Universal motor running DC...

Discussion in 'General Electronics Chat' started by Externet, Aug 23, 2014.

  1. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    Hi.
    Once upon a time there was an article on the web explaining very clearly how and why a 120VAC universal motor could run at nearly the same performance/power with much lower DC instead. Unable to find such elegant tutorial now.
    I think was the text part of a web page from some knowledgeable engineer who installed a 120VAC circular saw on a bicycle powered with 12 or 24? V batteries.
    Does anyone here has the same rationale ? It was something like comparing the motor impedance at 120VAC with its resistance at DC, yielding near power figures, but really not sure remembering the details.

    If you know of such tutorial please advise, or go ahead with yours.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    From my experience the universal motor is just that and the rated voltage applies to both AC and DC.
    I especially do not see a 120vac motor running with the same torque and rpm on 12 or 24vdc?
    This would in particular apply to the max rpm, where a series motor operates in a runaway condition for a particular voltage and rpm is limited by friction and windage.
    There have been attempts to run on lower voltage but this has usually involved converting the motor to a shunt field version.
    Max.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    A series motor effectively rectifies AC into DC (using brushes and segmented commutator), so runs equally well on either AC or DC of the same magnitude.

    That is why they are called "universal Motors"
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    When fed with AC there is still AC flowing through the rotor and field, just that the same reversal occurs in the rotor the same time as it does in the field.
    Max.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    I agree with Max that the voltage rating of the motor, whether on AC or DC, would be essentially the same. The inductance of the motor likely generates only a small part of the total back EMF when the motor is running on AC under load.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    They run a bit better on DC because the torque is constant.

    Running on AC they get pulses of torque at double the mains frequency. At higher RPMs they have enough inertia so the pulsed torque doesn't matter that much, but at low RPMs (and/or with low inertia loads) the performance can be significantly better with a constant torque from a DC supply.
     
  7. MaxHeadRoom

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    Jul 18, 2013
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    I agree they run slight better and quieter on DC, but not to the extent mentioned by the OP as powering a circular saw with 12/24vdc.
    Max.
     
  8. BillB3857

    Senior Member

    Feb 28, 2009
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    Maybe I'm wrong, but I thought the OP was talking about using a circular saw motor as the motive force for a bicycle with 12/24V supply voltage. Not using the saw as a saw with a bicycle powered source.
     
  9. crutschow

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    Mar 14, 2008
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    A universal motor is not a good choice for a battery powered vehicle. The are less efficient than the permanent magnet motors typically used for that purpose.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Agreed, and they probably won't even spin up on 12v to 24v.

    I've done extensive testing on 240v router motors and they need at least 50v DC to spin up, and have close to zero torque. ;)
     
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