Presentation on theme: "Motors & Motor Starters"— Presentation transcript:
1 Motors & Motor Starters Prepared By:Erik Redd&Jeremy Roberts
2 Motors AC-Motors Parts of an Electric Motor A. Stator : Stationary FrameB. Rotor : Revolving PartThe 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.
3 Diagram of an ac-motor This shows a three phase, two pole stator. Where A, B, and C are the three phases
4 Diagram of the Three Phases Fig Pg. 244Poles 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.
5 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
6 Polyphase Squirrel-Cage Induction Motors The 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 * Syn. rpm
7 Example.A 2 pole, 60 Hz motor runs at a full-load speed of 1760 rpm.What is the slip?
9 Single-Phase Motors Supplied by single source of ac voltage Rotor 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
10 Resistance Split-Phase Motors Has 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%
11 Capacitor-Start Motors Has 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 VLEfficiency is between 50-65%Capacitor controls the inrush current
12 Permanent Split-Capacitor Motors Winding 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%
13 Shaded Pole Motors Simple construction, least expensive Has 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%
14 DC Motors Consists of an armature winding and a stator winding Armature 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
15 Armature Voltage Control Is used for motor speeds below base speedOutput torque= T=k*ø*IAk is machine constantø is the main pole fluxIA is the armature current
16 Shunt Field Control Is used for motor speeds above base speed Horsepower, (HP)= Torque*rpm5252Where torque is in lb-ft
17 Speed Regulation Speed Regulation (IR)= no load rpm- full load rpm
18 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 BussThe 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
19 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
20 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
22 Motor Control Starters Motor will draw high inrush current while the starter will slow current downStarter reduces the amount of torque needed to start the motor
23 Magnetic Motor Starter Normally 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
24 Full-Voltage Starter Contains one set of contacts Motor is directly connected to the line voltage
25 Reversing Motor Starter Contains 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
26 Reduced-voltage Motor Starter Applies 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
27 Autotransformer Starter Two contactors are used:1) Start contactor- Closes first and connects motor to the linethrough an autotransformer- Deenergizes2) Run contactor- Motor switches to this contacter which hasfull voltage
28 Primary Resistor Starter Two contactor1) Line contactor- First to energize connecting motor to theline voltage through a resistor- After preset time, contactor opens2) Accelerating contactor- Energizes- Causes smooth acceleration to full voltage
29 Wye – Delta Starter Three contactors are used 1) Line contactor and start contactor- Energizes first and connects motor in wyeputting about 58% of line voltage acrosseach motor phase- Contacts open after preset time2) Run contactor- Energizes connecting motor in delta andputting full voltage on the motor
30 Part Winding StarterStarter supplies about 48% of normal starting torqueNot truly a reduced-voltage meansTwo Types1) Two-Step - one winding connected tofull voltage line and, after a preset time,the other connects2) Three-Step – one winding is connected in serieswith a resistor to the voltage line; after interval, resistoris shorted out and then second line is connected tofull voltage line
31 Solid-State Motor Starter For lower starting torque and smooth accelerationUsed on conveyors, pumps, compressors, etc.
32 Standard Modes of Operation Motor 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