Troubleshooting Hermetic Motor Failures

August 24, 2000
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The purpose of this article is to illustrate the cause and magnitude of the various types of motor failures and its effect on the entire system.

Expressions such as, “They don’t build them like they used to,” and “It was a normal failure” do not apply to today’s hermetic motors. The bearings, insulation, and overall efficiency are far superior to those built three decades ago.

Unlike most electrical devices, motors do not self-destruct. Hermetic motors running within their design limits can outlast all the other components in the system. But when you do have a hermetic motor failure, you’d wish the compressor was belt-driven.

The extent of the cleanup procedure after a motor breakdown is usually a judgment call. It would be a waste of time and effort to clean a system that wasn’t contaminated, and it would be unwise not to do a thorough cleaning job on a system that was fouled up with the residue from burned insulation.

Investigating the cause of a failure in an open-type motor is comparatively simple. You remove the end bell, and the entire winding can be examined. Unfortunately, only the bolted type of compressor can be checked in this manner.

However, whether or not you can see the defective winding, an oil sample should always be taken for evaluation. Compressor oil will absorb and retain the faintest scent of burned insulation.

There are three classifications of motor failures:

1. Factory defects;

2. Start winding failures; and

3. Run winding burnout.

Factory Defects

When a motor leaves the factory with defective winding insulation, the breakdown of the motor can occur suddenly and without warning. This is unlike most motor failures, which are preceded by “brown outs,” hard starting, or overloaded conditions that trip the circuit breakers.

A typical result of defective insulation would be a short circuit between a few turns in one of the stator fields. This type of breakdown may increase in either one of two directions:

1. If the heat of the short circuit destroys the insulation to ground, a sudden high surge of current will “blow” the overload protector and possibly the circuit breaker as well, before either had an opportunity to shut off the current.

A technician who comes upon this type of failure with a blown overload and a grounded motor, usually prepares for the worst type of burnout. The tech would be very surprised, after taking an oil sample from the compressor, to discover that the oil is clear, just a trace of that burned odor and no indication of acid.

You do not have to look at the winding to conclude that this type of motor failure (a grounded motor and clean oil) was caused by defective wire insulation. This class of breakdown doesn’t require any cleanup in the system.

2. The second path that the short-circuited field wires can take would be where the short doesn’t spread too rapidly to the adjacent wires because they are on the periphery of the field, and in enough open area where the refrigerant vapors slow down the conductive heat.

However, when there are enough shorted wires to initiate a bucking field, the reduced speed will increase the amperage. Now the entire field (which is only one section of the whole winding) starts to overheat, causing more turns to short to each other.

The eventual result will be a stalled motor when the loss of motor torque can no longer deal with the load. We now have a locked-rotor situation that can be easily handled by the overload protector.

If you could examine this motor, you would find that only one section of the field is burned. The extent of damage to the rest of the system will be reflected in the color, odor, and acid content in the oil.

As with all motors that cannot start, if the overload protector fails to do its job, the entire motor will burn out and the reason for the initial breakdown will never be known.

The remaining types of motor failures are caused by adverse conditions that are initiated remote from the motor.

Start Winding Failures

This type of motor breakdown doesn’t occur as often as a run winding burnout. Although the start winding draws less current than the running winding in locked-rotor condition, the temperature rise is almost three times as fast as the run winding.

A good question would be, “If the start winding heats up so rapidly, why do you have a higher percentage of run winding burnout?”

The answer is, when both windings are in a locked-rotor situation, the combined amperage of both windings activates the overload faster than if only the run winding was in the circuit. And since the start winding heats up at a faster rate, the overload is designed to cut out before the temperature of the start winding reaches a breakdown level. The start winding is never in the circuit by itself.

The conditions that are most damaging to the start winding occur when it remains in the circuit while the motor is running. This situation develops when:

  • The contacts in the starting relay are welded closed.

  • A low-voltage condition prevents the potential relay from reaching its pick-up level, leaving the contacts in a closed position. In both circumstances the start winding can be saved if the start capacitor overheats and “blows,” opening the starting circuit.

  • The run capacitor is shorted.

These situations can destroy the start winding in the fastest time if a short circuit develops while the unit is running. You would get the same results if you placed a wire across the start and run terminals. The running motor is drawing less current than if it was in locked rotor, but the start winding is drawing a high amperage and heating up fast. Only a very sensitive motor protector would be able to save this type of start winding burnout.

If the running capacitor has an internal fuse that opens because of the short, you have a few different problems, but they are not related to start winding hazards.

A PSC (permanent split capacitor) unit will not be able to start because the open circuit in the run capacitor prevents any current from going into the start winding.

A capacitor-start, capacitor-run unit will be able to start, but will run at a very low efficiency level if the run capacitor is not in the circuit. Even at low-level loading the motor will overheat and cause an insulation failure if the overload does not react in time.

Run Winding Burnout

The most intense type of burnout is “the running burnout.”

This is the scenario: It’s a hot day and the air conditioning unit is running continuously. The condenser is not efficient. The motor is running at 15% above its full-load ampere rating. The overload is on vacation.

After operating for about 5 hrs, if we could look inside the compressor, we would see a wisp of smoke coming from the running winding. During the 6th hr, the smoke is getting denser and it doesn’t stay in the crankcase. As fast as the smoke is being generated, it is rapidly pumped into the condenser.

We now have an ongoing chemical laboratory.

As a myriad of potent gases are produced, they are being blended with a variety of acids that are also being manufactured. The condensing unit, which has been transformed into a chemical plant, is now distributing this muck throughout the entire system.

In about 7 hrs, a short circuit develops that finally energizes the circuit breaker and shuts down the system.

But this isn’t the end of the mess. The combination of all the gases, acids, refrigerant, and oil under pressure has converted the crankcase into quite an efficient shellac factory. This type of burnout can simplify the clean-up procedure, because in most cases there aren’t any cleanup procedures.

To begin with, depending upon the quantity of refrigerant in the system, there may not be enough that can be saved. If you look inside the inlet tube to the condenser, you will find that the wall is coated with a dark, sticky substance. You will probably find the same condition at the outlet tube of the condenser.

Prudent business practice dictates replacing the entire high side.

Any symptom of acid in the oil that is in the evaporator, should stimulate a desire to replace it; especially if it has a header with multipass tubes. Why jeopardize the new compressor? Of course, the installation tubing has got to go if it has the same coating on the inside.

Before you think about a cleanup procedure for this type of burnout, take into consideration that most refrigerants have cleansing characteristics. What-ever you use to wash out the system, the new refrigerant will continue the process, and in time may contaminate itself by the residue that could not be removed by the initial cleanup.

Since the Montreal Protocol, the method to clean a contaminated system has been drastically changed. The best guidelines can be obtained from manufacturers of refrigerating and air conditioning equipment.

Ehrens is a long-time refrigeration expert at Sealed Unit Parts Co. Inc., 2230 Landmark Place, Allenwood, NJ 08720; 732-223-6644.

Publication date: 09/28/2000

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