So I am looking at an adapter board that is designed to operate 3 different types of motors. The motor I am interested in has inductors that are external to it that are placed in series with each coil.

I am trying to understand the purpose of this. I understand basic motor theory and how the coils inside the motor are inductors. I do not understand the external purpose. Is this related to back-EMF generated from non-active coils? Can anyone shed any light?

The motor I am interested in is: http://www.micromo.com/datasheets/Brushless DC motors/0206_B_DFF.pdf

 

How about posting info on the adapter board. I'm curious, too.

 

Can't find any datasheet on the test board I am using. This is the logic board that is attached to it however. What I can say is that there are three coils that are in line with each coil of the motor.

After poking around with a scope, before the coil there is a normal PWM signal. Afterwards, there is an overshoot and undershoot each time it is driven high or low. This is with no motor connected. Connected, I just see what appears to be a discrete sine wave. on both sides of the inductor.

http://www.micromo.com/datasheets/Drive Elect/BLD05002S_MMA.pdf

 

My guess would be that they are just smoothing chokes.

 

Sorry, but I am not familiar with smoothing chokes. I googled but still a little hazy. Is it a sort of filter to achieve the desired waveform? A quick answer and a link to a detailed article would be nice

Sidenote, probably not useful but I snapped a picture of the test board. The three coils in question are right above 'tors' in 'mymotors'

 

I really couldn't find a link to a good definition, but you are correct in your thought about it helping to achieve the proper waveform. In most BLDC applications, very high frequency PWM signals are used to approximate the POWER curves of the three phase sine waves used to drive the motors. The inductors tend to cause current to bridge the gaps between on and off of the PWM voltages, much the same as they do for helping to remove ripple in a power supply. From the physical size of your inductors, I would say that the PWM frequency of your drive is quite high.

 

I see. And yes, the board supports PWM frequencies up to 500 kHz according to the datasheet they provided.

Last question (maybe), why do I only see these inductors on the lines for the motor I am using? The other two motors this test board supports do not have this and probing shows a normal PWM signal going into the motor plugs.


Other 2 motor datasheets:

http://www.micromo.com/datasheets/Brushless DC motors/0308_B_DFF.pdf
http://www.micromo.com/datasheets/Brushless DC motors/0515_B_DFF.pdf

 

I really can't answer that question accurately, but a guess would be because of the inductance of the motor windings being lower in the smaller motor.

 

quick bump.

Is there any literature that describes how to determine when external power chokes are required? I need something for citation for a paper I am writing on this topic.

 

Although I didn't read the entire paper, look at Theory and Simulation on first page. The external inductors help reduce inrush current to the motor windings.