# Questions about electric motors

Ok, so I have some questions about electric motors. Specifically about amps, volts, rpm, load, torque, all that good stuff.

I want to power a ducted fan unit that has these stats:

95N of thrust @ 38,000 rpm, requiring 5.7 kilowatts of energy. The fan is a Schubeler DS-94-DIA 3-ph

The recommended motor for this fan unit seems to be the lehner 2280 with 9 windings. Here is a chartshowing some data for this fan/motor combo.

According to the chart on the right-hand side of the motor web page, the 2280 with 9 windings provides 822 rpm per volt. So this means that I’ll need to apply 46 volts to get it to spin at 38,000 rpm, correct? And according to the graph I linked, the current draw will be somewhere around 130 amps, correct?

What are the implications if I switch to a motor with 18 windings, giving half the rpm per volt, but then I double the voltage? Is torque at a given rpm halved? Is amperage draw halved?

Since rpm is determined by voltage, what happens when you increase the load and slow the motor down? Does the amperage draw increase in an effort to sustain the target rpm?

Can anyone link a good electric motor faq/theory website?

Thanks alot!

I think these motors are of a new kind, containing internal circuitry to do commutation and maybe other things. It may not be easy to figure out. The web pages say “brushless”, which implies an electronic form of a DC commutated motor having electronic switches instead of brushes. I think this would behave as a “shunt wound DC motor”, and the no load speed would approximate a multiple of the voltage, the torque would approximate a multiple of the current, and the drop in speed with load would approximate a multiple of the motor’s stationary resistance.

But then the web pages also refer to star and delta wiring, which are typical of three phase induction motors. These motors do not have or need brushes, but are not usually called “brushless” because the term usually implies a motor type in which the otherwise necessary brushes are replaced with electronic substitutes. Induction motors have no-load speeds proportional to the frequency you drive them at, and loaded speed drops that are proportional to torque, and more or less constant current but a phase angle with respect to voltage that depends on torque. Then again, I think the page said “no longer available in star or delta”, so not sure what that means.

Hope this helps.

But if they

The problem with doubling the voltage is that very few (if any commercial) ESCs can handle 92v. Also, the lower Kv motor will have smaller diameter windings that will increase the resistance of the windings. It’s generally better to run higher voltage and lower amps, but then it’s a trade off with the increased resistance.

Lehner are some of the finest motors out there, but hard to get in the states (and getting kind of expensive with the exchange rates). A brand that is very popular is the Neu Motor, made in San Diego. They are the premier high perfomance brand in the US (and are world class, the owner competes in different world championship classes), they cater to extremely high power applications in all forms of RC.

Neu Motors

For the DS 94, they recommend the 1527/1.5Y on 12S lipo that’s about 45v nominal, close to the 46v you mentioned.

EDF Page with Calculator

You won’t get but maybe 70~75% of Kv x V under load for actual RPM as a WAG.

A good site for EDF talk is RCGroups.

The reason the 2280 Lehner is no longer listed as both star and delta is because they have gone to the Hi Amp configuration where the taps are connected directly to the windings, before they were connected to a solder bridge on the rear of the motor that allowed user swapping to change the Kv. That configuration was limited by the current the solder could handle.

I don’t have any experience with EDFs, FWIW.

Current (amperage <twitch>) is halved for a given torque. Torque @ RPM will be near constant for given power.

Beyond resistance considerations mentioned, doubling the number of windings will increase the inductance by a factor of 4. At 38,000 rpm, the windings are operating at 633 Hz. Depending on the speed controller design, the inductance may, or may not waste power…it will, however, absolutely require additional (beyond your 2x assumption) voltage be supplied in order to reverse the current in the winding in the required time. The inductance of the motor would have to be known in order to quantify the effect.

Thank you all for your input, you’ve given me a lot to chew on. I’ve learned that rpm is a function of voltage and that torque is a function of amperage, and that the less the actual rpm (due to load) is compared to theoretical rpm, the greater the amps drawn. I assume the load rpm is determined where the power into the motor equals the physical resistance provided by the load.

Looking at the motor I mentioned previously, the 2280/9, I don’t think it could actually run that fan at the fan’s max power. It would draw 130 amps, which I think might fry that motor. I guess I gotta step up to the 30 series motors…

For what duration? The 2280/9 would handle 130A for a while with cooling. Of course the 30 series can be ordered with water cooling jackets and the Schulze 40.160 controller is also water cooled and can do 160A.

So what are you building?

Well, I’m just brainstorming with the idea of adding these fans/motors to ultralight aircraft. With the new technologies being developed, like this battery, electric flight is becoming more feasible.

Also, I found some stats for the 2280 and it listed max amps at 250, but that sounds like gross overconfidence.