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AC motor help

  1. Sep 7, 2008 #1
    how can i get greater performance from an standard inverter duty motor( marathon black max). I am in the process of buiding an inverter/controller to use as part of an EV conversion.
     
  2. jcsd
  3. Sep 7, 2008 #2

    russ_watters

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    Staff: Mentor

    You mean greater than the nameplate horsepower? Run it over 60 hz with a variable frequency drive (or just add load, depending on what the load is) and keep it cool so you don't burn it out. Typically, motors are rated for 15-20% under their actual capacity.
     
  4. Sep 7, 2008 #3
    Using modern inverter controllers can allow you to use motors well beyond their original specification without overheating, provided you do it with some care. For example, I can take a 2-pole 3-phase 1.5kW motor rated to run at 2850rpm (50Hz) to run safely at more than 4000 RPM (75Hz). The torque will drop off severely at 4700RPM, but that is well into very bad inefficient territory for other reasons anyway.

    Typically, inverter controllers can manufacture their own version of 3-phase, and so can rotate the field at frequencies other than the standard 50 or 60 Hz. Also, the current drive is delivered as PWM (pulse width modulated) using carrier frequencies that can be chosen as high as 15kHz (depending on lead lengths). They use various options for speed and torque control, and can even work well for many applications without encoder pulse feedback by using what is known as flux vector measurement.

    I have used such motors in unwinders, delivering full torque one direction while being dragged by the material in the other direction, using just the provided torque control mode. The only caution here is not to let the motor spend too much time at speeds less than about 15% of rated, or you risk inadequate cooling.
     
  5. Sep 7, 2008 #4
    I am looking at the ac induction motors that some of the major ev cars use and they weigh only 200lbs or so but are rated at 50-60kw constant and 100-125kw for short periods of time. Is there any way i can take a widely available inverter duty motor(i.e. Marathon Black Max - 30hp) and get even cose to these kind of numbers. I will be using a custom inverter that will be programmable to whatever specs i choose. I was also hoping to make new custom motor case with liquid cooling added. Why does this motor that weighs close to 750lbs produce such inferior numbers caompared to the Siemens motors used at www.metricmind.com that weigh only 200lbs
     
  6. Sep 7, 2008 #5

    russ_watters

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    Note, the torque may depend on the load. If you're driving a fan, upping the rpm will increase the torque. If you take a 1.5 kW motor running a fan at it's rated power and increase the frequency by 50%, you'll increase the torque by 225% and the power output to 5 kW, with a corresponding increase in amperage -- almost certainly burning out the motor.

    Caveat - I'm not familiar with a "torque control" mode and what kind of load that would apply to. The OP didn't mention the application.
     
  7. Sep 7, 2008 #6

    russ_watters

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    Not sure, but it could be a matter of cooling.

    What is the application?
     
  8. Sep 8, 2008 #7
    The appllication will be for an EV vehicle.
     
  9. Sep 8, 2008 #8
    For Russ & yikes

    The nameplate power specification is normally the power the motor can output, and for the designed input frequency, trying to go beyond this is asking for trouble. In this case, we are using high speed switching electronics in a pulse-width-modulated mode, delivered with a mix of high-bandwidth analogue and microprocessor supervisory circuits. While the "overclocked" motors I have tried are delivering energies beyond their nameplate rating, I do it conservatively.

    The main increase comes from the higher speeds. The actual currents and shaft torques I use are, if anything, somewhat less than when working the motor at maximum load from 50/60Hz mains. The air resistance (windage) is a non-linear increase, and there are other things there to scare me. When working properly, the 2850 motor is running over 4000 rpm.

    To clarify about "Torque Control Mode". The usual input is a speed reference demand to a control loop. Feedback derived from flux vector measurements or (as in my case), encoder pulses, are used to close the loop. The entire speed control loop can be fed within a position control loop should the application require it.
    Inside the speed control loop, the current control also has a closed loop feature, the feedbacks including all sorts of safety shutdowns, but importantly, the input is a torque demand. Instead of using speed control, my reference demands how much current will be delivered to develop torque. Should the speeds go beyond reasonable, these other factors win. There is a maximum setting. I use the motor as a "tensioner", driving in the opposite direction to the way it is being pulled. If the reel is big and heavy enough, the motor will oblige by reversing the force to help accelerate it.

    There are many competing products for this market. I happen to be using Omron brand, but there are many Japanese, German, and USA manufacturers. The inverter has a PLC add-on card to allow programmed operation.
     
  10. Sep 8, 2008 #9
    I am impressed with your knowledge of motors and their controls. It is helpful information going forward. What I am uncertain of is what can be done on this particular motor model (Marathon Black Max - 30hp - inverter duty) to achieve greater performance. From what i have read inverter duty motors are a little tougher thatn your standard run of the mill induction motors ansd i was hoping this would allow for greater performance. Perhaps it does not make any difference from one motor to another. Just wandering.
     
  11. Sep 8, 2008 #10
    Do you know the formulas for figuring hp and torque for a motor if i run it at higher frequencies and voltage.
     
  12. Sep 9, 2008 #11
    My guess is that a motor claimed to be built for "inverter duty" will have magnetics construction features that minimise iron losses, and allow it to support currents at high frequency. Some motors actually have deliberately chosen lower inductance coils that will not limit the risetime of switching waveforms.

    The use of these inverters can risk high harmonic currents and EMC interference from distorting the supply waveform, so often come with a weighty filter although the better ones minimize the filtering needs by clever power factor correction stages. A 22kW motor is something that the supplier will have lots of data on, and should be able to provide you with lots of ways to figure how to use it.

    I had excess capacity in the provided motors, so I took the lazy way known as "Autotuning" This is a short test run with the shaft not connected to any gearbox or load. Some parameters are given, such as the voltage available for driving it, and the maximum rated current, and the number of poles, etc. The "tuning" test determines things like the motor line-to-line resistance, the motor leak inductance, the motor iron saturation coefficient, the motor field rotation slip, and a truly astonishing array of other stuff.

    Once this basic stuff is known, you can do test runs to optimize the motor performance. It can make alarming, but harmless sounds as you take it into instability, but once done, it becomes possible to make the shaft turn real slow at full torque, set ramp rates, and manage active braking, with a few uncontrolled currents finally making into a braking resistor. For me, now that this new stuff has come along, A.C. motor control is the mode of choice for cost and flexibility reasons.

    The list of parameters, compensations, control modes, and configurations runs into hundreds. Also, there is a certain amount of supplier lock-in in the way they tend to make their parameter sets relate to their control gear. Do explore the set-up and programming instructions and help manuals for more than one product. It gives a feel for what features are supported as standard.

    I know I got there without tangling with motor equations. It was a pragmatic approach, but I appreciated it could be done without risking the motors. There are just so many safety lockouts.
     
  13. Sep 9, 2008 #12
    PS. russ_watters is absolutely correct when he cautions about the actual energy extracted. In my case, I am running 2850RPM motors at 4000RPM, but at the same time, the actual pull torque required for the task, and hence the currents required are only some fraction of the maximum.
     
  14. Sep 11, 2008 #13
    I am now looking at an ac induction servo motor rated @18.5kw,105Nm of torque, 2000rpm, 69 amps up to peak torque of 300,8000 rpms. It is 260v motor. What is the realtionship between torque and hp. This motor wil have plenty of torque for my needs, but is it possible to use space vector control to increase hp without have torque that is disproportionate. i amtrying to figure the relationship controlling torque,rpm,Kw,amps in connection to space vector control
     
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