Understanding series wound DC motor waveform ? - Page 1
Forum section: electronics / beginners

author: nurbster
author_rank: Contributor
timestamp: September 30, 2018, 03:20:29 pm
content: Hi Forum. Can somebody explain what is wrong with this motor based on voltage and current waveforms ? This 12V motor should draw somewhere around 10A, don't have specs, but I do know that 24A+ is definitely not right.Thanks, nurber

author: nurbster
author_rank: Contributor
timestamp: September 30, 2018, 03:21:39 pm
content: I don't have a reference of what a good series wound DC motor should look like so would appreciate anybody that can provide a link.

author: Benta
author_rank: Super Contributor
timestamp: September 30, 2018, 03:23:17 pm
content: The waveforms look fine for a series wound motor. The big question is: what's your mechanical load?

author: nurbster
author_rank: Contributor
timestamp: October 01, 2018, 12:38:20 pm
content: Thanks for replying. This is without a load and motor seems to spin easily enough so I don't think it's an issue with bushing, bearings, etc.

author: Benta
author_rank: Super Contributor
timestamp: October 01, 2018, 01:07:23 pm
content: NEVER run a series motor without load. It will accelerate towards infinity, worst case the anchor will disintegrate.As it is now, it's running at far too high rpm, which explains your excessive current draw.Another point: series DC motors are very often timed for a certain rotation direction and will not perform well if reversed.

author: jmelson
author_rank: Super Contributor
timestamp: October 01, 2018, 07:55:30 pm
content: Quote from: Benta on October 01, 2018, 01:07:23 pmWhile, in THEORY, the above is true, it neglects windage and armature iron losses, which both increase with speed.So, unloaded series motors will generally not explode or be damaged without load, but they will run at the highest speed where those losses reach equilibrium with the energy coming into the armature.QuoteSeries motors will ALWAYS run in the same direction, regardless of power supply polarity!Unless you have switching to reverse the respective polarity between the field and armature, they will always run in the direction designed by the manufacturer.Jon

author: jmelson
author_rank: Super Contributor
timestamp: October 01, 2018, 07:58:38 pm
content: You don't give the time scale for your scope trace, so we can't figure out how fast it is spinning. But, there is a roughly 2:1 fluctuation in current, which sounds VERY strange. I'm guessing there may be a shorted turn in the armature. Since the motor is drawing about 200 W by averaging the current and voltage, it does sound like something is wrong.Jon

author: Benta
author_rank: Super Contributor
timestamp: October 01, 2018, 08:06:49 pm
content: Quote from: jmelson on October 01, 2018, 07:55:30 pmTo the first point: I said "worst case". As neither of us has the slightest idea of which motor this is, let's leave it at that.To the second point: My bad! You're absolutely right, my apologies.

author: xavier60
author_rank: Super Contributor
timestamp: October 01, 2018, 09:55:06 pm
content: An unloaded Series DC motor speeds up because of the reduced current draw which weakens the field. The armature needs to spin faster for its generated EMF to almost match the supply voltage.

author: nurbster
author_rank: Contributor
timestamp: October 02, 2018, 02:26:34 pm
content: thanks for informative replies. I uploaded an uncropped version of original image in first post that shows the timescale.

author: nurbster
author_rank: Contributor
timestamp: October 06, 2018, 01:46:02 am
content: I used an insulation tester on this motor measuring approximately 50 megaohms @ 250V between leads and shaft as well as between leads and enclosure. I'm not experienced enough with such a tester, but I believe this is indicative of a fault ? I then opened up motor to reveal windings that looked darker than normal ( perhaps heat damaged ) and continued with insulation testing showing the same 50 megaohms readings between leads and lamination stack. Resistance measurements between commutator bars ( 180 degrees and adjacent ) seemed normal. Can anybody comment on insulation testing measurements ? What is the methodology experienced people would use to troubleshoot this motor in the field ? The insulation tester can reveal interesting things because you don't even have to open up the motor to get to individual commutator bars and so it seems valuable to try when first diagnosing, but I would appreciate knowing how others would approach this situation.

author: amyk
author_rank: Super Contributor
timestamp: October 06, 2018, 04:14:26 am
content: 50MR seems OK for a 12V motor.If that timescale is in seconds, that is reallyslowfor this type of motor... I count ~55 commutations in 1.2 seconds, which even if assuming you have a tiny 6-bar commutator is only around 500 RPM.We need more information about the motor, but based on what you've given so far it doesn't sound right for a series DC motor to be turning at that speed with no load. I'd expect well over 1000 RPM.