The rotating magnetic field of the resistance-start induction-run motor is produced by the out-of-phase currents in the run and start windings. Since the run winding appears more inductive and less resistive than the start winding, the current flow in the run winding will be close to 90 degrees out-of-phase with the applied voltage. The start winding appears more resistive and less inductive than the run winding, causing the start winding’s current to be less out-of-phase with the applied voltage, as shown in Figure 11–5. The phase-angle difference between current in the run winding and current in the start winding of a resistance-start induction-run motor is generally 35 to 40 degrees. This is enough phase angle difference to produce a weak rotating field, and consequently a weak torque, to start the motor. Once the motor reaches about 75% of its rated speed, the start winding is disconnected from the circuit and the motor continues to operate on the run winding. In nonhermetically sealed motors, the start winding is generally disconnected with a centrifugal switch. A centrifugal switch is shown in Figure 11–6. The contacts of the centrifugal switch are connected in series with the start winding, as shown in Figure 11–7. When the motor is at rest or not running, the contacts of the centrifugal switch are closed and provide a circuit to the start winding. When the motor is started and reaches about 75% of its rated speed, a counterweight on the centrifugal switch moves outward because of centrifugal force, causing the contacts to open and disconnect the start winding from power. The motor continues to operate on the run winding.
When the start winding is disconnected from the circuit, a rotating magnetic fi eld is no longer produced in the stator. This type of motor continues to operate because of current inducted in the squirrel cage windings in the rotor. Squirrel cage rotors are so named because they contain bars inside the rotor that would resemble a squirrel cage if the laminations were removed, as shown in Figure 11–8.
A squirrel cage is a device that is often placed inside the cage of small pets such as hamsters to permit them to exercise by running inside the squirrel cage. A squirrel cage rotor that has been cut in half clearly shows the bars and motor shaft, as shown in Figure 11–9. The bars of the turning squirrel cage rotor winding cut through lines of magnetic flux, causing an induced voltage in the rotor. Since the rotor bars are shorted together at each end, current flow through the rotor bars produces a magnetic fi eld in the rotor. Alternate magnetic fields are produced in the rotor, causing the motor to continue operating, as shown in Figure 11–10. This is the same principle that permits a three-phase motor to continue operating if one phase is lost and the motor is connected to single-phase power. The main difference is that the split-phase motor is designed to operate in this condition and the three phase motor is not. Resistance-start and capacitor start induction-run motors are rugged and will provide years of service with little maintenance. Their operating characteristics, however, are not as desirable as those of other types of single-phase motors. Due to the way they operate, they have a low power factor. They will draw almost as much current when the motor is running at no load as they will when the motor is running at full load. Typically, if the motor has a full-load current draw of 8 amperes, the no-load current may be 6.5 to 7 amperes.