Regulators or VFDs

This is not the place for basic...(or basic guys as you wrote)..... Please everybody, do not entertain this type of booze support questions.

Dear Sir,

Using a regulator ( stepped or stepless) to control fan is good idea where

you need a blind control. You use a VFD where you need precise control of the

system with a feed-back loop ( like precise positioning of lift landing).

Do not be spoon fed if you want to grow in your carrier.

S.Poornam.

The speed of DC and certain types of (usually small single-phase) AC motors can be controlled by voltage (potentiometer). The speed of most industrial motors can be controlled by frequency (VFD). It has to do with the design characteristics of the motor.

First of all, it seems to me a valid question and the question has also been indirectly answered in the recent thread http://cr4.globalspec.com/thread/55009.

It is correct that the speed of the induction motor can be controlled by controlling the motor supply voltage but the concern is the torque developed by the motor with reduced voltage. The torque developed by the induction motor is directly proportional to the square of the motor terminal voltage. So, with reduced voltage, the torque developed by the motor is drastically decreased and the motor is not able to drive the load that requires constant torque (which is the case for most of the motor loads). However for certain loads, which are not constant torque load (such as fan and blower), voltage control method can be used to control the speed.

In addition, the motor draws more than the rated current to keep the load running up to its breakdown torque with reduced voltage with the possibility of motor burn or overload trip.

Also see:

http://cr4.globalspec.com/thread/54011/Torque-Squirrel-Cage-Induction-Motor

http://sureshks.netfirms.com/article/stator/stator.htm (read "2. Stator Voltage Control for Speed Control of Induction Motor")

Hope it is clear now.

- MS