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Making Motors Dance Our Tune © 2012 Dr. B. C. Paul Global Spec.com Thomasnet.com Tricky Things We Like Motors To Do • Go at what ever speed we need them to turn at the moment. • Apply torque gently and bring things up to speed without breaking things into pieces • Turn our load at any speed we desire even if it does not match the motor The Problem of Speed Matching • Motor speeds are fixed by number of poles in the motor and current frequency (assuming an AC motor) • For 60 cycles per second – 3 phases – one rotor pole you will get about 3600 rpm • For 6 phases in the stator and for a two pole rotor you will get about 1800 rpm • For a 3 pole rotor you will get 1200 rpm • Further increases in phases in the stator or poles on the rotor usually don’t make much sense • The Chances that what we want to drive needs one of those 3 speeds is pretty remote. Chain and Sprocket Speed Reduction Will get you about 93% efficiency In the open. About 95% efficiency in an oil Tight enclosure. V Belt and Sheave Speed Reduction About 94% efficient. Many Sheaves have multiple sizes so you Can shift belts from one sheave to another for Easier changes of speed. Spur Gear Speed Reduction Cut Spur Gears can get About 90% efficiency Cast Spur Gears only About 85% Gears can be noisy and Require either multiple Gear reduction phases or Some pretty big gears. Helical Gears Single reduction About 95% efficient Double is about 94% Triple is about 93% Worm Gears Speed reductions of about 20:1 or less 90% efficient Speed Reduction up to 60:1 About 70% Reductions of 100:1 down to 50% Sizing Sheaves and Sprockets • I would like to use an induction motor turning at 3500 rpm to run a pump using a belt drive at 850 rpm • To avoid excessive bending of my belt I need the smaller sheave to be 9 inches in diameter. • How big does the other sheave need to be? • Which Sheave is on the motor and which on the pump shaft? A More Friendly Coupling • Magnetic Coupling Take two disks – put magnets in them – when you spin one disk the other disk will try to spin.to Keep the magnets lined up I Can Get a Gear Ratio by adjusting the number of magnets in each wheel Two Basic Designs Radial Design Axial Design Now Why Would I Want to Do That? • There is no joint contact to have to lubricate • I can transmit power through a wall or seal without having a hole in it • I will not be pushing on the bearings on one side trying to keep two sides of a gear or belt assembly tight • Vibration on one side does not transmit to the other side More Whys • It is forgiving of misalignment up to about 2 degrees Can Provide Torque Control • Can design magnetic fields to slip if torque goes above a design tolerance • Can prevent things from breaking • Can cause it to go about 30% slower on driven side compared to the drive side by adjusting the air gap. How Efficient Can This Be • With good alignment, small gaps and little slippage you can get about 99 to 99.9% efficiency • Even on bad day you should be above 97% • Unlike other systems you don’t have frictional losses So What is the Catch? • Optimized magnetic coupler systems are not an off the shelf item • Magnetic fields do induce electrical fields and flux flow through solids does produce heating • Magnets don’t like heat (loose magnetism) • Can combat with expensive magnets (get up to about 300 F) • Can put air blown cooling systems or such on the coupling • Speed or torque limits are generally a design characteristic of the product and do not change on the fly Next Chapter How Can I Vary the Speed of a Motor on the Fly? How to Speed Control A Motor • Use a DC motor • I can control its speed with a variable resistor that controls the voltage I apply to the electro magnetic poles on the stator. • Use a synchronous AC motor • The motors speed is a function of the number of poles on the rotor and the frequency of the AC current. • Use a solid state AC frequency controller to produce what ever AC frequency I need. • Use a Switched Reluctance motor and switch the DC current to poles to make the rotor turn at the desired rate. Problem of DC Speed Control • Where do I get enough DC current to run a 1000 HP motor? • That’s a whole lot of batteries in series • We need a source of DC current to run the motor • Since Thomas Edison lost the fight to make the US a DC electric system there isn’t much big DC current around to be had How About Our Own DC Generator • Turns out DC Generator is not to hard to come by - It’s a DC Motor • If I turn the shaft on a DC Motor I force the armature to turn • Remember that the current loops on the armature generated a back voltage • vg = Køη • Now I just tap the voltage off at the ring and brush contacts and I have my current • Note that DC voltage is direct function of the speed at which the armature is turned Just One Little Problem • As the armature coil rotates through the magnetic field the current direction alternates as different ends of the loop line up with the fixed poles • I didn’t want an AC generator • Electricity is tapped off the loop by rings contacted with brushes • break the rings so that positive and negative reverse whenever the current reverses • broken ring system is called a commutater The Result Voltage Produced by One Loop on the Armature after Commutation Because we have several loops that peak at different times (and capacitors) we can get an almost perfectly smooth DC voltage Just One Little Problem • Need to find enough constant speed hamsters to turn a DC generator to produce a constant voltage for a DC motor • How About Using an AC motor instead • We have lots of AC voltage around • We’ve met the Synchronous AC motor so we can get a constant DC current from a constant speed AC motor So Lets See How this Works • We need speed control so we get a DC motor • We now need big DC voltage to run the DC Motor so we get a DC generator • We now need something to turn the DC generator at a constant speed regardless of load so we get a synchronous AC Motor Ward Leonard • The combination of a synchronous AC motor to turn a DC generator is called a Motor Generator Set • Also called a Ward Leonard System after the people who developed it • Through most of the 20th Century it was the only way to get speed controlled systems large enough for mining applications Strengths of Ward Leonard • It is a fully reversible system • AC and DC motors could just as easily be generators if you turn the motor with the shaft instead of the motor turning the shaft • For a lot of large machines that have inertia to change directions you can recapture the power from the inertia • It is very robust against fluctuations in the power supply • It has a very large spinning reserve • If voltage is below 70% of rated then you have a problem. Weaknesses of Ward Leonard • Motors are loaded with rings and brushes that mean maintenance cost and down down. • Everything in the system is an inductor so it will throw your power factor off – especially at low load • Can put extra capacitors all over the lines to try to correct the power factor • Inertia – those spinning rotors add the inertia of big machines and make them respond slowly Alternatives to Ward Leonard • Solid State Electronic have semiconductor diodes that only let current flow one way • Arranged in proper circuits these diodes can be used to reverse the oscillations of an AC Current Simplified Thyristor Circuit + - AC Source DC Motor Load Simplified Thyristor Circuit + Limitation - Note that if the load becomes a generator and tries to feed power back into the line it can’t Thyristor Controls • Also called SCR for Silicon Controlled Rectifier • Also called DC Static • Thyristor Controls allow us to directly produce DC current from an AC line • This allows us to use DC speed controlled motors without having to install a motor generator set • Barrier for many years was getting thyristor systems big enough for a competitive price Advantages of DC Static • Solid State Electronics replaces lots of moving parts • Less maintenance • Fast change out of solid state boards • Less inertia and faster machine motion response Disadvantages • It can be less forgiving of variations in line power • It works fast to moderate the problem • It Dislikes Dirt and Moisture – ouch – this is a mine • Means some sort of isolation cabnet. • It is not reversible to generate power back • You can put in another reverse thyrister circuit but this can be very costly compared to the recovered power (done for little electronics – usually not big machines) • It too lowers the power factor angle – especially at low speeds AC Frequency Control for Speed • AC current is brought into a thyristor system and rectified to DC • The DC current is then sent to another solid state device called an inverter • An inverter reverses the + - value of a DC current • It can be made to turn inversion off and on at a regular frequency • We have now simulated an Alternating Current with any frequency we feel like Controlled Frequency AC • With our AC frequency controlled to any value we can now speed control an AC motor using solid state controls • Only about the 1990s that we got to AC frequency control large enough to run mine hoists • Mid 1980s saw Bucyrus Erie introduce the first Freq Controlled AC motor mining shovels AC Freq Control • Strengths • 0.95 Power Factor is almost constant • NO RINGS OR BRUSHES improves availability • Most area under speed torque curve - great power and response of motions • Electronics are amenable to modularization with board change outs (though there are more of them than with DC static) • Limits or eliminates brushes and high wear parts AC Freq Continued • Weaknesses • Can’t regenerate power back into the line • Need to keep those electronic isolated from a dirty environment • More vulnerable to dirty disrupted AC power supply • Power utilization usually not quite up with DC static • Can still be a little pricey (but those cheap AC motors can save weight and offset some of electronics cost) • AC Freq Control will probably come to dominate increasingly in the future Switched Reluctance • A variation of AC frequency control • AC current is rectified to DC • Now instead of using an inverter to create a specific frequency of AC current • We use switch electronics to turn electromagnetic fields off and on on the motor • Often use PLC to try to control a very non-linear motor and reduce torque ripple. Variations on the AC Frequency Control Theme • AC Frequency control with a synchronous AC motor gives strict speed control • But AC syncronous motor is most high wear and complex. • AC Frequency control with an induction motor • Simple with fewer moving parts • But speed control is not strict • Induction motors run a slower speed than current in order to generate torque Switched Reluctance • Its really switched DC • Computer controlled motor • Lots of electronics to worry about • Has good starting torque but not best for low speed operation. So What Am I Likely to Pick • Cheap and simple is the first choice • AC induction motor is cheapest and simplest • Motor does slip relative to frequency of current but if we have variable frequency control that may not be important most of the time. Now for Trying to Start a Motor • And bring something up to speed without jerking it to pieces • Means both speed and torque control • Conveyors can be a classic problem statement Getting Starting Force for a Conveyor • Starting Force is the Lift required to a fully loaded belt • (I don’t want to shovel the thing if I have to start a loaded belt) • Plus double the Frictional Force • Static friction is always greater than rolling friction • Practice is to use a factor of 2 to be safe A Motor Sizing Example • The lift Force is 66667.04 lbs tension to lift up slope • took it out of the tension calculations on an old example problem • The frictional Force is • Total Effective Tension - 73367.16 lbs effective tension • Minus 66667.04 lbs tension to lift up slope • Net Friction - 6700.119 lbs tension Continued • Frictional Force is 13,400 lbs • Lift Force is 66,667 lbs • Total Starting Force is 80,067 lbs Converting To Starting Torque • Need to know pully sizes and diameters • our assumed case is 4 ft. • Had I picked motor before belt I wouldn't have known Force Torque = Force * Lever arm Lever Arm Getting Starting Torque • Suppose the pully is 4 ft diameter • 80,067 lbs * 2 ft = 160,134 ft-lbs torque • We usually need a gear reducer • belt speed is 400 fpm • pully radius for 4 ft diam ¶ * D = 12.56 ft per revolution • pully speed is 31.85 rpm • Big motors usually 1800 or 3600 syc speed (3500 and 1750 common with slip) Getting our Gear Reducer • 1750 rpm / 31.85 = 54.94 • say 55 to 1 gear reduction • Torque on Motor Shaft is • 160,134/55 = 2912 ft-lbs • 2912 ft-lbs/ .94 (for gear reducer efficiency) • 3098 ft-lbs Checking Our Full Speed Torque Requirements • By same calculation Full Speed Torque • 146,734 ft lbs at pulley shaft • 146,734/ 55 / 0.94 = 2838 ft-lbs on motor shaft • Motor Sized on Full Speed • HP = 2838* 1750/5250 = 946 HP • (5250 is a conversion constant for ft-lbs to horsepower) • Note that Starting Torque is 3098 ft-lbs • Running Torque is 2838 ft-lbs • Need 9.1% more torque to start (can be a lot more) This brings us back to Motor Ratings and Starting Heating • Motor heating is related to the current through the stator – big current =big heat • We have also seen that starting torque is high • What about starting HP? • HP = Torque * rpm/5250 • How many rpm does the motor turn when it is standing still? • Starting HP must be ZERO! The Hot Issue • Remember that the back voltage induced in the stator windings corresponds to the HP output of the motor Applied Voltage If the back voltage is zero, what happens to the stator current? Starting Impacts • The heat produced by resistances in the stator is directly proportional to the stator current. • Result #1 - Motor starting produces tremendous heat • Implication - you can only try so many starts (usually 2 an hour) or you will overheat • Result #2 - Motor draws high starting current - high current draw increases the voltage drop on the feed lines • Can cut starting voltage back and loose starting torque depending on mine wiring Speed Torque Relations . Locked Rotor Torque Breakdown Torque Rated Torque Pull-Up Torque Torque Speed Torque - HP Relations • A and B type Motors • Little 1 HP motor may have Lock Rotor of 275% of rated torque • By 50 HP that surplus is only 140% • At 200 HP there is no surplus locked rotor toque above rated • Really big ones above 200 HP may only give you 80% of rated torque Checking Whether Our Motor Will Start the Belt • We needed about 960 HP to run the belt • This suggests a 1000 HP standard size motor • Check out the starting torque (Locked Rotor) • Rated Torque = 1000 * 5250/1750 = 3000 ft-lbs • Locked Rotor at 80% of rated 2400 ft-lbs • Need also to consider the impact of loss of starting voltage due to excess current draw Voltage Torque Relations • Remember the field strength and thus the torque varies with the square of the applied voltage • Thus • Torque Actual = Torque Rated * (Applied Voltage2)/(Rated Voltage2) • Most mine electrical systems are designed to allow no more than 10% voltage drop when a big motor kicks in Reduced Voltage Impact on Locked Rotor Torque • Actual Lock Rotor Torque = 2400 * (0.92)/(12) • 1944 ft-lbs of starting torque • To start a loaded belt we need 3098 ft-lbs • This thing isn’t going to start What to do about our problem • Use a bigger motor • 1000 HP * 3098/1944 = 1594 HP • Oh Boy I get to tell the boss we need a 1750 HP Motor instead of a 1000 HP - that will go over really well Our Little Belt Starting Problem • Get a Class C motor • Nema only rates these to 200 HP • Since Nema only rates motors to 500 HP we are already in a manufacture specific range • Class C motor usually have 50% to 100% more starting torque than a B • depending on drop in applied voltage and exact rated Locked Rotor Torque 1000 to 1250 HP would work Pull Up torque • For many large motors about 90% of Locked Rotor 90% of Locked Rotor Checking to Make Sure the Conveyor doesn’t stall out on Start Up Speed Torque Relationship 10000 ft-lbs 8000 6000 Motor Load 4000 2000 0 0 50 100 % of Synchronous Speed Line for 1750 HP B type motor 150 Plot a line from starting torque to running torque. Make sure it doesn’t dip below the pull-up torque. Bringing the belt up to speed Torque available above the load line can accelerate the belt load. Speed Torque Relationship 10000 ft-lbs 8000 6000 Motor 4000 Load 2000 0 0 50 100 150 % of Synchronous Speed This load will make a slow start - slow down even more and then take off. The Breakdown Torque Problem • As the load gets moving the stator current drops into line and voltage comes back to normal • This motor produced a breakdown torque of 9,450 ft-lbs • Torque on the pulley • 9,450*55/1 = 519,750 ft-lbs • Translated to belt tension • 519,750 ft-lbs/ 2 ft = 259,875 lbs tension So that’s Why they call it Breakdown Torque • 259,875 lbs/ 24 inches = 10,828 lbs/inch width • We can see that this breakdown torque will force us to drastically over-design the belt to avoid it being torn in two. • Generally we would like a belt that could handle about 5 to 10% more tension than required for start up The Problem of Taming the Speed Torque Curves The Ideal Belt Motor would have a fairly flat speed torque curve about 10% above the line for load torque Nema class C type motors have fairly flat curves - but are usually well above 10% extra for the load torque line Dealing with the Jerks (Not you the motor starting) • Use reduced voltage starting • Torque = Lock Rotor * (voltage used/rated voltage)2 • Can make not start • Most even uses SCR system Why Reduced Voltage Starting? • NEMA C type motors have the right shaped curve - its just too much more than what is needed • The same Torque = Rated Torque * (V applied2)/(V rated2) lowers the Torque curve during start-up and reduced big voltage drops from excess current draw How is Reduced Voltage Starting Accomplished? • Simple Method • Put a line of resistors in in front of the motor and the short them out as motor speed comes up Solid State Starters • Usually SCR based • One common design limits the stator current - just raises and lower resistance to keep a controlled stator current • (More difficult to reshape speed torque curve but you can raise and lower) • Second Design monitors and accelerates belt in a linear fashion • fancier but can reshape curve A More Elegant Solution Impeller Wheel Motor I can have a motor turn an impeller wheel in the air. It will impart a small amount of momentum to the air. Now Lets Mount a Similar Wheel to the Drive Shaft of Something Turbine Wheel Conveyor Pulley Now Lets Put an Impeller Wheel and a Turbine Wheel Face to Face With an Air Gap Between them so they don’t Touch When the impeller wheel spins a little of the air Momentum will transfer to the other wheel. Of course the efficiency sucks Maybe We Can Jack Up Efficiency if We Slap an Enclosure Housing Around Them Now we can make our fluid circulate around In a coupling loop. Of Course Air is a Pretty Thin Fluid – How About Something Thicker An oil might be a good choice Now Lets See About Starting Something We start and standstill. Our motor begins rotating Our impeller. Our impeller is pretty much sitting in Thin air so our motor is starting at as close to zero Load as possible – well that should solve any Problems we have with an induction motor having Wimpy starting torque and low pullup torque As the Motor Speeds Up Our Fluid Starts to Circulate You begin to see turning pressure applied to The turbine side of the coupling. Of course if We have a stubborn load on the turbine side The load can hold the turbine still. As the Motor continues to speed up our fluid circulation And coupling become more complete Our Start Up Torque Looks Like This Oh Wow – There is a God – This is just what we wanted! The torque builds up until the load starts to turn. Which color curve the coupling torque follows depends On the amount of fluid in it (which gives us yet another Idea on how to control things). Additional Fluid Coupling Benefits • No direct contact between parts means low frictional wear • No direct contact means low vibration transfer Do We Have A Replacement for Magnetic Couplings? • Not really • The impeller and turbine wheels must have fluid circulation between them – no transmitting power through walls • The impeller and turbine will spin at the same speed (less a slight amount of slip) – you can’t set a speed ratio by changing the number of magnets on each wheel • You need a steady distance between the impeller and turbine – no using the coupling for misalignment tolerance Can I Use My Fluid Coupling as a Torque Limiter? • Yes - like the magnetic coupling increasing load causes increasing slip – until the slip goes to 100% • In a fluid coupling this critical slip point can be adjusted on the fly by pumping fluid into or out of the coupling What Happens with Slip • The amount by which the turbine turns slower than the impeller is the slip – it is about 1 to 6% • Thus when the load turns at 96% of the drive speed the slip is 4% and the efficiency is 96% • If the load goes to 100% slip the energy into the coupling is converted to heat • The coupling has a heat plug that will blow and drain the fluid when things get hot enough Typical Curve for Sizing a Fluid Coupling