With the ohmmeter set on its highest scale, test for continuity between the terminals and the shell. This procedure is shown in figure 14-33. If continuity exists between the terminals and the shell, there is a ground.
The procedure for conducting a test lamp continuity check is as follows:
Disconnect the power and remove the wires from the motor terminals.
Ensure the lamp is connected in the hot side of the line. Plug the test lamp into a receptacle.
Connect the hot-line probe to a motor winding terminal.
Touch the free probe to the cleaned spot on the shell. Ensure that a good connection is made. If the light illuminates, there is a grounded winding.
Figure 14-34. - A typical megohmmeter (megger). Two leads are supplied, one of which is marked Earth or Ground.
The procedure for making the megohmmeter (high-resistance) test is as follows:
Disconnect power and remove the wires from the motor terminals.
Place the megger probe marked Earth or Ground on the motor or compressor frame. Ensure there is a good metal-to-metal contact.
Place the free probe on terminals C, S, and R in sequence. If any reading of low resistance is obtained, the motor is grounded.
You should always refer to the manufacturer's instructions when using a megger. It is also a good idea to request assistance from a Construction Electrician.
Electrical components in hermetic motor compressor circuits that give trouble include starting relays, overload protectors, and capacitors. It is essential that a refrigeration and airconditioning service member be able to identify these components and test them using the proper equipment and procedures.
Basically there are three types of starting relays in use. They are the current relay (magnetic type), the voltage relay (magnetic type), and the thermal relay (hot-wire type). In the hermetic motor control circuit, a starting relay allows electricity to flow through the starting winding until the motor reaches two-thirds to three-fourths of its rated speed. At this time, about 3 to 4 seconds after starting, it disconnects the starting circuit.
A current relay is an electromagnet, similar to a solenoid valve, that employs a weight and spring to hold the contacts open when the circuit is idle. In operation the instantaneous surge of starting current actuates the magnetic coil, causing the start winding contacts to close. This closure allows starting current to the winding;Continue Reading