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.The main difference is that a capacitor is connected in series with the start winding, as shown in Figure 11–11.Inductive loads cause the current to lag the applied voltage.
The stator winding of both motors contains both a start winding and a run winding.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. The contacts of the centrifugal switch are connected in series with the start winding, asshown in Figure 11–7.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.
Applications of capacitor start single phase induction motor
The stator is constructed in thismanner to produce a phase shift between the cur- rent flowing through the run winding and the current flowing through the start winding.Both the resistance-start and capacitor-start induction- run motors start rotation by producing a rotating magnetic field in the stator winding.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 field in the rotor.Capacitor-start induction-run motors are very similar to resistance-start induction-run motors.
The design of the stator winding is basically the same.
If the capacitance of the start capacitor is too great, it will cause the start winding current to shift more than 90 degrees out-of-phase with the run winding current and starting torque will be reduced.
When replacing the start capacitor for this typeof motor, the micro-farad rating recommended by the manufacturer should be followed.
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.