# How does the comparator set the motor speed at a fixed level?

How does the comparator below sets the motor speed at a fixed level?

I can think that the comparator output will turn ON if the speed proportional generator output voltage is lower than the zener reference voltage; and the comparator output will turn OFF if the generator output voltage is higher than the zener reference voltage.

But how does this makes the motor speed constant? Is there a step by step logical explanation?

• It just reduces power to the motor when the speed is too high and increases power when the speed is too low. Since the motor has inertia it will smooth everything out to in between "too fast" and "too slow" which is the speed you want. Just calling that thing an "amplifier" though is really loaded. Commented Apr 15, 2019 at 21:32
• The system is obviously a linear one. There should be "error amplifier' instead of "comparator". You have a bad textbook. Commented Apr 15, 2019 at 21:32
• But wouldn't it cause on off all the time an erratic movement? also wouldn't it need hysteresis? Commented Apr 15, 2019 at 21:34
• @Ale..chenski Im not into much these sorry for my ignorance. But do you mean by "linear" this type of control is for temperature but not for very smooth applications?. Commented Apr 15, 2019 at 21:35
• @user1999 Looks like on/off control to me. If so, I think the author expected the reader to realize that the comparator would have hysteresis (to whatever degree is wanted) built in and that the reader would see the amplifier stage as just whatever it takes to convert from the on/off comparator output to whatever is needed to drive the motor on and off. The generator must somehow magically produce the right range of signal, I guess, too. Entirely behavioral (except the zener and resistor.)
– jonk
Commented Apr 15, 2019 at 21:39

The so-called "comparator" outputs a voltage proportional to the difference between the generator voltage (some kind of ideal generator that outputs a smooth voltage) and the reference voltage across the Zener. That difference signal is amplified and fed to the motor.

This is a simple proportional control system and will have a steady state error inversely proportional to the gain, if it is stable.

Calling it a "comparator" is just wrong, at least in English.

I can think that the comparator output will turn ON if the speed proportional generator output voltage is lower than the Zener reference voltage; and the comparator output will turn OFF if the generator output voltage is higher than the Zener reference voltage.

Correct.

It's a bit like pressing the accelerator to the floor until you get to 60 kph and then taking it off until the speed drops to 59 kph whereupon you floor the pedal again. I knew one bus driver who's technique was similar and it wasn't pleasant.

But how does this makes the motor speed constant? Is there a step by step logical explanation?

The inertia of the bus - and in this case the inertia of the motor - will limit the fluctuation in speed to some amount that might be acceptable in certain situations and totally unacceptable in others.

• I had a friend who drove like that, his transmission didn't last very long. Commented Apr 16, 2019 at 16:31