An electric motor is an electromechanical device that converts electrical energy to mechanical energy.
The mechanical energy can be used to perform work such as rotating a pump impeller, fan, blower, driving a compressor, lifting materials etc.
BASIC WORKING PRINCIPLE
TYPES OF MOTOR LOADS
CLASSIFICATION OF MOTORS Electric Motors Alternating Current (AC) Motors Direct Current (DC) Motors Synchronous Induction Self Excited Separately Excited Three-Phase Single-Phase Series Shunt Compound
TYPES OF AC MOTORS * Electrical current reverses direction * Two parts: stator and rotor Stator: stationary electrical component Rotor: rotates the motor shaft * Speed difficult to control * Two types Synchronous motor Induction motor
AC MOTOR: INDUCTION MOTOR Most common motors in industry Advantages: Simple design Inexpensive High power to weight ratio Easy to maintain Direct connection to AC power source
COMPONENTS OF INDUCTION MOTOR A3-phase induction motor has two main parts: • A stator – consisting of a steel frame that supports a hollow, cylindrical core of stacked laminations. Slots on the internal circumference of the stator house the stator winding. • A rotor – also composed of punched laminations, with rotor slots for the rotor winding.
COMPONENTS OF INDUCTION MOTOR contd… There are two-types of rotor windings: • Squirrel-cage windings, which produce a squirrel-cage induction motor (most common) • Conventional 3-phase windings made of insulated wire, which produce a wound-rotor induction motor (special characteristics)
Induction Motor: Operating Principle Operation of 3-phase induction motors is based upon the application of Faraday’s Law and the Lorentz Force on a conductor. Consider a series of conductors (length L) whose extremities are shorted by bars A and B. A permanent magnet moves at a speed v, so that its magnetic field sweeps across the conductors.
Operating Principle Contd… The following sequence of events takes place: 1. A voltage E = BLv is induced in each conductor while it is being cut by the flux (Faraday’s Law) 2. The induced voltage produces currents which circulate in a loop around the conductors (through the bars). 3. Since the current-carrying conductors lie in a magnetic field, they experience a mechanical force (Lorentz force). 4. The force always acts in a direction to drag the conductor along with the magnetic field. Now close the ladder upon itself to form a squirrel cage, and place it in a rotating magnetic field – an induction motor is formed!
Induction Motor: Rotating Field Consider a simple stator with 6 salient poles - windings AN, BN, CN. The windings are mechanically spaced at 120° from each other. The windings are connected to a 3-phase source. AC currents Ia, Ib and Ic will flow in the windings, but will be displaced in time by 120°. Each winding produces its own MMF,which creates a flux across the hollow interior of the stator. The 3 fluxes combine to produce a magnetic field that rotates at the same frequency as the supply.
Induction Motor: Stator Winding In practice, induction motors have internal diameters that are smooth, instead of having salient poles. In this case, each pole covers 180° of the inner circumference of the rotor (pole pitch = 180°). Also, instead of a single coil per pole, many coils are lodged in adjacent slots. The staggered coils are connected in series to form a phase group. Spreading the coil in this manner creates a sinusoidal flux distribution per pole, which improves performance and makes the motor less noisy.
INDUCTION MOTOR : SLIP The difference between the synchronous speed and rotor speed can be expressed as a percentage of synchronous speed, known as the slip. s = (Ns – N) Ns Where s = slip, Ns = synchronous speed (rpm), N = rotor speed (rpm) • At no-load, the slip is nearly zero (<0.1%). • At full load, the slip for large motors rarely exceeds 0.5%. For small motors at full load, it rarely exceeds 5%. • The slip is 100% for locked rotor.
Induction Motor: Frequency induced in the rotor The frequency induced in the rotor depends on the slip: fR = s f fR = frequency of voltage and current in the rotor f = frequency of the supply and stator field s = slip
Induction Motor: Active Power Flow Efficiency – by definition, is the ratio of output / input power: η = PL / Pe Rotor copper losses: PJr = s Pr Mechanical power: Pm = ( 1-s)Pr Motor torque: Tm = 30Pr πNs Where: Pe = active power to stator Pr = active power supplied to rotor PL = Shaft Power
Induction Motor: Relationship between Load, Speed and Torque At 80% of full speed: highest “pull-out” torque and current drops At start: high current and low “pull-up” torque At start: high current and low “pull-up” torque At full speed: torque and stator current are zero