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08.22.2008, 10:51 PM

Slotted vs Slotless motor explanation

Originally Posted by BrianG View Post
Speaking about motor efficiency, I've heard it said that a slotless stator design (basically an air core) is generally more efficient, but the Neus are slotted. I would think because of the slotted nature (windings wrapped around a "core") that the magnetic field would be much more focused, and wouldn't have as much flux loss. I can see that the width of the magnetic field would be a lot less in a slotted vs slotless. What are your thoughts on this?
Depends on who you talk to -- there are those who think that slotless is better, and those who think that slotted is better. I just think you should use whatever motor design fits your application...

Slotless motors have a larger air gap, and so, are generally less torquey than slotted motors. But they also have more space for copper, so typically have lower copper losses. Slotless motors have less inductance, so they switch at high frequency (higher RPM) better than slotted motors. Slotless motors also have virtually no torque ripple, so deliver power smoother than a slotted motor. The CM20 and CM36 motors (our standard Mamba and Mamba Max motors) are slotless because we wanted maximum spool-up speed, fast response, and moderate torque. IMO, Slotted motors don't do as well as slotless in 1/10 scale applications because the buggies and cars are lightweight, and accelerate very quickly without high torque -- and the high RPM performance and quick response make up for the lack of low-end torque (although our slotless motors generate a LOT more torque than our competitor's slotted 1/10 scale motors -- -but that's due to poor rotor design, rather than a slotted vrs slotless tradeoff.)

Slotted motors can have very small air gaps, and so are very torquey. They can generate much higher peak torques as well, but have the advantage of higher inductance at low RPM, which helps to limit peak currents and keep temperatures lower at low RPM. Our 1515/1Y and 1512/1Y Monster motors are slotted because we wanted maximum low RPM grunt and shaft twisting torque. And IMO, slotless motors just don't do as well in a Monster Truck or big buggy as a slotted motor. Once you get above about 3 pounds or so, the slotted motors have a slight advantage.

So, for smaller, higher RPM motors (low torque, high horsepower), slotless motors usually have the advantage. For larger, lower RPM motors, slotted motors usually have the advantage.

So you can think of it this way: Slotless motors are like motorcycle engines -- high RPM, lower torque, high horsepower. Slotted motors are like automobile engines, low to medium RPM, high torque, high horsepower. Outrunners are like Diesel engines... lower RPM, lower horsepower, very high torque.

All of these comparisons are relative -- the difference in practice is fairly small. We have run Monster trucks on slotless, slotted, and outrunner motors with good results. We really went with the Neu slotted design simply because it had the highest efficiency of any motor in that size that we had tried. And because of the high efficiency, it really performed extremely well.

There are trade offs for every type of motor design. You can build a good slotted motor and it will compare very well with a good slotless motor. Or you can build a bad slotted motor, and it will compare very well with a bad slotless motor. I've seen both good and bad motors of every type.

So the real answer is: Efficiency is the single most important thing. Efficiency is directly related to power to weight ratio. And power to weight ratio is performance. So the better the efficiency, the better the performance. Whatever motor type gives you the best efficiency in your application is the one you should use...

I'm sorry for the long ramble...

Patrick del Castillo
President, Principle Engineer
Castle Creations

Last edited by BrianG; 08.25.2008 at 09:52 AM.
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