Why it is necessary to have speed controllers in dc motors?

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DaveElectronics6XT

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In a electric motor, a determined thrust, torque or speed is necessary to achieve a precise function. Let's say the thrust needs to be changed. Both thrust, torque and speed are function of applied voltage.

Why it is necessary to have speed controllers in dc motors? (Closed loop control) Shouldn't given a precise voltage output a precise rotor speed? From my point of view measuring the rotor speed is not necessary since it is already known by the applied voltage.
 
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The speed of a dc motor varies if the torque varies even for a fixed supply voltage.

A dc motor will generate a back voltage as it runs, and that voltage will oppose the supply voltage. The difference between the two voltage, divided by the resistance of the windings, gives the current taken by the motror.

The torque is approximately proportional to the current.

If the load on the motor needs more torque, the motor needs to take more current, and that needs either a larger supply voltage or a lower voltage generated by the motor. If the supply voltage doesn't change, the only way to get larger current is for the motor to slow down and produce less back voltage.

So to keep a constant speed with varying loads, as speed controller is needed.

However, the speed may be consistent enough with a fixed voltage that setting the voltage is a good enough way of controlling the speed.

Also many devices called "speed controllers" for dc motors are just voltage controllers, and they don't actually have any feedback from the actual motor speed. The devices are often switch-mode controllers, that rely on the inductance of the motor.
 
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Resistance.

The wiring and motor winding have inherent resistance, so the effective voltage changes with load current - caused by changing mechanical load on the motor.

Even temperature changes will cause some slight variations in resistance.
 
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Thank you very much. It is very clear now. At first what I didn't understand was the fact that I was thinking at a propeller where torque and thrust are bonded. In a more general way there are various other loads that adds up to the trimmed/desired ones that needs to be controlled, sensing and adjusting speed/voltage. The nature of additional loads can be inertial, electromagnetic inertial(inside the motor) and, directly applied to the shaft like friction. Thank you again for you answer.
 
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If a very precise speed control is not required, instead of a closed loop, a technique known as IR compensation may be used.
As the name implies, the controller senses the load current and increases the voltage proportionally, to compensate for the resistive losses as explained previously.
 
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This is an outline diagram of a typical real-world industrial DC motor speed controller, using a "dual loop" system - typical in machine tools back in the 1970s - 80s:



The incoming speed "setpoint" is compared to the appropriately scaled value from the tachometer W, for the outer "Speed loop".

The speed loop output (error or difference) is compared to the (again appropriately scaled) motor current feedback I, and that output, via a PID (or just an integrator in some drives) controls the current to the motor, via PWM duty, phase or voltage depending on the drive type.

[Diagram grabbed from here:
https://www.researchgate.net/figure...Control-of-the-motor-using-the_fig1_320420951 ]
 
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Thank you very much it is very helpful. So there is like a double check both on current, that may vary due to load or back emf, and on the rotor speed that may oscillate due to inertial effects r exernal loads. Seems like a very good control technique! thank you again!
 
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I have searched a bit on the internet, but i wasn't able to find it. Or rather they were from the electrochemistry. Can you provide more details on how does it work or link a reference? thank you again
 
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IR compensation is when the controller supplies more voltage as the current taken by the motor goes up.

For instance if an unloaded motor takes 1 A at 12 V, and under load it takes 2 A, but its speed drops by 20%, the controller would apply more voltage, probably around 14 V, when the motor current goes up to 2 A from the no-load condition.

The rate of increase of voltage as the current goes up would have to be adjusted for each application.

As an alternative, some systems measure the frequency of the ac component of the current that the motor takes. That ac component comes from the switching of the windings by the commutator, so the frequency is proportional to the motor speed. That can be used as the feedback in a speed control system.
 
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IR compensation is usually used with "Armature feedback", where the back EMF from the motor is used to measure it's speed for closed-loop control, rather than a separate tachometer.

The back EMF is measured during the intervals when no current is being fed to the motor, either part of a PWM cycle or phase control cycle before the next power pulse.

It is a user-set feature in all the drives I've worked with, as setting it too high can cause instability.
 
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I would say that it depends.

There are many ac and dc motors that don't have speed controllers. An ordinary dc motor connected to a fixed supply voltage will give a reasonably fixed speed. Also an ac induction motor runs at a constant speed and only slows down a bit under load.

There are still many applications where variable speed isn't needed and motors are run without speed control. Things like fans, conveyors, compressors or pumps often have no speed control in industrial applications. In automotive use, window, lock and seat movement motors are usually single speed.

Having said that, the situation is slowly changing.

As electronics get cheaper and cheaper, more and more worthwhile to add speed control so appliances like electric drills often have them now where they would have been rare 20 years ago.

Also dc brushed motors are often being replaced with brushless motors for reliability, and the control systems for those are effectively speed controllers.
 
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The applied voltage does not indicate the motor RPM by any means, it will vary with load.
This is why closed loop (PID) control, as shown in post #6 is used in CNC and other closed loop systems where very precise control of a motor is required.
.
 
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