By Craig Van Batenburg
Toyota first used the word “inverter” in 2000 to describe a silver box located above the Prius transmission. The Honda Insight’s “inverter” was under the hatch floor and not visible. Honda called it the “motor drive module” or MDM. That was a much more descriptive name, but it never caught on.
The inverter acts as a three-phase motor controller. It is located between the capacitors and the three-phase brushless motor. It looks like a circuit board with big switches attached. You will also notice a high-voltage DC connection of metal bars (never cables) from the capacitors to the inverter board. That connection is there so that the switches always have enough current to operate at high speeds. You will always see three outputs from the inverter, often with two or three current sensing devices that monitor the three-phase output. Usually there are three (or more) orange cables attached to the case that the inverter is located in. Sometimes the inverter is located in the unit with the high-voltage drive motor, such as in the 2012-15 Ford Escape.
Each three-phase brushless motor needs its own inverter, so it makes sense to refer to an inverter as a motor controller. The inverter is relatively simple. It uses its own processor that has instant feedback from various sensors to switch six high-powered transistors on and off. When the hybrid or electric vehicle needs that motor to power something, like the wheels or maybe the A/C compressor, the inverter is commanded to make that happen.
The inverter has two jobs. One, of course, is to power a three-phase motor. The other job, if the motor is spinning during the slowing down of the vehicle or if the internal combustion engine powers the HV motor, is to make DC out of three-phase AC. In those cases, the motor also acts as a generator, and the inverter must also convert three-phase AC to a useful DC. That DC can be used to recharge the HV battery or power something else. The inverter powers a motor using DC voltage but AC current.
The construction of an inverter
Inverters are housed is a box often with other components: capacitors, boost converter, DC-DC converter, a cooling system, HV cables, 12-volt connections, current sensors, temperature sensors, computer circuit boards, snubbers, etc. On the outside may be a crash sensor that will shut down the HV system but not set off the airbags.
The capacitors supply DC to three types of transistors, which for simplification I will call insulated gate bipolar transistors (IGBT). The IGBTs are controlled by a computer, usually located outside the inverter because inverters get hot, and engine control units do not like heat. Typically there will one or more low-voltage connector with multiple pins attached to the inverter box. The end result of the inverter is to switch high amps and volts rapidly — and in the correct order — to produce AC power from DC.
How an “inverter” makes AC out of DC
Inside an inverter is a circuit board along with six diodes and six IGBTs. As stated before, every electric HV motor that runs on AC power requires its own inverter. The inverter is the controller for the AC electric motor. The “control” means that the power (measured in Kw) is delivered as needed (based on load), the speed of the rotating part (rotor), rotation direction (forward or reverse) is all done within the inverter. In order for the inverter to know what to do, it needs inputs. The most important input is the “resolver,” a sensor.
The high-voltage DC power in the HV battery is constantly charging the capacitors, and in turn, the capacitors power the IGBTs. The system is not unlike the six diodes in a conventional alternator, but with a by-pass around the diodes.
The HV motor is being controlled with a “duty cycled” DC that also is reversing the positive and negative leads from the DC supply (capacitors), but the current is a perfect AC wave form. So the motor is a “DC motor,” running on AC current; it is both a DC and AC motor. During motor application, the IGBT are switched on and off rapidly and in the process make a square wave form of DC voltage that is “duty cycled.”
Think of it this way: Picture a scope connected to a 200-volt battery (do not try this live). You have set the zero line at the middle of the scope with 250 volts above and below the center line. The time base is in 0.5 second intervals. Once you have the scope cables connected to the HV battery in the correct fashion, you will see a flat line at about 200 volts above the center zero line.
Now disconnect and reconnect the cables in reversedorder, positive lead to the negative battery cable and “vice versa.” What do you see on the scope? 200 negative volts, below the line. That’s what is happening hundreds of times per second. Remember, we are only looking at voltage. The reversing of the polarity is changing what is known on a scope as “frequency.” The speed of the “rotor” in the motor is controlled by the frequency. What you are also doing is creating what looks like a high frequency AC source.
Power can be measured in watts: 746 watts equals 1 horsepower. To calculate watts, Ohms law tells us to multiply voltage and current. More voltage will increase “watts” and more “amps” will also increase watts. When you “up the voltage,” the cables do not need to be larger and additional heat is not a factor. When you increase “amps” the cables will need to be bigger along with every other component that the current must travel through. So knowing this it may explain why newer vehicles use a high-voltage system.
Getting back to the inverter, power output to the motor must also be controlled. That is done with “duty cycle.” After the frequency (per phase) has been created by closing the proper two IGBTs, one of the IGBT can be duty cycled. This will not change the voltage level per duty cycle but will interrupt the current flow. So if you change polarity and keep the IGBTs closed 100 percent of the time, the maximum amount of current will flow to the motor windings. But if you use a 50 percent duty cycle during that same event, the current will be half. So simply stated, frequency controls speed and duty cycle controls power.
As stated before, the inverter is supplied power from the HV capacitors, not directly from the high-voltage battery. A bank of capacitors are constantly being charged (or refilled) by the HV battery pack. Both the battery and capacitors are at the same voltage level all the time when in “READY to DRIVE” mode. Once the HEV or EV is turned off, the capacitors discharge their stored energy.
In the case of early Honda IMA systems, the DC voltage in the three capacitors were hard wired to the DC-DC converter, and once the contactor opens (Honda only used one contactor years ago), the capacitors discharge to a safe level in about 45 seconds. A safe level according to Honda and makers of DVOMs is 30 volts DC. You will notice that a “lightning bolt” on your meter will go off when the voltage is under 30 volts DC. Remember, most OEMs will never ask you to test a known live circuit if the voltage is over 30 volts. Make sure you test the voltage on the “capacitor bank” because they store enough voltage and current to injure or kill you. You will be looking for “zero volts,” but in reality you will see maybe one volt or milli-volts as the capacitors are still releasing their last bit of stored energy. •
Craig Van Batenburg is a former repair shop owner who is the CEO of Automotive Career Development Center (www.fixhybrid.com), which offers training and consulting related to electric and hybrid vehicles; he can be reached at Craig@fixhybrid.com.
