Measuring resistance in HVAC equipment
Measuring the electrical resistance (ohms) of motors, compressors, and other electrical devices in HVAC systems may seem straightforward. But when the measurements of zero (0) or “OL” (overload) come up in discussions, it becomes clear that there is quite a bit of confusion around this topic.
The answer to this one question holds the key to understanding resistance measurements.
What does electrical resistance resist? Almost everything has the property of electrical resistance. This includes electrical devices such as motors, switches, wires, etc., and even the human body. This property of resistance is measured in units called “ohms, named after German physicist Georg Simon Ohm. It can be measured with a standard digital multimeter.
Electrical resistance resists the flow of electric current. Things that have a very high amount of electrical resistance — aka a large number of ohms — resist the flow of current almost completely. These are known as insulators. Substances like rubber, plastic, glass, ceramic and air are good insulators because they have a very high property of electrical resistance.
Things that have a very low amount of electrical resistance (small number of ohms) such as gold, silver, copper, nickel, and even steel provide very little resistance to the flow of electrical current. These are known as conductors and they are used to build electric current carrying devices like wires.
Typical wire you are familiar with is actually a conductor (copper) encased in an insulator (plastic or rubber). This allows current to flow where it is wanted and prevents current from flowing where it is not wanted.
Somewhere in between an insulator (very high ohms) and a conductor (very low ohms) is an electric load. Motors, solenoids, contactor coils, lightbulbs and heater elements are examples of electric loads. Electric loads are constructed with a carefully calibrated amount of electrical resistance. When the proper amount of voltage is applied to this specific resistance, a certain amount of work will be performed and a certain amount of electric current will flow.
When a motor experiences damage or malfunction, frequently the motor’s electrical resistance will deviate from what its normal value is. This is where confusion often sets in.
“Ohming out a motor” is the process of measuring the electrical resistance of the motor windings and comparing that resistance to normal values. One problem with this is that “normal values” will vary widely depending on what type of motor you are looking at. You may not have access to what the normal values are supposed to be for your particular motor.
Two measurements that often come up are zero and OL (overload). Each of these measurements is very different from one another and they mean entirely different things.
A measurement of zero or very close to zero (less than 0.5 ohms) indicates a very low resistance to current flow. Applying voltage to this low level of resistance will result in extremely high current flow. In fact, the power supply will happily provide all the current it possibly can when there is very little to resist it. The result will usually be a blown fuse or breaker, melted wires or something similar.
Note that very high power motors that normally draw high levels of current will naturally have windings with very low resistance.
While you may not know exactly what the resistance of your particular motor should be, know that it should be more than zero. A resistance measurement of less than 0.5 ohms usually indicates a short circuit in the motor winding. A short circuit is literally an alternative path for current to flow that “shortcuts” the usual path and avoids all the normal resistance.
A measurement of OL is something else altogether. Most technicians use an auto ranging meter. These meters automatically adjust themselves to the correct scale range depending on what they are measuring. When your meter is adjusted to measure ohms or the symbol Ω, and you don’t have your test leads connected to anything, your meter automatically adjusts itself to its highest scale of measurement and reads “OL.”
It is easy to think that you aren’t measuring anything at this point, and you are not measuring any ohms. In truth, you are. Remember that air is a very good insulator. You are now measuring the resistance of the air between the tips of your two test leads and the electrical resistance of that air is very high.
It is so high that it is more resistance than your meter is capable of measuring. It is literally off the scale. Your meter is experiencing so much electrical resistance that is literally overloaded. That means that the meter is experiencing more ohms than it can count.
The other thing you know about your motor is that under normal circumstances, it does flow some current, so its windings must have a normal amount of resistance that can actually be counted by your meter. A measurement of OL when ohming out a motor can indicate that one of the fine wires that create the winding is broken. No current can flow under this condition and the motor cannot run. Motor replacement is then required.
Another possibility is that the internal overload protector in the motor is open. If the motor is hot, the overload protector should close (automatically reset) when the motor cools with time. If you find this condition when the motor is cool, the overload is stuck in the open position (broken) and motor replacement is required.
When measuring between any motor terminal to ground, or to the case of the motor, or to a refrigerant pipe of a compressor, you always want to see “OL.” This means that there is no path for current to flow to ground.
If you see zero or anything other than OL, there is some path for current to flow to ground. That motor is experiencing a short to ground and must be replaced.
Many standard high-quality digital multimeters measure resistance up to about 50 megohms. (Meg is short for million.) To measure higher levels of resistance accurately, you will need a megohm meter. Megohm meters (aka a “megger”) are essentially ohm meters that have a much higher scale of measurement. They are used to test the integrity of insulation. No insulator is perfect, and as insulation begins to break down, it can start literally leaking small amounts of current to ground. This is a useful test in very large horsepower electric motors and compressors.
There is a lot more to discover about measuring resistance, but for now, this should clear up any confusion between zero and OL.