The article covers the process of wiring transformers in parallel to increase their total VA rating while ensuring proper phasing to prevent voltage doubling. It also includes troubleshooting techniques for transformer-related issues in HVAC systems, emphasizing safe voltage testing practices, diagnosing short circuits, and understanding transformer protection features like fuses and circuit breakers.
Transformers can be wired in parallel to increase the total VA rating. This is also known as “phasing transformers.” Two 24-V, 40-VA transformers wired in parallel will have an 80-VA rating. As the load on a transformer increases, the VA rating needs to increase or the transformer will overheat and burn out. You can change out the transformer and install a higher rated VA transformer or wire another transformer in parallel.
It is important to understand how to wire the two transformers in parallel. Wiring the transformer incorrectly will double the secondary output voltage. For example, a 24-V output can become 48 volts. The higher voltage output will burn out coils and other loads that require 24 V. Here are the steps to successfully wire the secondary windings without creating a double voltage condition. Figure 1 illustrates how two transformers are wired in parallel.
1. Hook up the appropriate primary voltage to the two transformers.
2. Hook up one secondary wire of transformer 1 to one lead on transformer 2. This can be done on a test basis using a set of jumper cables (alligator clips with connecting wire) prior to the final wiring job.
3. Turn on the power to the transformers and read the voltage between the remaining unconnected leads, as shown in Figure 1. When powered up, do not allow the secondary wires to touch each other or ground. This will damage the transformer.
4. If the voltmeter reads “0 V” when connected, the connection is correct. Remove the voltmeter, turn off the power, and connect the final two secondary transformer leads together.
5. If the voltmeter in Figure 1 reads double the rated voltage, reverse the secondary leads. You will now read 0 V. Hook the leads together and measure 24 V. Connect the two secondary leads to the control circuit.
6. Finally, double check the power to the paralleled secondary winding. If the voltage is 48 V, disconnect the primary power and switch the wiring on secondary.
Figure 1 Phasing or wiring the secondaries of transformers in parallel is the intermediate step in measuring the final connection.
Tech Tip
Do not “spark” a transformer. Sparking a transformer is the practice of quickly touching the bare wires of the secondary side of a transformer together to see if it creates an electrical spark. Some techs use this as a way of determining that the transformer has secondary voltage. This may be a way to bypass using a voltmeter to measure the secondary voltage, but it is not a good practice. A transformer with an internal or external fuse will blow even with a quick touch or spark. Sparking a transformer is a poor substitute for a voltmeter. A spark can be created at lower, unacceptable voltage. Be a professional and use a professional’s instrument for measuring voltage.
Tech Tip
Some transformers have a built-in secondary transformer fuse that will not be obvious to the tech. This type of transformer will be identified as “fused” or “internally fused.” In some instances you can remove part of the winding cover, which is paper or plastic, and expose an open fuse link. Wire in an external fuse and continue to use the transformer. This should be considered a temporary fix. Replace the transformer with a new one. Some protected transformers have a built-in circuit breaker. Reset the breaker with the power off. Power up and try to determine the reason the breaker tripped. A temporary low-voltage situation will create a high-amperage condition that can trip the breaker. By the time you arrive at the job site, the low-voltage condition may have disappeared, but the breaker has been tripped. Reset and test.
Service Ticket
You are called to troubleshoot a package unit (see Figure 2) that is not cooling. You quickly determine that the control transformer is burned out. The transformer appears overheated and the secondary winding is open. Replacing the transformer, you install a temporary, 5-amp, inline circuit breaker similar to one shown in Figure 3. The purpose of the inline breaker is to protect the secondary of the transformer in case there is a short or overload in the control circuit. You plan to remove the temporary inline breaker after an operational check. As soon as the power is applied, the inline breaker trips. An inspection of the control wiring and control components does not reveal an obvious problem. Troubleshooting any HVACR system with a control voltage short is always a challenge. You start with these steps:
1. For the package unit shown in Figure 2, the first step in determining a short in the control voltage circuit is to disconnect everything from the left side of the transformer, at point 1. Powering up the primary of the transformer without it being connected in the circuit should not short out the secondary unless this transformer windings is already damaged.
2. Using the diagram in Figure 2 as our example, connect one control voltage circuit at a time until the inline circuit breaker trips. The inline circuit breaker will be installed, in series, between the left side of the transformer and the R connection on the thermostat. On the left side of the diagram, at point 1, hook up the R wire to the transformer. The right side of the transformer will also be connected. The circuit breaker does not trip.
3. Next, close the system switch at point 2 that follows the R connection. This is the On and Off switch found on some thermostats. The circuit breaker does not trip.
4. Now switch (close) the fan switch to Auto, point 3. The circuit breaker does not trip.
5. The IFR coil and contactor coil C have been disconnected as part of the troubleshooting process. Next, you turn the thermostat temperature low enough so that the cooling circuit is closed, point 4. The circuit breaker does not trip.
6. When you attach the IFR coil, at point 5, the breaker does not trip.
7. Finally, connecting the contactor C circuit, at point 6, trips the breaker. You have established that the short circuit is in the contactor circuit. “Ohm out” the contactor coil or look for shorted wiring in this line of the diagram. The ohmmeter indicates that the contactor coil measures 2 Ω, which is very low for a contactor coil. Replacing the contactor solves the problem. Finding a shorted component or wire can be a real test of your “figuring it out” skills. We just completed an exercise in finding a short-circuit problem. Eliminate as many circuits as possible by disconnecting the circuit from the transformer. Reattach one circuit at a time and apply power after each circuit is applied.
Figure 2 The package unit shown here has a short in the control voltage side of the system. The diagram does not show the short. Troubleshooting steps are necessary.
Figure 3 This is a temporary, 5-amp, inline circuit breaker. This should be placed on the “hot side” of the secondary of the transformer to protect it against overcurrent or short-circuit conditions.
Tech Tip
On older air handlers and furnaces, transformers are combined with a relay. This is called a fan center. A fan center is shown in Figure 4. The transformer may be permanently mounted to a plate on the fan center. A burned-out transformer will need to be replaced with a new fan center or by mounting another transformer near the fan center. The relay is usually plugged into the fan center and can easily be replaced if defective. The diagram in Figure 5 shows the electrical hook-up for a fan center showing the transformer and fan relay.
Figure 4 The fan center has a combined transformer and fan relay.
Figure 5 This diagram of a fan center shows the symbols for the transformer, relay coil, and relay contacts. The R, C, G, W, and Y screw connects are on the transformer.
How to Wire Transformers In Parallel Key Takeaways
In conclusion, understanding transformer wiring, troubleshooting, and protection mechanisms is crucial for ensuring the safe and efficient operation of HVAC systems. Properly wiring transformers in parallel increases their VA rating without causing dangerous voltage spikes that could damage connected components. Additionally, following systematic troubleshooting methods helps identify and resolve short circuits, preventing further system failures. Using appropriate testing tools, such as voltmeters instead of unsafe practices like “sparking,” enhances safety and accuracy in diagnosing transformer issues.
