Testing Salvaged Motors

[darren adcock]

Hi. Just want to make sure I'm checking correctly how much current is being drawn from motors I salvage, included is a drawing of how i'm testing (free running). And do i just stall the motor to check it's stall current? seems correct, but best to check. Mostly the motors are from printers i find that people have thrown out.

Am I reading the decimal point correctly in that this reads as 50mA?

Also to make sure i'm reading datasheets correctly for MOSFETS (particualarly TIP120 and IRF540N) ;
Is the Max collector current the current the rating I need to pay attention to?
And the collector dissipation is the power rating to pay attention to, to make sure the transistor stays within a safe temperature?
Power is I x V =p (watts)?
Thanks in advance

 

[Minder]

Depends on the motor, e.g. stepper motors use the same (rated) current whether at zero or high rpm.
DC motors the current depends on load and is also maximum at switch on, if/when full voltage is applied.
They have continuous rated current (torque) and peak current, the latter should only be used for very brief periods.
M.

 

[darren adcock]

Depends on the motor, e.g. stepper motors use the same (rated) current whether at zero or high rpm.
DC motors the current depends on load and is also maximum at switch on, if/when full voltage is applied.
They have continuous rated current (torque) and peak current, the latter should only be used for very brief periods.
M.

OK, thanks, more complicated than I assumed. I'm only using DC motors, not got round to stepper motors yet.

So to make sure i'm supplying enough power I need to take a reading at switch on and under load? I assumed under load would be to stall the motor, but of course this doesn't tell me what the motor is happy with.

So i can't per-se apply a test condition to determine what the motors specs are?

I will start taking note of where i get the motors from and find the info from there

Darren

 

[Minder]

The absolute maximum current of a DC brushed motor is when stationary and when full voltage is applied, which should be very brief if the motor is allowed to come up to rated speed within the continuous current range.
You could find this value by taking a resistance measure of the armature.
To do this accurately you should apply a small DC voltage with the armature locked and read the current, the resistance of the armature can be calculated from this.
A few readings on the armature should be taken at different points and the lowest reading is the valid one.
M.

 

[darren adcock]

The absolute maximum current of a DC brushed motor is when stationary and when full voltage is applied, which should be very brief if the motor is allowed to come up to rated speed within the continuous current range.
You could find this value by taking a resistance measure of the armature.
To do this accurately you should apply a small DC voltage with the armature locked and read the current, the resistance of the armature can be calculated from this.
A few readings on the armature should be taken at different points and the lowest reading is the valid one.
M.

Cheers M. I'm a little lost now, but will go and do some research.

 

[Minder]

With the likes of a PWM controller etc, the initial current can be controlled/limited by gradually bringing the motor up to speed.
M.

 

[darren adcock]

Thanks M. I'm reading your replies to the thread "motor stronger than power supply voltage" alongside this thread. I'm using my Modular Synthesiser via a mosfet circuit to control motors and have found the PWM function on the SQR wave cv outs to be very useful, especially when starting motors that are under a larger load.

 

[hevans1944]

Am I reading the decimal point correctly in that this reads as 50mA?

Yes, 50 mA using the digital muiltimeter on its 10A range.

Is the Max collector current the current the rating I need to pay attention to?

ALL the specifications are important, not just the maximum collector current, or in the case of MOSFETs the maximum drain current. You should NOT be using MOSFETs or BJTs in their linear mode with motors. Either type work well as switches however, dissipating essentially zero power when turned OFF and very little power when turned ON if driven "hard enough," for example into saturation for BJTs. This means you use pulse-width modulation (PWM) to drive your DC motors, applying voltage (or current) pulses at a constant high frequency (usually several kilohertz) and varying the duty cycle (width) of the pulses from zero or off to full-on and anything in between. The motor inertia will "integrate" the pulses and resolve them into an "average" current that is proportional to the duty cycle, no filtering necessary at the motor. There are plenty of PWM modules available "really cheap" from Asian vendors. Buy one and use it to "test" or characterize your salvaged motors on your bench.

 

[darren adcock]

Yes, 50 mA using the digital muiltimeter on its 10A range.


ALL the specifications are important, not just the maximum collector current, or in the case of MOSFETs the maximum drain current. You should NOT be using MOSFETs or BJTs in their linear mode with motors. Either type work well as switches however, dissipating essentially zero power when turned OFF and very little power when turned ON if driven "hard enough," for example into saturation for BJTs. This means you use pulse-width modulation (PWM) to drive your DC motors, applying voltage (or current) pulses at a constant high frequency (usually several kilohertz) and varying the duty cycle (width) of the pulses from zero or off to full-on and anything in between. The motor inertia will "integrate" the pulses and resolve them into an "average" current that is proportional to the duty cycle, no filtering necessary at the motor. There are plenty of PWM modules available "really cheap" from Asian vendors. Buy one and use it to "test" or characterize your salvaged motors on your bench.

Cheers Hop.

In terms of not using the MOSFETS or BJT's in linear mode; It appears that I am, for example I am using various low frequency control voltage wave forms from my synthesizer via a buffer to drive a motor via a TIP120. With varying results to be fair, depending on motor and load. I'll stop doing this.

So if i'm understanding correctly, I should be using PWM, which I should be able to strip board (I'll breadboard something up today). Then I find a circuit which allows the duty cycle to be CV controlled so I can have the motors working in sync with my Synthesiser, which should give the effect i'm after.

 

[hevans1944]

Sounds like you have the concept nailed. The PWM waveform, after low-pass filtering, should resemble the CV controlled synth output. Not sure what effect you are trying to create, but make sure the PWM frequency is much larger than the highest synth frequency. The motor response will be limited by the inertia of its rotor, but this can nevertheless extend up into the kilohertz range, at least for small shaft rotations, so you want the PWM frequency to be at least ten times the upper frequency limit of the synth output for "reasonable" accuracy.

Maybe you could post a "sound bite" here or a short video after you finish the project.

 

[darren adcock]

Sounds like you have the concept nailed. The PWM waveform, after low-pass filtering, should resemble the CV controlled synth output. Not sure what effect you are trying to create, but make sure the PWM frequency is much larger than the highest synth frequency. The motor response will be limited by the inertia of its rotor, but this can nevertheless extend up into the kilohertz range, at least for small shaft rotations, so you want the PWM frequency to be at least ten times the upper frequency limit of the synth output for "reasonable" accuracy.

Maybe you could post a "sound bite" here or a short video after you finish the project.

Hey Hop. Effect was probably lazy wording, easily get overwhelmed by information, anyhow the "effect" is simply to have the motors work in time with the synthesiser. In terms of the having the LFO's modulate the synth and at the same time control motors, hopefully extend this to all modulating signals from synth, but seems sensible to start with the LFO's.

I should have some early experiment vids soon. Today's bread boarding was reasonable i think. Managed to get close to what we have discussed. I'll post a schematic asap as I know I don't understand circuit design very well and tend to try and bodge circuits together without understanding how they interact properly.

I'll look into the frequency range of my LFO's and work out how to achieve what you suggest, I fear Mathematics

Quick break for me now, but will get the schematic here shortly.

 

[darren adcock]

Rather than me posting a big clumsy circuit I'm going post each section at a time, as it will make correcting much easier. Here is the intial clock circuit. My LFO at max range reads at 264 Hz so I tried to get as close 3KHZ as possible, but didn't have a 51k resistor for R2, as specified via an online 555 timer astable calculator to get 3KHZ. This circuit reads as 2.98 KHZon its output. The only frequency counter I have is on a cheap UNI T UT120C DMM so don't know how accurate it is. I should learn to be able to read this from my Hitachi V-222 Oscilloscope, i'm assuming this is possible. Or should buy a dedicated frequency counter, will look into this.

 

[darren adcock]

Here is the full schematic of what I have bread boarded.

A couple problems I am having;
-Can't seem to get more than 3.2v at on the emitter of TIP120, maybe the motor should go between +12v and Collector?
-This next problem may well because of placing the motor in the wrong place. There seems to be a very short window when the motor initially turns on and reaches it's max speed. It seems to act more logarithmically than linearly, I tried putting a 1uf capacitor between output of the 2nd 555 timer and the base of the TIP120 and this helped somewhat but didn't fix the problem. I tried swapping in and out components, following on from you mentioning the need for a low pass filter but I don't really understand what is needed.

Seems like i'm close to getting what i need but I don't understand the circuit too well to adjust it accordingly. Am I going in the right direction here?

I'm away now until Sunday night, but wanted to get this drawn and posted before I leave.

 

[Minder]

Using the IRF540 you would use the motor on the Drain side, the Source would connect to Common.
M.

 

[darren adcock]

Using the IRF540 you would use the motor on the Drain side, the Source would connect to Common.
M.

Ok thanks.

 

[darren adcock]

More testing today and an overhaul of schematic, essentially getting rid of second 555 timer and adding a Low pass filter (I expect I've got this wrong). I added the filter as the motor was making a screaming sound and was also amplified through some of the synths modules, mainly the wave folder, I got it as quite as i could by
swapping in and out values of R3 and C2 but is by no means solved the problem so I expect i'm misunderstanding something. Anyhow it's working better than it has before. Here's the schematic and a video of the circuit modulated by a sine wave, scoped from the 555's PWM output . Initially it was a square wave when i first had it bread boarded, but now it has changed and I haven't been able to figure out why yet. The modulation comes in towards the end of the short video.

 

[hevans1944]

You are still placing the motor winding in series with the emitter of the TIP120 BJT. Try placing the motor winding between the collector and the +12 V DC supply.

Because you are driving the motor with square waves, you need a diode in parallel with the motor winding to suppress the back emf (electromotive force) that occurs when the TIP120 suddenly turns off the motor current. Any general-purpose rectifier, such as 1N400x series will work. The higher reverse voltage capability of, say, a 1N4007 compared to a 1N4004 used to command a premium price, but today any diode in that series will be adequate.

Place the cathode on the +12 V DC power supply line and the anode on the TIP120 collector. That will reverse-bias the diode while the TIP120 is conducting current through the motor winding. The diode only becomes forward-biased when the TIP120 turns off and the collapsing magnetic field of the motor winding generates a back emf potential. Without the diode, the back emf can reach a level high enough to damage the collector-base junction of the TIP120.

You should remove the capacitor between the base of the TIP120 and ground to ensure fast switching of the current in the TIP120.

 

[darren adcock]

You are still placing the motor winding in series with the emitter of the TIP120 BJT. Try placing the motor winding between the collector and the +12 V DC supply.

Because you are driving the motor with square waves, you need a diode in parallel with the motor winding to suppress the back emf (electromotive force) that occurs when the TIP120 suddenly turns off the motor current. Any general-purpose rectifier, such as 1N400x series will work. The higher reverse voltage capability of, say, a 1N4007 compared to a 1N4004 used to command a premium price, but today any diode in that series will be adequate.

Place the cathode on the +12 V DC power supply line and the anode on the TIP120 collector. That will reverse-bias the diode while the TIP120 is conducting current through the motor winding. The diode only becomes forward-biased when the TIP120 turns off and the collapsing magnetic field of the motor winding generates a back emf potential. Without the diode, the back emf can reach a level high enough to damage the collector-base junction of the TIP120.

You should remove the capacitor between the base of the TIP120 and ground to ensure fast switching of the current in the TIP120.

Thanks Hop. I will do as you suggest today and report back later on.

 

[darren adcock]

Hey, that all worked much better thanks with a really detailed response. When i work out how to transfer video from an iphone 4 via bluetooth to my computer I'll post a video, most likely tonight when i get back to where my USB cable is.

I'm getting a lot of interference from the circuit to my sound system/or synth i'm unsure which. There's a high pitched squealing sound that comes through when i turn up amplification on the synth or the modules with gain or the powered speakers i use. I disconnected the motor and there's is still a faint squealing, so unsure which part of the circuit i need to research to rectify the problem, if you don't mind pointing me in the right direction please? Here is how the circuit looks now after your help correcting it. Or maybe this problem needs the video i made to help explain.

 

[darren adcock]

I made some adjustments to the circuit after looking at various websites which have seemed to help. Mainly I was looking at decoupling articles. http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html, do I need to add a choke as in the last example of the article?

-A 1000uf electrolytic across ground and +12v helped reduce the interference/audible noise.
-Then I placed a 100nf across the motor which didn't seem to make a difference.
- Then I placed a 100nf across -12v and +12v near the op-amp and this helped.
-Then I disconnected the ground side of the patch lead coming to the input of the circuit and the sound is almost gone, very faint squeal.

Possibly also it is because i am testing it on breadboard with jumper wires, will transferring it to a strip board layout help?