Stall torque - BLDC 1

[hyboxis79]

Looking on a BLDC motor catalog for motors with the same iron there are different motor windings but similar maximum power ratings. Other than that I know the operating current, no load current, armature resistance, terminal voltage
1. Just to be sure since it has a permanent magnet the no load current doesn't rely on the voltage (the no load current is given at 8.4 volt) right?
2. Since I have gotta choose from one of the motors is there a way to predict the stall torque / stall current or is that information only accessible through the manufacturer?

 

[MotorExpert]

A different motor winding means that the torque constant (torque that you get per Ampere current) is different. The overall power rating will be the same so this means the one motor will have higher torque/lower speed and the other one lower torque/higher speed.

If you know very little other parameters you should at least know the torque constant to predict the current.

But what is the torque and speed and duty cycle that you require? And what catalog are you looking at?


Edward Hage

http://www.specamotor.com

 

[hyboxis79]

Motorexpert,

I am looking at a R/C motor company called Hacker motors Catalog. They don't have a lot of specs in their brochures.


Regrading the maximum power the differences. Although I would achieve similar power outputs I would notice the differences in power inputs. Since there's a difference between the currents of low windings motors and high windings motors while voltage stay the same.
I can get the torque constant through the Voltage constant just by multiplying K_v by 1.345

I still don't know exactly the load I need. The nature of the load is to give cyclic vibrations or hits to a membrane structure. We made an experiment with a motor we already had - the hacker B40-19L (I am enclosing the catalog pages) but since we didn't reach the nominal operating current and voltage I can't really say what is the load.

 

[hyboxis79]

Here is the catalog:

http://files.engineering.com/getfil...ad8308e5fa8e&file=Hacker-2008-WEB_28+29-1.pdf

 

[MotorExpert]

I could not open the enclosed file.

What you may want to do is measure the current in the setup you have and multiply that with the torque constant to get the torque.

Also measure voltage and resistance (when the motor is not connected).

Now do: voltage - current * resistance = Eelectr = voltage constant * speed. so you can derive the speed from that (or measure it directly if you have such a device).

Than you have torque and speed. You mentioned you already have the torque constant and voltage constant of the new motor so you can calculate the current and voltage of the new motor and check if these values are within spec. If so this motor will probably suffice. Always good to check it with the seller if you can.

I assume the new motor is smaller than the B40-19L . This would be the way to check if it is not too small.

BTW measuring the current and deriving the torque constant. Be aware that given torque constant has +/- 10% or more deviation and add some 30% margin if you measure the motor in a cold condition (temperature influences torque constant; usually it is given in a warm condition but every vendor makes up their own rules about this).



Edward Hage

http://www.specamotor.com

 

[Clyde38]

I was not able to open the attachment as well.

1. Just to be sure since it has a permanent magnet the no load current doesn't rely on the voltage (the no load current is given at 8.4 volt) right?
2. Since I have gotta choose from one of the motors is there a way to predict the stall torque / stall current or is that information only accessible through the manufacturer?...

1. No load current will somewhat relate to the voltage applied. No load current includes the losses of the rotating armature itself (bearing friction, Foucault, windage, etc.) so the higher the voltage, the higher the no load speed of the motor. Higher speed is more loss, so current will increase some.
2. If you have EXCEL try the attached spreadsheet. Modify the values in blue. When you measure the resistance of a PM DC motor, be careful of the method. A simple ohmmeter measurement will not be quite right due to the commutation system (brushes etc.) and the low current of the meter, especially if the brushes are some form of graphite. Precious metal is less of a problem. It is best to apply a constant current (high enough to overcome the brush drop ~1 amp or so) and manually rotate the armature and measure the average voltage to calculate the resistance (R=V/I).
I hope this helps.

 

[MotorExpert]

Nice calculation sheet Clyde.

Very thorough, taking into account the temperature-dependance of the resistance and also the torque constant. I don't see it much being used and I am glad to see I am not the only one.

Looks a bit like my paper

http://www.specamotor.com/pdf/overheating_white_paper.pdf

where I calculate the temperature dependancy in a similar fashion. The difference is that I use torque and speed to calculate the final motortemperature and than I can check if this is under the maximum allowed temperature,
and you calculate the maximum allowed torque for the given maximum allowed temperature. Your method is more simpel (which is always good), but you miss the final temperature might you want to know this.


Edward Hage

http://www.specamotor.com

 

[Clyde38]

Hi Edward,
On the contrary, I've read your paper etc. I've looked at the results of your analysis (see attached). I thought this was more than hyobxis79 was interested in. Nice work.

 

[benta]

Clyde, the OP is asking about a *brushless* motor. Stall current can be calculated by just measuring the resistance of the windings.

Benta.

 

[roydm]

So that's what BLDC means, I wish some of you would explain your acronyms at least once in the post, I'm a bit slow!
Roy

 

[Clyde38]

Benta, that's correct. Sometimes my mind wanders (but not very far).
Man's mind, once stretched by a new idea, never regains its original dimensions. ~Oliver Wendell Holmes

 

[hyboxis79]

thanks Clyde38 this excel sheet is really useful.I havn't understood all of the equations but I am on the way.

The excel sheet that motorman sent is a bit above my current level of understanding but thanks anyways

 

[hyboxis79]

Clyde 38 I have another question for you. The stall torque in the spreadsheet you gave me is calculated by finding the current running at nominal voltage when no emf is induced. From what I understand you refer to the current as the starting load. Can you explain why? what about the friction toque then?

thanks

 

[Clyde38]

hyboxis79,
The stall torque is nothing more than the available current times the torque constant (torque per amp). In the spreadsheet, the nominal voltage divided by the resistance equals the available current (I=V/R). If you have a different voltage, the stall current (and torque) will be different (if the resistance stays the same). Any time the rotor is moving, there is EMF. The EMF is proportional to the speed (higher speed=higher EMF). I'm not sure what you are referring to with the statement "From what I understand you refer to the current as the starting load" however let me say that the maximum current that you can supply to the windings times the torque constant of the motor is theoretically the maximum torque at stall or at start (zero speed). Once rotation begins, the EMF will oppose the applied voltage and reduce the amount of voltage that contributes to desired torque. Friction torque will not vary much (hopefully) across the range of speed (0 to no-load). Rotational losses will normally increase as speed increases. I hope this answers your questions.

 

[hyboxis79]

thanks clyde38
this helps with the theory
But the bottom line as you said is that I can't be sure of the stall torque because it depends on current that the controller of the bldc motor allows me draw and on the manufacturer.

 

[Clyde38]

You can be sure of the motor's torque capabilities (I haven't discussed things like phase advance, nonlinearities, etc.) and you should have control over the operational characteristics of your drive. It may be time to get some additional help for your application. Usually the motor/drive manufacturer is more than willing to assist you. Best of luck.

Fortune brings in some boats that are not steered. ~William Shakespeare