Difference between motors

Hi:

I recently noticed that some dc motors have a very smooth rotation (removed one from al old casette recorder and another one from a cd-rom unit). On the other hand I bought a couple of motors

http://www.mabuchi-motor.co.jp/cgi-bin/catalog/e_catalog.cgi?CAT_ID=rk_370ca

and when you turn the shaft, you can feel some little bumps (6 to be precise), very similar to a stepper.

Has this something to do with the quality of the motors? I have another motor with an optical encoder attached and I'm trying to implement position control but this one also has those bumps and I think that could affect the control performance.

Some DC motors use magnets to create the stator field, others use winding that have to be energized to create the stator field. The 'cogging' you are feeling is from the magnets inside the motor.

retrolefty: Some DC motors use magnets to create the stator field, others use winding that have to be energized to create the stator field. The 'cogging' you are feeling is from the magnets inside the motor.

If you buy dc motors over the internet, is there any way to differentiate this?, some kind of specification, do you think this could affect position control?

Gilgamesh90: If you buy dc motors over the internet, is there any way to differentiate this?,

I have been doing that to much and to often lately. There are quite a lot of different motor types and there is the motor slang and also many different markets, all with their own slang and habits. When you do not know much about motors it is not easy to "go in and find what you need at the right price". On the other hand if you only need a "fairly low power standard motor" getting ripped off is not to bad.

To give an example of how confusing it can be. Motors normally have a watt rating. This is the power the motor can handle. You have "max rating" and "rated rating". Rated is the value you can have on "for ever and always". the max rating will destroy the motor in milliseconds to seconds (you need to read the datasheet to know the time). So though the watt rating is pretty much technically very specified you have to know what you are looking at.

Even then. Look at these 2 motors This 36mm BLDC motor has 550 watts http://www.hobbyking.com/hobbyking/store/__27867__Turnigy_Trackstar_1_10_17_0T_2400KV_Brushless.html and costs about 23$. If you look at this site (a respected company in industry) http://benl.rs-online.com/web/c/automation-control-gear/electric-motors-motor-controllers-peripherals/dc-motors/?searchTerm=dcmotor You will see they don't sell anything higher than 50 watt and you are talking 10 times the price. They do have AC 500 watt motors. Look at this one to see the differences http://benl.rs-online.com/web/p/ac-motors/7332180/

I'm an engineer and to me it is plainly impossible that the hobbyking motor has 550 watts. I don't even think it has 50 watts. So I don't know how these "watts compare". Note that I'm using a hobbyking motor as an example but generally any BLDC engine in the RC world has "wierdly high watts" to me. If anyone can bring me the key on how and when to convert please bring it.

Best regards Jantje

I'm an engineer and to me it is plainly impossible that the hobbyking motor has 550 watts. I don't even think it has 50 watts. So I don't know how these "watts compare". Note that I'm using a hobbyking motor as an example but generally any BLDC engine in the RC world has "wierdly high watts" to me. If anyone can bring me the key on how and when to convert please bring it.

Those BLDC motors as used in R/C vehicles are not being dishonest. They utilize powerful neo- something type magnets and utilize 3 phase PWM to the windings, so they have more in common with AC motors then in standard brushed DC motors. They do run indescribable power relative to their size, but often are not run a full output power capacity, but PWM down to a desired RPM or speed command and their operating time is limited to the battery duration and current capacity of the batteries. If there had not been such big advancements in Li-po batteries then the current BLDC motors would not have been created in the first place. It turns out the maximum power limit of such motors like those is it's maximum operating temperature as above a certain value the magnets are damaged because they loose their magnetic strength. So heat management comes into play,

So bottom line, I think their maximum wattage ratings are honest.

Well your an engineer so you know that because it is a 3-phase motor that means 550/3=183 W /per phase and if the model airplane or helicopter is running a 5S lipo then (5*4.1)=20.8V Motor Power in and 183W/20.8V = 8.7 A per phase @ 21V. 8.7A (per phase) is not a particularly high amount of current but that's what that 550W translates to. On top of that, if you pull open up the motor you will see that they have 9 to 12 poles (some more, some less) so when you devide that 8.7A between 3 or four separate poles now your talking about 3 or 4 amps per pole per phase and it doesn't sound as rediculous as 550W. And that's without even getting into the discussion about duty cycle and the fact that the current we are talking about is always an average because it is never on 100 % . It is always some fraction , and at full throttle it may approach 100% duty cycle but I doubt it is actually 100% so there is some cooling time, which also brings up the fact that the friggin model airplane is screaming through free air at 40, 50 or 60 mph (or more) with all that air flowing over the motor to keep it cool. If you strapped it down to the bench and ran it at full throttle the prop wash would still cool it a lot unless it was a "pusher " and then you would probably fry the motor in about a minute at full throttle with no air flow.

Seems I hit a soft spot. 8) I don't think I said it is "dishonest" I just see it as "slang" or a different way to look at watt. For instance both the responses above talk about the wattage consumed. In industry we typically speak about the wattage delivered. Typically for a motor +80% return is considered good. so that can account for some differences. BLDC motors are considered high efficiency (90% or higher) so that does not explain the difference.

My experiencing with a high watt RC motor. Not so long ago I saw someone with one of these high power watt RC motors spinning a wheel with leds. He had to help to get the wheel spinning going .... (500 watt? well not at stand still for sure.)

As to heat and magnets. Simplified: any motor just dies because it becomes to hot and burns. Heat has influence on the resistance of the wires and on the effectiveness (and life time) of the magnets. If you replace the magnets by the electromagnets you produce more heat to take away the ill effect of heat on magnets and you can increase control over the motor.

As far as I understand: if it was not for heath you could make a 1000 watt motor with a 40AWG wire in a 10 by 10 by 10 box. It is basically heat that makes the difference between "rated" and "peak" power. The better the return of investment of the motor the less heat so the less heat you have to dispose off so the closer rated and peak are together.

Assuming a 90% return a 550 Watt rated motor needs to dissipate 55 watt continuously. To be honest I think this is possible in an airplane with the rotor directly attached on the motor. After all when the motor is using power there is a big fan blowing air from a non closed cooling system. Add to that the BLDC construction (having the wires directly connected to the case) and you'll cool it down enough But what if I put that same motor to another use that does not have this cooling benefit?

Look at it this way? If this motor really has 550 watt rated why would a mill constructor use a 200$ motor (both without controller off-course)?

Again I am not screaming dishonest or unfair. I just wanted to explain someone to be careful wen comparing figures.

@raschemmel As explained above: I'm not concerned with getting the power in. I'm concerned about getting the heat out in continuous operation.

@retrolefty

and their operating time is limited to the battery duration and current capacity of the batteries.

This is a nice example of what I called "slang and habits of a market". You need to be in the RC world (which I'm definitely not) to know that "continuous operation = 1 battery load" 100% fair but not the same "continuous operation" which is used in industry.

Best regards Jantje

As explained above: I'm not concerned with getting the power in. I'm concerned about getting the heat out in continuous operation.

That's why we fly "balls to the wall" (full throttle) , to get more airflow (and because it looks cooler to traversed the field in 3 seconds)

The phrase balls to the wall, meaning an all-out effort, comes from the world of aviation. On an airplane, the handles controlling the throttle and the fuel mixture are often topped with ball-shaped grips, referred to by pilots as (what else?) balls. Pushing the balls forward, close to the front wall of the cockpit increases the amount of fuel going to the engines and results in the highest possible speed.

The earliest written citation is from 1967, appearing in Frank Harvey’s Air War—Vietnam:

You know what happened on that first Doomsday Mission (as the boys call a big balls-to-the-wall raid) against Hanoi oil.1

Gilgamesh90:

retrolefty: Some DC motors use magnets to create the stator field, others use winding that have to be energized to create the stator field. The 'cogging' you are feeling is from the magnets inside the motor.

If you buy dc motors over the internet, is there any way to differentiate this?, some kind of specification, do you think this could affect position control?

All small DC brushed motors are permanent magnet. The amount of cogging depends on the shape and skew of the various laminations and is not normally considered important except in servo motors.

Larger DC motors with field-windings are available, and they can be of two types, series (low resistance field winding) or shunt (high resistance field winding).

Universal motors also have field windings, but the resistance and inductance of the windings is optimized for 50 or 60Hz, not DC use.

All small DC brushed motors are permanent magnet. The amount of cogging depends on the shape and skew of the various laminations and is not normally considered important except in servo motors.

So, is it really a factor to consider when controlling position?

Gilgamesh90:

All small DC brushed motors are permanent magnet. The amount of cogging depends on the shape and skew of the various laminations and is not normally considered important except in servo motors.

So, is it really a factor to consider when controlling position?

If (precise) positioning is important brushed DC motors would not be my first choice. A stepper is better at that. Best regards Jantje

Lack of cogging is indeed a selling point for servo motors, presumably it improves
performance at low torques. Good servo motors are highly flexible performers,
wide range of torque and speed and positioning to the accuracy of the encoder,
which can be seconds of arc… Not cheap though.

MarkT: Good servo motors are highly flexible performers, wide range of torque and speed and positioning to the accuracy of the encoder

Agreed! that's why I'm trying to understand motor and encoders. I would like to build a robot using motors and encoders, I've seen hobby robots made out with RC servos or steppers, but don't like the way they work (torque can't be controlled properly and the movement is not very smooth). Unfortunately this is a very difficult endeavor that requires a lot of knowledge and skills.

One way of expressing the power output of a motor, is the product of torque and speed.

The small bldc have implausibly high power outputs because they run very fast. When you calculate the torque and the speed, the 500 watt figures don't look so implausible. For an aircraft propellor, the torque required to start them moving is very low.

Contrast that to a wheeled vehicle which has to start rolling from scratch, and the torque requirement is very different, as it also is for a dc motor driving a servo or needing to hold in a particular stalled position.

The faster you can make a motor run, the smaller you can make it, for a given power output, as long as you can provide the cooling. That's how those screaming dyson hand-dryers work.