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Old May 21, 2004, 03:02 PM
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How to measure no load current of a brushless motor?

I would like to know standard procedure to do this?
My guess - full throttle, but what woltage to use?
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Old May 21, 2004, 08:43 PM
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Depends on the motor, but 6V would probably be OK.
..a
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Old May 21, 2004, 10:36 PM
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What confuses me is that i get higher no load current as voltgae goes up, but at the same time it should be the same at any voltage in theory , and motocalc for example shows same current at different voltages too....?
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Old May 21, 2004, 10:37 PM
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I have always assumed that no load current is constant, and does not change with voltage, if its not so - then its meaningless since we all use different voltages ,,....
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Old May 22, 2004, 12:54 AM
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The no load current will change with voltage as the higher the voltage the faster it's tuning the motor. There won't be a big change though. The no load efficiency of the motor will be greatest at one particular voltage. Be carefull running brushless motors no load. Some manufacturers say for their motors it's a big no no. If you're only after the no load amps you should be able to get it off either the box it came in or a web site somewhere.

Cranky
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Old May 22, 2004, 01:09 AM
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Quote:
Originally Posted by Hovertime
I have always assumed that no load current is constant, and does not change with voltage, if its not so - then its meaningless since we all use different voltages ,,....
The current will rise as the source voltage goes up. How much it goes up depends on the motor. The cheap motors usually see a much greater change in current. The better motors will have much less change.

Steve
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Old May 22, 2004, 01:41 AM
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Quote:
Originally Posted by Cranky1
The no load current will change with voltage as the higher the voltage the faster it's tuning the motor.
Cranky
I though that needed increase in power is coming from increased voltage, as W = V x A ......

Quote:
Originally Posted by Cranky1
Be careful running brushless motors no load. Some manufacturers say for their motors it's a big no no.Cranky
Well thats true for any motor, or engine, but as long as I do not exceed max rpm i think I'm safe.
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Old May 22, 2004, 01:44 AM
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Quote:
Originally Posted by sneu
The current will rise as the source voltage goes up. How much it goes up depends on the motor. The cheap motors usually see a much greater change in current. The better motors will have much less change.

Steve
That might be the case as I'm having trouble with a cheap one

Then it is time for new question-what voltage is considered standard for no load current testing, and what voltage do manufacturers publish?
Is it some preset voltage - say 6V or voltage at best efficiency or what?
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Old May 22, 2004, 04:23 AM
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Hastings, New Zealand
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One would presume that Io was measured at the motor's nominal operating voltage, but who knows? Some manufacturers don't quote an operating voltage, and some don't specify Io at all!

The problem with brushless motor 'constants' is that they can change depending on the controller used, and its programming. I suppose a manufacturer could use that fact as an excuse for not publishing the constants...

When making your own measurements, the most accurate way is to use a voltage slightly less than your expected application, eg. measure Io at 6V if you will be using a 7.2V battery. In most cases Io has only a weak dependence on voltage, so the exact test voltage is not very important. Just bear in mind that if you operate the motor at much higher voltage than usual, Io may be a bit higher (and efficiency a bit lower) than expected.

As to why Io increases with more voltage, my understanding is that the motor needs some current to magnetise the stator - which is constant, and some to overcome friction and eddy current losses- which increase with speed. At low to medium speed the magnetic component dominates, so Io remains relatively constant.
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Old May 22, 2004, 06:24 AM
FLL
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Towaco, NJ
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As Andy, and Astro Bob B. in his book, say, 6 volts is a good source to check no load current and set timing. I've found below that the Io reading will be too low. At 6v up to 8v the Io will level out until too much voltage will force the Io upward. Any no load measurements must be made quickly as the motor will heat up rapidly and change your readings. The higher the voltage used the quicker the heat up.

Frank
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Old May 22, 2004, 09:01 AM
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Quote:
Originally Posted by Hovertime
That might be the case as I'm having trouble with a cheap one

Then it is time for new question-what voltage is considered standard for no load current testing, and what voltage do manufacturers publish?
Is it some preset voltage - say 6V or voltage at best efficiency or what?
A good rule would be to make the measurement at the normal operating voltage.I have not seen any standard for making measurements.

Steve
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Old May 22, 2004, 09:01 AM
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It's my understanding the the no load current for brushed motors is relatively constant regardless of applied voltage, because the majority of this current is the result of brush friction which is independent of RPM. There will be a smaller variable component from windage and the magnetizing current.

In brushless motors, the story is different and it appears that there are a number of other factors which influence Io, which is usually smaller in magnitude than the Io of brushed motors.
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Old May 22, 2004, 11:38 AM
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You don't get something (more rpms) for nothing--even at "no load". It seems reasonable that (milli)amps would go up as rpms go up. Among other things, HP is related to RPM and it takes 746 watts to make one hp at 100% eff. The formula I found on google is hp = rpm x torque / 5252 (constant). Granted torque goes down as rpm goes up (at the higher revs), but I don't believe they cancel eachother out.

Edit: The above is all wrong. I forgot that the voltage is being increased. DOH!

That said, I have seen the rate of increase in milliamps go down at a repeatable place for each motor.

JMO


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Old May 22, 2004, 11:49 AM
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You are not actually getting more RPM for nothing.

You are applying more voltage and therefore more power, it's just that the current is approximately the same.

This is frequently analyzed in larger motors as a constant current source.

In a motor which was 100% efficient, Eg would equal the applied voltage and there would be no current under no load conditions.
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Old May 22, 2004, 12:35 PM
Dimension Engineering
Akron, Ohio
Joined Jan 2002
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The reason it goes up somewhat is frictional damping, which the classic model of an electric motor doesn't touch, as it's usually a pretty small component in the motor itself, especially with a ball bearing motor. Friction increases with velocity, which creates a torque about the shaft. Asking the motor for more torque means more amps.
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