An isolation transformer is a transformer in which the input and output windings are electrically isolated. The isolation transformer is to isolate the current of the primary winding and the secondary winding respectively.

I What is an Isolation Transformer?

Isolation transformers are safety power supplies and are generally used for machine repair and maintenance for protection, lightning protection, and filtering.

The principle of the isolation transformer is the same as that of an ordinary transformer一the principle of electromagnetic induction. Isolating transformers generally (but not all) refer to 1:1 transformers. Although the secondary is not connected to the ground, there is still a potential difference and a phase difference of 180 degrees between any line of the secondary and the ground, which is dangerous.

The power supply for the control transformer and electronic tube equipment is also an isolation transformer. Power supplies such as valve amplifiers, valve receivers and oscilloscopes, and lathe control transformers are all isolation transformers. For example, a 1:1 isolation transformer is commonly used for the safe maintenance of color TVs. It is also used in air conditioners.

Figure 1. Isolation Transformer

First of all, we usually use one line of AC power voltage to connect to the ground, and there is a 220V potential difference between the other line and the ground, which causes an electric shock. However, the secondary of the isolation transformer is not connected to the earth, and there is no potential difference between any two wires and the earth. People will not get an electric shock if they touch any line, so it is safer.

Secondly, the output end of the isolation transformer is completely open and isolated from the input end, so that it has a good filtering effect on the input end of the transformer (the power supply voltage supplied by the grid), so as to provide a pure power supply voltage to the electrical equipment.

Another use is to prevent interference. It can be widely used in subways, high-rise buildings, airports, stations, docks, industrial and mining enterprises, and tunnels for power transmission and distribution.

II How does an Isolation Transformer Work?

The principle of the isolation transformer is the same as that of the ordinary transformer. They both use the principle of electromagnetic induction. The isolation transformer generally refers to a 1:1 transformer. Because the secondary is not connected to the ground, there is no potential difference between any secondary line and ground, which is safe to use, and often used for power supply maintenance.

Not all isolation transformers are 1:1 transformers. The power supply for the control transformer and electronic tube equipment is also an isolation transformer. Power supplies such as valve amplifiers, valve receivers, oscilloscopes, and lathe control transformers are all isolation transformers. A 1:1 isolation transformer is commonly used for the safe maintenance of color TVs and in air conditioners.

III Purpose of Isolation Transformer

Generally, although the primary and secondary windings of the transformer also have the function of isolating the circuit, in the case of higher frequencies, 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 core columns to reduce the capacitance between the two. The primary and secondary windings also can be placed concentrically, but there is an electrostatic shield placed between them to obtain high anti-interference.

Electrostatic shielding is an unclosed copper sheet or non-magnetic conductive paper between the primary and secondary windings, which is called a shielding layer. The copper sheet or non-magnetic conductive paper is connected to the shell with a wire. Sometimes in order to obtain a better shielding effect, a shielding shell is also covered throughout the transformer.

A Shield is also added to the lead terminal of the winding to prevent other external electromagnetic interference. In this way, there will be only the main 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 all kinds of interference from other circuits.

IV How to use Isolation Transformer

1. Connect the input terminal of the isolation transformer to the corresponding power supply voltage (380V), and connect the output terminal of the isolation transformer (660V) to the input terminal of the equipment to be used.

2. Turn on the power supply and it can work normally.

V Classification

1. Ordinary Isolation Transformer

Ordinary isolation transformers have no direct electrical connection between the primary and secondary windings, so they 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 range, but cannot suppress differential mode interference.

Signal and pulse isolation transformers are also widely used in the audio to video range, to interrupt the ground loop, and achieve functions such as coupling AC, isolating DC components, and impedance matching. Due to the distributed capacitance between the primary side and the secondary side, the isolation transformer ground loop impedance is reduced.

2. Shielded Isolation Transformer

A metal shielding layer is inserted between the primary side and the secondary side of the shielding isolation transformer. The shielding layer divides the capacitance between the primary side and the secondary side into two, which plays a shielding role.

Figure 2. Shielded Isolation Transformer

If the metal shielding layer is connected to the ground terminal of the transformer, the common-mode interference from the primary side is bypassed by the shielding layer impedance before reaching the secondary side.

If the metal layer is connected to the primary side input terminal of the transformer (when there is a tap charger, the tap charger, or the ground terminal and the neutral terminal), the differential mode interference from the primary side is also short-circuited by the shielding layer before reaching the secondary side.

3. Double Shielded Isolation Transformer

When common mode and differential mode interference appear on the primary side at the same time, a shielding layer is connected to the primary side to reduce differential mode noise, and another shielding layer is connected to the reference plane or ground of common-mode interference to reduce common-mode noise.

The housing of the isolation transformer is also connected to the safety ground. The connecting wire of the shielding layer must be short and reliable, otherwise, the shielding effect will be significantly reduced at high frequencies.

4. 3 phase Isolation Transformer

(1) Principle of 3 phase isolation transformer

It mainly uses electromagnetic induction to reduce the capacitance of the primary and secondary lines. Generally, it‘s used in air conditioners to facilitate maintenance with the 1:1 specification. Under normal circumstances, there is an isolation circuit between the primary winding and the secondary winding of the transformer. But if the frequency is higher, there will be electrostatic interference on both sides.

Figure 3. 3 Phase Isolation Transformer

This kind of interference can be avoided by putting the primary and secondary windings of the isolator transformer on unused columns. In this way, the capacitance between the two can be reduced. The placement between them has certain principles and functions. If they're placed concentrically, electrostatic shielding will be added, and the anti-interference ability is very high.

(2) The function of Isolation Transformer

1) It can completely insulate the electrical on the primary side and the secondary side and also isolate the circuit. In addition, the high-frequency loss of its iron core can prevent high-frequency noise from entering the control loop. Only on occasions with a small power supply range and short lines, an isolation transformer is used to suspend the secondary to the ground. At this time, the capacitance current to the ground of the system is too small to cause personal injury.

2) Electric Isolation. The output and input capacitance coupling of the isolation transformer is small, and it has a restraining effect on the interference caused by lightning, discharge, grid switching, motor startup, and other grid noise. It is a relatively effective power supply noise suppressor.

From this point of view, the isolation transformer is more effective in protecting equipment. The so-called protection of personal safety refers to the protection of people who operate 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 earth), so it will not cause electric shock.

3) Change the voltage. The three-phase isolation transformer can change the 220V AC voltage to AC 15V, it can even separate the 220V GND from the 15V GND, which is very important.

We know that when the human body holds the 220V neutral wire and then touches the earth, no current will flow, but if the human body holds the live wire and then touches the earth, it will be life-threatening.

The isolation transformer can isolate the reference point (ie, ground). After the isolation of the transformer, for example, if the voltage is still 220V, as long as the two wires are not in contact with the earth, then there is no problem if the human body holds any one of them and connects to the earth.

5. Isolation Transformer in UPS

The UPS isolation transformer mainly has the following functions:

(1) Reduce the zero-ground voltage and optimize the UPS terminal power supply network

The installation of an isolation transformer on the inverter output of the UPS can isolate the electrical connection between the input and the output, thereby effectively reducing the zero-ground voltage of the output.

Since the secondary winding of the isolation transformer adopts the Y-type connection, a new neutral line is generated after the neutral point is grounded, thereby reducing the zero-ground voltage.

Figure 4. Isolation Transformer in UPS

In fact, the minicomputers of HP, IBM, and SUN have extremely high requirements on the zero-ground voltage to ensure precise computing capabilities and highly reliable data processing and transmission capabilities. The installation of an isolation transformer can completely solve the problems caused by high zero-ground voltage.

(2) Filter out the harmonics on the load side and improve the quality of power supply

The isolation transformer itself has inductive characteristics. It can filter out a large number of low-order harmonics at the load end, reduce high-frequency interference, and can greatly attenuate high-order harmonics. The use of power isolation transformers can effectively suppress the noise interference entering the AC power supply and improve the electromagnetic compatibility of the equipment.

(3) Enhance the overload and short circuit protection capability to protect the load and the UPS host

Due to its own characteristics, the isolation transformer is the most stable device in UPS. During the normal operation of the UPS, if it encounters a large short-circuit current, the transformer will generate a reverse electromotive force, delay the impact of the short-circuit current on the load and the inverter, and protect the load and the UPS host.

(4) "Passing AC and blocking DC". Protect the load when UPS fails.

VI Isolation transformer VS. Switching Power Supply

The isolation transformer refers to a transformer with electrical isolation between the input winding and the output winding. Its function is to safely isolate people and electrical appliances. So what is the difference between the isolated transformer and switching power supply?

1. The input and output of the isolation transformer are all alternating current, while the switching power supply is transforming an alternating current to direct current

2. An isolation transformer is a device that uses the principle of electromagnetic induction to change the voltage, current, and impedance. The primary isolation transformer is used in AC circuits. The switching power supply is a power supply that uses modern power electronic technology to control the on and off time ratio of the switching transistors to maintain a stable output voltage. Switching power supplies have two types, AC to DC and DC to DC.

Figure 5. Switching Power Supply

3. Efficiency: 400W isolation transformer has high efficiency, while a 400W switching power supply has low efficiency;

4. Temperature rise: The temperature rise of the 400W isolation transformer is lower than that of the 400W switching power supply.

5 The cost of an isolation transformer above 500W is lower than that of the switching power supply.

6. The isolation transformer itself has no interference, but the switching power supply itself is a high-frequency interference source with high-frequency electromagnetic radiation;

7. The service life of the isolation transformer is longer than that of the switching power supply;

8. The isolation transformer can work normally at -30℃, but the switching power supply is already difficult to use in the cold north;

9. The volume of the isolation transformer is flexible and customized, without independent and opening mold, and the switching power supply has a long customized cycle;

10. The isolation transformer can be used for obvious overload in a short time, but the switching power supply can not.