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Old Feb 15, 2012, 09:07 PM
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Differences between rc and dc brushless motors?

What are the differences between these two motor types?
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Old Feb 16, 2012, 02:56 AM
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Letchworth, Great Britain (UK)
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Not sure what your question is

All electric motors operate by having a set of fixed magnets repel another set of magnets that are free to rotate around a shaft, and are fixed to the shaft so that they turn it. Both sets of magnets can be electro-magnets, but in our hobby motors one set is permanent magnets (don't need any power) and the other set is electro-magnets (need power).

In the simplest of motors, the power to the electro-magnets needs to be reversed twice every revolution so the magnets will always repel each other whatever position the moving ones are in, and keep the motor spinning. In these simple motors, the electro-magnets are rotating, and they get their power through a pair of "brushes" rubbing on a metal ring on the shaft. Reversal of the power during every revolution is achieved because that ring is in two halves, so as it rotates it's alternating which half is in contact with which brush. One weakness of this system is the brushes themselves, which can burn out and, even when new, are a source of electrical inefficiency. These motors are referred to as "brushed" motors or, sometimes, "DC" (direct current) motors because they will run if you simply apply a battery to their two input wires.

Most flyers these days use "brushless" motors. These have the electro-magnets on the static part of the motor, with the permanent magnets on the moving part, so the electro magnets can be connected directly to the power supply without using brushes. But, without any brushes we need another way to reverse the power every revolution, and this is done using an electronic speed controller (ESC) which electronically switches the power in sync with the rotation of the motor. These motors are sometimes referred to as "AC" (alternating current) motors because the power that's supplied to them is alternating. They cannot be run by simply applying a battery to their input wires.
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Old Feb 16, 2012, 03:38 AM
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Staffs, UK
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I think you might need to ask your question again.

The brushless motors we use for RC (radio/remote control) models are DC (direct current) brushless motors. More accurately they are electronically commutated motors where the external ESC does the commutation and is supplied with the DC power.

So there's no difference at all unless you meant something else by rc and/or dc ?

Steve
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Old Feb 16, 2012, 03:56 AM
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Bruce Abbott's Avatar
Hastings, New Zealand
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Do you mean, the difference between AC and DC brushless motors?

An AC brushless motor uses AC (Alternating Current) power to generate a rotating magnetic field, which drags the rotor around with it. This can be done synchronously or asynchronously. A synchronous AC motor runs at a fixed speed, locked in phase with the AC power frequency. One problem with this design is that it is not self starting (it needs some other mechanism to get it up to synchronous speed) and if too much load is applied then it may drop out of sync and stop.

Another type of AC brushless motor is the Induction motor. This has copper wires embedded in the rotor, in which currents are induced by the rotating magnetic field. This current produces magnetism in the rotor itself, which then interacts with the stator field to produce a torque which turns the rotor. Due to this induction process, the rotor does not have to stay in sync with the supply frequency, but can 'slip' to a lower speed under load.

A DC motor has to produce its own rotating magnetic field, by switching current through coils in step with its rotation. In a brushed DC motor this is achieved using a commutator and brushes. A Brushless DC motor works just the same way, but with electronic commutation. A sensored BLDC motor has magnetic or optical sensors that transmit the rotor position to the controller. Essentially it is the same as the brushed motor, except that it uses solid-state switches (transistors) rather than a mechanical switching mechanism (commutator/brushes).

A sensorless BLDC motor has no sensors (obviously!) but the controller manages to read the rotor position by detecting the voltage generated on the third wire (each of the 3 wires is powered alternately in a 6 step sequence, but only two wires are powered at any one time). Unfortunately this doesn't work at very low speed or at rest, because the voltage is too low/zero. Therefore on startup the controller just tries to run the motor asynchronously at a low speed, then checks to see if the generated voltage is in sync. If not then it resets and tries again, which may cause the rotor to jiggle back and forth a few times before starting up.
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Old Feb 16, 2012, 10:40 AM
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Originally Posted by abenn View Post
Not sure what your question is

All electric motors operate by having a set of fixed magnets repel another set of magnets that are free to rotate around a shaft, and are fixed to the shaft so that they turn it. Both sets of magnets can be electro-magnets, but in our hobby motors one set is permanent magnets (don't need any power) and the other set is electro-magnets (need power).

In the simplest of motors, the power to the electro-magnets needs to be reversed twice every revolution so the magnets will always repel each other whatever position the moving ones are in, and keep the motor spinning. In these simple motors, the electro-magnets are rotating, and they get their power through a pair of "brushes" rubbing on a metal ring on the shaft. Reversal of the power during every revolution is achieved because that ring is in two halves, so as it rotates it's alternating which half is in contact with which brush. One weakness of this system is the brushes themselves, which can burn out and, even when new, are a source of electrical inefficiency. These motors are referred to as "brushed" motors or, sometimes, "DC" (direct current) motors because they will run if you simply apply a battery to their two input wires.

Most flyers these days use "brushless" motors. These have the electro-magnets on the static part of the motor, with the permanent magnets on the moving part, so the electro magnets can be connected directly to the power supply without using brushes. But, without any brushes we need another way to reverse the power every revolution, and this is done using an electronic speed controller (ESC) which electronically switches the power in sync with the rotation of the motor. These motors are sometimes referred to as "AC" (alternating current) motors because the power that's supplied to them is alternating. They cannot be run by simply applying a battery to their input wires.
Quote:
Originally Posted by slipstick View Post
I think you might need to ask your question again.

The brushless motors we use for RC (radio/remote control) models are DC (direct current) brushless motors. More accurately they are electronically commutated motors where the external ESC does the commutation and is supplied with the DC power.

So there's no difference at all unless you meant something else by rc and/or dc ?

Steve
Quote:
Originally Posted by Bruce Abbott View Post
Do you mean, the difference between AC and DC brushless motors?

An AC brushless motor uses AC (Alternating Current) power to generate a rotating magnetic field, which drags the rotor around with it. This can be done synchronously or asynchronously. A synchronous AC motor runs at a fixed speed, locked in phase with the AC power frequency. One problem with this design is that it is not self starting (it needs some other mechanism to get it up to synchronous speed) and if too much load is applied then it may drop out of sync and stop.

Another type of AC brushless motor is the Induction motor. This has copper wires embedded in the rotor, in which currents are induced by the rotating magnetic field. This current produces magnetism in the rotor itself, which then interacts with the stator field to produce a torque which turns the rotor. Due to this induction process, the rotor does not have to stay in sync with the supply frequency, but can 'slip' to a lower speed under load.

A DC motor has to produce its own rotating magnetic field, by switching current through coils in step with its rotation. In a brushed DC motor this is achieved using a commutator and brushes. A Brushless DC motor works just the same way, but with electronic commutation. A sensored BLDC motor has magnetic or optical sensors that transmit the rotor position to the controller. Essentially it is the same as the brushed motor, except that it uses solid-state switches (transistors) rather than a mechanical switching mechanism (commutator/brushes).

A sensorless BLDC motor has no sensors (obviously!) but the controller manages to read the rotor position by detecting the voltage generated on the third wire (each of the 3 wires is powered alternately in a 6 step sequence, but only two wires are powered at any one time). Unfortunately this doesn't work at very low speed or at rest, because the voltage is too low/zero. Therefore on startup the controller just tries to run the motor asynchronously at a low speed, then checks to see if the generated voltage is in sync. If not then it resets and tries again, which may cause the rotor to jiggle back and forth a few times before starting up.
Thanks for all of your replies, to be more specific I was curious to what the major differences in these two types of motors were from the perspective of them being used in rc aircraft and scooter/bike applications.
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Old Feb 16, 2012, 12:25 PM
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Can you be a bit more precise about what two types of motor you're talking about...perhaps links to where you've seen the terms "dc brushless motor" and "rc brushless motor" used ?

Steve
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Old Feb 16, 2012, 02:13 PM
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elecfryer's Avatar
Ventura CA
Joined Aug 2007
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To be clear,

Do a search on DC brushless and you will get results for motors!

Do a search on RC brushless and you get NO RESULTS regarding electic motors.

To the best of my knowledge, NO SUCH THING as a "RC BRUSHLESS MOTOR"

For your application, I am sure a Brushless motor will work fine, you just need to find the correct motor.

Michael
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Old Feb 16, 2012, 04:24 PM
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United States, AK, Fairbanks
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Quote:
Thanks for all of your replies, to be more specific I was curious to what the major differences in these two types of motors were from the perspective of them being used in rc aircraft and scooter/bike applications.
A lot of electric bike conversions actually use large brushless motors that are sold as RC aircraft motors. The motor doesn't know what it's powering, obviously, so as long as it's used within its electrical and mechanical capabilities it'll do whatever you tell it to. Since most of these conversions use sensorless motors, folks often add Hall effect sensors to gain control at the very low RPMs that these motors may need to run at, namely when starting from a dead stop without pedaling.

For dedicated bike motors such as the brushless hub motors that are gaining popularity, you'll generally see them with lower Kv values and higher pole counts than what most RC applications use. This is because it takes a heck of a lot of torque to run something like a 26" bike wheel in a direct-drive configuration, so very low Kv values and are desirable. Weight is another factor; they'll usually be built with the focus set more on structural strength and durability because bike wheel hubs need to stand up to a lot of abuse. RC planes are a relatively cushy home for them as far as mechanical stress, but we do regularly push them to the edge of their electrical abilities.

From an operational standpoint, they're identical. Both RC aircraft and bike/scooter motors require a source of external commutation in the form of an electronic speed control and both involved fixed windings and rotating permanent magnets.
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Old Feb 16, 2012, 05:10 PM
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I'm still wondering why people call the RC brushless motors DC (which I do as well)- they're 3 phase AC!

( tongue only a little in cheek- the differences between brushless DC and brushless AC 3-phase are subtle but there )
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Old Feb 16, 2012, 05:25 PM
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DC motors are really mechanically commutated ( commutator & brushes ) AC motors.

That is why there is a armature & brushes. To cause switching & repulsion.forces.

DC flows ONLY in 1 direction AND is held at a constant potential..voltage.
AC is everything else.

Edit & a question
Is there REALLY a DC motor ? 1 guess
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Old Feb 16, 2012, 05:37 PM
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Old Feb 16, 2012, 08:49 PM
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Quote:
Originally Posted by C₄H₁₀ View Post
A lot of electric bike conversions actually use large brushless motors that are sold as RC aircraft motors. The motor doesn't know what it's powering, obviously, so as long as it's used within its electrical and mechanical capabilities it'll do whatever you tell it to. Since most of these conversions use sensorless motors, folks often add Hall effect sensors to gain control at the very low RPMs that these motors may need to run at, namely when starting from a dead stop without pedaling.

For dedicated bike motors such as the brushless hub motors that are gaining popularity, you'll generally see them with lower Kv values and higher pole counts than what most RC applications use. This is because it takes a heck of a lot of torque to run something like a 26" bike wheel in a direct-drive configuration, so very low Kv values and are desirable. Weight is another factor; they'll usually be built with the focus set more on structural strength and durability because bike wheel hubs need to stand up to a lot of abuse. RC planes are a relatively cushy home for them as far as mechanical stress, but we do regularly push them to the edge of their electrical abilities.

From an operational standpoint, they're identical. Both RC aircraft and bike/scooter motors require a source of external commutation in the form of an electronic speed control and both involved fixed windings and rotating permanent magnets.
This is what I was looking for, so wouldn't scooter motors being built to handle the weight of the contraption and the rider themselves while rc plane motors are basically designed for the plane itself? Would the large weight difference cause the rc plane motors used on a scooter/bike to over work itself? Is one more preferable than the other for it's respective application?
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Old Feb 16, 2012, 11:27 PM
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The only major difference is that a motor designed primarily for use in planes will probably need to power your scooter through a chain or belt drive to prevent the motor's bearings from having to support the scooter's weight and absorb shocks from bumps amd such. They can't take physical abuse like that, but electrically the motor will work fine as long as you don't force it to draw more power than it can handle.

Depending on the motor's Kv and the battery used, the drive setup may need to incorporate some reduction gearing.
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Old Feb 17, 2012, 10:39 AM
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Originally Posted by C₄H₁₀ View Post
The only major difference is that a motor designed primarily for use in planes will probably need to power your scooter through a chain or belt drive to prevent the motor's bearings from having to support the scooter's weight and absorb shocks from bumps amd such. They can't take physical abuse like that, but electrically the motor will work fine as long as you don't force it to draw more power than it can handle.

Depending on the motor's Kv and the battery used, the drive setup may need to incorporate some reduction gearing.
Excellent thanks for the explanation!
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Old Feb 17, 2012, 01:16 PM
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Do any of these threads help if it's a bike orientated question -

Electric Bike

Help! Is this battery suitable for my E-bike?

RC Motors/controllers for E-bike

What would be a good power system for a mountain bike?
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