Continous power dc motor

Hello to all, can any kind soul point me in the right direction to measure or mathematically calculate the continuous power of a brushed (dont know if that is relevant or not) dc motor. The motor manufacturer only lists its power output as peak power and does not reply to my queries sent. I have a motor rated at 350 Watts peak which i wish to use in a pic based project but under uk law i am allowed 250 Watts continuous.
Many thanks for any suggestions
oscar
(2 spelling mistakes, old english teacher would go mad)

anonymouse, W = I x E, or Wattage = Current x Voltage...

Dave Purola,
N8NTA

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Originally Posted by Dave

anonymouse, W = I x E, or Wattage = Current x Voltage...

Dave Purola,
N8NTA

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Thanks for the reply, I was interested to know how continuous power of a motor is derived (formula perhaps).Is the peak power of a motor measured under un/loaded conditions, if their are other parameters in calculating the continuous power of a motor, i.e derating factors for continous use,heat dissipation etc. Similar factors to working out the csa of a conductor, but using the motors rated peak power as that is all I have to work from.
cheers

Are you able to run the motor under the given load as a test?
If so then measure the amps.

The load will determine the amps.

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Originally Posted by mackrackit

Are you able to run the motor under the given load as a test?
If so then measure the amps.

The load will determine the amps.

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My problem is Dave that by the nature of a dc motor the motor current reduces to zero as the rpm increases due to the back emf, so the current is non linear (motor will be at various pwm from 0 to 100). Thus I am not sure how manufacturers come to derive the continuous power. I understand that peak power can be upto 5x continuous power depending on manufacturer, and I cannot work back to the motor constant (to see if their are equivalents) as I am missing peak torque data.
Thanks for your reply

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as I am missing peak torque data.

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350 watt seconds (Wsec) is equal to 350 newton meters (N-m)

Al.

Does the load change or is it constant ?? are you going to continually read power or just make some estimations for your project ??
The motor current is related to the loading of the motor (how much effort the shaft is requiring ) , so if the current goes to zero, there is no power being consumed (E X I) by the motor at that time, in fact if the load tries to turn the motor shaft faster then it would normally run at that voltage, the motor then becomes a generator and current tries to flow back to the supply.
Power,
....you have instanteanous power, the E X I at an instant
....average power, a sumation over a period of time say 1 second
....and watt/hours or kw/hours

for PWM and power, I think you can relate the PWM % to the voltage, for example 12 volt DC at 50% PWM would be 6 (for E) X I (current). take 10 or 100 readings per second and do some calculations to obtain power usage.

don
amgen

Thanks for the replies, what i have is a dc motor which only lists its peak power of 350 Watts @ 24 Volts on its rating plate. To conform to legal specs the maximum allowable "continuous "power of the motor equals 200 Watts, this is my dilema, to convert the peak to continuous power to see if this motor complies. I am interesting in finding out how the motor manufacturers come to a continuous rating, being that the peak power can be a multiple of e.g 2 to 5 times continuous power. Under a steady load the motor current will ramp up under start, usually clamped via a controller to improve motor efficiency ratings, then ramp down to zero as the rpm rises to maximum (back emf). Whether the manufacturer maps these figures @ set temperature @ set load and finds the mean value or uses an algorithm containing other data is where I am stuck at.
Many thanks

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but under uk law i am allowed 250 Watts continuous

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As an aside... what particular UK Law are we talking about here? Is this something akin to the Disabled Mobility Scooter kind of installation where politicians have decided you can't give little old ladies trundling along the pavement a turbo boost for fear of over-stressing their heart pacemakers?

250W continuous is kinda ambiguous... define 'continuous'... kinda says nothing about 'peak' or how long that 'peak' could last for... you could design something that was 250W continuous but 2kW peak... hmmm F1 Mobility Scooter Racing...

But back on-topic, you as a designer can define your own 'continuous'. You might find that your 350W peak motor may not even have the guts to run 100W continuous let alone 250W (which is probably why the manufacturer is embarassingly silent). Typical comparison is the 1kW PEP PC Speaker Systems you see for sale (which run from a dinky 10W wall-wart)!!!!

So if it's a 12V Motor, feed it 12V and then load it untill you get 250W power consumption... come back a week later and see if it's still running or if it's burnt the house down... I'd call that a reasonable 'continuous' test. But really, you have insufficient Data from the manufacturer to even begin any kind of mathematical calculation.

Here is another non-answer to your question,,,,,
Motors in US are usually rated in HP (horse power) which is also 748 watts. I think in Europe they rate motors in W's or KW's mostly.
Your motor would be then have a peak hp of about 1/2 HP, so if you consider de-rating to 75% for continous duty, that would be .375 HP (3/8 hp) or about 280 WATTS. For a 24 volt motor, that is about 11-12 amps continous at that loading.
Also, back EMF , which is the voltage created in the turning motor and opposes the applied voltage changes with load,so for example,if your running motor tries to pull something up a hill, the motor will try to slow down - reducing the back EMF (opposion voltage) and more current will flow thru the motor (provided the supply can give it) which reflects in the fact that it takes more HP (E X I) to pull up-hill .
If you can't get more info from the manufacturer, you may have to test the motor under load to make sure it doesn't overheat like Melanie suggested.
don

Thank you for your replies, Melanie it was more of a 2 wheel design but now it might incorporate a Biddy Bingo Button for the 4 wheeled variety. I had the equation in mind that a motor constant is proportional to the ratio of peak torque to peak power at stall etc, for a loose association, but summised the law as always would be even looser than this. At least now I can cover my bottom, many thanks to all for your input