Copyright Goodheart-Willcox Co., Inc. Chapter 13 Electric Motors 259 and the start winding is removed from the active cir- cuit. See Figure 13-12. When the motor is turned off and it begins to slow, the weights return to the rest position and allow the contacts to close. Split-phase motors can operate on two differ- ent voltages. They are called dual-voltage motors and have two run windings and one start wind- ing, See Figure 13-13. When the run windings are wired in series, they are wired for high volt- age. The first run winding drops half the applied voltage. The other half is dropped by the other run winding that is parallel to the start winding. When all three windings are in parallel, low volt- age is applied. Motors draw more current on initial start. This is because stator current is initially not impeded by inductive reactance, thus restricting current flow to only the wire’s pure resistance. This also induces rotor current to the maximum value. This current is called locked rotor amperage (LRA), or also referred to as inrush or starting current. LRA can be two to six times greater than the normal operating current and lasts typically a fraction of a second. The speed of the motor is called synchronous speed, and it is determined by the motor’s rotating magnetic field. The speed is measured in revolutions per minute (RPM). Synchronous speed can be calculated in the following formula: Ns = 120 × F _____ p where Ns = synchronous speed measured in revolutions per minute (RPM) 120 = constant F = frequency measured in hertz (Hz) p = number of stator poles (run winding) Electrical contacts Spring Weights attached to spring Motor shaft rotation Motor shaft Motor frame Plate Weights move outward as shaft rotates Goodheart-Willcox Publisher Figure 13-12. The weights of a centrifugal switch move outward as the motor reaches a set speed to open the contacts. L1 L2 240 V Numbered terminals Run Run L2 120 V L1 Goodheart-Willcox Publisher Figure 13-13. Dual voltage single-phase motor. Voltage is selected by changing the numbered connections.