Basic knowledge, selection, and usage of isolation transformers
Working principle of isolation transformer
Isolation Transformers provide electrical isolation between AC power lines (main power sources) and electrical equipment. This means that there is no DC path between the two windings. They are mainly used for three purposes:
Firstly, isolate the secondary winding from the ground (grounding)
Next is to provide the operation of raising and lowering the line (main) voltage
The third is to reduce the line noise transmitted from the primary winding to the secondary winding, and vice versa
If the number of turns in the primary winding is greater than that in the secondary winding, the voltage on the secondary winding will be lower than that on the primary winding. This is a voltage reduction configuration. If the number of turns in the primary winding is less than that in the secondary winding, the voltage on the secondary winding will be higher than that on the primary winding, resulting in a boost configuration. Most isolation transformers have the same number of turns in their primary and secondary windings, so the primary and secondary voltages are also the same.
The energy in a transformer is conserved, so if we ignore losses, the product of VP and primary current (IP) will be equal to the product of VS and secondary current (IS). The rated power of a transformer is determined by multiplying the RMS voltage of the primary winding by the RMS primary current. This is 'apparent power', measured in volt amperes or VA.
The points on the schematic are phase determination points, indicating the direction of primary and secondary currents. As shown in the figure, the current flowing into the primary point side of the winding will cause the secondary current to flow out from the secondary point side of the winding. This is important if the windings are to be placed in series or parallel. Not following the phase determination of the winding may result in errors.
Faraday shielding is an electrostatic shielding that reduces the capacitance between the primary and secondary windings, and is typically grounded. This shielding can reduce the amplitude of common mode noise and transients passing through the transformer.
The primary and secondary windings in the isolation transformer are highly insulated to minimize direct conductivity between each other. The standard for measuring insulation effect is leakage current. Most isolation transformers are also tested using high potential or withstand voltage testers. When checking for leakage current, these instruments will apply high voltage at both ends of the insulation.
Faraday shielding can be a foil layer or a tightly wound winding, as shown in the figure. Grounding is usually located on the primary side and connected to the earth. Due to the use of enameled wire in the primary and secondary windings, this structure is called "double insulation".
Alternatively, the windings can be placed side by side on the magnetic core, which is called a "multi slot wire frame" structure, or wound around a ring-shaped magnetic core.
Commercial isolation transformer
Isolation transformers can adopt an open structure or be enclosed in a shielding structure. Hammond Manufacturing's 171E isolation transformer uses a shielded shell structure. The end cover shield contains the magnetic field of the transformer and is also used to minimize the interference of the external magnetic field of the transformer. This 500 VA 1:1 transformer also includes leads NEMA、 Three wire grounded input and output connectors, as well as integrated overload Circuit Breakers.
Although the grounding wire is connected to the secondary output connector, it is not used in most isolation transformer applications. At rated input voltage, the leakage current between the primary and secondary of the transformer is less than 60 microamperes (µ A).
The rated voltages of the primary and secondary windings are the same, which are 0, 104, 110, and 120 volts, respectively. This allows for series or parallel connection on the primary or secondary windings.
Therefore, for 110 or 220 volt inputs, the nominal 1:1 ratio can be maintained. Moreover, it is also possible to configure a step-up transformer from 110 volts to 220 volts, or a step-down transformer from 220 volts to 110 volts.
In addition, multi tap windings allow for intermediate rated voltages, such as 208 volts, 214 volts, or 230 volts
If both the primary and secondary windings are connected in series, the transformer has a 220 volt input and a 1:1 voltage ratio. If both the primary and secondary windings are connected in parallel, the input voltage is 110 volts, with a 1:1 voltage ratio, and the available current is twice that of a single winding. If the primary winding is placed in series and the secondary winding is placed in parallel, the primary voltage will drop to half. If the secondary winding is in series and the primary winding is in parallel, a 2:1 boost can be achieved.
Medical isolation
The isolation transformer used in medical applications must meet stricter requirements for leakage current. For ground leakage, shell leakage, and patient leakage, there are maximum leakage current specifications. Grounding leakage refers to the leakage current in the equipment grounding wire. The shell current describes the current flowing from a bare conductive surface to the ground through a conductor other than the grounding wire. Patient leakage refers to the current flowing from the patient to the ground when connected to the device normally. Most devices in this category have passed UL/IEC 60601-1 certification.
Typical applications of isolation transformers
The most common application of isolation transformers is to isolate equipment from AC line grounding. To illustrate the necessity of doing so, we take a switch mode power supply (SMPS) as an example.
This is a line powered power supply using a flyback topology structure. The primary side of the circuit is highlighted in yellow, which performs full wave rectification on the line (power) input and applies it to the primary rail. This means that if a 120 volt line is used, the voltage level between the high and low voltage rails is about 170 volts, and if a 240 volt line is used, the value is about 340 volts. The voltage of this rectification line will be stored in the primary energy storage capacitor C2.
Please note that the primary and secondary sections of the power supply will be electrically isolated using flyback transformer L2 and optocoupler Q4. The secondary section is grounded at the negative (-) output terminal, while the primary section is not grounded. If a grounded input instrument (such as an oscilloscope) is used for troubleshooting, this grounding method will be problematic. Connecting the grounding terminal of the oscilloscope probe to the components on the primary side of the power supply may cause a short circuit and damage the main components and oscilloscope.
The low-voltage primary rail in the power supply is connected to the neutral line of the AC line. Although the neutral wire is grounded at the inlet, when it reaches the input of the SMPS, it may be several volts higher than the ground voltage, making it an unsafe connection point for oscilloscope probe grounding.
Isolation transformers are mainly used for electrical isolation of the primary section of SMPS. Once isolated, it can be connected to the grounding terminal of any part of the probe in the primary circuit. In this way, regardless of which point the ground clamp is connected to, the grounding reference will be placed here, thereby eliminating the possibility of a short circuit in the primary circuit.
When multiple devices (each with its own grounding return path) are connected together, this same grounding isolation capability makes isolation transformers very useful for diagnosing and correcting grounding circuits.
Transformers allow for isolated grounding to identify which equipment is the source of ground leakage current.
Isolation transformers can also reduce high-frequency noise transmitted from the line to the connected equipment or from the equipment back to the line. This is due to the series inductance of the transformer and the Faraday shielding of the grounding, which reduces the capacitive coupling at both ends of the transformer.
summary
Isolation transformers can isolate devices connected to the secondary winding from the AC power supply on the primary winding, thus allowing for the redefinition of the reference plane on the secondary equipment. This also allows for redirection and control of leakage current.
Meanwhile, they will minimize the transmission of high-frequency harmonics and noise to the greatest extent possible. These devices are very useful for testing power related equipment.