Neu motors: RMP/V : loaded or unloaded????

[Van Aelst Jozef]

Hello guys, have put following question to manufacture or distributor of Neu motors concerning given motor data.

Quote:
Have seen technical details from above listed motors.
Given RPM/Volt are under idle or under load conditions??? If given under idle , please what should be values under load?
Can these motors ordered with an internal cooling fan?? If so, what is the corresponding ordering code? Motors will be used in RC speedboats.
Can you give some motor diagrams for different voltages (refer to Lehner diagrams).?
Unquote

Received following answer:

Quote:
Jef,
RPM/Volt is a constant value, it will not change under a load. The wattage (W) will change, however, and the continuous watts should be what the motor will draw under normal use, the surge watts will be what the maximum amount of power the motor can draw safely. Use the given watts divided by your input voltage (W/Volts) to get the amp draw you should expect from your system. Most of the motors can be ordered with an internal fan, but that is not recommended for powerboat applications. Those kids of boats should have their own type of water cooling built in, and the motors with fans are meant mainly for helicopter use. If you want to order them that way, you can, the option is an extra $25 USD, but will probably not fit correctly in your motor mount. I do not have detailed motor diagrams to give you.

Greg Wright
Product Specialist
Castle Creations

Unquote

I thought that loaded RMP/V were 0.93- 0.85 of unloaded RMP/V:confused::confused:
Should appreciate your commence. Has anyone some motordiagrams of Neu motors??

Regards.
Jef.

[GunnarH]

Hello Jef,
this is a kind of misunderstanding. The so called kV (in rpm/V) is in fact a constant independet from current as it refers to the internal voltage (Supply voltage - internal voltage drop), as the idle current is very low the kV is approximately same as rpm/V in no load condition, so often both terms are being confused.
But from my experience the Neu Motors have a rather high variation in rpm/V, some have higher rpm, some have lower rpm, so the values on the website are only a rough landmark.
For the rpm-drop under load you can calculate with 10% in the point of best effciency compared to no load condition.
Best Regards
Gunnar

[MiSt]

Indeed one of the two REAL CONSTANTS describing a DC-motor with permanent magnets inside is the rpm/V constant. The other is the torque/A constant.

The percentage of no-load rpms you mention is just a rule of thumb, the real physics in behind is the inner resistance of the motor (for BL, including wiring and MOSFET-losses): Every DC-motor works as a generator, creating a voltage slightly less than the voltage applied to its clamps. The current flowing through the internal resistance causes this difference in voltages. Depending on the load - the torque - the current varies from idle current (max. rpm) to stall current (zero rpm), resulting in decreasing rpms, proportionally decreasing with the current. You can watch this in the LEHNER calculator, for example, or in the various motor simulators available mainly for aircraft setup design.

Common but confusing practice is to divide the actual rpms by the actual voltage, resulting in non-constant rpm/V values in the mentioned rule-of-thumb range of 85-93% or even extending this, depending on the real motor characteristics and the suitability of the motor for a given purpose. LEHNER motor rpms are specified this (wrong ...) way at maximum efficiency.

To summarize: An electric DC motors has as many rpm outputs as it has torque outputs and voltage inputs.

Congrats, Gunnar, you were faster ;-)

[Sika]

Its most likely a misunderstanding.

1. I have no idea what that Castle Product Specialist is trying to say.

2. The KV numbers listed on the NEU page are idle rpm - more or less. They can vary from batch to batch by about 3-5% and worst, not all of them are correct. So be careful and check the number from one type against other winds from the same type.

3. Usually motors drop rpm from idle to "load at best efficiency" by the efficiency factor, thus 7-13% drop is common, depending a bit on the size.

4. Finally, it is correct that the KV (rpm/V) of a motor is a constant, even though that was not your question. What changes is the voltage feed into the motor under load. It's the voltage drop over the internal resistance of the motor that will create the lower rpm under load - given a constant voltage power supply.

For example, if you got a motor with a KV of 1500 rpm/V that runs at 20V (=30.000 rpm) and the internal resistance of the motor is 10mOhm, then the rpm drop of the motor alone at 100A is equal to the voltage drop: 20V - 100A * 0.01Ohm = 20V-1V = 19V. Thus the motor will spin at 19V * 1500 = 28.500 rpm. You need to add internal resistance for the ESC and the cable too, so expect a bit more drop.

Joerg

P.S. Gunnar AND Michael have been faster...

[Van Aelst Jozef]

Thanks guys, we learn every day as we get older.
Long time ago (40 years) since we learned of source voltage(E), internal resistance( ri ) drop and effective voltage((U) available at terminals of the source.

Greets. :thx:

Jef.