Operation and troubleshooting of electric motors - Part II.
March 1, 2015 by Dave Demma
The typical HVAC/R system will utilize motors for three distinct functions: condenser fan motor, evaporator fan motor and the compressor motor. Since it is an electro-mechanical device with moving parts, a motor will have a finite life. At some point, accumulated wear will cause the device to fail. Yet, for a variety of reasons there are many motors that fail long before the point that accumulated wear-induced failure will occur (see sidebar).
When a motor fails, it is imperative to determine the cause of failure, or you can expect that a repeat failure is likely to happen. So, let’s start with a few basics.
Motors that experience premature bearing failure can be the result of a lack of maintenance. They can also be the result of too much maintenance. On motors with sleeve type bearings, or larger motors with ball bearings, care must be taken to not over lubricate the bearings. While it might seem contrary to reason, over lubricating a sleeve bearing will cause the oil to bypass the bearing finger and dissipate into the motor, resulting in inadequate lubrication and premature bearing failure.
In larger motors with ball bearings, over-greasing results in the balls sliding along the race rather than turning. The over abundance of grease will start to churn and will result in the base oil contained in the grease bleeding out, leaving only the grease’s thickener behind. The bearings will suffer a premature failure because the thickener has a limited ability to lubricate. An additional problem with over greasing the bearings is that the excessive grease will work its way into the motor’s stator/rotor assembly, ultimately distributing the grease into the windings. The grease acts as an insulator, preventing the motor windings from properly cooling, which then results in overheating. It is always best to follow the motor manufacturer’s recommendations regarding the type of bearing lubricant to use and the frequency of bearing lubrication.
Motor electrical failures are often the result of motor winding overheating. In three phase motors voltage/current imbalance is one of the more common causes of overheating. Voltage imbalance is typically the cause for current imbalance and a mere two per cent imbalance will cause a current imbalance in the 15 per cent range (see Table 1). The resulting overheating from the current imbalance will lead to a short motor winding life.
If the voltage supply has no imbalance it is still possible to have a current imbalance. If any of the three motor phases has a greater resistance than the others, it will cause a current imbalance. This might be caused by a loose wire connection, a severely pitted contact in the motor starter, a loose wire nut, and so on. If the motor starter contacts are pitted, it is time to replace them. Cleaning will not provide a long-lasting solution to this situation. A discoloured wire on either the line/load connections of the breaker or motor starter is a good indication of a loose lug connection, with the resulting current imbalance. Current imbalance will cause motor winding overheating, leading to a reduced motor life.
A single phase condition is an extension of the current imbalance described above, but instead of merely increased resistance in one of the legs, the leg is open at some point. This means that one leg of the three phase power supply is lost. This may be due to a failed contactor leg. If this occurs when the motor is operating, the two active phases will attempt to pick up the load that the lost phase was carrying. The current draw for these two phases will increase to approximately 150 per cent of normal. In a full load condition this will likely cause the motor’s over current protection to trip. An idle motor will not start if a single phase condition exists, with the over amperage in the two active phases resulting in an over current protection trip.
If a system has experienced a refrigerant leak and the compressor short cycles in its low pressure control as a result, this can lead to motor overheating as well. It takes several minutes of compressor runtime to dissipate the heat generated by the elevated current draw necessary for the motor to start.
If the motor starts, runs for a brief moment and then cycles off because of the low system charge, the motor will never have a chance to cool off.
Single-phase motors present more of a challenge, as their circuits include relays and capacitors. As such, it is necessary for the technician working on single-phase motors to have a good understanding of capacitors.
Run Capacitor The run capacitor is used to improve motor efficiency. It is placed in series with the motor start winding and remains in the circuit all of the time. Current flowing through the capacitor results in a phase shift of the motor’s current, resulting in an improved power factor. A capacitor will generate a fair amount of heat when it is in continuous operation. Run capacitors are oil filled, which aids in their ability to dissipate heat.
A defective run capacitor will result in higher amp draw, causing motor winding overheating and possible overload trips. When considering the replacement for the defective capacitor, the manufacturer’s microfarad and operating voltage rating should be followed. If the exact replacement is not available, a microfarad rating of +/- 10 per cent would be acceptable. A higher voltage rating (440V versus 370V) can be safely used, but a lower voltage rating should never be used.
Run capacitors normally fail open circuited. This can be verified with an ohm meter, with the resistance reading showing infinite resistance. Occasionally, the capacitor will short and this can be verified with a zero-resistance reading.
Start Capacitor Designed for intermittent duty only, start capacitors typically have a high microfarad rating. A bleed resistor is normally fixed between the two terminals on the start capacitor, allowing the capacitor’s charge to dissipate during the off cycle.
A voltage or current relay is used to take the start capacitor out of the circuit when the motor reaches its rated RPM.
To identify which terminal is which on a single-phase compressor, use an ohm meter to read the resistance between the various terminals. The resistance between the start and run terminals will be the highest, the resistance between the common and start terminals will be the second highest and the resistance between the common and run terminals is the lowest.
Certainly, proper operation of the motors in the HVAC/R system is required for the equipment to deliver its rated capacity. Ensuring that motors are lubricated properly and that any undue resistance (loose terminals, pitted starter contacts, and so on) is addressed will go a long way to achieving that proper operation. <>
• Approximately 30 per cent of all compressors pulled from the field, returned to manufacturers, and torn down for failure analyses do not have observable defects. (Copeland – Emerson Electric)
• 60 to 70 per cent of returned failed compressors are the result of system/service related issues, or the result of misdiagnosis. (Carrier Corporation)
• 80 per cent of compressors returned for electrical motor failure were system caused mechanical failures that progressed into an electrical failure. (Carrier Corporation)
• 95 per cent of alleged warranty failures turned out to be caused by external influences from the refrigeration system itself – that is to say that the compressor was not at fault. (Bitzer Corporation)
• .25 per cent (that is ¼ of one per cent, or one out of every 400) of total UK compressor sales resulted in actual valid warranty claims. (Bitzer Corporation)
Elevated motor amperage readings are typically not the result of a faulty motor. It is more likely the result of a mechanical failure in progress or abnormally high system pressures. Bearings that are worn, leaking discharge valves (causing one or more cylinders to be operating at substantially higher pressures in the downstroke) are two examples of a mechanical failure. Abnormally high system pressures may be caused by a dirty condenser, inoperative condenser fan motor, high load condition or high ambient temperature. The resulting increase in motor amperage from any of these conditions will lead to motor overheating, breakdown of the motor winding insulation and will ultimately result in a failed motor. But these easily fall into the “80 per cent of compressors returned for electrical motor failure were system caused mechanical failures that progressed into an electrical failure.”
An interesting aspect of being a HVAC/R technician is that the cause of failure is not always easily perceived. Technician must search for all possible causes, sniffing out evidence and clues, and then make a logical and educated deduction as to the cause of failure. Many compressor motor failures require this approach. Neglecting to fully investigate may result in the root cause of failure being missed and the customer suffering another expensive loss in a relatively short amount of time.