DC Motor Control Basics

You have a DC Motor you want to control it. But how? And what does DC Motor control means? Pretty much when we are talking about DC motor control, there are 4 aspects we need to get our minds busy with:

  1.  Direction of rotation
  2. Motor Speed
  3. Motor Torque
  4. Motor Start and Stop

Direction of Rotation

A DC Motor has two wires. We can call them the positive terminal and the negative terminal, although these are pretty much arbitrary names (unlike a battery where these polarities are vital and not to be mixed!). On a motor, we say that when the + wire is connected to + terminal on a power source, and the – wire is connected to the – terminal source on the same power source, the motor rotates clockwise (if you are looking towards the motor shaft). If you reverse the wire polarities so that each wire is connected to the opposing power supply terminal, then the motor rotates counter clockwise. Notice this is just an arbitrary selection and that some motor manufacturers could easily choose the opposing convention. As long as you know what rotation you get with one polarity, you can always connect in such a fashion that you get the direction that you want on a per polarity basis.

DC Motor Rotation vs Polarity

DC Motor Rotation vs Polarity

  • DC Motor rotation has nothing to do with the voltage magnitude or the current magnitude flowing through the motor.
  • DC Motor rotation does have to do with the voltage polarity and the direction of the current flow.

DC Motor Speed

Whereas the voltage polarity controls DC motor rotation, voltage magnitude controls motor speed. Think of the voltage applied as a facilitator for the strengthening of the magnetic field. In other words, the higher the voltage, the quicker will the magnetic field become strong. Remember that a DC motor has an electromagnet and a series of permanent magnets. The applied voltage generates a magnetic field on the electromagnet portion. This electromagnet field is made to oppose the permanent magnet field. If the electromagnet field is very strong, then both magnetic entities will try to repel each other from one side, as well as atract each other from the other side. The stronger the induced magnetic field, the quicker will this separation/attaction will try to take place. As a result, motor speed is directly proportional to applied voltage.

Motor Speed Curve

Motor Speed Curve

 One aspect to have in mind is that the motor speed is not entirely lineal. Each motor will have their own voltage/speed curve. One thing I can guarantee from each motor is that at very low voltages, the motor will simply not move. This is because the magnetic field strength is not enough to overcome friction. Once friction is overcome, motor speed will start to increase as voltage increase.

The following video shows the concept of speed control and offers some ideas on how this can be achieved.


 Motor Torque

In the previous segment I kind of described speed as having to do with the strength of the magnetic field, but this is in reality misleading. Speed has to do with how fast the magnetic field is built and the attraction/repel forces are installed into the two magnetic structures. Motor strength, on the other hand, has to do with magnetic field strength. The stronger the electromagnet attracts the permanent magnet, the more force is exerted on the motor load.

Per example, imagine a motor trying to lift 10 pounds of weight. This is a force that when multiplied by a distance (how much from the ground we are lifting the load) results in WORK. This WORK when exerted through a predetermined amount of time (for how long we are lifting the weight) gives us power. But whatever power came in, must come out as energy can not be created or destroyed. So that you know, the power that we are supplying to the motor is computed by

P = IV

Where P is power, I is motor current and V is motor voltage

Hence, if the voltage (motor speed) is maintained constant, how much load we are moving must come from the current. As you increase load (or torque requirements) current must also increase.

Motor Loading

Motor Loading

One aspect about DC motors which we must not forget is that loading or increase of torque can not be infinite as there is a point in which the motor simply can not move. When this happens, we call this loading “Stalling Torque”. At the same time this is the maximum amount of current the motor will see, and it is refer to Stalling Current. Stalling deserves a full chapter as this is a very important scenario that will define a great deal of the controller to be used. I promise I will later write a post on stalling and its intricacies.

Motor Start and Stop

You are already well versed on how to control the motor speed, the motor torque and the motor direction of rotation. But this is all fine and dandy as long as the motor is actually moving. How about starting it and stopping it? Are these trivial matters? Can we just ignore them or should we be careful about these aspects as well? You bet we should!

Starting a motor is a very hazardous moment for the system. Since you have an inductance whose energy storage capacity is basically empty, the motor will first act as an inductor. In a sense, it should not worry us too much because current can not change abruptly in an inductor, but the truth of the matter is that this is one of the instances in which you will see the highest currents flowing into the motor. The start is not necessarily bad for the motor itself as in fact the motor can easily take this Inrush Current. The power stage, on the other hand and if not properly designed for, may take a beating.

Once the motor has started, the motor current will go down from inrush levels to whatever load the motor is at. Per example, if the motor is moving a few gears, current will be proportional to that load and according to torque/current curves.

Stopping the motor is not as harsh as starting. In fact, stopping is pretty much a breeze. What we do need to concern ourselves is with how we want the motor to stop. Do we want it to coast down as energy is spent in the loop, or do we want the rotor to stop as fast as possible? If the later is the option, then we need braking. Braking is easily accomplished by shorting the motor outputs. The reason why the motor stops so fast is because as a short is applied to the motor terminals, the Back EMF is shorted. Because Back EMF is directly proportional to speed, making Back EMF = 0, also means making speed = 0.

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