Author Topic: Coreless motors weak at low rpms on PWM controllers  (Read 1077 times)

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peteski

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Re: Coreless motors weak at low rpms on PWM controllers
« Reply #15 on: January 06, 2023, 10:37:43 AM »
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So those photos of the scope's screen were all taken back in 2019, or its internal time clock is  not set correctly?

I have an old Tektronix 465B scope that is large and bulky, but the Rigol digital scope I acquired several years ago is only a size of a woman's purse.

If you curious how the decoder's do a good job powering the coreless motors, why not hook up your scope to their motor output, and see what the waveform looks like?  But I think to get most accurate results (since the decoder normally uses BEMF feedback from the motor), you would have to monitor the motor output with the motor connected.
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JeffB

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Re: Coreless motors weak at low rpms on PWM controllers
« Reply #16 on: January 06, 2023, 11:05:19 AM »
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So those photos of the scope's screen were all taken back in 2019, or its internal time clock is  not set correctly?

I have an old Tektronix 465B scope that is large and bulky, but the Rigol digital scope I acquired several years ago is only a size of a woman's purse.

If you curious how the decoder's do a good job powering the coreless motors, why not hook up your scope to their motor output, and see what the waveform looks like?  But I think to get most accurate results (since the decoder normally uses BEMF feedback from the motor), you would have to monitor the motor output with the motor connected.

The internal clock was correct...  I looked at the timestamp on the photos locally and it shows 01/23/2019.  I had a bit of a modelling lull for a few years and I've only been back at actively building things for about a year.

The scope was one that I used here at work.  Not sure if our electrical engineer even has it, or any scopes now days.

Jeff

mmagliaro

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Re: Coreless motors weak at low rpms on PWM controllers
« Reply #17 on: January 06, 2023, 03:41:00 PM »
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Have you tried turning the BEMF on the decoder OFF?  That is the one huge thing that the decoder is doing that
the simple PWM board is not.
And yeah, I agree with Peteski, at this point, I'd be so curious that I'd open the engine, tack on some test wires, and
look at the decoder output on the scope while you actually run the engine.  It would be great to see if the
wave form is noticeably different from a simple PWM square pulse, or if their frequency is super-high or low.

Oh!  Were you scoping that PWM output WHILE you were running the engine?  That could also be a huge factor.  It could be
that the PWM doesn't look like that at all when you throw an inductive load across it.

Anyway... if one were curious... these are the things I'd be doing.

JeffB

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Re: Coreless motors weak at low rpms on PWM controllers
« Reply #18 on: January 09, 2023, 06:53:31 AM »
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Have you tried turning the BEMF on the decoder OFF?  That is the one huge thing that the decoder is doing that
the simple PWM board is not.
And yeah, I agree with Peteski, at this point, I'd be so curious that I'd open the engine, tack on some test wires, and
look at the decoder output on the scope while you actually run the engine.  It would be great to see if the
wave form is noticeably different from a simple PWM square pulse, or if their frequency is super-high or low.

Oh!  Were you scoping that PWM output WHILE you were running the engine?  That could also be a huge factor.  It could be
that the PWM doesn't look like that at all when you throw an inductive load across it.

Anyway... if one were curious... these are the things I'd be doing.

The engine runs great with the decoder, so I have no real need to mess with it and I'm not curious enough to do any more analysis.  The scope I used was at work.  Not sure we (the company) even has it any more.  We've done a significant equipment purge here in the lab in the last few years.

I'm just happy that I was able to figure out what the problem was...  It made no sense why a handful of mechanisms with anywhere from 67.5:1, up to 90:1 with 4.5:1 and 6:1 worth of compounding, had no low end torque. 

Running off my old Troller, they have a ton of power, even at the slowest speeds...  Same with the locomotive that runs off a DCC decoder.  No problems there.

Jeff