Transformer and Power Phasing - Reversed Polarity

    As more and more electronics are introduced to the HVACR industry, polarization of incoming power and phasing of primary to secondary voltage on transformers is becoming more of an issue.

Polarization has been observed in the Appliance industries since the introduction of the three-prong plug, however, the HVACR industry does not use a plug for incoming power, but is hard wired.

Many of the electronic boards being used today will not function properly for flame rectification, logic functions or possibly not at all without polarization of incoming power. Many also require phasing between the primary and secondary sides of step-down transformers.

We recommend that these two items be checked during normal installation and/or service calls. See as follows:

1) Measuring Line Voltage Polarity 

2A) Transformer Phasing - On Board Check (L1 to R)

2B) Transformer Phasing - Across Transformer Check (L1 to R)

• Subtractive Polarity = (120VAC - 24VAC = 96VAC+/-), then the phase/polarity IS CORRECT

• Additive Polarity = (120VAC + 24VAC = 144VAC+/-), then it is out of phase/reversed polarity
  • Swap the R and C on the secondary side of the transformer to correct the issue

• *NOTE: For fame rectification the common side of the secondary voltage (24 V) is cabinet grounded. If you were to bench test a transformer the primary neutral and secondary common must be connected together for testing purposes.

3)  Multi-Transformer Phasing - On Board Check (R to R)

1. To verify transformer phasing, shut off power to the equipment and remove the wires from the 24v output of the transformer on its secondary side.
2. Then utilize a properly rated voltmeter and place leads on both transformers 24V "R" output terminals with the equipment energized.
   • Properly phased circuits should register 0-2VAC.
   • An improperly phased parallel is normally found to be around 50VAC.
3. If found to be out of phase it is advised to de-energize the equipment and swap the two “line” primary power leads on one transformer. Then perform the same test.
4. If voltage is now at 0-2VAC, verify equipment operation.

It is for stability of the communication system and for flame rectification why circuit phasing is considered a necessary step of a quality install and quality service visit. Phase checking is simply measuring the difference in the "peak-to-peak" of the voltage sine waves.

A smaller number than 120 VAC indicates correct polarity

A larger number than 120 VAC indicates incorrect polarity