ELECTRIC MOTOR CONTROL 3

[Ethnan]

Hello good people!

I would appreciate if anyone could help me with a circuit available in the market that can help me realise the drawing attached.

The purpose of circuit is to keep the electric motor speed constant as the mechanical load on the motor keeps varying.

What electrical circuit is available in the market to achieve this?

 

[waross]

It would save a lot of time dragging out information if you told us your application.
What kind of motor?
Series?
Shunt?
Compound?
Permanent magnet?
How constant?
eg: The speed of many industrial induction motors varies by about 2.2% from no load to full load.
This is assumed to be a constant speed for most applications.
If closer control is required it is possible at a cost. (VFD)
Series motor: Used for special applications such as engine starting. Speed varies greatly with load.
Don't consider a series motor.
Shunt motor: Good speed regulation. Speed drop under load depends mostly on the armature circuit resistance.
Compound motor: Very good speed regulation. An over compounded motor will increase speed slightly as the load is increased.
Permanent magnet motor: Similar to a shunt motor. A PM motor may have more speed loss under low voltage conditions than a shunt motor.
BrushLess DC motor. (BLDC) this is actually an AC motor with an integral inverter to develop AC from the supplied DC for the windings.
Inherently stable speed.
Methods of speed control.
1. Tachometer or speed sensor. Gives very good control of the speed.
2. Current control. The load is inferred from the current drawn. As the current increases the applied voltage is increased.
A shunt motor may use field weakening. to trim the speed.
A permanent magnet motor must have a circuit that reduces the voltage at lower currents. This works from the full load speed and trims the speed as the load decreases.
So, what are you driving at a constant speed?
How constant is constant?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

You will need what's called a "DC Drive". There are plenty of small 12VDC drives available, but they rarely (I've never seen one) have a closed loop speed feedback option, they generally just have a speed input signal (from something like a potentiometer) and whether the motor can maintain that set speed is load dependent.

If you want it to maintain a specific speed with any reasonable accuracy under variable load conditions, you will need one with the ability to accept a feedback loop from a tachometer or encoder. But for a 12VDC motor, that may not be easy to find. You will need to define the current rating you need and then you would be best served to mention what part of the world you are in. Suggestions of suppliers that you have no access to are somewhat pointless.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Ethnan]

@Waros. Thanks so much for your time. The motor is a permanent magnet motor,(that's the best way to describe it) but quite customized for my project. I did not want to bore you guys with details of the project. Essentially, the motor drives a mechanical carriage which carries items along a path. The carriage can house up to 100 cans.But the user can put only 10 or 20 or 34 or any number at any time. But it is IMPORTANT that the speed of the carriage remains constant no matter the number of cans inside the carriage.

@Jraef, I have seen some closed loop DC drives but the maximum I have seen is about 15A, too low for my application. I may have to adjust the voltage for my project but the current may then go up to 1000Amps with higher voltage. I may be able to adjust the entire project if high current closed loop DC drives is not available in the market. I really hope they are! My current project would need a DC drive that can handle current upto 100Amps. Higher currents would also be desirable. I am an Electrical Engineer in Nigeria.

 

[waross]

From the skimpy information that you posted, the motor would drop about 1% speed for every Amp. But depending on how the armature resistance was measured possibly much less.
Again, how constant must the speed be?
If you are attempting to determine position based on a constant speed and elapsed time, there are much better ways to determine position.
The current through a spinning DC armature is determined by the applied voltage minus the back EMF divided by the armature resistance.
So, Applied voltage = 12 volts.
10 Amps x .12 Ohms = 1.2 Volts
Back EMF = 12 Volts minus 1.2 Volts = 10.8 Volts back EMF.

I did not want to bore you guys with details of the project

Please, bore us.
It will save so much time dragging details out of you.
Details matter.
We still don't know the HP.
We don't know the power source.
We don't know the speed tolerance.
If you want answers without providing pertinent details you may soon be on your own.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Please, bore us. ...

Yes, please, because many of us are sitting at home under isolation orders looking for something interesting to do, besides on-line meetings where everyone just talks about the same thing; coronavirus... what to do about coronavirus, what not to do because of coronavirus, how to work at home because of coronavirus (even as the work we CAN do is drying up), etc. etc. etc.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Ethnan]

Thanks guys! Appreciate your time. The risk with too much details is that you might end up confusing your readers rather than providing better clarity. So the motor needs to maintain steady speed with +/-5% tolerance. Proximitors or position sensors wont cut this for me. Its not about position or time. The motor is in a chain of proceas where its load keeps varying but the next process must start where the motor drops the carriage of cans. My team have thoroughly analysed this and the best solution seems to be to ensure the motor moves at constant speed. Now you may suggest, why not have a position sensor that senses when the carriage arrives at its position, to activate the next process? For this peculiar process, this wont work as some other feedback needs the speed of the motor to perform some complex iterations,a part of the automated system. I haven't measured the HP, power factor ,efficiency of the motor as I dont have the tools to do that. The coil resistance is about 0.04ohms and the wire gauge can take upto 300amps actually. The question I think that needs answer is how to get closed loop DC drives of high current rating and low voltage. Kind Regards

 

[waross]

The big question.
WHY DC???
An AC induction motor, out of the box will give about +0 -2.2% speed regulation.
Do you have a nameplate on the motor?
Do you have any specs for the motor?
Do you have the motor?
Do you have a picture of the motor?
Is this a reworked PM starter motor?
Between us, Jeff and I have over 100 years experience solving problems.
Go ahead and confuse us.
I, and probably Jeff are getting a little frustrated trying to drag details out of you.
We can't solve a problem if we don't know what we have to work with.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Ethnan]

Thank you guys. I do respect your wealth of experience and will sure need it again soon. However, I do not want to do a lot of re-designing of the project. I have intimated earlier that the motor is custom made, not bought from the market. It is just an arrangement of heavy coils of wire and magnets that produce some heavy motion.

Good news is that I got a higher current DC drive in Aliexpress. Here is one of 30Amps.

https://www.aliexpress.com/item/33001322329.html?spm=a2g0o.cart.0.0.115d3c00FTOErA&mp=1

and below is 16Amps

https://www.aliexpress.com/item/32905241478.html?spm=a2g0o.cart.0.0.7f8e3c00eoJuSB&mp=1

I am discussing with the sellers for 100 Amps and above.

I really do appreciate your time on my posts. Lets stay away from the virus!

God bless!

Kind Regards