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Old Feb 10, 2005, 01:52 AM
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San Carlos, California, United States
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Slotted vs Slotless motors - advantages?

I've been looking around the web looking for comparisons of slotted vs. slotless motors. Unfortunately, it mostly turns up searches of motor companies touting the advantages of their slotless motor technology.

The only webpage I could find which would even mention a few of the advantages of slotted motors was:

http://www.designnews.com/article/CA87122.html

As I understand so far:

Advantages of slotted motors:

o More powerful (more torque) due to smaller air gap between iron and magnets

Advantages of slotless motors:

o No cogging
o Larger rotor size
o Reduced damping losses due to eddy currents

If anybody has more info, could they post? Also, I'm interested in efficiency comparisons at partial throttle for slotless vs. slotted motors.

Toshi
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Old Feb 10, 2005, 02:15 AM
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For our application whether a motor has cogging or not is neither an advantage or a disadvantage. For some precision applications it does.

As for partial throttle efficiency the biggest factor here is the design of the controller not whether a motor has slots or not. The controllers that are presently on the market for us could be improved a lot as far as part throttle efficiency goes.

Quiet Flyer Magazine a while back had comparisons of different controllers at part throttle in the PowerOn column.

Chuck
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Old Feb 10, 2005, 02:39 AM
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Quote:
Originally Posted by rcelectfly
For our application whether a motor has cogging or not is neither an advantage or a disadvantage. For some precision applications it does.

As for partial throttle efficiency the biggest factor here is the design of the controller not whether a motor has slots or not. The controllers that are presently on the market for us could be improved a lot as far as part throttle efficiency goes.

Quiet Flyer Magazine a while back had comparisons of different controllers at part throttle in the PowerOn column.

Chuck
The cogging isn't a disadvantage for R/C planes, but it's a disadvantage for R/C cars. There are a few car forums here.

Toshi
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Old Feb 10, 2005, 04:16 AM
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1 disadvantage with strong cogging motors shows up when used with the smaller and lower C batterys: When the throttle is slowly advanced from zero, the motor dosn't turn untill the starting current is reached. Then the motor runs fast because the no load current is lower than the starting current. During the time the throttle is being slowly advanced and the motor is not turning, the current is very high and can exceed the C rating. I= (batt volts -bemf)/r .
GB
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Old Feb 10, 2005, 09:49 AM
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Quote:
Originally Posted by TMorita
There are a few car forums here.
True, but this topic is posted in the "Airplanes - Electric" forum.

Cogging in a slotted motor depends a lot on the rotor design. So, if cogging is a major issue for your application, you can minimize it by changing the rotor. Magnet width, magnet strength and air gap are primary factors.

-David
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Old Feb 10, 2005, 01:18 PM
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There are very good slotless and slotted motors, and there are not so good of each type as well.

Kontronic, hacker, and lehnor are examples of very high quality slotless.

Aveox, plettenburg, neumotor, and acro (outrunner) are examples of high-end slotted.

The only real difference from a pragmatic standpoint is that slotted motors tend to have lower kvs, so they need lower ratio gearboxes than slotless. All Direct drive motors (inrunner or outrunner) are slotted because slotless do not have enough torque to runn dd except in ducted fans.

Just for fun, look up kontronic "tango" motors. These motors are sort of in between slotted and slottless, as they have a unique rotating iron ring to reduce iron losses. These motors are ridiculously efficient, really nice if you want a motor in the 800-1400 watt category.
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Old Feb 11, 2005, 02:39 AM
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Quote:
Originally Posted by TMorita
The cogging isn't a disadvantage for R/C planes, but it's a disadvantage for R/C cars. There are a few car forums here.

Toshi
Yes, I was commenting as regard to use in airplanes since that is what this forum is about.

Why is cogging not good for cars?

Chuck
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Old Feb 11, 2005, 03:33 AM
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Quote:
Originally Posted by rcelectfly

Why is cogging not good for cars?

Chuck

It acts like a break... when you come of the throttle you want some free wheeling any breaking should come from the speed controller so that you have better controll over it..
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Old Feb 11, 2005, 06:23 AM
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Quote:
Originally Posted by twest
The only real difference from a pragmatic standpoint is that slotted motors tend to have lower kvs, so they need lower ratio gearboxes than slotless. All Direct drive motors (inrunner or outrunner) are slotted because slotless do not have enough torque to runn dd except in ducted fans.
I was thinking about this earlier.

It seems slotted motors are better for low Kv/high torque applications, and slotless are better for high Kv/low torque.

I'm primarily interested in helicopters, actually, and in most helicopters we run motors at around 20k-24k RPM. Many people use slotless motors (Hackers and Kontroniks), but these motors seem like very poor choices for this RPM range.

It seems like below 30k RPM, slotted is better due to more torque, but above 30k RPM, slotless is better because of lower losses due to not having an iron core.

Toshi
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Old Feb 11, 2005, 10:48 AM
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Quote:
Originally Posted by PaulVi
It acts like a break... when you come of the throttle you want some free wheeling any breaking should come from the speed controller so that you have better controll over it..
Once the motor is spinning the drag is the same for both types.

Steve
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Old Feb 11, 2005, 10:54 AM
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Quote:
Originally Posted by TMorita
I was thinking about this earlier.

It seems slotted motors are better for low Kv/high torque applications, and slotless are better for high Kv/low torque.

I'm primarily interested in helicopters, actually, and in most helicopters we run motors at around 20k-24k RPM. Many people use slotless motors (Hackers and Kontroniks), but these motors seem like very poor choices for this RPM range.

It seems like below 30k RPM, slotted is better due to more torque, but above 30k RPM, slotless is better because of lower losses due to not having an iron core.

Toshi
Not really right--there are many issues that go into the designs and the details really determine how a motor will perform. Both motor types can work well at the same RPM ranges if designed for the application. Slotted motors can and do run at very high RPM--as do slotless motors. In most cases the stator materials and rotor design really determine the suitablity for high RPM.

Steve
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Old Sep 20, 2014, 11:35 PM
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I imagine since I want to make a slotless inrunner with something like 100kv it'd need to have many poles on the rotor and the rotor would also need to be wide. How would the windings be different with the intention of a low kv high torque inrunner? I imagine the winding poles would need to also increase and the wire thin. And then a flux ring or back steel behind the windings.
If electrical steel were ground to a powder or filings and mixed with epoxy and reformed would it still keep it's desirable electrical properties?
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Old Sep 21, 2014, 04:17 AM
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Staffs, UK
Joined Nov 2003
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Rather than resurrecting this almost 10-year old thread you'd probably do better with your queries in the Electric Motor Design and Construction forum http://www.rcgroups.com/electric-mot...struction-361/

Steve
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