Published on February 16, 2014
DC Motors AC Motors By Ms. Nishkam Dhiman Assistant Professor -EEE Deptt. Chitkara Institute of Engg. & Technology
DC Motors 1. Series Motor 2. Shunt Motor 3. Compound AC Motors 1. Induction Motors 2. Synchronous Motors Servo Motors
When current flows in a conductor it produces a magnetic field about it. ◦ when the current-carrying conductor is within an externally generated magnetic field, the fields interact and a force is exerted on the conductor. Therefore if a conductor lies within a magnetic field: ◦ motion of the conductor produces an electric current ◦ an electric current in the conductor will generate motion The reciprocal nature of this relationship means that, for example, the DC generator above will function as a DC motor
1) Stator – The static part that houses the field windings and receives the supply and 2) Rotor – The rotating part that brings about the mechanical rotations.
3) Yoke of dc motor.(cast iron or steel, provides protective cover) 4) Poles of dc motor.(cast iron, slotted) 5) Field winding of dc motor.(copper wire) 6) Armature winding of dc motor.( Copper wire) 7) Commutator of dc motor. (copper segments stacked together, it commutates or relay the supply current from the mains to the armature windings housed over a rotating structure through the brushes of dc motor.) 8) Brushes of dc motor. The brushes of dc motor are made with carbon or graphite structures, making sliding contact over the rotating commutator. The brushes are used to relay the electric current from external circuit to the rotating commutator form where it flows into the armature windings.
Commutator Brush Armature Armature Winding
Shunt Field 100 Sh u n Series Field t 80 Series 60 Se 40 Co rie s m po un d Series Field 20 0 0 100 200 300 Percent of Rated Torque 400 Shunt Field Percent of rated Speed 120 Shunt Compound
Series Motor: Armature and field connected in a series circuit. Apply for high torque loads that do not require precise speed regulation. Useful for high breakaway torque loads. locomotives, hoists, cranes, automobile starters Starting torque 300% to as high as 800% of full load torque. Shunt Motor: Field coil in parallel (shunt) with the armature. Current through field coil is independant of the armature. Result = excellent speed control. Apply where starting loads are low fans, blowers, centrifugal pumps, machine tools Starting torque 125% to 200% full load torque (300 for short periods).
Performance is roughly between series-wound and shuntwound Moderately high starting torque Moderate speed control Inherently controlled no-load speed safer than a series motor where load may be disconnected e.g. cranes
AC motors can be divided into two main forms: ◦ synchronous motors ◦ induction motors High-power versions of either type invariably operate from a three-phase supply, but single-phase versions of each are also widely used – particularly in a domestic setting.
Stator As its name indicate stator is a stationary part of induction motor. A three phase supply is given to the stator of induction motor. Rotor The rotor is a rotating part of induction motor. The rotor is connected to the mechanical load through the shaft. The rotor of the three phase induction motor are further classified as • Squirrel cage rotor • Slip ring rotor or wound rotor or phase wound rotor
Advantages of squirrel cage induction rotor 1. Its construction is very simple and rugged 2. as there are no brushes and slip ring, these motors requires less maintenance. Applications: Squirrel cage induction motor is used in lathes, drilling machine, fan, blower printing machines etc
Slip ring or wound three phase induction motor : In this type of three phase induction motor the rotor is wound for the same number of poles as that of stator but it has less number of slots and has less turns per phase of a heavier conductor.The rotor also carries star or delta winding similar to that of stator winding. The rotor consists of numbers of slots and rotor winding are placed inside these slots.
Advantages of slip ring induction motor 1. It has high starting torque and low starting current. 2. Possibility of adding additional resistance to control speed Applications: Slip ring induction motor are used where high starting torque is required i.e in hoists, cranes, elevator etc
When a 3 phase electric conductors are placed in a certain geometrical positions (In certain angle from one another) there is an electrical field generate. The rotating magnetic field rotates at a certain speed, that speed is called synchronous speed. Now if an electromagnet is present in this rotating magnetic field, the electromagnet is magnetically locked with this rotating magnetic field and rotates with same speed of rotating field. Synchronous motors is called so because the speed of the rotor of this motor is same as the rotating magnetic field. It is basically a fixed speed motor because it has only one speed, which is synchronous speed and therefore no intermediate speed is there or in other words it’s in synchronism with the supply frequency. Synchronous speed is given by Ns=120f/P
Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it. It’s stator winding which consists of a 3 phase winding is provided with 3 phase supply and rotor is provided with DC supply. The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux. The rotor carrying DC supply also produces a constant flux. At a particular instant rotor and stator poles might be of same polarity (N-N or S-S) causing repulsive force on rotor and the very next second it will be N-S causing attractive force. But due to inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition. Hence it is not self starting.
• Synchronous motor having no load connected to its shaft is used for power factor improvement. Owing to its characteristics to behave at any power factor, it is used in power system in situations where static capacitors are expensive. • Synchronous motor finds application where operating speed is less (around 500 rpm) and high power is required. For power requirement from 35 kW to 2500KW, the size, weight and cost of the corresponding induction motor is very high. Hence these motors are preferably used. Ex- Reciprocating pump, compressor, rolling mills etc
Servomotor is normally a simple DC motor which is controlled for specific angular rotation with help of additional servomechanism (a typical closed loop feedback control system). A Servo is a small device that incorporates a three wire DC motor, a gear train, a potentiometer,an integrated circuit, and an output shaft bearing (Shown in Figure). Of the three wires that stick out from the motor casing, one is for power, one is for ground, and one is a control input line. The shaft of the servo can be positioned to specific angular positions by sending a coded signal. As long as the coded signal exists on the input line, the servo will maintain the angular position of the shaft. If the coded signal changes, then the angular position of the shaft changes.
Servos are constructed from three basic pieces; a motor, a potentiometer (variable resister) that is connected to the output shaft, and a control board. The potentiometer allows the control circuitry to monitor the current angle of the servo motor. The motor, through a series of gears, turns the output shaft and the potentiometer simultaneously. The potentiometer is fed into the servo control circuit and when the control circuit detects that the position is correct, it stops the motor. If the control circuit detects that the angle is not correct, it will turn the motor the correct direction until the angle is correct. Normally a servo is used to control an angular motion of between 0 and 180 degrees. It is not mechanically capable (unless modified) of turning any farther due to the mechanical stop build on to the main output gear. The amount of power applied to the motor is proportional to the distance it needs to travel. So, if the shaft needs to turn a large distance, the motor will run at full speed. If it needs to turn only a small amount, the motor will run at a slower speed. This is called proportional control.
A very common use of servo motors is in Radio Controlled models for controlling direction of motion like cars, airplanes, robots, movement of the tray of a CD or DVD player and puppets. They are also used in powerful heavy-duty sail boats.
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