# realtion between voltage and torque in DC motor with gearbox

Hi
I want to control a DC motor with gearbox ,the input signals in control program is torque but in practical the input of the motor is voltage so I want to know a relation between the desire torque and voltage

Torque is directly related to current. The relationship between current and voltage in an electric motor is complicated by the inductance of the motor coils and the back-emf (an opposing voltage) that increases as the motor speeds up. A motor reaches its maximum speed when the back-emf nearly matches the driving voltage and the net current (and torque) is barely sufficient to overcome friction at that speed.

If you want a motor to maintain a specific speed you need a feedback mechanism - for example a detector that allows the Arduino to calculate the time for each revolution. Then it can vary the power going to the motor in response to changes in speed.

...R

thanks for your answering , you mean that there isn't any way to obtain a desire voltage for a special tracking? ,actually I need a simple relation between voltage and torque and speed of the motor to have a special tracking in other word I want a way to create a desire torque in a dc motor in practical, please told me a way

koronus:
thanks for your answering , you mean that there isn’t any way to obtain a desire voltage for a special tracking? ,

You need to give the full details of your requirements.

At the moment this is a XY-Problem.

…R

To control motor torque you need to sense current and use a feedback loop to modulate the motor
drive to maintain desired torque.

Alternatively a constant-current chopper circuit can be used directly to control current (as its
typically used for stepper motors).

Robin2:
You need to give the full details of your requirements.

At the moment this is a XY-Problem.

...R

OK , I am working on a robotic arm so for having tracking of the end effector from A to B the motor needs some special torques and I obtain these torques but in practical I couldn't give these torques to the motor I could give the voltage to the motor for having different speeds of motor so I need a way to convert these torques to voltages

koronus:
OK , I am working on a robotic arm so for having tracking of the end effector from A to B the motor needs some special torques and I obtain these torques but in practical I couldn’t give these torques to the motor I could give the voltage to the motor for having different speeds of motor so I need a way to convert these torques to voltages

May I rephrase your statement ? You are varying the voltage to the motor to increase torque, but the current to the motor does not change. If so, what are you using for a power supply for the motor?

Paul

Paul_KD7HB:
May I rephrase your statement ? You are varying the voltage to the motor to increase torque, but the current to the motor does not change. If so, what are you using for a power supply for the motor?

Paul

the maximum voltage that I have is 12V and I use PWM

koronus:
the maximum voltage that I have is 12V and I use PWM

Thank you! That means you are increasing the CURRENT for each pulse when you raise the voltage.

Paul

Paul_KD7HB:
Thank you! That means you are increasing the CURRENT for each pulse when you raise the voltage.

Paul

so is there any linear equation between the CURRENT and the torque?

koronus:
so is there any linear equation between the CURRENT and the torque?

Not unless the data sheet for the motor shows it. The torque curve relates directly to the the magnetic property of the material used in the motor.

Paul

koronus:
so is there any linear equation between the CURRENT and the torque?

I think if you don't allow the speed to change the torque will approximately double when the current doubles. However when (as usual) the speed changes things get very complicated.

If the torque exerted by the arm is the critical thing then you need a strain gauge to measure the force and provide data for a feedback system.

...R

ok thanks a lot (Robin2 ,Paul_KD7HB, MarkT)

koronus:
OK , I am working on a robotic arm so for having tracking of the end effector from A to B the motor needs some special torques and I obtain these torques but in practical I couldn't give these torques to the motor I could give the voltage to the motor for having different speeds of motor so I need a way to convert these torques to voltages

Not possible, as I said you need current sensing for torque sensing or control, there's no way round this for
a permanent magnet DC motor.