To troubleshoot non-functioning or poorly performing HVAC motors, there are several key steps to take regardless of the motor type, as well as some additional steps depending on the specific type of motor you’re dealing with.
General Motor Troubleshooting
To begin troubleshooting any electric motor, first verify that electrical power is reaching the motor at the correct voltage. Corroded contacts and loosened connections throughout the electrical network can prevent the motor from starting, making it look as if the motor is at fault. Find the motor’s control board or terminals and use a multimeter to verify the voltage.
When working around high voltages, take every necessary precaution to avoid touching live wires or contacts.
Once you’ve checked that the correct voltage is available to the motor and it’s still not starting, it’s time to shut the power off and examine the motor. Be sure to completely disconnect power in a way that prevents you or anyone else from accidentally turning it back on while the motor is being examined. Double-check after disconnecting to make sure there is no voltage. If the motor is hot, allow it to cool before handling it.
Rotate the motor shaft by hand to check if it is jammed. The load may be stuck, or the motor bearings faulty or worn, preventing the shaft from rotating. A loose or wobbling shaft is another indicator that the bearings are damaged, preventing the motor from operating correctly.
If the shaft rotates freely and smoothly, visually inspect any thermal protection devices and check that they are not tripped. If possible, use a multimeter to ensure that the resistance across them is zero, allowing the free passage of electricity to the motor windings.
Many HVAC motors use one or more capacitors to provide additional torque and improve the performance of the motor. Capacitors can fail in either open-circuit or short-circuit states, both of which prevent the motor from functioning correctly.
To begin, read the specifications on the capacitor and match them to the manufacturer’s recommendations, which are typically found on the nameplate of the motor. A capacitor that is too small may not provide enough starting torque to move the load, especially after wear and tear have set in and increased the amount of work that the motor must do to rotate the load.
Before handling and testing a capacitor, it is extremely important to fully discharge it, as capacitors can store an electrical charge for some time after they are disconnected from power. Typically, a 10-15K ohm power resistor held across the capacitor terminals for 5-10 seconds will do the job.
With the capacitor removed, visually inspect it for signs of damage and deterioration, including cracks, swelling and leakage of fluid. Any of these are indicators that the capacitor is faulty and will need to be replaced.
If the capacitor looks fine, turn your multimeter to the capacitance setting and hold the probes against the capacitor’s terminals for 10 – 15 seconds, or until the reading has stabilized. The resulting reading should match the rated value on the capacitor shell, within the specified tolerance. If the reading is low, or the multimeter reads either open-circuit or short-circuit, the capacitor is defective and will need to be replaced.
Troubleshooting Motor Windings
Once you have checked both the power supply and capacitors, it is time to check the condition of the motor windings by testing their resistances. If a motor winding has deteriorated, it can fail in either the open-circuit or short-circuit state, both of which prevent the motor from running.
Many HVAC motors come with multiple preset speeds, each of which corresponds to a wire coming from the motor that is attached to a specific point on one of the motor’s windings. Test the resistance between each speed wire and the common neutral wire. The resistance of a winding depends on the size and power of the motor, so check the manufacturer guidelines for the correct value. If the multimeter reads OL (Open Loop), the windings have burnt apart, and a replacement motor will have to be sourced. If the reading is 0.00 (short circuit), the winding insulation has likely melted, fusing the windings, and again the motor must be replaced.
Finally, if the windings seem fine, make sure that each of the speed wires is not grounded to the motor frame. The multimeter should read OL with one probe on the speed wire and the other on the motor frame.
Troubleshooting ECM Motors
An ECM (Electronically Commutated Motor) uses a rectifier to convert the AC input power to DC, as well as a microprocessor to manage the commutation. These are typically found in a module inside the back end of the motor. When ECM motors fail, it is common for this electronics module to be the culprit, so it should be tested before looking at the motor windings.
First, leave the module to sit disconnected from power for a couple of minutes to allow any electrolytic capacitors to discharge. During this time, visually inspect the module and all the components on it for obvious signs of damage. Blackened, cracked or swollen components, often accompanied by a burnt smell, indicates the motor has failed and will need to be replaced. In this case, where a component such as a thermistor may have been damaged by an electrical event such as a lightning strike, a new one can often be soldered in to complete a repair.
ECM motors have a 24V DC common source wire that supplies power to each of the speed wires attached to the windings. Check that the correct voltage is present at this source before inspecting the rest of the motor. If it checks out, connect each speed wire in turn to the 24V common and power up the motor to see if it will run. If it does not run, check the motor windings as detailed in the previous section.
There are multiple indicators that an ECM motor module is bad. If the motor starts but ramps up and down in speed, or if the motor continues running after the speed wire is disconnected from power, the module has likely deteriorated and will have to be replaced.
By following the steps provided in this article, you should be able to quickly troubleshoot an HVAC motor and diagnose the most common reasons for motor failure.
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