# same voltage and adjustable current for dc motor

#### meshcurrent

Joined May 28, 2022
16
Hello friends, first of all sorry for my bad English. I am an electrical and electronics engineering student. I have a problem in my control systems lesson project. The project is about controlling a dc motor with adjustable current and same voltage on the dc motor. The professor gave us a clue. It is current mirror circuit. I did some research but I did not find the correct circuit design. There is a circuit consisting of 2 transistors. But I don't undersand very well. Could you help me please?

#### Alec_t

Joined Sep 17, 2013
14,313
Welcome to AAC!
Post your best effort at the solution and then we can help. This isn't a "homework done for you" forum.

#### crutschow

Joined Mar 14, 2008
34,428
Yes, we can, but you need to post the circuit and what you have done to try to solve the problem.

#### meshcurrent

Joined May 28, 2022
16
Yes, we can, but you need to post the circuit and what you have done to try to solve the problem.
Hello, thank you for your reply. My professor only gave a hint about the circuit. And he said that the circuit could be a mirroring circuit. I set up a simple switching circuit with 2 transistor circuits. This circuit mirrors the current, but the voltage falling on the motor increases as the current increases. What I want is to keep the current variable and the voltage constant on the motor. I don't know exactly how to research or what to research. I would really appreciate if you can help.

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#### dl324

Joined Mar 30, 2015
16,917
Welcome to AAC!
The project is about controlling a dc motor with adjustable current and same voltage on the dc motor.
This isn't a good way to vary motor speed.

#### meshcurrent

Joined May 28, 2022
16
AAC'ye hoş geldiniz!

Bu, motor hızını değiştirmenin iyi bir yolu değildir.
[/ALINTI]
Teşekkür ederim. Bana verebileceğin bir tavsiye var mı?

#### Papabravo

Joined Feb 24, 2006
21,225
Hello, thank you for your reply. My professor only gave a hint about the circuit. And he said that the circuit could be a mirroring circuit. I set up a simple switching circuit with 2 transistor circuits. This circuit mirrors the current, but the voltage falling on the motor increases as the current increases. What I want is to keep the current variable and the voltage constant on the motor. I don't know exactly how to research or what to research. I would really appreciate if you can help.
There is a problem and it is called OHM's Law. Take another look at that law, consider the implications, and tell me what needs to change to get the behavior you are describing.

#### meshcurrent

Joined May 28, 2022
16
There is a problem and it is called OHM Law. Take another look at this law, think about the consequences, and tell me what needs to change to achieve the behavior you describe.
I know Ohm's law. Didn't you read what I wrote? I didn't say that your circuit is correct anyway. I just put it as an example. I want to change the current of a motor so that its voltage remains constant. I'm researching what kind of circuit I should build for this.

MOD NOTE: Translated to English. @meshcurrent: Please be sure to translate your posts to English before posting.

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#### dl324

Joined Mar 30, 2015
16,917
"Bana verebileceğin bir tavsiye var mı? "

Is there any advice you can give me?
I won't offer any advice. This is schoolwork and I don't know what your professor intends for you to learn from this exercise.

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#### Alec_t

Joined Sep 17, 2013
14,313
I want to change the current of a motor so that its voltage remains constant.
Have you been taught about the back-emf generated by a motor?

#### Papabravo

Joined Feb 24, 2006
21,225
I know Ohm's law. Didn't you read what I wrote? I didn't say that your circuit is correct anyway. I just put it as an example. I want to change the current of a motor so that its voltage remains constant. I'm researching what kind of circuit I should build for this.

MOD NOTE: Translated to English. @meshcurrent: Please be sure to translate your posts to English before posting.

#### LesJones

Joined Jan 8, 2017
4,190
Assuming the motor is a permanent magnet brush motor (You have NOT told us what kind of motor it is.) The the current consumed is roughly proportional to the torque that the motor is providing. If the motor is not under any mechanical load then it's speed is roughly proportional to the voltage. With a fixed voltage to the motor the effective voltage it sees is reduced by the resistive effect of the current through the windings as the load on the motor is increased which causes the current to increase.
Exactly how was the problem presented to you. I think we may be trying to solve a different problem than the problem presented to you.

Les.

#### boostbuck

Joined Oct 5, 2017
514
What I want is to keep the current variable and the voltage constant on the motor
It seems that even though you 'know' Ohm's Law you don't understand it in a practical sense. Perhaps you could describe how you propose to keep the voltage constant as the current varies?

Joined Jul 18, 2013
28,685
The project is about controlling a dc motor with adjustable current and same voltage on the dc motor.
I too would like to know what method is going to be used to alter the current while maintaining a constant voltage.
There is one obvious way.

#### BobTPH

Joined Jun 5, 2013
8,958
Is the voltage constant? Why would that matter?

#### boostbuck

Joined Oct 5, 2017
514
Is the voltage constant? Why would that matter?
It seems to be in the requirements, but I suspect there is a miscommunication somewhere.

#### Irving

Joined Jan 30, 2016
3,885
With a fixed voltage to the motor the effective voltage it sees is reduced by the resistive effect of the current through the windings as the load on the motor is increased which causes the current to increase.
The effective voltage seen by the motor winding is the supply voltage - the back EMF. The effect of the motor resistance is tiny by comparison unless the motor is stalled

@Papabravo
Not sure why you think Ohm's Law is relevant? DC Motors are non-linear devices, so Ohm's Law does not apply to a motor except under stall conditions.

#### meshcurrent

Joined May 28, 2022
16
Hello, I am an electrical and electronics engineering student. I'm working on a project. This project controls DC motor. I want to adjust the current going to the motor so that the voltage falling on the motor remains constant. For example, let's say 12V falls on my dc motor and I want this motor to draw different currents such as 2A, 3A, 5A. I learned that current mirrors can be used for this control. I can adjust the current, but this time the voltage falling on the motor also changes. However, I want the voltage falling on the motor to remain constant. How can I do that. I've been researching for about a week. Can you help me or give me a suggestion? I've been researching for about a week. I would be very pleased if you answer.

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#### Papabravo

Joined Feb 24, 2006
21,225
Hello, I am an electrical and electronics engineering student. I'm working on a project. This project controls DC motor. I want to adjust the current going to the motor so that the voltage falling on the motor remains constant. For example, let's say 12V falls on my dc motor and I want this motor to draw different currents such as 2A, 3A, 5A. I learned that current mirrors can be used for this control. I can adjust the current, but this time the voltage falling on the motor also changes. However, I want the voltage falling on the motor to remain constant. How can I do that. I've been researching for about a week. Can you help me or give me a suggestion? I've been researching for about a week. I would be very pleased if you answer.
I still don't think there is a way to do this as you have stated it. Apparently, there are few if any other bright ides out there.

#### WBahn

Joined Mar 31, 2012
30,055
Hello, I am an electrical and electronics engineering student. I'm working on a project. This project controls DC motor. I want to adjust the current going to the motor so that the voltage falling on the motor remains constant. For example, let's say 12V falls on my dc motor and I want this motor to draw different currents such as 2A, 3A, 5A. I learned that current mirrors can be used for this control. I can adjust the current, but this time the voltage falling on the motor also changes. However, I want the voltage falling on the motor to remain constant. How can I do that. I've been researching for about a week. Can you help me or give me a suggestion? I've been researching for about a week. I would be very pleased if you answer.
You would have to also control the load on the motor.

In a typical DC motor (like a permanent magnet motor), the voltage is primarily related to the speed of the motor and the current is primarily related to the torque of the motor. So as you change the current, you change the torque. But to keep the voltage the same, you need to then change the load so that the new torque value results in the motor spinning at the prior speed.

Be very careful running these types of motors with a constant current source, you can get a catastrophic overspeed condition pretty easily.

I found this out the hard way when I was a junior. A friend and I were working on a demo in which we bounced a laser beam off of two rotating mirrors and onto a screen, producing Lissajous patterns. The patterns weren't too stable, so I got the bright idea of running them in constant current mode instead of constant voltage mode. As I slowly turned up the current limit, nothing happened until the developed torque became sufficient to overcome the static friction. At that point, within just a second or two, the motor spun up so fast that the mirror grenaded, sending glass shards everywhere in the plane of rotation. A few minutes reflection was all it took to realize our mistake.