Chemical Engineering - Apllied Mechanics

Introduction motors part 2

Divided by topic

INDUCTION MOTOR A. Vania Politecnico di Milano, Dept. of Mechanical Engineering , Milan, Italy APPLIED MECHANICS –Prof. Andrea VANIA INDUCTION MOTORS 2 An induction or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor therefore does not require mechanical commutation, separate -excitation or self -excitation for all or part of the energy transferred from stator to rotor, as in universal, DC and large synchronous motors. An induction motor's rotor can be either wound type or squirrel -cage type . Three -phase squirrel -cage induction motors are widely used in industrial drives because they are rugged, reliable and economical. Single -phase induction motors are used extensively for smaller loads, such as household appliances like fans. Although traditionally used in fixed -speed service, induction motors are increasingly being used with Variable -Frequency Drives (VFDs) in variable -speed service. VFDs offer especially important energy savings opportunities for existing and prospective induction motors in variable -torque centrifugal fan, pump and compressor load applications. Squirrel cage induction motors are very widely used in both fixed -speed and VFD applications. APPLIED MECHANICS –Prof. Andrea VANIA ASYNCHRONOUS MOTOR 3 1 stator 2 rotor 3 motor shaft 4 air gap 5 rotor bars 6 rings APPLIED MECHANICS –Prof. Andrea VANIA PRINCIPLES OF OPERATION 4 The induction motor is an electrical motor that is composed of a stationary part, the stator, and a rotating part, the rotor .The rotor is rather massive . The stator contains a suitable even number of grouped windings (stator windings , stator poles ). An alternate current AC (varying with a harmonic law) flows through the stator windings . These poles are mounted at diametrical opposite positions over the circumferential internal surface of the stator, in order to form a pair of opposite poles .All poles are equally spaced . The windings of a pair of opposite poles are connected in a series configuration . The currents that flow through the pairs of poles are characterized by a suitable phase shift . A three -phase motor has a minimum number of six poles (3 pairs of poles) while a two - phase motor has a minimum number of four poles (2 pairs of poles) . In a three -phase motor the pairs of poles have a physical (and an electrical) phase shift of 120 ° degrees, while in a two -phase motor the poles have a physical (and an electrical) phase shift of 90 ° degrees . A three -phase power supply provides a rotating magnetic field in an induction motor APPLIED MECHANICS –Prof. Andrea VANIA PRINCIPLES OF OPERATION 5 In both three -phase and two -phase motors the shifted AC currents cause a rotating magnetic field ,whose rotational speed is ns. The rotor is equipped with a suitable number of poles (rotor windings) that are short - circuited .Owing to magnetic induction phenomena, a further magnetic field is generated by the rotor windings . This magnetic field rotates with a rotational speed, nr,that is lower than that of the magnetic field generated by the stator windings . The rotational speed, ns, of the magnetic field generated by the stator windings is constant as it depends on : The currents induced in the rotor windings cause a magnetic field that rotates with a rotational speed equal to ns– nr(absolute slip ,S ). As the short -circuited rotor windings are placed inside a rotating magnetic field and they are flowed by an electric current, they are subjected to forces that are exerted in the tangential direction (with respect to a rotor cross - section) .These forces cause a drive torque and a rotary motion of the rotor .The rotational speed of the rotor is just nr. • the frequency fof the power supply (50 Hz or 60 Hz depending on the country) ; • the number p of pairs of pole s F F F F 60 s f n p = APPLIED MECHANICS –Prof. Andrea VANIA PRINCIPLES OF OPERATION 6 This means that the magnetic field of the rotor rotates with a relative rotational speed nswith respect to the stator, so that it is synchronous with the magnetic field of the stator .Owing to this synchronous condition the motor generates a constant torque (for any given rotational speed of the rotor) . When the two rotational speeds nsand nr,are equal, the drive torque nullifies as no induced current are generated in the rotor windings . Therefore, the rotational speed of the rotor is commonly a little lower than that of the synchronous rotational speed ns. The difference between these two rotational speeds is proportional to the resistance torque applied to the drive shaft by the Motor -Driven -System, MDS . The relationship between the frequency of the power supply, f,and the number p of pairs of poles is : (e .g.: for a three -phases motor with 3 pairs of poles and f = 50 Hz : ns= 1000 rpm ; or for a two -phases motor with 2 pairs of poles and f= 50 Hz :ns= 1500 rpm) . In rated operating conditions the rotational speed of the rotor is always lower than the synchronous speed of about 3-6% . This is the phenomenon called slip (scorrimento ) that allows the drive torque to be generated . The rotational speed of the rotor can be obtained from the expression of the relative slip ,s:60 s f n p = ( ) s r s s n n n=− APPLIED MECHANICS –Prof. Andrea VANIA PRINCIPLES OF OPERATION 7 The actual value of the slip depends on the resistance torque applied to the drive shaft . This torque is never null as friction forces between parts of the system subjected to relative motions, as well as fluid -dynamic forces, always exist . Therefore, these resistance actions prevent the motor to rotate at the synchronous speed . The synchronous rotational speed is constant, for given values of the parameters f and p, however, the use of inverters allows one to change and control the frequency of the current that actually supplies the motor . Typical speed -torque curves for different motor input frequencies as for example used with variable -frequency drives APPLIED MECHANICS –Prof. Andrea VANIA PRINCIPLES OF OPERATION 8 In both induction and synchronous motors, the AC power supplied to the motor's stator creates a magnetic field that rotates in time with the AC oscillations . Whereas a synchronous motor's rotor turns at the same rate as the stator field, an induction motor's rotor rotates at a slower speed than the stator field . The induction motor stator's magnetic field is therefore changing or rotating relative to the rotor .This induces an opposing current in the induction motor's rotor, in effect the motor's secondary winding, when the latter is short -circuited or closed through an external impedance .The rotating magnetic flux induces currents in the windings of the rotor ; in a manner similar to currents induced in a transformer’s secondary winding(s) . The currents in the rotor windings in turn create magnetic fields in the rotor that react against the stator field .Due to Lenz’s law, the direction of the magnetic field created will be such as to oppose the change in current through the rotor windings . The cause of induced current in the rotor windings is the rotating stator magnetic field, so to oppose the change in rotor -winding currents the rotor will start to rotate in the direction of the rotating stator magnetic field . The rotor accelerates until the magnitude of induced rotor current and torque balances the applied load . Since rotation at synchronous speed would result in no induced rotor current, an induction motor always operates slower than synchronous speed . The difference, or "slip ," between actual and synchronous speed varies from about 0.5 to 5.0% for standard Design B torque curve induction motors . The induction machine's essential character is that it is created solely by induction instead of being separately excited as in synchronous or DC machines or being self -magnetized as in permanent magnet motors . APPLIED MECHANICS –Prof. Andrea VANIA PRINCIPLES OF OPERATION 9 For rotor currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field (ns); otherwise the magnetic field would not be moving relative to the rotor conductors and no currents would be induced .As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque . The ratio between the rotation rate of the magnetic field induced in the rotor and the rotation rate of the stator's rotating field is called slip . Under load, the speed drops and the slip increases enough to create sufficient torque to turn the load . For this reason, induction motors are sometimes referred to as asynchronous motors . An induction motor can be used as an induction generator, or it can be unrolled to form a linear induction motor which can directly generate linear motion . A three -phase power supply provides a rotating magnetic field in an induction motor APPLIED MECHANICS –Prof. Andrea VANIA SYNCHRONOUS SPEED 10 An AC motor synchronous speed, ns, is the rotation rate of the stator magnetic field, which is expressed in revolutions per minute as : where fis the motor supply’s frequency in Hertz and p is the number of magnetic poles. That is, for a six -pole three -phase motor with three pole -pairs set 120 ° apart, p equals 6 and equals 1,000 rpm and 1,200 rpm respectively for 50 Hz and 60 Hz supply systems . Typical speed -torque curves for different motor input frequencies as for example used with variable -frequency drives120 s f n p = APPLIED MECHANICS –Prof. Andrea VANIA SLIP 11 Slip, s, is defined as the difference between synchronous speed and operating speed, at the same frequency, expressed in rpm or in percent or ratio of synchronous speed. Thus: where nsis the stator electrical speed and nris the rotor mechanical speed. Slip, which varies from zero at synchronous speed and 1 when the rotor is at rest, determines the motor's torque. Since the short -circuited rotor windings have small resistance, a small slip induces a large current in the rotor and produces large torque. At full rated load, slip varies from more than 5% for small or special purpose motors to less than 1% for large motors. These speed variations can cause load -sharing problems when differently sized motors are mechanically connected. Various methods are available to reduce slip, VFDs often offering the best solution .sr s nn s n − = 120 s f n p = APPLIED MECHANICS –Prof. Andrea VANIA TORQUE CHARACTERISTIC CURVE 12 rpm Effect of the supply voltage on the torque characteristic curve Torque characteristic curve Effect of a variable rotor resistance on the torque characteristic curve Resistance torque characteristic curve APPLIED MECHANICS –Prof. Andrea VANIA MOTOR & GENERATOR UNIT 13 The torque curve is emi -symmetric APPLIED MECHANICS –Prof. Andrea VANIA BRUSHED DC MOTOR 14 A simple DC electric motor . When the coil is powered, a magnetic field is generated around the armature .The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation . The armature continues to rotate . When the armature becomes horizontally aligned, the torque becomes zero . At this point, the commutator reverses the direction of current through the coil, reversing the magnetic field . The process then repeats .0 2 00 0 4 00 0 6 00 0 8 00 0 1 00 0 0 1 20 0 0 1 40 0 0 1 60 0 0 0 20 40 60 80 1 00 1 20 1 40 R otatio nal s pe e d [rpm ] Torque [Nm ] - Power [kW ] D C e le c tric mo tor T orq ue P ower