equipment, and power transformers for safety lighting.
12. What losses occur during transformer operation? How can these losses be reduced?
The losses during transformer operation include two parts:
(1) It is caused by the iron core. When the coil is energized, the magnetic lines of force are alternating, causing eddy current and hysteresis losses in the iron core. This loss is collectively called iron loss.
(2) It is caused by the resistance of the coil itself. When current flows through the primary and secondary coils of the transformer, energy loss will occur. This loss is called copper loss.
The sum of iron losses and copper losses equals transformer losses, which are related to transformer capacity, voltage, and equipment utilization rate. Therefore, when selecting a transformer, the equipment capacity should be matched with the actual usage as much as possible to improve equipment utilization rate, and care should be taken to avoid operating the transformer under light load.
13. What is a transformer nameplate? What are the main technical data on the nameplate?
The nameplate of a transformer indicates its performance, technical specifications, and application, which helps users make their selections. The main technical data to consider when selecting a transformer typically includes:
(1) Rated capacity in kilovolt-amperes. That is, the output capacity of the transformer under rated conditions. For example, the rated capacity of a single-phase transformer = U_line × I_line; the capacity of a three-phase transformer = U_line × I_line.
(2) Rated voltage (volts). Indicate the terminal voltage of the primary coil and the terminal voltage of the secondary coil (when no load is connected). Note that the terminal voltage of a three-phase transformer refers to the line voltage U<sub>line</sub> value.
(3) Rated current in amperes. This refers to the line current I<sub>line</sub> value that the primary and secondary coils are allowed to carry for a long period of time under rated capacity and allowable temperature rise conditions.
(4) Voltage ratio. This refers to the ratio of the rated voltage of the primary coil to the rated voltage of the secondary coil.
(5) Wiring method. Single-phase transformers have only one set of coils for high and low voltage, supplying power only for single-phase use, while three-phase transformers have a Y/Δ configuration. In addition to the above technical data, there are also the transformer's rated frequency, number of phases, temperature rise, and impedance percentage, etc.
14. How to select a transformer? How to determine the appropriate capacity of a transformer?
First, it is necessary to investigate the power supply voltage at the location where electricity is used, the actual power load of the user, and the local conditions. Then, refer to the technical data marked on the transformer nameplate to select one by one. Generally, the transformer capacity, voltage, current, and environmental conditions should be considered comprehensively. Among them, the capacity selection should be based on the capacity, nature, and usage time of the user's electrical equipment to determine the required load, and the transformer capacity should be selected accordingly.
During normal operation, the electrical load on the transformer should be approximately 75% to 90% of its rated capacity. If the actual load on the transformer is less than 50% during operation, a smaller capacity transformer should be replaced; if the load exceeds the rated capacity, a larger transformer should be replaced immediately.
Meanwhile, when selecting a transformer, the primary coil voltage should be determined based on the power supply line, and the secondary coil voltage should be selected based on the electrical equipment. Ideally, a low-voltage three-phase four-wire power supply should be chosen. This allows for the simultaneous supply of power for both mains and lighting.
When selecting the current, it is important to ensure that the load can meet the requirements of the motor when it starts (because the starting current of the motor is 4 to 7 times larger than that during the initial running phase).
15. Why can't a transformer be operated under overload?
Overload operation refers to the operation of a transformer exceeding the current value specified on its nameplate.
Overload is divided into two types: normal overload and accidental overload. The former refers to the increase in electricity consumption by users under normal power supply conditions. It often causes the transformer temperature to rise, promotes the aging of transformer insulation, and reduces service life. Therefore, transformer overload operation is not allowed.
Under special circumstances, the overload operation of a transformer for a short period of time shall not exceed 30% of the rated load (in winter) and shall not exceed 15% in summer.
For the latter, the requirements for accident overload and permissible time are shown in the table below.
16. What types of tests should be performed on a transformer during operation?
To ensure the transformer operates normally, the following tests should be performed regularly:
(1) Temperature test. The temperature is very important in determining whether a transformer is operating normally. The regulations stipulate that the upper oil temperature should not exceed 85°C (i.e., a temperature rise of 55°C). Most transformers are equipped with dedicated temperature measuring devices.
(2) Load Measurement. To improve transformer utilization and reduce energy loss, it is essential to measure the transformer's actual power supply capacity during operation. This measurement is typically conducted during peak electricity consumption periods in each season, using a clamp meter for direct measurement. The current value should be 70-80% of the transformer's rated current; exceeding this indicates overload and requires immediate adjustment.
(3) Voltage Measurement. The regulations require that the voltage variation range be within ±5% of the rated voltage. If it exceeds this range, a tap changer should be used to adjust the voltage to the specified range. Generally, a voltmeter is used to measure the voltage at the secondary coil terminals and the terminal voltage at the end user.
(4) Insulation resistance measurement. To ensure that the transformer is always in normal operating condition, the insulation resistance must be measured to prevent insulation aging and accidents. During the measurement, the transformer should be stopped from operation. The insulation resistance value of the transformer should be measured using a megohmmeter. The measured resistance should not be lower than 70% of the previously measured value. When selecting a megohmmeter, a 500-volt voltage rating can be used for the low-voltage coil.
