An adjustable Linear speed drive is a device that controls speed, and direction ofan AC or DC motor.
Some high-performance drives are able to run in torque regulationmode. DC Drives1. DC Drive ControlSystemA basic DC drive control system generally containsa drive controller and DC motor as shown in The controls allow the operator to start, stop, and change direction and speed of the motor byturning potentiometers or other operator devices. These controls may be an integral part of the controller or may be remotely mounted.The drive controller converts a 3-phase AC voltage to an adjustable DC voltage,which is then applied to a DC motor armature. Fig.
1.1-FinalDC Drive Control System The DC motor converts power from theadjustable DC voltagesource to rotating mechanicalforce. Motor shaft rotation and direction are proportional to the magnitudeand polarity ofthe DC voltage applied to the motorThe tachometer (feedback device) shown in Figure 1.1 converts actual speed to an electrical signal that is summed with the desired reference signal.
The outputof the summing junctionprovides an error signal to the controller and a speed correction ismade.2. DC MotorsThe following are the four basic types ofDC motors and their operating characteristics:3. Shunt WoundShunt-wound motors have the field controlled separately fromthe armature winding.
With constant armature voltage and constant field excitation, the shunt-wound motor offers relativelyflat speed-torque characteristics. The shunt-wound motor offers simplified control for reversing,especially for regenerative drives Fig.1.2- Shunt Wound graphicalform 4. Series WoundThe series-wound motor has the field connected in series with the armature. Although the series-wound motor offers high starting torque, it has poor speed regulation.
Series-wound motors aregenerally used on low speed, very heavy loads. Fig.1.3 – Series Wound 5. Compound WoundThe compound-wound DC motor utilizes a field winding in series with the armature in addition to theshunt field, to obtain a compromise in performance between a series and a shunt wound type motor. The compound-wound motor offers a combinationof good starting torque and speed stability.
Fig.1.3 – compound woundPermanent MagnetThe permanent magnet motor has a conventional wound armature with commutator and brushes.
Permanent magnets replace the field windings. This type of motor has excellentstarting torque, with speed regulationslightly less than that of the compound motor. Peak starting torque iscommonly limited to 150% of rated torque to avoid demagnetizing the field poles. Typically thesearelow horsepower. Fig.1.4– permanent magnetArmature voltagecontrolled DC drives are capable of providing rated current and torque at anyspeed between zero and the base (rated)speed of the motor.
These drives use a fixed field supply and give motor characteristics as seen in Figure 1.2. The motor output horsepower isdirectly proportional to speed (50% horsepower at 50% speed).
Fig.1.5 – torque-HP graphThe term constant torque describes a load type where the torque requirement isconstant over thespeed range.Horsepower at any given operating point can be calculated with the following equation:Torque is measured in Lb-FtSpeed ismeasured in RPM.Armature and Field Controlled DC DrivesThe motor is armature voltagecontrolled for constant torque-variable HP operation up to basespeed. Above base speed the motor is transferred to field current control for constant HP -reduced torque operation up to maximum speed. Fig.
1.6- Operation above Base Speed One characteristic of a shunt-wound DC motor is thata reduction in rated field current at a givenarmature voltage will result in an increase in speed and lower torque outputper unit of armaturecurrent Fig.1.7- Motor Speed and load charactersticsAC DrivesThe speed ofan AC motor is determined for the most part by two factors: The applied frequencyand the number of poles.
Where:N = RPMf = frequencyP = number ofpolesSome motors such as in a typical paddle fan have the capability to switch poles in and out tocontrol speed. In most cases however, the number ofpoles is constant and the only way to varythe speed is to change the appliedfrequency. Changing the frequency isthe primary function of an AC drive. However, one must consider that the impedance ofa motor in determined by the inductive reactance of the windings. Refer to the equation below.Where:XL = Inductive reactance in Ohms f = Line frequencyL = inductance This means that if the frequency appliedto the motor is reduced, the reactance and therefore impedance ofthe motor is reduced. In order to keep current under control we must lower the applied voltage to the motor as the frequency isreduced.
This is where we get the phrase “volts perhertz”. The most common method of controlling the appliedvoltage and frequency is with apulse width modulated”PWM” technique. With this method, a DC voltageis applied to the motor windingsin time controlled pulses in order to achieve current that approximates a sine wave of the desired frequency.
IGBTs or Isolated Gate BipolarTransistors are the latest technologyand offer the ability to switch the PWM pulses very fast. This allows several thousand pulses to be appliedin one cycle of the applied motor frequency. More pulses in a given cycle result in a smoother current waveform and better motor performance.6. AC Motor TypesAC motors can be divided into two main types: induction and synchronous. Induction motors aremost common in industry. Synchronous motors are special purpose motors that do not requireany slip and operate at synchronous speed.
The induction motor isthe simplest and most rugged ofall electric motors. The induction motor isgenerally classified by a NEMA design category. Before a meaningful discussion on NEMA type motors can be had, we should first look at what makes up a torque speed curve.7. Anatomy of a Speed Torque CurveGenerally speaking the following can be said about a speed torque curve when starting across theline.
Starting torque isusually around 200% even thoughcurrent is at 600%.This is when slip is the greatest. (Starting torque isalsocalled Blocked Rotor Torque, Locked Rotor Torque or Breakaway Torque.) Such a large inrush ofcurrent may cause the supply voltageto dip momentarily, affecting other equipment connected to the same lines. To prevent this, large motors will connect extra resistors to inductors in series with the stator during starting.
Extra protectivedevices are also required to remove the motor from the supply lines if an excessive load causes a stalled condition.As the motor beginsto accelerate, the torque drops off, reaching a minimum value, called Pull-upTorque, between 25-40% of synchronous speed (Point B). Pull-up Torque iscaused by harmonics thatresult fromthe stator windingsbeing concentrated in slots. If the windingsare uniformlydistributed around the stator periphery, Pull-up Torque isgreatly reduced. Some motor design curves show no actual Pull-up Torque and follow the dashed line between points A and C.As acceleration continues, rotor frequency and inductive reactance decrease.
The rotor flux moves more in phase with the stator flux and torque increases. Maximum Torque (or BreakdownTorque) is developed at point C where inductive reactance becomes equal to the rotor resistance. Beyond point C, (points D, E and F) the inductive reactance continues to drop off but rotor currentalsodecreases at the same rate, reducing torque.
Point G issynchronous speed and proves that if rotor and stator are at the same speed, rotorcurrent and torque are zero.At running speed, the motor will operate between points F and D, depending on load. Howevertemporary load surges may cause the motor to slip all the way back near point C on the “knee” of thecurve.
Beyond point C, the power factor decreases faster than current increases causing torque to dropoff.On the linear part of themotor curve (points C to G), rotor frequency is only1 to 3 hertz –almost DC. Inductive reactance isessentially zero and rotor power factor approaches unity.Torque and current now become directly proportional – 100% current produces 100% torque. If a1HP motor has a nameplate current of3.
6 amps, then when it draws 3.6 amps (at proper voltage and frequency) it must be producing 100% of it’snameplate torque. Torque and current remaindirectly proportional up to approximately 10% slip.
Notice that as motor load increases from zero (point F) to 100% (point E), the speed drops only45-55 RPM, about 3% ofsynchronous speed. This makes the squirrel cage induction motor very suitable for most constant speed applications (such as conveyors) where, in some cases, 3%speed regulation might be acceptable. If better speed regulation is required, the squirrel cage motor may be operated froma closed loop regulatorsuch as a Rockwell Automation variablefrequency drive.The locked rotor torque and current, breakdown torque, pull-up torque and the percent slip,determine the classificationsfor NEMA design motors. The speed-torque curve and characteristics of each design are as follows:Design A — motors have a low resistance, low inductance rotor producing low starting torque andhigh breakdown torque. The low resistance characteristic causes starting current to be high.
It is a high efficiency design; therefore, the slip isusually 3% or less. Fig.1.7 – Design A ACInduction MotorPerformane:Linear induction motors are often lessefficient than conventional rotary induction motors; the end effects and therelatively large air gap that is often present will typically reduce the forcesproduced for the same electrical power. The larger air gap also increases theinductance of the motor which can require larger and more expensive capacitors.
However, linear induction motors can avoid the needfor gearboxes and similar drivetrains, and these have their own losses; andworking knowledge of the importance of the goodness factor can minimisethe effects of the larger air gap. In any case power use is not always the mostimportant consideration. For example, in many cases linear induction motorshave far fewer moving parts, and have very low maintenance. Also, using linearinduction motors instead of rotating motors with rotary-to-linear transmissionsin Motion Control systems, enables higher bandwidth and accuracy ofthe Control systems because rotary-to-lineartransmissions introduce backlash, static friction and/or mechanical compliancein the control system.Uses: Because of these properties, linear motors are often used inMaglev propulsion, as in the Japanese Liminio Magnetic levitating trainline near Nagoya. The world’s first commercial automated maglev system waslow speed Maglev that ran from the airport terminal of Birminghaminternational airport to the nearby station between 1984–1995. Thelength of the track was 600 metres (2,000 ft), and trains “flew”at an altitude of 15 millimeters (0.
59 in), levitated by electromagnets,and propelled with linear induction motors. It was in operation for nearlyeleven years, but problems with the electronic systems made it unreliablein its later years. One of the original cars is now on displayat Peterborough, together with the RTV31 hover train vehicle.
However, linear motors have been usedindependently of magnetic levitation, Tokyo is an example of an automatedsystem that utilizes LIM propulsion; the longest Transit system systememploying such technology is Vancouver sky train, with approximately 60 km(37 mi) of track compatible with Innovia Metro trains. Linear induction motor technology is alsoused in some Roller coster At present it is still impractical on streetrunning Trams, although this, in theory, could be done by burying it in aslotted conduit.Outside of public transportation, verticallinear motors have been proposed as lifting mechanisms in deep Mines, andthe use of linear motors is growing in Motion control applications. They arealso often used on sliding doors, such as those of low floor trams such as thecitadis and the eurotramDual axis linear motors also exist. Thesespecialized devices have been used to provide direct X-Y motionfor precision laser cutting of cloth and sheet metal, automated Drafting,and cable forming. Also, linear induction motors with a cylindrical secondaryhave been used to provide simultaneous linear and rotating motion for mountingelectronic devices on printed circuit boards. Most linear motors in use are LIM (linearinduction motors) or LSM (linear synchronous motors). Linear DC motors are notused as it includes more cost and linear SRM suffers from poor thrust.
So forlong run in traction LIM is mostly preferred and for short run LSM is mostlypreferred.Linear induction motors have also been usedfor launching aircraft, the Westinghouse Electropult system in 1945 was anearly example and the (EMALS) was due to be delivered in 2010.Linear induction motors are also used inlooms, magnetic levitation enable bobbins to float between the fibers withoutdirect contact.A motor rated for 60hz operation may be run at higher frequencies when powered by RockwellAutomation AC Drive.
The top speed dependsupon the voltagelimits of the motor and it’s mechanical balancing. 230V and 460V motors normally employ insulationrated for as much as1600V,so the voltage limit is notusually a problem. An average 2 pole industrial motor can safely exceed its base speed by 25%. Many manufacturers balance their 3 pole and 4 pole rotors to the same speed – 25% over the 2 pole base speed. A 4-pole motor may therefore operate up to125% over base speed before reaching its balance limit. A 60hz 4-pole motor might run up to135hz, whereas a 60hz 2-pole motor would reach its balance limit at 75hz. Both motors would runat the same RPM.
Always contact your motor manufacturer if you plan to operate at these speeds. Constant Voltage OperationWhat happens to the volts per hertz ratio above rated frequency? If output frequency is increased to 120hz with 100% voltageapplied to the motor; the Volts per Hertz ofthe drive is no longer7.6 but rather 3.83.
The same Volts per Hertz ratio results when a line started motor isoperated at60hz with only 50% voltage applied(for reduced voltagestarting). As might be expected the effecton torque is the same. Recall that torque varies as the square of theapplied voltage: 2 T K 1 xEAs such, maximum torque at 120hz is only25% of the maximum torque at 60hz.If AC drive output frequency is reduced from120hz to 90hz at a constant voltage,the Volts per Hertz ration improves from3.83 to 5.1 V/Hz.
This isthe same as providing 66% voltage at 60hz to a line-started motor. Torque will be 0.662 or 44% ofthe full voltagetorque at 60hz. Below illustrates the peak torque curve for constant voltage operation from base speed to 4 times basespeed.