I am modelling a motor with a little simulation script. The principle is:

  • Input is the traction force needed to be provided by the motor
  • Output is the current to be provided to the coil of the motor (DC, rotationnal motor)

But with my simulation, I am getting a lower current when the traction force is higher. I think that speaking of energy, it does not make sense.

I don't know yet how to calculate the Voltage needed with the current, so maybe it could explain something?

Thanks in advance for insights

EDIT: I replace "solenoid" by "coil" Thank you for the answer in comment, but then I have another question to solve my problem:

  • What are the numerical relation between Voltage and Speed, and the numerical relation between Current and Torque?
  • 1
    \$\begingroup\$ I can't VTC, but does this answer your question? How are current and voltage related to torque and speed of a brushless motor? \$\endgroup\$
    – Ron Beyer
    Jan 2, 2020 at 20:30
  • 1
    \$\begingroup\$ In DC motors the current is proportional to torque. So no, it does not make sense. \$\endgroup\$
    – Eugene Sh.
    Jan 2, 2020 at 20:32
  • 1
    \$\begingroup\$ Ampere's law is the irrevocable physical fact that determines how the magnetic field is generated from a current (and hence determines the magnetic forces/torques acting between permanent magnets and a coil). According to ampere, curl(B)=mu*J, so if you double the current (while leaving the coil geometry the same) you will get twice the magnetic field. If it were reverse, you'd get infinite flux density for zero current, which makes no sense. Unless we know what your simulation does, nobody can say what you could have done wrong. \$\endgroup\$
    – oliver
    Jan 2, 2020 at 20:40
  • \$\begingroup\$ A motor doesn't have a "solenoid". It has one or more windings or coils. This sounds like a translation problem. If you add your location or language into your user profile we can help you out. \$\endgroup\$
    – Transistor
    Jan 2, 2020 at 20:48
  • \$\begingroup\$ Can you edit your question and show the script or post an image of the simulation tool? \$\endgroup\$
    – Huisman
    Jan 2, 2020 at 21:07

1 Answer 1


Some elementaries should be kept clear if one tinkers with electromechanics. One of them is how the simplest possible fixed magnetization DC motor behaves. If we assume the DC motor doesn't suffer poor commutation nor magnetic non-idealities like iron saturation, but accept there's some resistance in windings and brushes, the motor has very simple math model. It is even more simple if we use SI units. That means for ex. we measure rotation speed as angular velocity in radians per second and rotation torque is in Newtonmeters instead of obscurities like ounzeinches. The model is in the scanned paper:

enter image description here

Rotating motor acts simultaneously as DC generator which generates internal voltage or "electromotive force" E which is simply a constant k x rotation velocity. The induction constant k contains all material& construction dependent things which connect electricity to mechanical phenomenas.

Motor accelerates after it's connected to a DC source (U) (assuming there's enough torque to win the friction). The induced voltage E increases, so the current (I) decreases because there's less voltage over the internal resistance.

The total power taken from the source goes partially to dissipation in the resistance, but the rest is converted to mechanical energy. As electrical quantity that power must be =IE.

Electrical power converted to mechanical has several targets. The speed increase needs a part as kinetic energy of the rotating masses (=the motor itself + the load). After the initial acceleration has ceased the motor keeps up constant torque T which is the sum of friction in the motor and how much the load needs to keep rotating at the achieved rotation speed.

The total rotation power in mechanical terms is rotation speed x torque. It must be equal with electrical rotation power. From that fact we get a formula for torque: T=kI.

With formula T=kI we can for ex. calculate what is the rotation speed with given k, U,R and torque.

Your simulation showed that the current decreased when the torque increased. Your simulation either is wrong or the motor is substantially more complex than the elementary fixed magnetization DC motor.

One part of the mechanical torque can grow at the same time when the motor current decreases in a simulation run. That's the torque which is needed to win the friction. It generally grows as the speed increases. When the acceleration decreases gradually, less torque is needed to accelerate rotating masses, so the total torque decreases but what's transferred out of the motor by the axle can still grow. That's the case when the inertial mass (more exactly "the moment of the inertia") is mostly inside the motor and the friction is mostly in the load.

  • \$\begingroup\$ Thank you @user287001 Is there any classic formula for the motor you described, to give the value of the constant K? I think it is related with R and the coil/solenoid's inductance L in Henri.m-1? \$\endgroup\$ Jan 3, 2020 at 11:34
  • \$\begingroup\$ There's no simple formula for k, every dimension and the used materials in the magnetic structure count. Winding resistance doesn't affect, but the number of turns do. As well one must take into the account how the commutation works and how the magnetization is done. To get approximations for k you must learn a long section of electric motor theory. Sorry. \$\endgroup\$
    – user287001
    Jan 3, 2020 at 12:16
  • \$\begingroup\$ Still thanks for the insight \$\endgroup\$ Jan 3, 2020 at 12:33

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