Adjustable Speed Drives, Motor Torque And Hp

Transcript

Adjustable Speed Drives, Motor Torque and HP Important considerations when applying Adjustable Speed Drives By Russ Safreed, PE IEEE WV Section Meeting - held at WVU Tech, Montgomery, WV February 25, 2014 Abstract  The goal of this presentation is to present practical considerations when applying Adjustable Speed Drives in real world applications. A Real Life Project (things are never as easy as what they seem to be) Project Basic Data  Difficult material to pump  Owner had experiences with their installation     that did not work well. Installation in Germany worked really well. Wanted to copy it exactly. Owner was very reluctant to change any part of the Germany design. Fast tracked project… very short schedule… little engineering required.. Pressure to just copy the installation and get it installed! Electrical Data provided from Installation in Germany  Motor:  600hp,  1500rpm (4 pole)  driven by Adjustable Speed Drive Initial Project Flow Owner wanted to spec and buy motor separately from pump Process guys worked on pump spec which was a long process We began work to specify and obtain quotations on motor and ASD. Electrical Progress  Specified 600hp Motor  4 pole machine, 1800rpm in US  Can slow the motor down with the ASD – so no problem there..  All this work was done without having the manufacture data on our pump yet… Pump Data Sheet Arrives  Pump requires 420HP at 1150rpm  OK… no problem… we’ve got a 600hp motor… exactly what they used in Germany.  Let’s move the project on… remember… copy the design and keep the project on schedule!! Wait… Let’s think about this  Why a 600hp motor in Germany? The pumps      are identical. Why did they use such a large motor?? Maybe it was an extra motor they had laying around?? Is the pump really the same? Are the flow rates the same? Maybe the process guys didn’t specify the exact same pump? Maybe we need to understand this better before we move on… Additional Data  Additional conference calls with Germany revealed:  The pumps they were using were obtained from the US on a project that had been cancelled.  Load data indicated that power consumption was in the 400hp range… not 600hp.  So why such a large motor??? Some Clarity begins to emerge  Germany’s power system is 50hz. US is 60hz     (Humm?? Let’s think about that. How could that be affecting this… I mean, 420 hp of work in Germany will still be 420hp of work in the US) Wait.. Here’s a thought. Since the pump was from a US project, it would be designed for a commonly available motor speed in the US.. which was about 1150rpm. German’s available motor speeds are 3000, 1500, 1000. Maybe the German’s had no choice but to buy 1500rpm motor and slow it down to 1150 to match pump. Could this have something to do with it??? Reviewed the Germany data again and found that they are operating the pump around 1150rpm… We may be onto something.. Do we really understand HP and Torque?  Torque – a force measured in ft-lbs  HP is a measurement of Power  HP = (Torque X RPM) 5252 Torque  Torque is a measurement of force.  Torque can be multiplied or divided through a gearbox to achieve any output torque regardless of input torque available.  For motors, measured in ft-lbs  Notice that the unit of time is not involved with torque Horsepower HP cannot be multiplied through a gearbox. HP = (Torque X RPM) 5252 HP is always a consideration when “time” is involved. ie. If you are trying to pump a liquid and trying to achieve a given GPM, then you are going to be concerned about HP. The Motor Nameplate Motor Nameplate  A motor’s nameplate will tell you the horsepower the motor can provide given certain input parameters.  It is basically the HP output at one point on “the curve”.  If any of the input parameters change, the horsepower will not be what the nameplate says. Let’s do some research…  Time to REALLY read the ASD manual!!  Not really exciting stuff!! …300+ pages…  But… we find this statement… “the ASD can provide constant torque for speeds below the baseline frequency and constant hp for speeds above baseline frequency”  Wow… that is easy to skim over and not really think about what is it saying… Torque Horsepower Lets put that statement in a graph nameplate freq nameplate freq Motor RPM Motor RPM HP = Torque x RPM 5252 We are on to something..  Remember that HP formula???  HP = Torque X RPM 5252 What happens to HP when you slow a motor down? Lost HP  So let’s evaluate Germany’s motor HP.  600hp = (T x 1500)/5252  T = 2100 ft-lbs Now lets change speed to 1150 and plug the Torque back into the formula HP = (2100 x 1150)/5252 HP = 460hp at 1150rpm Same motor in US  4 pole motor = 1800rpm  600hp = (T x 1800)/5252  T = 1750 ft-lbs AT 1150 RPM  HP = (1750 x 1150)/5252  HP = 383  Wow… this motor that we have specified isn’t even going to work!! We are numb..  Let’s read the manual again… run the numbers again… does this make sense…  Yes it does… Our conclusion is that Germany had to oversize the motor because they had to slow it down from 1500 to 1150 to satisfy the pump..  We would need to oversize even more!  So what would you do?? Changed mindset from “copy design” to “lets engineer this right”  Since the pump requires 1150rpm, the obvious      solution is to provide a 1200 rpm motor. Motor size will be 450hp All of a sudden we are heros: Avoided an installation that would not have worked (this would have been really bad!) 450hp motor and ASD was significantly less expensive than 600hp motor and 600hp ASD. Win-Win situation… (those don’t happen often!!) Let us think about this a little further outside the project.  What concerns are there when applying ASD’s to centrifugal pump loads.  Need to consider pump affinity laws and the ASD’s torque-speed curves (loosely follows the Torque-HP formula) Pump Affinity Laws HP1 HP2 FLOW1 FLOW2 RPM1 RPM2 3 RPM1 RPM2 HP REQUIRED INCREASES EXPONENTIALLY (CUBED) AS RPM GOES UP FLOW RATE IS DIRECTLY PROPORTIONAL TO RPM Learnings  When operating a pump above design speed, the HP requirement goes up exponentially  Whereas, the ability of the ASD will not allow any additional HP output above motor nameplate.  If you plan to run a motor above nameplate speed, you will have to significantly oversize the motor and ASD  Not usually any issues when operating a motor below nameplate rpm because hp requirement will drop faster than the ASD output will drop. Additional thoughts  How does this affect other types of loads.  Does this scare you away from ASD design.  Don’t let it… there are plenty of resources out there to help you…  You just need to never quit asking yourself the question… what am I missing here…  Donald Rumsfield made a quote during the second Gulf War …. It also applies in engineering… Rumsfield Quote There are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the ones we don't know we don't know. Conclusions  How do we protect ourselves from the “unknown      unknowns” Don’t ever think you know or understand Never stop participating in training Get help from the experts (equipment manufacture in many cases) Read magazines or web articles on subjects you are involved with. Be careful you are not led down a path that doesn’t follow good engineering practice