Modifiying motor armature - Page 1
Forum section: electronics / beginners

author: gkmaia
author_rank: Frequent Contributor
timestamp: September 29, 2020, 07:41:53 pm
content: I got a 12vDC motor that at NO LOAD runs at 8000RPM and takes 0.5A. I would like to modify it to take 0.25A at 4000RPM.If I double the windings I will roughly double the resistance. Will it cut the current and RPM by half at 12vdc?As resistance has increased I could then reduce the thickness of the wire?

author: Zero999
author_rank: Super Contributor
timestamp: September 29, 2020, 07:55:31 pm
content: Quote from: gkmaia on September 29, 2020, 07:41:53 pmYes, that's true, but it has nothing to do with the resistance. More turns, means more back EMF will be generated, given the same speed, so the speed will be slower.Why bother? Just use PWM to reduce the speed. Gears are another option and a 1:2 ratio will give double the torque.

author: gkmaia
author_rank: Frequent Contributor
timestamp: September 29, 2020, 08:07:43 pm
content: Thanks for your reply.It is because it is my Pace MBT motor and I cannot source the RS555-SH here in NZ and I need it asap. As I have winded quite a few transformers and I have bunch of winding wires around my best option would be to modify a spare one I have as it is too aggressive for it in terms of speed and current. That would take a couple hours and will be fun.I think gear is out of question as it drives the pump directly. And I really don't want to hack the PSU etc...

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 29, 2020, 08:19:20 pm
content: Quote from: gkmaia on September 29, 2020, 07:41:53 pmhave you opened one such motor? simply put... its impossible to modify...Quote from: gkmaia on September 29, 2020, 07:41:53 pmdouble the nightmare, there is no more space there... better you take Hero's advice on using PWM to reduce speedQuote from: gkmaia on September 29, 2020, 07:41:53 pmi hope you have no relation with a guy that wanted to make transistorless oscillator...

author: gkmaia
author_rank: Frequent Contributor
timestamp: September 29, 2020, 08:59:51 pm
content: Quote from: Mechatrommer on September 29, 2020, 08:19:20 pmThat is very encouraging...I will definitively try to modify it now.

author: gkmaia
author_rank: Frequent Contributor
timestamp: October 02, 2020, 09:57:38 pm
content: Rewinding worked. PACE is back and working within spec.

author: james_s
author_rank: Super Contributor
timestamp: October 02, 2020, 10:23:39 pm
content: Quote from: Mechatrommer on September 29, 2020, 08:19:20 pmNonsense, I have disassembled and rewound small hobby motors on a few occasions, it can be a bit fiddly but it certainly is not impossible. Back in the days of brushed motors in RC cars and before that slot cars it was not uncommon for racers to rewind motors to alter their characteristics.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 03, 2020, 07:49:34 am
content: simply put... i tried to discourage him from wasting time... when today we have buck converter and cheap motors... cheers.

author: gkmaia
author_rank: Frequent Contributor
timestamp: October 04, 2020, 02:26:12 am
content: It was good to learn these motors can be re-winded and modified to a level.If I had done a chinese PWM i would have lost the chance of learning that and eded up with an annoying humming.The PWM would have a positive side as it would allow better control of the pump, therefor custom flow for the thermopik and thermojet.but I think i am happier with the result so far.

author: Zero999
author_rank: Super Contributor
timestamp: October 04, 2020, 09:19:04 am
content: Quote from: gkmaia on October 04, 2020, 02:26:12 amPWM doesn't produce any audible noise, if done at an ultrasonic frequency. There are plenty of different ways to do it, which might involve learning something new: 555 timer, comparator, MCU.

author: Yansi
author_rank: Super Contributor
timestamp: October 04, 2020, 09:29:54 am
content: Motor PWM shall always be done at a frequency, where the current ripple through the winding is negligible (e.g. well under 10% of rated current).fpwm>> fmin= Ub / (0.4*La*In)For most motors across all different powers and types the limiting minimum frequency lies in the few kHz region, but due to the audible noise at such frequencies, higher ones are used, if practicable.//edit: Ub is the bus voltage of the PWM stage (Ud in below image), La is the armature inductance (L below image) and In is the nominal armature current. 0.4 is a constant of 4*0.1, 4 comes out the equation if you do all the math and 0.1 is the 10%. All maths comes from the U = L*di/dt, assuming armature electrical time constant La/Ra is much higher compared to the PWM period - which in most cases it is. Equations assuming exponential shape of the armature current are much more complex and usually are not needed.

author: gkmaia
author_rank: Frequent Contributor
timestamp: October 04, 2020, 10:19:46 am
content: The previous experiences I had were with 220v/60hz PWMs. The noise was not coming from the PWM but from the motor.As if even at it's lower setting was still allowing for some current to flow, but not enough to spin the motor.I did a bit of a look on how timer based PWMs work and if I understood right the timer ones increase or reduce the frequency of which the driving transistor is biased allowing for more or less constant supply of power.If I apply this logic to my humming one it seems that the timer never completely stopped biasing the transistor.But why for example we need to use frequency to bias the driving transistor. And instead of a timer, just another transistor to control the bias of the driver transistor linearly?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 04, 2020, 10:44:54 am
content: Quote from: Yansi on October 04, 2020, 09:29:54 amNo, not necessarily.For a simple DC motor, torque is pretty much constant with motor current, and RPM is just a "side effect".If you use a low PWM frequency you just get a shift from a nearly constant current (and torque) to a pulsed torque, and this can even be beneficial to the control of the motor.The pulsed torque is integrated by a mechanical flywheel. If the PWM frequency is very low, an extra flywheel may need to be added, but for intermediate frequencies the armature of the motor itself is sufficient.The advantage that can be gained here is better controlability at low RPM and loads.With a high PWM frequency, the (average) current may not be sufficient to overcome static friction of the brushes, bearings etc. and you get a slip stick effect.With a very low PWM frequency, you get a high current / torque pulse, which starts moving the flywheel / motor, but is too short to cause significant RPM differences.It's not uncommon to have controlability over the motor this way at half or even a quarter of the RPM than a higher PWM frequency would have.I also find the squealing noise of a few kHz annoying, while a low humming sound of a few hundred Hz fits much better in with the rest of the motor noise.About rewinding the motor...For some motors the stator can be rewound relatively easily, but for a DC motor with brushes and permanent magnets in the stator it's not very practical and bordering madness. (although some people still do it)For a simple DC motor the no load current is relatively constant over it's rpm range.Maybe it goes from 400mA @ 1000rpm to 600mA & 10000rpm.For a DC motor, current is related lineairly with torque, and most torque for a DC motor with no load is the torque needed to overcome the static friction of the brushes, and a bit for the bearings.If you double the windings, then you will also double the constant with which current is turned into torque. Which means you only need half the current, and also can use thinner wire (half the cross section).The total (DC) resistance of the wire will have quadrupled (It's now twice as long, and twice as thin), but as he maximum current also has halved the total power loss due to the resistance of the copper wire stays the same! (Do the math).The end result is that you changed your 12V 10A motor, into a 24V 5A motor. (with same RPM and power rating).

author: Yansi
author_rank: Super Contributor
timestamp: October 04, 2020, 11:19:36 am
content: Quote from: Doctorandus_P on October 04, 2020, 10:44:54 amSo you will increase current ripple -> increase losses in both the iron and copper (even to a point the motor will burn out) and fuck up all gearing or machinery that the motor runs, due to torque ripple.QuoteBased on what? That is a nonsense. If you want to compensate the droop of the torque characteristic due to armature resistance, there are ways to properly do so, such as a small positive feedback from armature current to armature voltage (PWM duty cycle respectively). That way you can control the slope of the the characteristic and make it even overcompensated if the application can make use of.But certainlynot by increasing current ripple!That is an approach of an uneducated hack.

author: Zero999
author_rank: Super Contributor
timestamp: October 04, 2020, 02:16:41 pm
content: Quote from: gkmaia on October 04, 2020, 10:19:46 amIf your previous experience involves mains powered motors, then they most likely used phase control, rather than PWM, which is totally different.I don't understand your question, regarding the biasing of the transistor. With PWM the transistor is either on or off.

author: gkmaia
author_rank: Frequent Contributor
timestamp: October 04, 2020, 07:17:27 pm
content: Quote from: Zero999 on October 04, 2020, 02:16:41 pmInstead of using a pulse to bias the transistor use a variable resistor divider at the right range.

author: james_s
author_rank: Super Contributor
timestamp: October 04, 2020, 08:34:15 pm
content: I think you have a fundamental misunderstanding of what PWM is. The transistor is used as a switch, it toggles between cutoff and saturation at a frequency of typically several kHz to several tens of kHz, it does not need biasing. The ratio of on to off time is adjusted to control the average power, it works much like a buck converter.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 04, 2020, 08:58:04 pm
content: Quote from: james_s on October 04, 2020, 08:34:15 pmjust to add... PWM is either switching transistor or buck convertor. i prefer buck converter since it will give "constant" current instead of pulsed current to the motor, but bad thing is we need another big inductor for the buck converter...Quote from: james_s on October 04, 2020, 08:34:15 pmlike i am when i first got here 10 years ago i was not sure whats the meaning of transistor in saturation or linear mode... here if OP google for "mosfet switch driving motor" he will get many examples, such as ...

author: gkmaia
author_rank: Frequent Contributor
timestamp: October 04, 2020, 09:43:58 pm
content: Cutoff, the transistor appears as an open circuit between the collector and emitter terminals. That happens when the pulse from the timer is low state, 0V.Saturation is where the base current has increased well beyond the point that the emitter-base junction is forward biased. That happens when the pulse from the timer is HIGH state, 5V, therefor way above the +/-0.6 required to forward bias it.As you explained it just toggles the transistor on and off and depending on the rate you end up getting more or less RPMs. I understood that, but failed to explain as I do not have an academic background on electronics.My alternative to a PWM was to provide a constant voltage controlled by a POT and not a timer to the base of the transistor. This voltage would start at 0.4V, there for the transistor would not be conducting. With the POT I would increase the voltage till it reaches 0.7v where the transistor is forward biased but far from being saturated.I imagine my alternative may not work for reasons beyond my current knowledge.

author: james_s
author_rank: Super Contributor
timestamp: October 04, 2020, 09:48:43 pm
content: What you're describing is using the transistor in linear mode, behaving as an electrically controlled resistor. That will work to control the speed of the motor but it is inefficient, all of the excess voltage will be dissipated as heat in the transistor. It will also not perform very well because putting a resistor in series with the motor will control the torque, the voltage across the motor will depend on the load so the speed is not going to be steady. You may also find that by the time you give it enough current to start from a stop, it will run too fast.

author: gkmaia
author_rank: Frequent Contributor
timestamp: October 04, 2020, 10:11:29 pm
content: Thanks I understand it now.