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Electrical Power to Mechanical Power

  1. Mar 27, 2009 #1
    Hello! We (myself and the other folks at my company) have a couple of questions about the proper way to compare electrical energy to mechanical energy. Our application involves holding a load at zero speed, as well as moving it back and forth at varying speeds. Most of the time, our motor will be at zero speed, holding a constant torque. We have some profiles that we are expected to run, and we wanted to calculate the RMS power requirement of the profile, to ensure that it is below the RMS power that can be supplied to the motor from the drive.

    Our EE tells us that the power required to hold the load in place is very small, and probably won't contribute much to the RMS power. His claim is that the power draw is only what is required to overcome the electrical losses. Presumably, this is still P=VI, where I is the stall current. His argument is that there is no mechanical work being done (obviously), thus there will be little electrical work being done.

    My feeling is that this is not necessarily the case. Will the current increase as the motor starts to turn, even if the torque is constant? If there is a torque constant, does that mean the current will have to increase, or is stall a special case?

    Our EE performed a test on a small motor a little while ago and monitored the power while varying the load and the speed. He observed that the power requirement to hold a load at zero speed was small. When the speed increased and the load was reduced, the power draw increased significantly (this is how he justifies his above claim).

    Our ME has a story to support his argument as well: Many years ago, we built a machine that employed a motor to hold a load at zero speed as well as to move the load dynamically (very similar to what we're doing now). He said that the drive saturated the RMS power while holding the load in place, and when they commanded some acceleration after holding the load for some time, it didn't respond because the drive's overload protection was limiting the juice going to the motor.

    Who's wrong? Are they both right? I agree with some of what both of them say, but I can't quite form a complete opinion on it. I want to repeat the test that our EE performed, but I also want to know what is happening. Does it come down to efficiency/power factors, or is something else involved?


  2. jcsd
  3. Mar 27, 2009 #2


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    This all depends on real world numbers. Torque comes to mind first. It's easy to hold a couple of inch pounds or less at zero RPM. Naturally, low speed versus high speed with the same torque in both cases will take more power in the high speed case. The variable speed drive used in the case your ME supports his argument with may not have been made to drive such a load. A stalled motor that is holding against torque will draw current at a fairly low voltage. Depending on the type of drive, it may not be made to do something like this. That is not to say that the motor itself used lots of power to hold the load stationary, but the drive was simply not made to do it. Without knowing more about the drive it is nothing more than an educated guess.
  4. Mar 29, 2009 #3
    The behavior of a stalled motor is like to a short-circuited transformer. When secondary side of one transformer is shorted, the power source fronts the approx net inductance. In this case the primary current (apparent element) increased many times of ratted current, but this current includes two reactive and active elements which reactive element is higher than active element. Indeed, the electromotors in stalling situation will draw a lot of current from network for compensation of demagnetizations which caused due to reaction of rotor windings. When rotor start to moving, the magnetic reaction of rotor decreased and primary apparent current or contributed reactive power from source can be decreased. In this case if accelerated rotor will be front a resistive torque, necessary active power shall be drawn from network. For more information you can refer to power station active and reactive power control conceptions in special documents.
    Truly, there are a lot of conceptual subjects for thinking, my proposal is acceptation of energy survival law as said your EE; because general nature laws help us to front of high complex problems.
    In the mean time, if you like similar electrical-mechanical paradoxes; you can refer to General Electrical Riddle No.4 from http://electrical-riddles.com as a sample.
  5. Mar 29, 2009 #4
    Most motors are not designed to operate in a stalled situation. There are special motors for brake motors and torque motors.

    For a brushed dc motor, the torque is independent of RPM up to a significant fraction of its operating rpm, and drops off as the back emf increases. This is true for both series connected and shunt connected brush motors. Using the motor in a stalled situation can damage the commutator. For a sensor-type (Hall effect sensor) brushless motor with a permanent magnet rotor, it can operate indefinitely in a stalled situation as long as the stator current is limited to prevent coil overheating. Many motors have internal fans which obviously are not performing their function in a stalled situation, so the coils could overheat unless the current is limited.

    For ac motors, polyphase induction motors are better than single phase for operation at very low or stalled rpm, again if the current is limited. But they are designed to operate within about 2% of their synchronous frequency (e.g., 1740 rpm for 1800 rpm synchronous speed). Single phase motors with starting coils (split phase and capacitor start) are not rated for stall operation. Repulsion start ac motors have better starting torque, but have a commutator. Shaded pole motors are cheap motors and not recommended.

    My general rule is if the motor is too hot to touch, or if you can smell the hot coils, turn it off.

    (Text has been corrected per the next post.)
    Last edited: Mar 29, 2009
  6. Mar 29, 2009 #5
    I'm fairly certain Bob S actually meant "aren't", as in:

    "Many motors have internal fans which obviously "aren't" performing their function in a stalled situation, so..."
  7. Mar 29, 2009 #6
    Thanks for the input, this is interesting information. Historically, we have used brushelss DC motors for this application, although we are probably going to make the switch to AC (3-phase) pretty soon. Of course we always talk to the manufacturer when we're selecting motors to make sure they approve of the selection. They can tell us whether the motor/drive will do what we want and we have never had a problem after the machine is built, so long as we involve them in the design process, but I wanted to get a little more insight into how they are determining whether or not the system will work.

    Thanks again!

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