Chapter 13 AC Motors

Objectives Discuss the operation of an induction motor. Identify and explain the operation of various three-phase motors. Explain how a split-phase condition is created. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Objectives Discuss the purpose of ac motor protection circuits. List the common causes of motor failure. Explain basic troubleshooting techniques for ac motors. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Induction Motors Three-phase motor Rotation of magnetic field inside a stator Rotor inside stator housing Squirrel cage rotor © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Synchronous Speed Dependent on frequency and number of poles As the number of coils or poles per phase increases, the rpm of the motor decreases © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Slip Rotor travels slower than synchronous speed of magnetic field 4%–5% of motor’s rpm Rotor’s rpm depends on number and size of rotor bars As slip increases, greater current circulates, leading to greater torque © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Types of Induction Motors Three-phase synchronous motor Wound rotor motor Dual-voltage motor Single-phase induction motor Capacitor start, capacitor run motor Repulsion-induction motor Shaded-pole motor Stepping motor © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Three-Phase Synchronous Motors Motor and magnetic field rotate at same speed Rotor windings are energized, so rotor is an electromagnet No slip to maintain Used where constant speeds exist under changing mechanical loads © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Wound Rotor Motors Use three sets of windings wrapped around soft iron laminations Can be brought to near synchronous speed Varying induced current in rotor windings Rotor speed can be adjusted © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Dual-Voltage Motors Stator coils arranged in pairs Three-phase motors Economical motors © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Single-Phase Induction Motors Splitting phases Used in home appliances © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Inductive Reactance Phase Splitting Starting windings Running windings © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Capacitor Phase Splitting Larger phase displacement High starting torque © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Capacitor Start, Capacitor Run Motors Heavy mechanical load conditions Similar to capacitor start motors Single-phase air-conditioning equipment © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Repulsion-Induction Motors Resemble dc motors Brushes connected to each other, not power source Rotation started by repulsion Currents displaced by brushes New poles opposing stator poles © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Shaded-Pole Motors Shading the pole Shading coils Induced current opposes change in magnetic field of field coil © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Stepping Motors Rotate in steps Gear teeth inside motor Pulses of power, not continuous Speed of power pulses determines rotor speed Used in robots and disk drive systems © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Selsyn Units Synchros Two motor units Energized by ac power supply Transmitter and receiver © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Review What is the stator in a motor? Stationary windings in a circular pattern inside a housing © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Review What differentiates a squirrel cage from other rotors? It is common in the three-phase induction motor, used to produce rotation instead of just vibration © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Review What is synchronous speed? The speed of the stator’s rotating magnetic field © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Review What happens to the rpm of a motor as the number of coils or poles per phase increases? The rpm decreases © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Review Which kind of motor has stator coils arranged in pairs? The dual-voltage motor © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Review What is the single turn of wire in a shaded-pole motor called? Shading coil © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Motor Protection Dedicated line Branch circuit protection © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Motor Protection (Cont.) Run protection Service factor Based on percentage of normal current under full load condition (Percentage of current – 1) × 100 © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Motor Failure Excessive bearing wear Mechanical overload Single-phasing condition Locked rotor condition Low voltage © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Excessive Bearing Wear Most common cause of failure Some bearings should be lubricated Dry bearings cause friction Causes motor to go into overload condition © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Mechanical Overload Affects small ac motors Overheating can cause irreversible damage Can produce short between windings © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Single-Phasing Condition Affects three-phase motors Power supply to one line is lost to open or blown fuse Increased current will damage motor Thermo protection prevents overheating © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Locked Rotor Condition Causes severe damage Caused by mechanical failure in the system Excessive current in windings © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Low Voltage Excessive motor current in windings Causes the rotor to lock Motor should be under mechanical load © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Troubleshooting Motors Clamp-on ammeters Used only for ac circuits Circuit does not have to be opened for ammeter to be connected Current should be below motor’s rating Compare all three lines of a three-phase motor Megohmmeters © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Review What is a dedicated line? A circuit used for only one purpose © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Review What is the ability of a motor to withstand or avoid damage from an overload condition? Service factor © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Review In a case where the motor is energized, but the rotor is not turning, what component can be most severely damaged? The motor windings © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Review What is the most common cause of motor failure? Excessive bearing wear © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

For Discussion Why does an overloaded motor heat up? Why are motor starters necessary on heavy-duty motors? Compare an induction motor to a transformer. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Glossary Branch circuit protection Dedicated line Installation of a fuse or breaker to protect the individual circuit feeding a motor. Dedicated line Refers to a circuit that is used for only one purpose. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Glossary Locked rotor condition Rotor A rotor that is not turning while power is being applied to the motor because of a physical impedance. Rotor Rotating part of an ac generator. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Glossary Run protection Running winding Protects a motor from excessive heat damage or fire while in a normal mode of operation. Running winding Turn of heavy wire placed in a slot around the inside of a stator. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Glossary Selsyn unit Service factor A transformer application designed to relay information through motor units. Also called a synchro. Service factor The ability of a motor to withstand or avoid damage from an overload condition. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Glossary Shading coil Shading the pole A single turn of heavy wire placed in a slot that is cut in the face of the poles of a shaded-pole motor. Shading the pole Placing a winding wire in the slot of the starter of a shaded-pole motor. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Glossary Single-phasing condition Slip Occurs when the power supply to a three-phase motor loses one of its three lines due to an open or blown fuse. Slip The difference in the rotor speed and the magnetic field of the stator in ac motors, usually expressed as a percentage. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Glossary Splitting phases Squirrel cage Dividing a single-phase current into a polyphase current. Squirrel cage Made of bars placed in slots of the rotor core and joined at the ends. Used in induction motors. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Glossary Starting windings Stator Many turns of relatively fine wire in an induction motor. Stator Stationary coils of an ac generator. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Glossary Synchro Synchronous speed Electromechanical device used to transmit the angular position of a shaft from one position to another without mechanical linkage. Synchronous speed The speed at which the rotor speed equals the speed of the rotating magnetic field. © Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.