precise speed control Servo motors may be made from severalmotor types, the most common being:
- brushed DC motor.
- brushless DC motors.
- AC servo motors.
Basically, No Volt Release Coli ensures thatwhenever supply resumes after switching off or supply failure, themotor does not start on it's own, but starts only after theuser starts it and that too through current limitingresistors.
AC Motors are not self-starting,and thus, it requires some external equipment to start themotor initially. DC Motor are self-startingmotors. The Armature is stationary, and the magnetic fieldrotates in the AC Motor but in DC Motor the armaturerotates, and the magnetic field is stationary.
Various types of starters are as follows:
- Stator resistance starter.
- Star delta starter.
- Autotransformer starting.
- Rotor resistance starters.
- Variable stator frequency starting.
There need to be sufficient winding resistancewithin the field winding to restrict the current drift. However, itis necessary that the armature resistance is low because theDC generator generates excessive currents and any DCarmature resistance will cause decrease in generatorperformance.
The starter is there to help the lamp light. Thestarter (which is simply a timed switch) allows current toflow through the filaments at the ends of the tube. The currentcauses the starter's contacts to heat up and open, thusinterrupting the flow of current. The tube lights.
Drawbacks of a 3 PointStarter
The 3 point starter suffers from a seriousdrawback for motors with a large variation of speed byadjustment of the field rheostat. To increase the speed of themotor, the field resistance should be increased. Therefore, thecurrent through the shunt field is reduced.The Basic function of 2 point starter isto protect DC Series motor from high starting current. At the timeof starting large armature current is drawn by the motorwhich is limited by 2 point starter by connectionresistance is series with the armature.
The voltage produced across the armature is known ascounter or back EMF. The Back EMF in the DCmotor is expressed by the following mathematical expression.The back EMF is proportional to the speed of themotor.The governs the armature current and thus the backEMF maintains the speed and torque of themotor.
Consequently, AC motors are less efficientthan DC motors. In fact, the DC motor is 30% moreefficient than AC motors due to the secondary magneticfield being generated from the permanent magnets rather than copperwindings.
A.C. motors are powered from alternatingcurrent (A.C.) while D.C. motors are powered fromdirect current (D.C.), such as batteries, D.C.power supplies or an AC-to-DC power converter.A.C. induction motors do not use brushes; they arevery rugged and have long life expectancies.
The losses that occur in a DC Machine isdivided into five basic categories. The various losses areElectrical or Copper losses (I2R losses),Core losses or Iron losses, Brush losses,Mechanical losses, Stray load losses. Theselosses are explained below in detail.
A three phase Induction Motor is SelfStarting. When the supply is connected to the stator of athree-phase induction motor, a rotating magnetic field isproduced, and the rotor starts rotating and the inductionmotor starts. At the time of starting, themotor slip is unity, and the starting current is verylarge.
When a dc supply is provided to thetransformer primary no self induced emf will begenerated (no back emf). Therefore heavy current will flowin the transformer primary winding which may result inburning down the transformer primary winding.
Small DC motors are used in tools, toys,and appliances. The universal motor can operate on directcurrent but is a lightweight brushed motor used for portablepower tools and appliances. Larger DC motors are currentlyused in propulsion of electric vehicles, elevator andhoists, and in drives for steel rolling mills.
Three-phase electric motors usethree different electric legs with a 1/3-cycle lagbetween them. The motor's starter wires directly to themotor's wire terminals. Motor starters have a set ofcontactors. When the motor's switch is turned on, voltage flowsthrough the contactor's coil, creating a magneticfield.
A series resistor can certainly be used. It canbe a variable resistance (rheostat) or one or more fixedresistors. Larger DC motors are seldom used for fixedspeed operation, but electronic control or series resistance may berequired just for starting a larger DC motor to avoidexcessive starting current.
Why High Starting Currents:
When 3 phase voltage applied across the stator windingfor starting of induction motor, high inrush currentsmagnetize the air gap between the stator and rotor. An induces emfis generated in the rotor windings of the induction motor becauseof the rotating magnetic field.The starting current is high as there isno back EMF or counter EMF present in the armature circuit becauseat starting counter EMF is zero. It's armature has very lessresistance due to this it need more current atstarting time. Hence DC starters are used to limitthe starting current of motor.
A Direct Current Motor, DC is namedaccording to the connection of the field winding with the armature.Mainly there are two types of DC Motors.
An electric motor is an electrical machine whichconverts electrical energy into mechanical energy. The basicworking principle of a DC motor is: "whenever a currentcarrying conductor is placed in a magnetic field, it experiences amechanical force".
Starters are used to protect DCmotors from damage that can be caused by very high current andtorque during startup. They do this by providing externalresistance to the motor, which is connected in series to themotor's armature winding and restricts the current to anacceptable level.
This experiment aims to measure the windingsresistance of the stator (armature) windings. Everyarmature has its armature resistance (Ra). The realarmature resistance is greater than its resistancemeasured by direct current. The voltage drop across theinternal resistance can be calculated by using the Ohm'slaw.
In electrical engineering, an armature is thecomponent of an electric machine which carries alternating current.When the machine or motor is used as a motor, thisEMF opposes the armature current, and the armatureconverts electrical power to mechanical power in the form oftorque, and transfers it via the shaft.
A star delta starter is the most commonly usedmethod for the starting of a 3 phase induction motor. Whenmotor reaches about 80% of its full load speed, it willbegin to run in a delta connected stator winding. A stardelta starter is a type of reduced voltagestarter.
First, DC motor is not being used in modernelevators. It is VFD controlled induction motor doingduty. But if I were to choose type of DC motor forelevator, it would be DC shunt motor. It istrue that DC series motor is generating high startingtorque but its speed highly varies as per mechanical load onit.
Motors below 1 Hp is directly connect withoutstarter because their armature resistance is very high andthey have ability to afford the high current due to highresistance. Starter in series with motor ( I.e.Resistance) is reduce the high starting current and armature takesa low current and motor will be start.
Working Principle Of A DCMotor
A motor is an electrical machine which convertselectrical energy into mechanical energy. The principle ofworking of a DC motor is that "whenever a current carryingconductor is placed in a magnetic field, it experiences amechanical force".The direct current face plate starter is anelectrical switchgear that starts direct current motors by changingthe ohmic resistance in the armature and excitation circuit. DCface plate starters are being increasingly replaced withelectronic power converters such as thyristors.
When single-phase AC supply is given tostator winding. It produces alternating flux.i.e. which alternatesalong one space axis only. It is not synchronously revolving(or rotating) flux, as in case of 3 phase stator winding,the fed cannot produce rotation. Hence single phaseinduction motor is notself-starting.
