Motors & Motor StartersPrepared By: Erik Redd &Jeremy Roberts

MotorsAC-MotorsParts of an Electric MotorA. Stator : Stationary FrameB. Rotor : Revolving Part

The rotary motion in an ac-motor is caused by the fundamental law of magnetism. This law states that like poles repel and unlike poles attract.

Diagram of an ac-motor This shows a three phase, two pole stator.

Where A, B, and C are the three phases

Diagram of the Three PhasesFig. 13-2 Pg. 244 Poles 1 and 4 are at their greatest magnetic field at time equal to one, because phase A (red line) is connected to those poles, and the same for the other poles when their corresponding phases are at maximum current magnitude.

Synchronous SpeedSpeed at which it takes the motor to go one cycle and one revolution.

S=[120*frequency}] (# poles)

Example:For a three-phase, 60 Hertz, 2 pole motor:S=[120*60]/2=3600 revolutions per minute

Polyphase Squirrel-Cage Induction MotorsThe most common three-phase motorDoes not have solid polesInstead, it has laminations: numerous flat sheets held together in a package. They are insulated from each other (this reduces Eddy currents) making up the statorThe difference between induction and synchronous motors is that the rotor for an induction motor can travel at a different speed than the stator. This is called Slip. slip= Syn. rpm Motor rpm *100 Syn. rpm

Example.A 2 pole, 60 Hz motor runs at a full-load speed of 1760 rpm. What is the slip?

Ans. %slip= 3600-1760*100 3600=51.1%

Single-Phase MotorsSupplied by single source of ac voltageRotor must be spun by hand in either direction, does not have a starting mechanismHas no starting torqueThree different types of single-phase motors: split-phase, capacitor start, permanent split-capacitor, and shaded-pole motors

Resistance Split-Phase MotorsHas a start winding and a main windingWinding currents are out of phase by 30 degrees, this produces a flux field that starts the motorMain winding current (IM) and start winding current (IS) lags supply voltage (VL)Start (inrush) current is highNeeds centrifugal starting switch or relay to disconnect the start winding (protects it from over heating)Efficiency is between 50-60%

Capacitor-Start MotorsHas the same winding and switch mechanism arrangement as split-phase but adds a short time-rated capacitor in series with the start windingThe time shift phase between the main and start winding is close to 90 degreesIS leads VL Efficiency is between 50-65%Capacitor controls the inrush current

Permanent Split-Capacitor MotorsWinding arrangement is the same as the capacitor and split-phase motorsCapacitor can run continuously, rated in microfarads for high-voltage ratingsNo centrifugal switch is neededIM lags VL, while IS leads VLEfficiency is between 50-70%

Shaded Pole MotorsSimple construction, least expensiveHas a run winding only, shading coils are used instead of the start windingStator is made up of a salient pole, one large coil per pole, wound directly in a single large slotA small shift in the rotor causes torque and starts the motorEfficiency is between 20-40%

DC MotorsConsists of an armature winding and a stator windingArmature windings act as the rotorHas three different classifications: constant torque, constant horsepower, or a combination of the twoStandard industrial dc motors are shunt woundedModifications of the dc motor are: shunt wound, stabilized shunt exciting fields, compound wound motors, and series wound motors

Armature Voltage ControlIs used for motor speeds below base speedOutput torque= T=k**IA k is machine constant is the main pole flux IA is the armature current

Shunt Field ControlIs used for motor speeds above base speedHorsepower, (HP)= Torque*rpm 5252 Where torque is in lb-ft

Speed RegulationSpeed Regulation (IR)= no load rpm- full load rpmfull load rpm

Brushless DC MotorsThree phase ac power is converted into dc by the input side of the motor to charge up a bank of storage capacitorsThese capacitors are called the Buss The purpose of the buss is to store energy and supply dc power to transistors in the output side as the motor requires the power to start up

Brushless DC MotorsFigure 13-21, page 264 shows the input power sectionIt consists of three fuses, six diodes, a choke, and two capacitorsThe fuses protect the diodesThe choke protects against line transientsThe motor control may run at very low speeds at very high torques while drawing little current from the ac line

Brushless DC MotorsThis picture is a representation of the encoders (rotor part of the motor) telling the corresponding transistors (stator) to turn on in order to get maximum torque from the motor

Picture of a Brushless Motor

Motor Control StartersMotor will draw high inrush current while the starter will slow current downStarter reduces the amount of torque needed to start the motor

Magnetic Motor StarterNormally open contactsNot always possible to control amount of work applied to the motorHas overloadsMotor may be overloaded resulting in damage to the motorOpen due to excessive motor current, high temperature, or a combination of both

Full-Voltage StarterContains one set of contactsMotor is directly connected to the line voltage

Reversing Motor StarterContains two starters of equal sizeTwo starters connect to the motorInterlocks are used to prevent both starters from closing their line contacts at the same timeFigure 14-4A

Reduced-voltage Motor StarterApplies a percentage of the total voltage to start (50% - 80%)After motor rotates, switching is provided to apply full voltageTorque will be reduced when startingFour types:1) Autotransformer2) Primary Resistance3) Wye Delta4) Part Winding

Autotransformer StarterTwo contactors are used:1) Start contactor - Closes first and connects motor to the line through an autotransformer - Deenergizes2) Run contactor - Motor switches to this contacter which has full voltage

Primary Resistor StarterTwo contactor 1) Line contactor - First to energize connecting motor to the line voltage through a resistor - After preset time, contactor opens2) Accelerating contactor - Energizes - Causes smooth acceleration to full voltage

Wye Delta StarterThree contactors are used 1) Line contactor and start contactor - Energizes first and connects motor in wye putting about 58% of line voltage across each motor phase - Contacts open after preset time 2) Run contactor - Energizes connecting motor in delta and putting full voltage on the motor

Part Winding StarterStarter supplies about 48% of normal starting torqueNot truly a reduced-voltage meansTwo Types 1) Two-Step - one winding connected to full voltage line and, after a preset time, the other connects 2) Three-Step one winding is connected in series with a resistor to the voltage line; after interval, resistor is shorted out and then second line is connected to full voltage line

Solid-State Motor StarterFor lower starting torque and smooth accelerationUsed on conveyors, pumps, compressors, etc.

Standard Modes of OperationMotor voltage gradually increases during accelerationCreates a kick start pulse of 500% of full load amperage for high frictionUsed when necessary to limit currentUsed when motor requires a full voltage start