So far we have talked only about the kind of hermetic motors installed in residential units, the split-phase type. These are called hermetic because they are mounted inside an airtight container with the compressor. Basically, there are four types of hermetic motors in commercial use today:
1. Split-phase hermetic motor (as it has separate run and start windings) used mainly in residential units with limited use in small commercial units.
2. Capacitor-start, induction-run motor.
3. Capacitor-start, capacitor-run motor.
4.Permanent – split, capacitor-run motor.
1. Split-phase hermetic is the simplest kind, used mostly for household refrigerators since the compressor motors do not require a lot of starting torque. In these units, when the thermostat shuts the system down, the high- and low-side pressures equalize through the capillary tube. In heavy commercial units, the pressures do not equalize because they use TEV (thermostatic electric valve) or automatic valves (covered later) as well as capillary tubes. These valves isolate the high and low sides, and the pressures in the system do not equalize very easily in the off cycle. In these motors, a relay controls the engagement of the start windings. The starting relay used may be of the thermal type, the current type or the potential type, which will be covered later.
2.Capacitor-start compressor motor is a popular type of hermetic motor in refrigeration units. A capacitor is installed in series with the motor start winding 3.4.to produce more starting torque by providing more initial voltage. This capacitor is isolated from the circuit during the run cycle (see fig. 43). This type of compressor is normally used in walk-in coolers, salad bars, beverage coolers, ice machines, and similar commercial refrigeration units.
3. Capacitor-start, capacitor-run compressor motor is a very efficient type of motor. A start capacitor increases its starting torque, and a run capacitor increases its efficiency during the run cycle. It is used in commercial refrigeration for heavier applications such as larger walk-in coolers, heavier air conditioners, etc. Figure 44 illustrates how they are wired.
4. Permanent-split, capacitor-run compressor motor. This type of hermetic compressor motor is widely used in wall- or window-type air conditioners. It is not equipped with a start capacitor. Its starting torque is (almost) low. Consequently, it is sensitive to the fluctuation of the line voltage. Supply voltage should not exceed ±10% of the required voltage printed on the compressor nameplate. A run capacitor is installed between its start and run windings to provide more efficiency during the run cycle. Figure 45 shows a typical wiring diagram of these motor compressors.
