If you know who (or what) “ELI the ICE Man” is, you’re probably a commercial electrician or electrical engineer. If that applies, you might even consider skipping this article! Otherwise, what you don’t know can hurt you, damage your test equipment and fool your diagnostics. AC (Alternating Current) is found inside alternators, coming out of sensors, and totally rules the worlds of HEVs, PHEVs, and BEVs. In spite of AC’s varied uses in today’s cars and trucks, most of us in the wrenching world are far more comfortable with our knowledge level of DC compared to AC.
What exactly is AC, and why should you care?
We’ve all been focusing on DC (Direct Current) for most of our automotive service careers, and rightfully so. DC is the flow of electrons in one direction. The direction of flow displayed in schematics (conventional flow theory of positive to negative) or the scientifically proven theory of electron flow (negative to positive) is mainly trivial for those of us in the automotive service world. AC (Alternating Current) is electricity that moves in both directions. The speed (frequency) at which it switches back and forth for household AC electrical power is 60 Hz in North America and 50 Hz in certain other parts of the world. Residential and light commercial AC voltage levels (Figure 1) are 120 and 240 (+/- 10 volts), depending on whether the circuit is used for lower wattage appliances (i.e., lighting, televisions, etc.) or higher wattage appliances (i.e., electric water heaters, clothes dryers, etc.). But wait . . . this is “Motor Age Magazine” and not “Electrical Contractor Magazine,” right? Right . . . but the automotive world is taking a VERY fast turn to vehicle electrification. Knowledge of 3-phase AC power inverters, AC induction motors and off board charging equipment for PHEVs (Plug-in Hybrid Electric Vehicles) and BEVs (Battery Electric Vehicles) needs to become yet another area of expertise for the professional automotive repair technician! Bluntly put . . . if you shy away from learning some of the same concepts your buddies in the electrical contracting world already know, you’re going to fall behind in your ever-changing automotive world!
Where will you encounter AC circuits on today’s vehicles?
They’ve been all over the place, and you may not have thought much about them. But with vehicle electrification, they’re growing by leaps and bounds! Here are just a few examples:
Examples of Traditional Automotive Low Voltage AC Circuits with Easier to Understand Concepts:
Sensors
- Electronic ignition distributor pick-up coils
- 2-wire reluctance-style (older) wheel speed sensors
- 2-wire CKP Sensors
- Vehicle Speed Sensors
- Knock Sensors
Audio
- Amplifier outputs to speakers
- Low level inputs from head units, microphones, and playback devices
Composite video
- Infotainment
- Video game connections
- Simple analog back up cameras.
Examples of Newer Automotive AC Circuits with High Voltage / More Challenging Concepts:
- Low voltage 3-Phase BLDC in tank modular fuel pumps
- Electric motor resolvers (complicated position sensors)
- 3-Phase High Voltage AC Motor Generators (MGs on HEVs, PHEVs and BEVs)
- EVSE (Electric Vehicle Supply Equipment)
- Level 1 Charging Cable (120-volt cable that comes with every PHEV and BEV)
- Level 2 Charging Cable (optional 240-volt cable that must be installed in customer’s garage by a qualified electrician)
- Level 3 (technically level 2 “DC Fast Charge”) charging station for 1-hour BEV charges
Testing AC Power Circuits? Use Caution!
Basic Rules for Test Equipment and AC Powered DUTs (Devices Under Test):
Rule # 1
ALWAYS use a differential probe and/or isolation transformer to "electrically separate" your equipment from what you're testing to prevent damage and/or injury!
Reason: Vehicle grounds, equipment grounds, earth grounds and ground loops can be confusing. Electrical isolation can be your best friend.
Rule # 2
ALWAYS treat circuits as if they have potentially hazardous levels of voltage until proven otherwise.
Reason: OSHA rules designate any voltage (AC or DC) over 50-volts as potentially injurious or lethal. I’ve seen Fluke Instruments documents that put the number at 30 volts for AC power and 60 volts for DC. Using a Live/Dead/Live test is a good way to apply this safety rule. LIVE: First use your equipment to measure something you know is live (to make sure your connections and meter / scope set up is good. DEAD: Next move to the component / circuit you’re wanting to make sure has been powered down. LIVE: Finally, move back to the circuit that you originally used in the 1st “LIVE” test to make sure you’re not having a “stupid” moment!
Rule # 3
ALWAYS Follow OEM Recommended Safety Practices.
Reason: Auto and truck manufacturers have teams of engineers making sure their dealership service technicians stay safe. Safety is always a priority with vehicle manufacturers. When in doubt – read the manual! If you are prompted by a service manual to test Live AC voltage, (or opt to go beyond the manufacturer’s published diagnostics and scope an inverter output – Figure 2) make sure you protect your equipment (Figure 3) and yourself (with Class 0 / 1,000-volt safety gloves). If you’re dealing with residential / commercial 120-volt and 240-volt AC power and are in doubt, consult a professional electrician!
Rule # 4
Ohm’s Law Still Applies to AC Circuits (Kind of!)
Reason: Subtle differences between DC and AC can change the math!
When DC travels through an inductor (i.e., a coil winding) there are some strange things (current inrush, voltage spikes, etc.) but they typically only occur for an instant. When AC travels through the same inductor, the frequency of the AC (moving back and forth within that coil) causes some additional resistance depending on the inductance (measured in Henries) and the frequency of the AC. (Figure 4) When DC travels in a circuit with a capacitor involved, it simply charges the capacitor and stops flowing. The capacitor acts like a diode to stop the current flow. When AC is involved in a similar “capacitive natured” circuit, the AC will charge and discharge the capacitor. AC current (for all practical purposes) flows through the capacitor if the frequency of the AC and the capacitance (measured in Farads) allow. That’s where we can get the crazy differences in a scope pattern illustrated in Figure 5. Ohm’s Law still applies (Figure 6) but with some variations thanks to inductive and capacitive reactance. The Math to total circuit Impedance (Z) is complicated, (Figure 7) compared to simple math with Ohm’s Law. The good news is, there is NO need to memorize ANY of this to diagnose vehicles. Just understand why AC behaves differently (compared to DC) depending on the nature of the circuit!
Testing AC power the RIGHT way!
Some concepts of AC power using a meter are simple – use a meter that says “True RMS” on it. RMS stands for “Root Mean Square”. It’s simply 0.707 multiplied by the peak AC voltage. (Figure 8) When looking at an AC power (wall socket) waveform on a scope, you’ll see a VERY tall waveform with peak-to-peak voltage. Peak voltage is the highest voltage measured from the 0-volts level on the AC waveform. A battery powered multimeter has no issues with plugging into line power because it’s isolated from the power source. A scope, however, may need extra adapters and procedures (Figure 9) in order to capture accurate waveforms w/o damage to the scope or you!
Tips on EVSE – Electric vehicle supply equipment
Regarding EVSE, you might ask “why not let the customer deal their local electrician on the matters of charging cables / stations?” Take a long pause and think about that question for a moment. Ask yourself “how many electricians do you know who are knowledgeable on PHEVs and BEVs?” I think we’ll all agree the answer to this question will be “few to none!” Similarly, “how many automotive techs know the difference between Level 1 EVSE charging cable NEMA 15 and NEMA 20 plugs and wall outlets?” (Figure 10) The same answer applies . . . few to none! This will be an area where cooperation between those who manufacture and install EVSE will need to work closely with you, the professional auto tech. Otherwise, PHEV and BEV high voltage battery pack charging issues will become a game of “finger pointing” between the electricians / charging station providers and the auto techs. Not all EVSE components are created the same. The PHEV / BEV Level 1 / 120-volt charging cable in Figure 11 is has a composite body and rubberized cord that is rated to survive vehicle runovers and operate in extreme cold temperatures. Some cheaper and / or fancier (Bluetooth Controlled) charging cables entering the aftermarket may not last as long or have the same protection compared to more robust (and expensive) charging equipment. I like to compare the lesser quality PHEV / BEV charging equipment to the brand X inexpensive power supplies for laptop computers. I’ve always avoided using them out of the reasonable concern they might damage my laptop. An electrical incident in your shop and / or expensive ‘thermal incident’ with your customer’s PHEV / BEV would be an even greater concern compared to a damaged laptop. For the AC Power savvy shop of tomorrow, continued education and well researched tool and equipment choices (Figure 12) will be the key to success diagnosing and repairing vehicles that interact with AC power!
