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DC motor, consistent speed under different loads?

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diy549

New Member
Hello All, Is there some way to get a Dc gear motor to run at the same speed even under slightly different loads?

Thanks
 
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MikeMl

Well-Known Member
Most Helpful Member
Sure, use an electronic speed controller. Such circuits compare the actual motor speed to a set speed, and modulate the average current to the motor to maintain a constant speed. Search Key Words such as PID, PWM, feedback, servo, H-Bridge might be helpful.
 

diy549

New Member
Thanks for the response Mike, but a speed controller just sends out a specified voltage right? A motor under the same voltage but lighter load would move faster right?
 

MikeMl

Well-Known Member
Most Helpful Member
Thanks for the response Mike, but a speed controller just sends out a specified voltage right? A motor under the same voltage but lighter load would move faster right?
No, the ones I am familiar with use speed feedback from the motor. In some cases, the back emf is sensed to determine speed; if the motor slows down, the back emf decreases. In other cases a hall effect sensor is embedded in the motor winding to sense rotor speed. In others, a DC tachometer is used to sense speed, etc. In others, a photoelectric pick-up is used.

Running a motor on a fixed voltage causes the motor to slow down if more torque is called for. The controllers I am thinking of automatically increase current (indirectly voltage) if the motor slows down.
 
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diy549

New Member
The gear motor I have is just two wires, can the motor still give feedback to the speed controller?
 

MikeMl

Well-Known Member
Most Helpful Member
The gear motor I have is just two wires, can the motor still give feedback to the speed controller?
Can you see the shaft as it goes into the gearbox? Mount a mirror on the shaft and use an LED and a phototransistor to sense the rotation rate.

Permanent magnet rotor? Mount a Hall effect sensor on the motor to sense the rotation rate.

Use a PWM which chops the current to the motor. Look at the back emf during the Off periods. This one can be done with two wires, no mods, but it requires a PWM drive.
 

Hero999

Banned
The best way is to use a PLL (Phase Locked Loop).

The idea is that you synchronise the frequency from the tachometer to a highly stable oscillator. It doesn't matter whether you use PWM or alter the voltage to the motor, it will work just as well.
 

marcbarker

New Member
Tacho speed feedback controls, PLLs and optical pulse generators for motor speed control (along with the complicated circuitry) is for very accurate things like recording studio tape transport motor, VHS machine, floppy disk drives and printer head carriages to name a few.

Cassette tape player motor has a varying load, inside it they just have a simple 2-wire permanent magnet DC motor, along with a voltage regulator built as part of the motor on a round PCB. Break one apart and see. The speed is stable to single figure % over a considerable load torque range.

Just adding an adjustable voltage regulator to the motor will do it.
 

Hero999

Banned
Use a 32768Hz crystal for the oscillator and you should be able to set your watch to it.
 

marcbarker

New Member
Use a 32768Hz crystal for the oscillator and you should be able to set your watch to it.

PLL's really good. I once synchronised two floppy disk drives together (without a PC at all), using a PLL, phase locked using the 1 pulse per rev datum signal produced by the "index pulse" is it?. The master FDD set it's own rpm according to its XTAL and I had the slave FDD motor controlled by the PLL's VCO, comparing the master and slave FDD datum sensors. All this... so I could defeat copy protected diskette. Worked really well.
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

A relatively simple method is to measure the back emf of the motor and use
that as a feedback signal to regulate the speed to some set value. This can
be done using an analog technique too so doesnt require a microcontroller.

Using a voltage regulator alone unfortunately does not work because the
motor speed varies even with constant terminal voltage when the load
changes.
 

marcbarker

New Member
Hi,

A relatively simple method is to measure the back emf of the motor and use
that as a feedback signal to regulate the speed to some set value. This can
be done using an analog technique too so doesnt require a microcontroller.

Using a voltage regulator alone unfortunately does not work because the
motor speed varies even with constant terminal voltage when the load
changes.

Cassette tape drive motor is quite stable, is this the way they do it?
 

marcbarker

New Member
Some measure the motor current and increase the supply voltage as the current gets larger. There are special ICs that do that for cassette players.

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.
 

Hero999

Banned
That's for a two-wire PM DC motor right? How does measuring the motor current determine how much voltage to apply?
A DC motor doesn't draw a constant DC current.

It draws a DC current with some AC ripple on top and by measuring the frequency of this, you can work out how fast the motor is spinning.
 

marcbarker

New Member
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.

marcbarker-albums-postings-picture292-untitled.gif

https://www.datasheetcatalog.org/datasheet/kec/KIA6903P.pdf
 
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MrAl

Well-Known Member
Most Helpful Member
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.
 
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Hero999

Banned
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?
 

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Hero999

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

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  • 12V 0.5ms-d.JPG
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kinarfi

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