DC motor, consistent speed under different loads? - Page 2 - Electronic Circuits Projects Diagrams Free

marcbarker · 11th July 2009, 01:59 AM

KIA6903P IC for speed control of general purpose DC motors. controls by detecting back emf of DC motor. Only needs this IC plus a 75 ohm and a 1 k pot.

http://www.datasheetcatalog.org/data...c/KIA6903P.pdf

MrAl · 11th July 2009, 07:11 AM

Quote:

Originally Posted by marcbarker

That's for a two-wire PM DC motor right? How does measuring the motor current determine how much voltage to apply?

I always thought that circular shaped circuit board inside the motor was a voltage regulator, with an adjustment pot below the hole to trim the speed.

Just as a sanity check, I connected an ordinary DC PM motor to a variable-voltage supply here, the RPM is proportional to set voltage. The RPM doesn't change by much when loading the spindle. When the loading on the spindle makes the current exceed the power supply limit, then the revs drop. I suspect the voltage is dropping.

Hi,

The RPM does in fact change though, and in some systems this is undesirable.

Basically you measure Va, Ia, and knowing Ra the calculation for the
back emf is:
Vb=Va-Ia*Ra
This is then subtracted from a reference voltage Vr that sets the speed,
and the error voltage is then amplified and integrated and that sets
the new Va. That forms the complete feedback system. The final
value of the system w(inf) is Vr/Kb, which means constant w.

In the above:
Va is the armature voltage (measured)
Ia is the armature current (measured)
Ra is the armature DC resistance
Vb is the back emf
Kb is a constant of the motor
Rotational inertia is ignored.

Hero999 · 11th July 2009, 02:36 PM

I've 'scoped the current waveform through a small cheap Chinese DC motor at different voltages to change the speed.

The 'scope was set to AC, the current was measured by connected a 1Ω resistor in series with the motor. One division represents 50mA on all waveforms.

1) 3V 2ms/div
2) 6V 2ms/div
3) 9V 1ms/div
4) 12V 0.5ms/div

Sorry about my shaky photography, my hand isn't that steady and the waveform wasn't always stable.

Assuming the motor is three pole, what's the speed?

Hero999 · 11th July 2009, 02:38 PM

It'll only alow three attachments per post, so here's the last one.

kinarfi · 11th July 2009, 04:18 PM

After you deduct for I*R loss voltage is directly proportional to speed and current is directly proportional to torque.
Kinarfi

marcbarker · 11th July 2009, 04:34 PM

Yes I see what you mean now.

By my rough reckoning, @ 3 V = 5000 rpm
6 V = 10,000
9 V = 16,000
12 V = 19,000

Did by chance see of the RPM changed much with loading?

Hero999 · 11th July 2009, 04:47 PM

You're right.

I make it about 17000rpm@9V but it isn't easy to count the squares due to my shaky photography.

I make it 3.5 squares giving a period of 3.5ms, 60/0.0035 = 17,148rpm.

I'll see if I can count the pulses with my frequency counter, it might give be triple the frequency but I'll easily be able to figure that out.

Yes, the frequency does change with loading.

This is probably the simplest method of measuring the speed of a DC motor, it's fully compatible with PLL. It can also be made to be compatible with PWM, if a filter is added to get rid of the PWM, making what is essentially a buck converter.

Hero999 · 11th July 2009, 07:24 PM

Here's a better picture at 9V.

I've also noticed that the waveform changes when the motor is run in reverse which indicates that the motor isn't completely symmetrical. This normally happens because of the ways the brushes wear when the motor is normally only operated in one direction.

1) Better picture, 9V 1ms/div, 50mA/div
2) Waveform when connected in reverse, same 'scope settings as #1.

I've tried using a frequency counter but it didn't work. I measured a frequency of 30kHz which seemed to increase when I put a load on the motor. The counter is probably triggering at a harmonic and is changing when a load is applied (the waveform also changes shape when loaded). I tried adding a low pass RC filter 680R and 100nF but it didn't work. I also tried a 22µF capacitor in parallel with the motor and a common mode EMC choke to no avail.

The motor emits some pretty high frequency RF harmonics: I accidentally connected it to the 20MHz to 1.3GHz counter input and it read 300MHz which explains the TV and radio interferance generated by it. The harmonics probably extend well into the microwave band.

MrAl · 11th July 2009, 09:05 PM

Hi,

Yes, the arc when the commutator switches is going to generate all kinds of stuff.
It might be hard to filter out because the filter has to have wide bandwidth too.
Maybe mount an antenna next to the motor to pick up the RF bursts, then count
them (chuckle) :-)
Maybe high pass would help pick out the arc over points.

The back emf technique BTW has been used on various tape recording machines.
It's not perfect but it does work.
Hall effect sensors have also been used very effectively and there isnt much
of a problem with noise.

marcbarker · 11th July 2009, 10:35 PM

Quote:

Originally Posted by MrAl

Maybe mount an antenna next to the motor to pick up the RF bursts, then count
them (chuckle) :-)
Maybe high pass would help pick out the arc over points.

Good point. Looking at the waveforms, maybe get a good signal for freq counter by differentiating out that very fast edge I can see there on all the traces. Maybe try first putting the counter input probe near, but not touching, to try this out first.

I reckon differentiated pulse could be buffered up with a capacitively coupled 74HCsomething gate connected in linear mode (you know, with some neg feeback round it).

Of course with a brushless motor it's really easy to interface to one of the phases.

Hero999 · 11th July 2009, 11:59 PM

My previous post was supposed to have attachments.

Here they are:
1) Better picture, 9V 1ms/div, 50mA/div
2) Waveform when connected in reverse, same 'scope settings as #1.

With suitable filtering and Schmitt trigger such a system could easily be used with a PLL to form a very stable motor controller.

I've tried connecting an 8Ω speaker in series with a small DC motor before and it emitted sound proportional to the speed of the motor. Obviously the speaker will blow if it was connected for too long, with a load on the motor, to get round this an LM386 amplifier and speaker could be connected to the sense resistor. This could be used in a remote controlled car or robot for an engine sound.

MrAl · 12th July 2009, 08:15 AM

Hi,

Hey, that's better :-)

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11th July 2009, 01:59 AM	#16
marcbarker	KIA6903P IC for speed control of general purpose DC motors. controls by detecting back emf of DC motor. Only needs this IC plus a 75 ohm and a 1 k pot. http://www.datasheetcatalog.org/data...c/KIA6903P.pdf Last edited by marcbarker; 11th July 2009 at 04:51 PM.

11th July 2009, 02:36 PM	#18
Hero999	I've 'scoped the current waveform through a small cheap Chinese DC motor at different voltages to change the speed. The 'scope was set to AC, the current was measured by connected a 1Ω resistor in series with the motor. One division represents 50mA on all waveforms. 1) 3V 2ms/div 2) 6V 2ms/div 3) 9V 1ms/div 4) 12V 0.5ms/div Sorry about my shaky photography, my hand isn't that steady and the waveform wasn't always stable. Assuming the motor is three pole, what's the speed? Attached Thumbnails __________________ I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong. Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

11th July 2009, 02:38 PM	#19
Hero999	It'll only alow three attachments per post, so here's the last one. Attached Thumbnails __________________ I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong. Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

11th July 2009, 04:18 PM	#20
kinarfi	After you deduct for I*R loss voltage is directly proportional to speed and current is directly proportional to torque. Kinarfi

11th July 2009, 04:34 PM	#21
marcbarker	Yes I see what you mean now. By my rough reckoning, @ 3 V = 5000 rpm 6 V = 10,000 9 V = 16,000 12 V = 19,000 Did by chance see of the RPM changed much with loading?

11th July 2009, 04:47 PM	#22
Hero999	You're right. I make it about 17000rpm@9V but it isn't easy to count the squares due to my shaky photography. I make it 3.5 squares giving a period of 3.5ms, 60/0.0035 = 17,148rpm. I'll see if I can count the pulses with my frequency counter, it might give be triple the frequency but I'll easily be able to figure that out. Yes, the frequency does change with loading. This is probably the simplest method of measuring the speed of a DC motor, it's fully compatible with PLL. It can also be made to be compatible with PWM, if a filter is added to get rid of the PWM, making what is essentially a buck converter. __________________ I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong. Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer. Last edited by Hero999; 11th July 2009 at 04:51 PM.

11th July 2009, 07:24 PM	#23
Hero999	Here's a better picture at 9V. I've also noticed that the waveform changes when the motor is run in reverse which indicates that the motor isn't completely symmetrical. This normally happens because of the ways the brushes wear when the motor is normally only operated in one direction. 1) Better picture, 9V 1ms/div, 50mA/div 2) Waveform when connected in reverse, same 'scope settings as #1. I've tried using a frequency counter but it didn't work. I measured a frequency of 30kHz which seemed to increase when I put a load on the motor. The counter is probably triggering at a harmonic and is changing when a load is applied (the waveform also changes shape when loaded). I tried adding a low pass RC filter 680R and 100nF but it didn't work. I also tried a 22µF capacitor in parallel with the motor and a common mode EMC choke to no avail. The motor emits some pretty high frequency RF harmonics: I accidentally connected it to the 20MHz to 1.3GHz counter input and it read 300MHz which explains the TV and radio interferance generated by it. The harmonics probably extend well into the microwave band. __________________ I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong. Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

11th July 2009, 09:05 PM	#24
MrAl	Hi, Yes, the arc when the commutator switches is going to generate all kinds of stuff. It might be hard to filter out because the filter has to have wide bandwidth too. Maybe mount an antenna next to the motor to pick up the RF bursts, then count them (chuckle) :-) Maybe high pass would help pick out the arc over points. The back emf technique BTW has been used on various tape recording machines. It's not perfect but it does work. Hall effect sensors have also been used very effectively and there isnt much of a problem with noise.

11th July 2009, 11:59 PM	#26
Hero999	My previous post was supposed to have attachments. Here they are: 1) Better picture, 9V 1ms/div, 50mA/div 2) Waveform when connected in reverse, same 'scope settings as #1. With suitable filtering and Schmitt trigger such a system could easily be used with a PLL to form a very stable motor controller. I've tried connecting an 8Ω speaker in series with a small DC motor before and it emitted sound proportional to the speed of the motor. Obviously the speaker will blow if it was connected for too long, with a load on the motor, to get round this an LM386 amplifier and speaker could be connected to the sense resistor. This could be used in a remote controlled car or robot for an engine sound. Attached Thumbnails __________________ I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong. Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.