IT EXAM 2 (11-13)

Motor Protection Methods

- Dedicated circuit - Stop/Start Pushbutton Control - Run protection - Set to motor "service factor" (S.F.) - (125% or 1.25 for example) - S.F.: indicates how much extra load a motor is allowed to take.

Induction Motor Operation Principles

Adding armature coils and corresponding commutator segments increases torque. This is a function of the inverse square law. The closer unlike poles are to one another, the greater the magnetic force of attraction. When electric current passes through a magnetic field, the magnetic force produces a torque which turns the DC motor

Induction AC vs DC Motors

An AC Induction motor: first, induction of current by rotor, then magnetic Repulsion/Attraction. A DC motor: A simple magnetic Repulsion/Attraction machine

Overload Protection

• Locked rotor damage • Fuses • Circuit Breakers • Thermal-overload Devices • Bimetallic devices

Motor Torque

a specialized form of electric motor which can operate indefinitely while stalled, that is, with the rotor blocked from turning, without incurring damage. In this mode of operation, the motor will apply a steady torque to the load

Motor Construction

Armature: (rotor) consists essentially of coils of wire around a metal core and in which electric current is induced or in which the input current interacts with a magnetic field to produce torque. Commutator: a moving part of a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit. Brushes: a device which conducts current between stationary wires and moving parts, most commonly in a rotating shaft Field Magnet: (Stator) a magnet for producing a magnetic field

Universal Motors

Can operate on either AC or DC: • Typically series wound • Used for light loads due to large amount of sparking at the brushes • Small fans • Powered Drill

Motor Effect

Discovered by Michael Faraday circa 1821. the term used when a current-carrying wire in the presence of a magnetic field experiences a force. Two magnetic fields cannot occupy the same space at the same time. The field around the conductor flows with the permanent field above the conductor but opposes the permanent field below. The conductor will move toward the weakened field: creates motion (torque)

Back emf in AC Motors

Due to Lenz's Law: the magnetic field in the rotor opposes the magnetic field in stator. This called back emf (equivalent to cemf). Back emf reduces the speed of rotor.

Permanent Magnet Motors

Require armature current only. Are common in small "fractional motor" category. Application: Starter motor in automobiles, windshield wipers, washer, for blowers used in heaters and air conditioners, to raise and lower windows, it also extensively used in toys. Because of its permanent magnet, these motors are used where there is no need of speed control of motor by means of controlling its magnetic field.

AC Motor Construction

Rotor: the rotating electrical component. It also consists of a group of electro-magnets arranged around a cylinder, with the poles facing toward the stator poles. The rotor is located inside the stator and is mounted on the AC motor's shaft. Squirrel Cage: the rotating part of the common "squirrel cage" induction motor. ... In operation, the non-rotating "stator" winding is connected to an alternating current power source; the alternating current in the stator produces a rotating magnetic field. Stator: the stationary winding in an electric motor, either for rotary or linear. The stator in an AC motor is a wire coil, called a stator winding, which is built into the motor. When this coil is energized by AC power, a rotating magnetic field is produced.

Single Phase AC Motor

Single-phase motors are used in many home appliances where 3-phase electricity is not readily available. Single-phase has to be split into two phases to generate rotating magnetic field. (ex: kitchen hoods, elevators, escalators)

Speed Regulation %

Some motors develop full power under load. Other motors must be brought up to speed before a load is applied. Formula: % Speed Regulation = ((RPM no load - RPM full load) / RPM full load) x 100%

Synchronous speed: effect of frequency and # of poles on speed

Synchronous speed is the speed of the stator's rotating magnetic field. The Synchronous Speed of any AC motor is always a multiple of the input frequency (50hz vs. 60 hz) and # of paired poles. Standard motors have two, four, six, or eight poles. These poles play an important role in determining the synchronized speed of an AC motor.

Comparison of AC and DC motors

The biggest advantage of AC induction motors is their sheer simplicity. They have only one moving part, the rotor, which makes them low-cost, quiet, long-lasting, and relatively trouble free. (more reliable). DC motors, by contrast, have a commutator and carbon brushes that wear out and need replacing from time to time. The friction between the brushes and the commutator also makes DC motors relatively noisy (and sometimes even quite smelly). Speed control is simpler and accurate in DC motors. Unlike DC motors, AC motors can't be driven from batteries or any other source of DC power. This is because AC motors have a changing magnetic field to turn the rotor.

Operating Principles

The induction motor (or asynchronous motor) always relies on a small difference in speed between the stator rotating magnetic field and the rotor shaft speed called slip to induce rotor current in the rotor AC winding.

Phase and Phases

The more coils you have, the more smoothly the motor will run. The number of separate electric currents energizing the coils independently, out of step, is known as the phase of the motor. with either one, two, or four coils switched on and off together by three separate, out-of-phase currents.

Counter electromotive Force (CEMF)

The motor converts electrical energy into mechanical energy. However, when armature begins to rotate, the motor also becomes a generator. This counter current is called

Speed Control in AC vs. DC Motors

Thespeedofaninductionmotordependsonthe frequency of the alternating current that drives it and # of poles. It turns at a constant speed unless we use a variable-frequency drive. To control the speed of an AC motor(make it go faster or slower), you have to increase or decrease the frequency of the AC supply using what's called a variable-frequency drive.

Conditions when a motor fails

Unless equipped with brushes, AC electric motors are almost maintenance free. All however will eventually fail. - excessive bearing wear - mechanical overload - locked rotor - low voltage/ high voltage