1. What is a transformer?

In communication circuits, the device that increases or decreases voltage is called a transformer. The transformer can convert any value of voltage into the voltage value we need with the same frequency to meet the requirements of energy transmission, distribution, and use. For example, the electricity generated by a power plant has a low voltage level and must be raised in order to be transmitted to a distant electricity consumption area. The electricity consumption area must then be reduced to a suitable voltage level through voltage reduction to supply power equipment and daily electrical equipment.

2. How does a transformer change voltage?

Transformers are made based on electromagnetic induction. It consists of an iron core stacked with silicon steel sheets (or silicon steel sheets) and two sets of coils wound around the iron core. The iron core and coils are insulated from each other and have no electrical connection, as shown in the figure. We refer to the coil connecting the transformer and the power supply as the primary coil (or primary side), and the coil connecting the transformer and electrical equipment as the secondary coil (or secondary side). When the primary coil of a transformer is connected to an AC power source, a changing magnetic field line will be generated in the iron core. Due to the secondary coil being wound around the same iron core, the magnetic field lines cut the secondary coil, inevitably generating induced electromotive force on the secondary coil, causing voltage to appear at both ends of the coil. Because the magnetic field lines are alternating, the voltage of the secondary coil is also alternating. And the frequency is exactly the same as the power frequency.

The manager’s argument is that the voltage ratio between the primary and secondary coils of a transformer is related to the ratio of turns between the primary and secondary coils, which can be expressed as follows: primary coil voltage/secondary coil voltage=number of turns in the primary coil/number of turns in the secondary coil, indicating that the more turns there are, the higher the voltage. Therefore, it can be seen that there are fewer secondary coils than primary coils, which is the step-down transformer. On the contrary, it is a step-up transformer.

3. What are the types of transformer designs?

There are single-phase and three-phase transformers according to the number of phases. There are power transformers, specialized power transformers, voltage regulating transformers, measuring transformers (voltage transformers, current transformers), small power transformers (for low-power equipment), safety transformers, and two types of core and shell transformers according to their structure. The coil has dual windings and multiple windings, and autotransformers are divided into oil immersed and air cooled according to cooling methods.

4. What are the components of a transformer?

The transformer components are mainly composed of iron cores and coils, as well as oil tanks, oil conservators, insulation sleeves, and tapping heads.

5. What is the use of transformer oil?

The function of transformer oil is: (1) insulation effect. (2) Heat dissipation effect. (3) Eliminate the arc effect.

6. What is an autotransformer?

An autotransformer has only one set of coils, and the secondary coil is tapped out from the primary coil. Its electrical energy is transmitted not only through electromagnetic induction, but also through electricity. This type of transformer has fewer silicon steel sheets and copper wires than ordinary transformers and is commonly used for voltage regulation.

7. How is the voltage regulator regulated?

The structure of the voltage regulator is the same as that of an autotransformer, except that the iron core is made into a circular coil and wound around the circular iron core. The secondary coil tap uses a sliding brush contact to make the contact slide in a circular pattern along the surface of the coil, achieving a smooth voltage regulation effect.

8. What is the current relationship between the primary and secondary coils of a transformer?

When a transformer operates with a load, the change in secondary coil current will cause a corresponding change in primary coil current. According to the principle of magnetic potential balance, the current of the primary and secondary coils is inversely proportional to the number of coil turns. The current on the side with more turns is smaller, and the current on the side with fewer turns is larger. This can be expressed as follows: primary coil current/secondary coil current=secondary coil turns/primary coil turns.

9. What is the voltage change rate of a transformer?

The voltage change rate of the voltage regulator is one of the main performance indicators of transformers. When a transformer supplies power to a load, the voltage at the load end of the transformer will inevitably decrease. The voltage drop value is compared to the rated voltage value, and the percentage is taken as the voltage change rate, which can be expressed by a formula; Voltage change rate=[(secondary rated voltage – load terminal voltage)/secondary rated voltage] x 100%. A typical power transformer has a voltage change rate of 4-6% when connected to the rated load.

10. How to ensure that the transformer has a rated voltage output?

Voltage too high or too low can affect the normal operation and service life of transformers, so voltage regulation is necessary. The method of voltage regulation is to introduce several taps into the primary coil and connect them to the tap changer. The tap changer changes the number of turns of the coil by rotating the contacts. Just rotate the position of the tap changer to obtain the required rated voltage value. It should be noted that voltage regulation should usually be carried out after cutting off the load connected to the transformer.

11. What are the commonly used small transformers? In what situations is it applied?

Small transformers refer to single-phase transformers with a capacity of less than 1 kVA, mostly used as power transformers for electrical equipment control, electronic equipment power transformers, and safety lighting power transformers.

12. What are the losses of transformers during operation? How to reduce losses?

The losses during transformer operation include two parts; (1) It is caused by the iron core. When the coil is energized, the alternating magnetic field lines cause eddy currents and hysteresis losses in the iron core, which are collectively referred to as iron losses. (2) It is caused by the resistance of the coil itself. When there is current passing through the primary and secondary coils of the transformer, electrical energy loss occurs, which is called copper loss.

The sum of iron loss and copper loss is transformer loss, which is related to transformer capacity, voltage, and equipment utilization. Therefore, when selecting transformers, efforts should be made to ensure that the equipment capacity is consistent with the actual usage, in order to improve equipment utilization, and attention should be paid not to operate the transformer under light load.

13. What is the nameplate of a transformer? What are the main technical data on the nameplate?

The nameplate of the transformer indicates the performance, technical specifications, and usage situation of the transformer, which is used to meet the user’s selection. The main technical data that usually needs attention during selection are:

(1) Kiloamperes of rated capacity. The output capacity of the transformer under rated conditions. If the rated capacity of a single-phase transformer is U line x I line; Three phase transformer capacity=U line x I line.

(2) Rated voltage in volts. Indicate the terminal voltage of the primary coil and the terminal voltage of the secondary coil (when not loaded) separately. Note that the terminal voltage of the three-phase transformer refers to the line voltage U value.

(3) Rated current in amperes. Refers to the line current I value that primary and secondary coils are allowed to pass through for a long time under rated capacity and allowable temperature rise conditions.

(4) Voltage ratio. The ratio of the rated voltage of the primary coil to the rated voltage of the secondary coil.

(5) Wiring method. A single-phase transformer only has one set of coils for high and low voltage, and is only supplied for single-phase use. A three-phase transformer has Y/△ type. In addition to the above technical data, there are also the rated frequency, number of phases, temperature rise, and impedance percentage of the transformer.

14. How to choose a transformer? How to determine the reasonable capacity of a transformer?

Firstly, it is necessary to investigate the power supply voltage of the electricity consumption area, the actual electricity load of the user, and the conditions of the location. Then, refer to the technical data indicated on the transformer nameplate to select one by one. Generally, the transformer capacity, voltage, current, and environmental conditions should be comprehensively considered. The capacity selection should be based on the capacity, nature, and usage time of the user’s electrical equipment to determine the required load, in order to select the transformer capacity.

During normal operation, the transformer should withstand an electrical load of approximately 75-90% of its rated capacity. When the actual load borne by the transformer is measured to be less than 50% during operation, the small capacity transformer should be replaced. If it is greater than the rated capacity of the transformer, the large transformer should be replaced immediately. At the same time, when selecting a transformer, the voltage value of the primary coil of the transformer is determined based on the line power supply, and the voltage value of the secondary coil is selected based on the electrical equipment. It is best to choose a low-voltage three-phase four wire power supply. This can provide both power and lighting electricity simultaneously.

For the selection of current, attention should be paid to whether the load can meet the requirements of the motor during motor starting (because the starting current of the motor is 4-7 times larger than that during sinking operation).

15. Why can’t transformers operate under overload?

Overload operation refers to the operation of a transformer exceeding the current value specified on the nameplate. Overload is divided into two types: normal overload and accident overload. The former refers to the increase in user electricity consumption under normal power supply conditions, which often causes the temperature of the transformer to rise, promotes insulation aging, and reduces its service life. Therefore, transformer overload operation is not allowed. Under special circumstances, the overload operation of transformers in a short period of time cannot exceed 30% of the rated load (in winter), and cannot exceed 15% in summer. For the latter, the requirements for accident overload and allowable overload time are shown in the table below.

Multiple of rated load	Overload allowable time
	outdoor transformer	indoor transformer
1.3	2hours	1hour
1.6	30min	15min
1.75	15min	8min
2.00	7.5min	4min

 

16. What types of tests should transformers undergo during operation?

To ensure the normal operation of the transformer, the following tests should be conducted regularly:;

(1) Temperature testing. The operation status of the transformer is normal, and the temperature is very important. The regulations stipulate that the upper oil temperature shall not exceed 85C (i.e. temperature rise of 55C). Generally, transformers are equipped with dedicated temperature measurement devices.

(2) Load measurement. In order to improve the utilization rate of transformers and reduce the loss of electrical energy, it is necessary to determine the true power supply capacity that the transformer can bear during operation. Measurement work is usually carried out during the peak electricity consumption period of each season, using a clamp ammeter for direct measurement. The current value should be 70-80% of the rated current of the transformer. If it exceeds the limit, it indicates overload and should be adjusted immediately.

(3) Voltage measurement. The regulation requires that the voltage variation range should be within ± 5% of the rated voltage. If it exceeds this range, a tap should be used to adjust the voltage to reach the specified range. Generally, a voltmeter is used to measure the terminal voltage of the secondary coil and the terminal voltage of the terminal user.

Insulation resistance measurement. In order to keep the transformer in normal operation, it is necessary to measure the insulation resistance to prevent insulation aging and accidents. When measuring, efforts should be made to stop the transformer from running. The insulation resistance value of the transformer should be measured using a shake table, and the measured resistance should not be lower than 70% of the previous measured value. When selecting a shake table, the low-voltage coil can use a voltage level of 500 volts.