Brushless motor specifications question

It does

You would if the motor is driving its rated load, however the waveforms are often modified to take account of the motors back emf waveshape to give smooth output torque.

I would think a "specification" like that, particularly if on a name-plate would mean total RMS... at maximum speed. Is this a variable speed motor? ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

The question is about brushless motors in general, with three big wires sticking out for the three phases. Not a brushless with a built-in controller -- those are boring, unless I'm designing the controller.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

I have never seen it written any where but I have always reasoned that measuring the current of a 3-phase motor that you measure and sum each leg and then divide by two since you must have measured everything twice.

In practice (brushless model airplane motors) I just measure the DC input to the controller.

Come to think of it, I doubt that I have an instrument that will measure the AC current accurately on the brushless motor. I think my stuff is tuned to 60 HZ.

Would RMS current be different than average current?

John Ferrell W8CCW

Ask the manufacturer ;-)

The one (controller chip) I designed for Bosch/Mercedes... each winding was around 25A peak. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

What do you mean by "brushless AC motor"?

Assuming 3-phase:

It could be:

3-phase induction "squirrel cage" motor. The commonest.

3-phase induction motor with wound rotor and slip rings to control rotor current when starting. Usually only very big motors.

3-phase synchronous permanent magnet rotor motor. Found mainly associated with PWM vector servos.

3-phase synchronous wound rotor slip ring motor. Big specialized motors, and "synchronous capacitors" for PF control, which actually run unloaded.

Induction motors run at a speed slightly less than that predicted by the frequency and number of pole pairs, called slip, which increases with torque loading.

Synchronous motors run at f/n revs per second, where n is the number of pole pairs. Increasing load does not affect the speed up to a critical level where the motor falls out of sync, and stops, making nasty noises. This happens when the angle between the internal ("back") EMF and the applied voltage (the so-called "torque angle") reaches 90 degrees.

In all cases, current increases with load. Running unloaded, current will be low.

For a three-phase motor, plate details usually mean per-phase RMS current at full load, unless specified. The current in each phase should be equal, and all three should add "vectorially*" to zero, unless the motor is sick. Hence Kirchoff's laws still apply.

*I know they're not true vectors,but the some power engineering guys still call them so.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

I was thinking three phase permanent magnet brushless synchronous machines with sinusoidal back-emf, known in the trade as "AC brushless motors".

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

Plate conventions still apply. Rated current is per-phase at full torque.

That sort of motor generally has a stall torque and current stated, too.

I see 8-pole, 200Hz ones regularly.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

Two things...

(1) I'd hardly call the back EMF "sinusoidal". Only in your wet dream.

(2) I've never heard them called "AC"

Here's the one I designed to...

PM, with three power-NMOS drivers, driven from a chip that watched back-EMF for commutation. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

Permanent magnet brushless motors come (mainly) in two flavors: "DC", with a back-emf that's nominally trapezoidal, and "AC", with a back-emf that's nominally sinusoidal. I assume that yours had a trapezoidal back- emf.

The AC ones are more complicated to drive (particularly if you want to go down to zero speed), but are supposed to be smoother.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

I had to go very slowly and there was "soft" shaping on the drain currents. The primary test was the chief engineer sticking his head in the Mercedes air duct and listening for any "whine" ;-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

variable

=46or sine wave source the rated amperes at full load is for each phase wire, balanced. PWM sources may not read the same TRMS.

?-)