Electric Motor Speed Control

Thread Starter

friskyy

Joined Mar 27, 2011
13
Hi

What is the relationshipp between motor speed and average value of the armature input voltage, in a small electric motor controlled by Pulse-Width-Modulation( PWM) ?

Is it correct that by increasing the duty cycle of PWM ( the average time the motor is switched on) , we can only increase its average speed. But if we want to increase its maximum speed possible, we have to increase the armature input voltage, because Angular Velocity (w)= Vinput/(k*magnetic flux)?
 

PackratKing

Joined Jul 13, 2008
847
A[ny] motor has a maximum speed, based on rated voltage and drawing a "normal" current......a narrower PWM will control speed -- overvoltage, will only cause it to complain loudly, and release its magic smoke. :D
 

Thread Starter

friskyy

Joined Mar 27, 2011
13
Obviously, when the duty cycle of pwm increases, the average voltage across the armature of the motor increases and hence its speed increases.

Now, every motor has a safety rating. Say if I have a 200rpm,12V DC geared motor, it will rotate at 200rpm when 12V is applied. When the avg voltage goes down to, say 6V (maybe due to lower duty cycle of pwm), it will rotate at 100rpm.

Now, its maximum rating is at 12V. This doesnt mean that you cannot apply voltages greater than 12V. If you apply 15V, the corresponding rpm will increase, but its outside the safety limit. You never know when it will damage, or the so-called 'magical smoke' comes out.. ;)
Hi

So, this means that if we want to achieve a higher maximum speed we need a motor with higher max rating. Say, 18V Motor will be able to do 300rpm without overheating? Then the relationship w=Vin/machine constant* magnetic flux gives us the max possible speed and the duty cycle dictate if this speed is going to be achieved?
 

BillB3857

Joined Feb 28, 2009
2,570
You can't go by rated voltage alone when looking at a motor. The way it is wound and the strength of the magnets ultimately make the rated speed determination. You need to look at all the posted ratings.
 
Top