The first reaction of a servicemen when first viewing a motor burnout in a hermetic compressor is to assume that the motor or some component of the electrical system is at fault. Although it is sometimes difficult to prove and is usually not the case. The majority of motor burnouts that resulted from damage to the compressor and motor from system related causes, such as the areas previously discussed.
All attempts should be made to determine the mode of failure before assuming that the motor was bad. If the motor protection, circuit and control in functioning properly it is extremely difficult for a catastrophic failure to occur purely from electrical means.
The problems in this section are discussed to include mechanical and specific motor and electrical problems. Some of the common electrical-related compressor problems stem from the loss of power in one of the three phases (single phasing), low voltage, shorted motor windings, over- heating, dragging rotor and electrical control problems.
When a motor fails, it is strongly recommended that the windings be cleaned for inspection. Their appearance will generally lead to the apparent cause of the failure. Before proceeding, the stator windings of a three-phase, four pole motor will be examined.
The identification of the windings of each of the three phases is important when diagnosing a motor problem. Motors are available with 3, 6, 9 and 12 leads. However, regardless of the number of leads, the general appearance of the stator winding of these motors is the same.
(Figure 19)
This illustrates the stator coil arrangement of a three-phase, four-pole motor. For this purpose; the four coils, or poles, of each of the three phases are the same color for easy identification.
Note that the coils of the three phases appear in the color series yellow, which represents a phase silver, which represents a phase; and copper, which represents a phase and that this series is repeated at each 90 degree interval.
(Figure 20)
The phase windings of a typical motor are also easily identified because each winding is separated from the others by an insulating barrier.
(Figure 21)
A complete motor burnout is most apt to occur when the motor is i n the stalled position. At the moment the motor is energized. the electrical and physical stresses on the windings are the greatest. If at this time voltage is low or the compressor is mechanically locked up, the motor will burn unless the overloads are tripped within a very short time.
When a motor burns in the stalled position, the soot and other by-products of the burn are confined to the suction side of the system. This may help with your diagnosis.
With the absence of proper motor protection, another cause for burnout is inadequate motor cooling due to reduced flow or no flow of suction gas. Since this type of burn takes place when the compressor is operating, the by-products of the burn are frequently carried to the discharge side of the system.