I have been reading about motor controllers, and I found this article online (Introduction to Motor Drives and Encoders), but I am having a hard time understanding the diagram (figure below). Can someone explain the process of this control system in an algorithmic manner? I am really confused with the feedback part. I would appreciate any form of help.

Simplified diagram of a common type of electronic motor drive

  • \$\begingroup\$ That's not a schematic. \$\endgroup\$ Oct 12, 2014 at 2:12
  • \$\begingroup\$ how do i upload a snip \$\endgroup\$
    – user153322
    Oct 12, 2014 at 2:12
  • \$\begingroup\$ @IgnacioVazquez-Abrams - is it better if we just call that a diagram? \$\endgroup\$
    – Ricardo
    Oct 12, 2014 at 2:18
  • 1
    \$\begingroup\$ A block diagram, certainly. \$\endgroup\$ Oct 12, 2014 at 2:22

2 Answers 2


The intention of the diagram is to show that the feedback system compares the intended and actual speed of the motor and derives an error signal which is used to alter the power supply in such a way that the motor speed changes towards what it should be. Thusly:

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The diagram is confusing in that it seems to imply that the output of the error comparator is amplified and used to directly control power supply voltage or perhaps frequency or ... in order to linearly control the motor based on the error amplitude.

In fact, what happens is the amplified error signal is used to alter the power supply output in a manner which will reduce the subsequent error. eg if the motor is running faster than desired the sum of desired-actual will be negative and the power supply will be controlled in such a way as to reduce speed. I purposefully avoided direct statements about exactly how the error signal is used to affect speed. Because ...

Depending on the motor technology used and load characteristics the process of altering motor speed may be very simple or extremely complex. In a simple DC motor drive the system may reduce motor voltage. In some systems it may increase field excitation. In a process which involves complex dynamic effects a traditional PID control algorithm or a custom fuzzy logic algorithm (or both or more) may be applied.

  • 1
    \$\begingroup\$ The 4 lines into the motor controller are not labled, perhaps there is confusion there. One might guess the input signals are: enable/disable, Forward, Reverse, and speed control. Im not going to explain the algorythums for that type of motor control, but Here is book that helped me understand the subject: "Understanding Motor Controls" by stephen L herman, 2nd edition. Its probably more basic than your current book \$\endgroup\$
    – j0h
    Oct 12, 2014 at 4:15
  • \$\begingroup\$ @RusselMcMahon : Now I understand, wow, it really does make sense, error=desired-actual, and this error will be amplified to control our motor (plant). Thank you for such a great explanation, you have made my day. \$\endgroup\$
    – user153322
    Oct 12, 2014 at 5:39

The diagram illustrates what is called a proportional controller, one of the simplest closed- loop control schemes.

The set point signal is subtracted from the tachometer signal to give an error signal, which is amplified and fed to the motor. For example, suppose the set point signal is 0-10V.. Imagine it at 50% of top speed .. if the motor is not turning, the tach signal will be 0V so the error will be 5V. Now a gain factor is applied .. perhaps something like 0.1V = full power. So, the motor driver saturates at full power until the speed approaches the desired set point. It will settle out with a maximum error of 1% (depending on load and set point) plenty good enough for many applications, and if the gain is not set too high(!) it will be stable. There is a trade off between stability and maximum error since real closed- loop systems will be at least 2nd order.

In the above example, if the motor requires 100% power to turn at 50% of top speed (say it is heavily loaded) the error will be 0.1V or 1% of full speed (4.9V from the tachometer) so if top speed is 10,000 RPM, it will actually turn at 4,900 RPM.


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