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Thread: Motor Rating: Peak Power vs Rated Power?

  1. #1

    Question Motor Rating: Peak Power vs Rated Power?

    Hello,

    A motor on a chair I was looking at stated the power rating as 800w. But according to the manufacturers brochure the motor rating is 2,700w 'peak'.

    Is the 800w the continuous power rating? I.E. it can pull 800 watts of power all day without overheating, but under heavy load conditions such as going up a hill it can pull 2,700 watts but only for a short period of time?

    EDIT:
    Doing a quick google search before clicking the 'submit new thread' button I have just found this at: http://machinedesign.com/basics-design/dc-motors

    "Dc motors are often applied where they momentarily deliver three or more times their rated torque. In emergency situations, dc motors can supply over five times rated torque without stalling (power supply permitting)."

    Although related
    I thought torque and power were different? For example I thought car engines produce maximum torque at lower rpm and max power at higher rpm?

    I don't know I'm confused!


  2. #2
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    Really think about what you wrote in your post! The answers are right there. Yes maximum torque is used when simply breaking free and starting to roll, or more resistance to motion is realized, and yes again maximum power or design (AMPS) power at maximum speed. 800 Watts is equal to a bit more than one horse power design power output, DC motors allow variable power output which AC usually doesn't.

    Yes pulling a long steep hill at maximum power may well overheat your motors and the controller may fold back and deliver lower amperage to your motors, which in turn will slow you down to protect itself. Your controller is the adjuster of the amount of amperage (power) that can be delivered to your motors.

    PS the reference site is a good one for basic understanding of DC motor capabilities.
    Last edited by Bob Sullivan; 07-02-2016 at 09:49 AM. Reason: the PS

  3. #3
    Thankyou.

    I've been doing a bit more reading about powered mobility (I currently have a manual chair) and I didn't realise power output is a combination of the motor rating, the controller rating, and the battery size.

    Also I read somewhere that with battery technology it's not all about the Ah rating. Apparently the battery's internal resistance rating is important also? Does that mean that the max current that can be drawn from a battery is governed by it's internal resistance according to Ohm's law? I'm just guessing, I know a little bit about electrical theory but don't know if what I just said is correct? I think for example a 45Ah can produce in theory 45A for 1hr, and 5A for 9hrs. So would the max current output of the battery be Ah/R=A? Where R is the internal resistance.

  4. #4
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    I will be quite honest with you I know enough to be dangerous to myself and you. Yes, I do know lead/acid Batteries have a great degree of internal resistance. Some manufacturers product's have more than others. And a Lead/Acid deep cell can only deliver enough voltage to operate our machines to about 50% +/- of the manufacturers listed AH capacity, before the voltage drops below the controller's low voltage operating range. So the actual demand you place on the batteries does somewhat correspond to the demand by the motors and auxiliary equipment if your chair is so equipped. Also the Peukert value (immediate internal resistance) does control the motor output as well.

    The batteries we mostly use in our Power Chairs are gel cells and spill proof, but are still Lead/Acid (LA) so the best batteries are the largest you can install in the space you have available. Cheap Batteries are just that "CHEAP" and 24 volt systems since they operate in series must start out life together as perfectly matched in full charged voltage as possible. Or they will have an exceedingly short life span. This is because when the battery with the lowest voltage charges to its capacity, your charger senses it is fully charged. Then every succeeding time you charge it, this same lie is still present, this is a cause of plate sulfation. Until they are dead for your use in a power chair.

    Leaving you batteries for a length of time at low or no charge is a battery killer, and another cause of battery sulfation. In fact the should be recharged after each use no matter how long you use them. I have experimented with several chairs as I do have several, and the sooner after use they are charged, the better, and the longer the charger is kept on after the initial charge and is left on "float" charge the more battery life I have found "I" get. That is if your charger has float capacity.

    It also does no harm, to put your chair on a temporary charge, if you can, while you are in one place for a period of time. This is especially handy if you are extremely active, or in a working situation.

  5. #5
    Senior Member
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    Mr. Tippy; There are some mobility web sites that are less than honest about their knowledge about batteries and how they should be used. I have no use for some of these misleading sites. And the manufacturers representatives who promise you service after purchase and drop you flat immediately after they have your purchase money. Those mechanics that cannot see the difference between a 1/4-20 bolt and a 1/4-24. Visit WWW/WheelchairDriver.com or BatteryUniversity.com

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