Difference between resistence and capacitance start induction motors


Resistance-start induction-run and capacitor-start induction-run motors are very similar in construction. The stator winding of both motors contains both a start winding and a run winding. The start winding is made of smaller wire and placed higher in the metal core material than the run winding.

induction motor                                            Related image

Since the start winding is made with smaller wire than the run winding, it will exhibit a higher resistance than the run winding. Placing the run winding deeper in the metal core material causes it to exhibit a greater amount of inductance that the start winding. Electrically, the winding appears similar to the circuit

Resistance-Start Induction-Run Motor

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.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.

difference between starting and running winding resistance

Capacitor-Start Induction-Run Motors

Capacitor-start induction-run motors are very similar to resistance-start induction-run motors. The design of the stator winding is basically the same. The main difference is that a capacitor is connected in series with the start winding. Inductive loads cause the current to lag the applied voltage. Capacitors, however, cause the current to lead the applied voltage. If the starting capacitor is sized correctly, the start winding current will lead the applied voltage by an amount that will result in a 90-degree phase shift between the run winding current and the start winding current, producing an increase in the amount of starting torque.

difference between starting and running winding resistance

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 type

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