What is Isolation Transformer?

The Function and Working Principle of Isolation Transformer

  Isolation transformer refers to the transformer with electrical isolation between the input winding and the output winding. The isolation transformer is used to avoid accidental contact with the live body at the same time. The isolation of the transformer is to isolate the respective currents of the primary and secondary windings. In the early days, it was used in the power industry in European countries, and was widely used in the control power supply of general circuits, security lighting and indicator lights in the electronics industry or industrial and mining enterprises, machine tools and mechanical equipment.

  There is a special transformer with high insulation strength between the primary side and secondary side windings to isolate different potentials and suppress common mode interference. The transformation ratio of the isolation transformer is usually 1:1.

How an isolation transformer works?

  The principle of isolation transformer is the same as that of ordinary transformer. All use the principle of electromagnetic induction. Isolation transformers generally refer to 1:1 transformers. Because the secondary is not connected to ground. There is no potential difference between any of the secondary wires and ground. Safe to use. Often used as a maintenance power supply.

  Isolation transformers are not all 1:1 transformers. The power supply for the control transformer and valve equipment is also an isolation transformer. Power supplies such as tube amplifiers, tube radios and oscilloscopes and lathe control transformers are isolation transformers. For example, 1:1 isolation transformer is commonly used in the maintenance of color TVs safely. Isolation transformers are used more often, and they are also used in air conditioners.

  Generally, although there is an isolation circuit between the primary and secondary windings of the transformer, in the case of high frequency, the capacitance between the two windings will still cause electrostatic interference between the circuits on both sides. In order to avoid such interference, the primary and secondary windings of the isolation transformer are generally placed on different cores to reduce the capacitance between the two; Electrostatic shielding for high anti-jamming properties.

  Electrostatic shielding is to set an unclosed copper sheet or non-magnetic conductive paper between the primary and secondary windings, which is called a shielding layer. Copper sheets or non-magnetic conductive paper are connected to the casing with wires. Sometimes in order to achieve better shielding effect, a shielding shell is also covered throughout the transformer. The lead-out terminals of the windings are also shielded to prevent other external electromagnetic interference. In this way, there is mainly only residual magnetic coupling between the primary and secondary windings, and the equivalent distributed capacitance between them can be less than 0.01pF, thereby greatly reducing the capacitive current between the primary and secondary windings and effectively suppressing the power supply and other circuits. various distractions.

Classification of isolation transformers

  Because there is no direct electrical connection between the primary side and secondary side windings of ordinary isolation transformers, ordinary power transformers have the function of potential isolation regardless of the transformation ratio, while isolation transformers can isolate higher potential differences. It is widely used in AC power lines and communication lines to isolate ground loops and effectively suppress common mode interference in the low frequency and audio frequency range, but cannot suppress differential mode interference. Signal and pulse isolation transformers are also widely used in the range of audio to video to interrupt the ground loop and achieve functions such as coupling AC, isolating DC components, and impedance matching. The ground loop impedance is reduced due to the distributed capacitance between the primary and secondary sides of the isolation transformer. When the potential of point B rises and the interference voltage en occurs for some reason, the high-frequency common mode interference can be coupled from the primary side to the secondary side.

  The shielding isolation transformer inserts a metal shielding layer between the primary side and the secondary side of the isolation transformer, and the shielding layer divides the capacitance between the primary side and the secondary side into two, which plays a shielding role. If the metal shielding layer is connected to the transformer ground, the common mode interference from the primary side will be bypassed by the shielding layer impedance before reaching the secondary side. If the metal layer is connected to the input terminal of the primary side of the transformer (connect the voltage regulating tap or the ground terminal and the neutral terminal when there is a voltage regulating tap), the differential mode interference from the primary side is also short-circuited by the shielding layer before reaching the secondary side.

  Double shielding isolation transformer When common mode and differential mode interference occur on the primary side at the same time, connect one shielding layer to the primary side to reduce differential mode noise, and connect the other shielding layer to the reference plane or ground line of common mode interference to reduce common mode noise. The isolation transformer enclosure is also connected to safety ground. The connection line of the shielding layer must be short and reliable, otherwise the shielding effect will be significantly reduced at high frequencies.

  Triple-shielded isolation transformers When higher isolation requirements are required, triple-shielded isolation transformers can be used. The connection method of the three different shields depends on the installation method of the transformer and the grounding conditions. Generally, the transformer is installed on the partition of the equipment rack or the partition wall of the shielding room, and the rack is connected to the safety ground wire of the equipment, and the input power is safely disconnected and protected by an insulating tube.

  Requirements for isolation transformers The isolation transformers used in electronic equipment in substations and power plants should be selected with reference to the national standard requirements for equipment. The isolation transformers used in power supplies should meet the following indicators:

① Rated voltage: 220V ± 20 %;

② Rated current: selected according to the input maximum working current of the equipment plus 50%;

③ Insulation resistance: >10MQ;

④ Power frequency withstand voltage 2500V, 1min, leakage current <10mA;

⑤ High frequency interference suppression common mode 2500V ( 1MHz damped oscillation wave) suppression >60dB, differential mode 1250V (1MHz damped oscillation wave) suppression >40dB.

The magnitude of the leakage current of the isolation transformer and the shielding effectiveness should be selected according to the needs of different equipment applications.

The function of isolation transformer

1. Completely insulate the electricity between the primary side and the secondary side, and also isolate the circuit. In addition, the high-frequency loss of its iron core is used to suppress the introduction of high-frequency clutter into the control loop. Using an isolation transformer to suspend the secondary to the ground can only be used in situations where the power supply range is small and the line is short. At this time, the capacitance current of the system to ground is too small to cause personal injury.

2. Electrical isolation

  One is to protect equipment, and another very important role is to protect personal safety! Isolate dangerous voltages. The isolation transformer output and input capacitive coupling is small, and it has a more effective suppressing effect on the interference caused by lightning, discharge, grid switching, motor startup and other grid noises. It is a more effective power noise suppressor. From this point of view, the isolation transformer has the function of protecting the equipment. The so-called protection of personal safety is a protection for the person operating this equipment. Because the electromotive force of the isolation transformer is obtained through secondary induction, it does not form a loop with the primary side (which forms a loop with the ground), so it will not cause electric shock.

3. Change the voltage

  Such as changing the 220V AC voltage to AC 15V, but it is very important to separate the 220V GND from the 15V GND. We know that when the human body holds the 220V neutral wire and then contacts the ground, there will be no current flow, but when the human body holds the live wire and then contacts the ground, it will be life-threatening. The role of the isolation transformer in being able to isolate the reference point (ie, the ground) is great. After the isolation of the transformer, for example, the voltage is still 220V, as long as the two wires are not in contact with the ground, there will be no problem if the human body holds any one of them connected to the ground.

Selection of isolation transformer

1. Look at the appearance

  A good-quality transformer generally looks clean and pollution-free, the metal material is not warped, the insulating tape is neatly wound, the leads (or pins) are well fixed, and the labels are well printed and adhered neatly.

2. Look at the logo and packaging

  The identification of the transformer should include the trademark, model and rated electrical parameters, or it will be accompanied by a manual containing complete electrical parameters and clear wiring instructions.

3. Look at the parts list

  In addition to good workmanship, a good-quality transformer is inseparable from good materials. Whether it is winding, insulating tape, skeleton, iron core and lead (or pin), it is very important. Insulating paint is often used in transformer production, which has a good effect on moisture resistance, heat dissipation, insulation and fixed windings of transformers. Some enterprises may not use insulating paint or use poor-quality insulating paint, which will affect the performance and safety of the transformer, and may even release toxic substances.

Green Energy Electric Group has been specializing in the production of isolation transformers and autotransformers for decades. Welcome to purchase and inquire!