Effect of load on brushed/brushless motor speed

Discussion in 'General Electronics Chat' started by Anirban Raha, May 30, 2014.

  1. Anirban Raha

    Thread Starter Member

    Dec 4, 2013
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    Hi all,
    I don't know if this the right forum for posting this question...:confused: because my question is more of Electrical than Electronics. Now, we know that a PMDC motor is, ideally, a constant speed motor. But practically, when it is loaded, it slows down from it's no-load speed. My question is that, do brushed motors slow down more than brush-less motors of equivalent ratings(rated voltage, no-load speed, kv ratings) with the same amount of load?
    I know that brush-less motors are more efficient than brushed counterparts. But does that also imply that effects of load on speed is lesser for a brushless motor?
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
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    My thoughts only, but a brush type motor running open loop (no feedback) will slow down when loaded. The reduced speed reduces the back EMF which raises the current which increases torque in an attempt to keep the speed up. If, on the other hand, the brush type motor is running in a closed velocity loop having speed feedback, any reduction in speed is sensed and compensated for. A BLDC motor usually runs in a closed loop system which will compensate for any reduction in speed due to load. Other folks may have differing opinions.
     
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  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Without some kind of feedback in the form of PID loop control, they all basically operate in the velocity mode.
    There are basically two types of BLDC motor, one that has hall effect or equivalent commutation feedback to the drive or open loop.
    Similar to a DC motor with a Tach feedback to the drive that improves the 'tightness' of the velocity loop, so does the BLDC feedback hall devices improve the velocity loop, albeit down to a certain low rpm depending on pole count due to the coarse resolution of the H.E. devices or method.
    With BLDC without any feedback as in RC ESC or Fisher & Paykel washing M.C. motors, the loop is totally open.
    The next step up in control is the closed loop PID control as in CNC or motion control, the customary method now is for the drive to act in the torque mode, rather than the velocity mode (trans-conductance amplifier).
    There is also the AC sinusoidal that is identical in construction to the BLDC, except that it is driven with a 3ph sinusoidal signal.
    Max.
     
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  4. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I've never ran a side-by-side comparison, but I predict with a high level of confidence that the brushed motor will slow more than the brushless.
    No, not in & of itself.
    Cause & effect. The resistance to change in speed with the addition of load is an effect of the (cause: ) electronic commutation control. Higher efficiency is also an effect of the (cause: ) electronic commutation control. But resistance to change in speed with the addition load is not an effect of the (cause: ) higher efficiency, because higher efficiency is not the cause in the scenario; electronic commutation control is.
     
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  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Some other advantages of one over the other, BLDC apart from having no brushes, is smaller in total frame size for equal torque, BLDC has higher potential max RPM and also lower rotor inertia.
    Heat is produced in the stator and not the rotor, so cooling is more effectively handled.
    Max.
     
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  6. Anirban Raha

    Thread Starter Member

    Dec 4, 2013
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    But even after using a closed loop control in brushed motor can i increase its speed at the maximum votage for a given load? For eg, if a 12V 10000 rpm pmdc motor rotates at 1000 rpm at 12V with a load in open loop mode, will it rotate at a greater speed under the same conditions in closed lop mode? given the only thing the circuit can control is the supply voltage?
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you have a motor rated 10krpm at 12v, if the load at 12vdc causes a drop in rpm to 1krpm, you may be exceeding the continuous torque (current) rating.
    An increase in supply voltage to a DC brushed motor that exceeds the plate voltage is acceptable in order to increase the rpm within the motor maximum, as long as the continuous torque value is not exceeded.
    The simplest type of feedback in a DC velocity style drive is a DC tach, these are known to be as close as 1% error.
    Max.
     
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