Frequecy control for AC motor.

[mytech]

Hi i need to control a very small rated AC motor using frequency.
I thought of controlling it using UPS circuit... In normal system's UPS, the frequency would set for 50Hz in my country... I thought of putting the veriable resistor insted of fixed one..
I have not tried yet. would it work???.. Then one more circuit using 555 also availble.. Let me good circuit to control frquency.. thanks.

 

[crutschow]

Depends upon the amount of speed control you need. You shouldn't go much below 50Hz since the transformer would likely saturate and draw high input currents or kill the inverter power transistors. You could probably go above 50Hz. The limit would likely be how fast the inverter transistors switch and how hot they get.

 

[Hero999]

If you use a modified sinewave and reduce the duty cycle, it's possible to go under 50Hz but don't expect great performance.

 

[stevez]

Is the motor a type that can be adjusted by modifying the frequency ( is it's rotational speed synchronized with line frequency)? Some small AC motors are more universal.

If it will respond to changes in frequency the next question would be will it's power/torque characteristics meet or exceed your requirements at all of the speeds you have planned? If your application is constant duty - can the motor tolerate these conditions?

 

[mytech]

Thaks for your replies..
Actually i am just going to show the demo.. So i can use any type of AC motors..
I too have the same question.. When i reduce the frequency what would happen to torque & power.. Will it remains same or will give poor output??..

 

[Hero999]

If you have a choice then don't use any type of AC motor, use a universial motor and don't reduce the frequency, use phase control.

 

[stevez]

Power is more or less torque multiplied by rotational speed. If you reduce the speed you'd have to increase the torque of the motor to keep power constant - so it's likely that power will drop with speed. The change in torque is quite dependent on the motor and the drive but probably drops as well.

My work occasionally calls for varying the speed of a motor - usually a large (10 hp and up, 3 phase) industrial motor. There are some general rules that describe the behavior of these motors but the construction of smaller motors is such that these general rules may not be a good guideline. I am the mechanical engineer who has to work with the electrical engineer on motor and drive selection. It is not unusual to find that the capabilities of the motor are ok at full speed but fail to meet needs at reduced speeds.

 

[bloke_man]

If you have a choice then don't use any type of AC motor, use a universial motor and don't reduce the frequency, use phase control.

Thats the ticket! will do the trick nicely.