HVAC214 - Oil Electrical Motors Part 1

Motors Motors consist of a stationary part called the stator that surrounds a rotating part called the rotor. One or more windings are mounted on the stator. There are no windings usually on the rotor.

Parts of a motor

Motors The rotor, or the center part of the motor is not connected to any external source of voltage. When the winding on the stator is powered it generates a magnetic field. As the A/C changes from a positive to a negative this magnetic field will rotate.

Magnets Like poles of a magnet will repel each other. Unlike poles will attract each other.

Motors As you remember like poles of a magnet repel each other. As the magnetic fields of the stator windings change from positive to negative it “chases” the rotor as the poles on the rotor are positive and negative.

Motor This constant changing of the magnetic field will keep a motor running. The problem comes when you are trying to start a motor. If the magnets are lined up evenly between poles the motor would not start. This is the purpose of the start winding.

Start and Run Windings

Start Winding This is an additional winding in a single phase motor that allows the motor to start when the poles of the stator and rotor are not in the correct position. The current applied to the start winding is slightly out of phase with that applied to the run winding.

Run winding This is the winding on the stator that allows the motor to continue to operate. The run winding gets constant voltage until the motor shuts off.

Shaded Pole Motor The shaded pole motor is the simplest motor. It has one winding surrounding a laminated core. It uses a shaded pole to determine its rotation.

Shaded Pole Motor The shaded pole motor consists of a squirrel care rotor, an iron core and two or more rings (poles) with a coil and small copper inserts. The insert slows the buildup of the magnetic field just enough to cause rotation towards the magnetic field.

Shaded Pole Motor The action provides for induced magnetism into the rotor. The rotor will then turn as if it is attracted by the magnetism. Movement of the rotor will continue as long as there is change of the polarity of the poles and the rotor.

Shaded Pole Motors If the motor stalls or if the bearings freeze the motor windings will usually not burn out. When the motor is in a locked condition the windings draw very little current. The direction of a shaded pole motor is towards the shaded pole.

Shaded Pole Motor Shaded pole motors can be made multiple speed by inserting voltage taps into the motor winding. Shaded pole motors are low in cost but have a very low starting torque and a very low power factor.

Shaded Pole Motor Example

Shaded Pole Motor Shaded pole motors are built for very small sizes and are used for light duty in small appliances. In refrigeration shaded pole motors are frequently used for Evaporator fan motors.

Split Phase Motors Split phase motors are also called induction start - induction run motors. These motors use a current relay or a centrifugal switch to remove the start winding from the circuit. The start windings work on a phase shift principle.

Split Phase Motor The voltage from the start winding to the run winding is 10 degrees out of phase. Starting torque and running torque are relatively low in split phase motors because the phase shift is so low.

Split phase motors These motors are used for belt drive blower applications such as small appliances like: Washers Dryers Dish washers Some older furnace blowers

Split Phase Motors

Split Phase Motor The centrifugal switch removes the start winding from the motor circuit once the motor reaches around 75% of its rated speed. This is the part that most often fails when a motor is started and stopped many times.

Centrifugal Switch The switch is normally closed when the motor is not operating. Centrifugal Switch