Question : i am working on a project to make a device that controls the speed of three phase induction motor i talked to my instructor about it he told me that u can control the three phase motor speed by controlling the amount of power that is flowing to the motor, what i read in the books that i need to control the frequency of the motor in order to control its speed.
which method is the correct one & if the amount of power controls the speed then how the motor will have a high torque when its slow & ha..
Answer : Electronic variable frequency controllers with three-phase motors are the most widely sold variable speed system for motors larger than about 1/4 Hp. For lower power, various types of electronic controls are used with DC motors. Electronic variable frequency controllers have been used for about 45 years, but they only became the dominant type of speed control within the last 15 or 20 years.
The speed of an AC motor can be controlled by reducing the voltage, but that method is much less efficient and effective than frequency control. Reducing the voltage reduces the available torque and increases the slip of an induction motor allowing the load to slow down the motor. Voltage reduction is more effective as a speed control method for motors that have a high slip to begin with and have a load like a fan or pump that drops off when the speed is reduced so that the motor doesn t stall and the motor doesn t overheat when the voltage is reduces. This method is commonly used with ceilin..
Question : I recently bought a small lab mixer (the kind that works by vibrating a padded platform against which you hold a capped tube or the like) and would like to add the ability to control the speed of the thing by adding a rheostat-like control dial to the wiring.
Knowing that dimmer switches and fluorescent lights do not play well together, I am wondering if there is anything special I need to know before adding the control to the vortexer's wiring.
Answer : It depends on the type of motor in the mixer. If it is an induction motor, it will always try to run near the 60 Hz power line rate. If you reduce the voltage enough, the motor will stall . You will not be able to control the speed in a satisfactory manner.
If the motor is a 'universal' type (AC or DC) then a dimmer might work, but you should use a dimmer that is rated to control inductive loads.
Question : Any websites or links where I can find a picture, schematic or animation to show how a brushless dc motor works?
Answer : http://en.wikipedia.org/wiki/Brushless_DC_electric_motor
has a good write up but this link has a great animation that will make it all make sense to you,