Let's talk about the magic that is inverters and how to test the boards that drive this technology. Inverters play an extremely important role in household appliances, industrial machinery, and the rising popularity of household HVAC systems. Inverters consist of a wide variety of electronic circuitry, and there are many situations in which they need to be tested. This testing includes product performance testing, maintenance testing, and inspections in the event of a malfunction or failure. We will be focusing on testing inverter boards in this article. 

Let's start by talking about what an inverter is. We use inverters in a variety of settings where they’re needed to drive equipment ranging from electrical products to large industrial machinery. This is done by varying the speed of their motors. They play two principal roles: first is converting alternating current (AC) into direct current (DC). Second is using that direct current to create a simulated alternating current (AC) to vary the frequency or speed of a 3-phase motor.

Now that we know what an inverter is, we can dive into the components. Understanding components within inverter boards allows us to better understand where we should test these boards. 

Diode Bridge Rectifiers

The first part of the inverter process is taking the incoming power alternating current (AC voltage) and changing it to direct current (DC voltage). We do this using what is called a diode bridge rectifier. I know it sounds fancy, but it’s quite simple. Now let's take a walk down  “basic electronics lane” where we first learned what a diode is. 

A diode is basically an electrical check valve that only allows positive current to flow in one direction. Now, we take a series of these diodes in a specific order to create a configuration that creates a positive output regardless of the incoming current. This is how we take alternating current and create direct current. Remember this test is designed to ensure that the diodes are not stuck in the incorrect direction.

DC Smoothing Reactors

Now, let's talk about the second part of this process, which has to do with smoothing out this incoming current. DC smoothing reactors (also known as DC chokes) will reduce the charging current to the capacitor bank under a voltage surge condition. They help to protect the rectifier from current surges resulting from voltage surges/transients. Reactors also reduce the ripple quantity of current and improve the input power factor through the inverter board. This is displayed below in the diagram. The DC smoothing reactor is that giant, heavy device that looks like a transformer within the electrical cabinet. 

Think of the reactor as an inductor. In other words, the inductor offers high impedance to the ripples and no impedance to the desired DC components. Thus, the ripple components will be eliminated. When the rectifier output current increases above a certain value, it stores energy in the form of a magnetic field. It's this energy that is given up when the output current falls below the average value. Most if not all changes in current that occur in the circuit will be smoothed by placing the inductor in series between the rectifier and the load. This inductor will be connected to the inverter board through two connections, but some manufacturers use four.

We can test the reactor by removing the leads from the inverter board with the power off to the equipment and ohm out the reactor. With our multimeter set to ohms in auto range, not continuity. We should have 0.6 ohms between input and output connections on the reactor. A faulty reactor will read in the 500-ohm to 1-kohm range. A failing reactor will have an ohm reading that continually climbs until it reaches the 500-ohm range. A reactor in this range is considered faulty and should be replaced. 

Capacitors

Let’s move on to the next important step in the inverter process: the capacitors. Every inverter board has a specific number of capacitors built into the board to store direct current that has left the reactor. The purpose of these capacitors is to maintain a steady supply of DC voltage to the next part of the inverter process.

We can check this portion of the inverter board by typically checking for DC voltage between C+ and C- using a multimeter. Set the multimeter to read DC voltage in auto. When the unit has power with an active demand to run whether that is cooling or heating. You will find that the capacitors will be fully charged in preparation for operation. Based on the manufacturer of the unit you are working on, this voltage reading can be anywhere from 220 to 680v DC. I recommend that you read the applicable service manual of the equipment you’re working on. This will allow you to find what that value is supposed to be. 

What if the system is under or over voltage in this particular part of the inverter process? I recommended that you verify the incoming power for under/over voltage as well as phase imbalance. Remember that we need a steady amount of DC voltage created from our diode bridge rectifier in order to charge our capacitors. Most manufacturers recommend +/- 10% voltage differential between phases and 2% phase imbalance when equipment is operating.

Here is another example of what is taking place within this section of the inverter process. Remember that balance is key when supplying voltage to the IGBT section of the inverter board. 

Insulated-Gate Bipolar Transistors (IGBTs)

Now we have arrived at the final part of the inverter process, the IGBTs or insulated-gate bipolar transistors. You see, the inverter process of converting AC voltage to DC allows us to precise control. We do this by modulating the frequency and speed at which the motor runs at. This is done through a series of six IGBTs. P+ represents the positive DC voltage supplied to the IGBTs. Motor frequency is based upon how fast we switch the IGBT's on and off. Think of it as a light switch. The speed of this switching determines the simulated AC voltage that is created. Think of these IGBTs as fast-switching relays that can handle a high range of current. 

We test these IGBTs in two different ways. 

First Process

The first way is with the system’s power off, safety first. Once power is off we then remove the inverter compressor terminals from the inverter board. We check the IGBTs in a type of resting state to ensure that they are in the correct orientation and not stuck in the opposite direction. Stuck IGBT's would lead to issues of output-simulated AC waveform and cause high amperage as well as irregular operation.

When checking the IGBTs, we must place our multi-meter in diode check and follow the included diagram. We are checking the IGBT forward and reversed bias. This is to ensure that we get a reading in the correct orientation: red+ lead on P+ and black- lead on U, then V, then W reading OL. 

Next, we place our black- lead on N- and check the same connections: U, then V, then W reading OL. This is our way of showing they are closed in this orientation. 

Then, we place our red+ lead on the same N- terminal and check the compressor output U, then V, then W, reading 0.4 to 0.5. 

Once complete, we place our black- lead on the P+ terminal and our red+ lead on U, then V, then W, reading 0.4 to 0.5. If you have been following along, this is a 12-step check process ensuring that the IGBTs are not stuck in the incorrect orientation. 

Second Process

Now, the second process to check this is more of a manufacturer-specific test. Specifically with Daikin equipment you can place the machine into a “transistor check mode”. This is where the unit outputs the lowest possible voltage as if it was driving the motor. I would power down the equipment and remove the compressor leads. Remember this can only be done after the DC voltage from the capacitors have discharged. Capacitors located on the board need to drop below 34VDC to prevent electrical shock before touching any electrical wiring. Remember that these capacitors are fully charged at power off. This means they need time to discharge before we stick our hands inside the equipment. 

With compressor terminals removed and isolated to avoid touching the cabinet or grounding out. We then turn our power back on to the equipment. We place the system into transistor check mode Setting 2 – 28. It is at this step that we check using our multi-meter for a balanced output voltage between all three compressor wires. I created a detailed slide of information on how to perform this test listed below: 

There you have it, the inverter process laid out with what to check and where throughout the entire process. 

Conclusion

In conclusion, always reference the service manual of the equipment you are working on when checking these components. The numbers listed here are from Daikin inverter equipment and may be incorrect for the board that you are troubleshooting. However, the principles and components do not change. An inverter board will have each one of these components, regardless of who made it. With that said, there is another side to the inverter circuit that is not covered here, which is the communication circuit of the board that determines at what frequency to output and when. The length needed to cover the communication circuit of inverters and the associated boards will have to be saved for another tech tip. 

Last but not least, remember that in no part of this process should an inverter board measure to ground. This is true for any of the connecting terminals that we have checked in this article. There are several inverter board designs out there that have a designated grounding terminal. This terminal can be used for testing the internal components of the board to ground to find if there is earth leakage associated with a failure.

I hope you found the information presented here useful—and remember, when you understand the basic principles behind these boards, there is nothing you cannot overcome in the field.