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

Facts:
  • 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.

16 comments for “DC Motor Control Basics

  1. Sue Som
    June 24, 2010 at 7:25 pm

    Do you have any write-up where the DC motor inrush current limitations are discussed? Why DC motor inrush current, in typical industrial application, are typically limited to 2-3 times whereas the AC motor can typically can go to NEMA Letter G like 6-7 times the FLC?

  2. liyana
    April 2, 2014 at 8:58 am

    what equation are been produced, when we related current with torque?

  3. George
    January 27, 2016 at 1:06 pm

    In which case, what would be a suitable dummy load for a DC actuator motor

    • avayan
      January 27, 2016 at 1:15 pm

      Hi George,

      Do you mean a mechanical load to apply a dynamic torque or an electrical load which resembles a DC motor in terms of current and voltage?

      If it is a mechanical load, most people will use particle brakes or some kind of dynamo. These are quite expensive, though. The good thing about this venue is that you can control the load in terms of torque by applying a controlled current. For example, a dynamic load of this nature may give you an X amount of NM when you apply a 100 mA current across it. Of course this means the machine is characterized but that is one of the reasons they are so expensive. Plus it is one of those things which not everybody needs… You could also come up with your own version of a load by using belts, pulleys and so on. It is not the most scientific way of doing it, but you will get an idea of how the system responds.

      If you are talking about an electronic means to emulate a DC motor, then there is really nothing better than another DC motor! But at the end it depends on what you are trying to do. Sometimes all I need is to play with my current regulation in which case I will use a big inductor with a resistance in series. This will give me something similar to what a motor is when it is stalled and the BEMF is zero. If you want to emulate stuff like acceleration/deceleration and braking, then truly a DC motor is the only thing I can think of.

      Hope the info helps!

  4. Aashi
    February 12, 2017 at 4:03 pm

    Does shorting the 2 terminals of the motor, one time with the ground and one time with the positive voltage make any difference?

    • avayan
      February 19, 2017 at 8:30 am

      The only difference that I can think of is for thermal considerations. When you short the motor’s BEMF, in essence you have entered what we call Slow Decay mode. Current recirculates through the FETs and it takes longer to die down than on Fast Decay mode (when opposing FETs are being used). As a result, since the current takes longer to die down through the FETs, the I^2R component lasts a little bit longer. This is of course only a couple extra micro seconds, if at all, but do this a few thousand times per second (which we do!) and it adds up quickly! So in essence, you can imagine that if you alternate which FETs are being used, then they only see this heat component 50% of the time each (half time the high side and the other half time the low side). In reality, however, the gain obtained with this technique is so minimal it makes little sense to implement the complexity required to choose which FETs are being used during the Slow Decay time. So what you will see out there is that Slow Decay time is fixed to most likely the low side FETs. The other reason why the low side FETs are used (as opposed to the high side FETs) is because usually you can see the low side FETs having lower RDSON. This is done on purpose to save on cost and given the fact that it is the low side FETs the ones that will be taking the Slow Decay recirculation current. It also makes sense to use the low side FETs specially if you have dual SENSE resistors on the H Bridge’s low side. If you were to use the high side FETs, then you would not be able to measure current during the OFF time. Most H Bridges out there, however, only use a single SENSE resistor so for this purpose, it doesn’t matter whether high side or low side FETs are used during recirculation, as the current can’t be measured anyway. Hope the info helps!

  5. Luis Aguillon
    September 26, 2017 at 9:21 pm

    Hey, I’m really new into things like electricity, so could you please explain it to me as if I was a 10 year old. I’m actually 12.

    • Luis Aguillon
      September 26, 2017 at 9:25 pm

      NVM I actually understood it after reading more than once.

      • avayan
        September 27, 2017 at 1:41 am

        Hi Luis,

        Thanks for reading this blog post. Wanted to congratulate you on starting in such a fascinating world so early in life. Please continue gathering all of this information and do not give up this thirst for knowledge. Many things will seem hard to understand but will eventually sink in. A little patience leads to better understanding of mostly anything!

        Anyway, if there is anything about motion control that I can help you with, do not hesitate to let me know. Good luck on your electronic projects and endeavors!

  6. Luke
    September 30, 2017 at 1:27 pm

    i have a 48 volt club car motor that we pulled to test. It has 4 terminals, a1,a2, s1 and s2. S2 and a2 are linked with a bar of metal and s1 and a1 are linked aswell. There is ohms resistance between a2 and a1 and only those two posts. We have many burnt terminals and have been trying for a few hours to get it to turn, any help would be appreciated. Thanks

    • avayan
      October 1, 2017 at 5:55 am

      Hi Luke, I did a search for motors like what you describe and after seeing a few pictures I came to realize those are wound stator brushed DC motors. In essence, they are kind of similar to a brushed DC, except that instead of using permanent magnets, they use another winding to become an electro magnet. If I recall, in these motors you energize the stator directly from the battery to obtain the maximum magnetic field possible. Then, you PWM or speed control the rotor winding to generate the torque which moves the motor.

      Since you are only seeing resistance from one winding, that immediately tells us the other winding is burnt out. Most likely it is an open. Hard to say how this happened, but usually failure on motors are caused by overheating.

      In theory these motors can be re-wound for less than the cost of a motor, so you may want to check into that.

      Hope the info helps!

  7. Chris Walford
    December 8, 2017 at 7:06 pm

    If you run a PM dc motor in reverse, as a generator, does the EMF reverse also?

  8. Chris Walford
    December 8, 2017 at 7:07 pm

    Sorry – i meant back EMF

    • avayan
      December 9, 2017 at 4:35 am

      Hi Chris,

      That is correct. The voltage (and current, if the generator is loaded) being generated will depend on the rotation of the motor. This is much more meaningful if you have a brushed DC motor because the output is a DC signal where its magnitude is directly proportional with the speed of rotation and its polarity with the direction of rotation. This is why you see most brushed DC motors having a red and a black cable. It gives you a relationship to which terminal is positive and which terminal is negative.

      For example, imagine you rotate the motor Clock Wise (CW) and you see a positive voltage from red to black (+ to -). If you rotate the motor Counter Clock Wise (CCW), then you will see a negative voltage from red to black (+ to -).

      There are many ways to deal with this reality. You could either impose a mechanical restriction to ensure the generator only rotates in one direction. Or, the simpler one would be to use a bridge rectifier to ensure the system’s output is always of positive polarity. Of course you will have some losses on the rectifier, but there is just no way to win all the battles 😉

      Hope the info helps!

      JIQ

  9. jadeja jayrajsinh
    June 20, 2018 at 3:24 pm

    i have install the battery in oppostire way in my hysoung gt650 r from that day my bike is not starting . so what can i do ???

    • avayan
      July 6, 2018 at 12:41 pm

      That sounds like a major destruction event. Although most motor drivers out there should be designed to survive plugging the power source backwards, there is a possibility this feature was not available within your power stage. If so, I am afraid the electronics are quite cooked. It would not be worthwhile to try and fix it as usually most of the electronics are fried. You might as well get a new controller board, which hopefully can still be found. Good luck on your search!

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