I just have to ask this...
Is this an educational or hypothetical question? or are you actually planning on using this 'technique' in a real application?
If it's a real application, I'd definately use an AC Drive (Doing what it is DESIGNED to do), and a PLC with an analog output (doing what it is DESIGNED to do). If your heart is set on PWM for some reason, then use a stepper motor, and a stepper-motor drive that accepts PWM inputs (Gekko and others make very inexpensive stepper drives like this).
thank u for yr respond,
no this is not a hypothetical qustion, its for real application.
no . not allowed to purchase any helping drives such as vfd's only
Do you have a clue to what you are doing? Do you have any concept on how to control a VFD?
I think you need to study ALOT more before trying an endeavor of this magnitude.
NOTE: In most cases I am one of the most willing to assist but in this case I do not believe you have studied or learned enough in this area
thank u for ur reply
i m trying to do crude a motion control on a acmotor, 3pahse,
at first i thought it was just coding ladders, but then, it turns out implicitly, many constraints for me to forsee how it will work just yet.
Well, I have to ask this...
What PLC can provide an accurate PWM singnal? What drives accept a PWM signal as a reference? Why bother?
Cry Baby, for all practical purposes FORGET the PWM idea, and tell us about your application so we can stear you in the proper direction. What do you want your motor to do?
thank u elevmike,
i summarise my application as follows;
this application is to deal with induction motor-inverter fed control in 1 package design using PLC.
it require a switch mode inverter,
outcome= obtain a simple outcome result of speed, position and fast brake control for the induction drive...
I think I understand what Crying baby is trying to do. Maybe he/she can verify if I'm right.
First, the intent is to create a crude motion controller using an inexpensive GE Fanuc Micro PLC. The PLC does not have analog output capabilities, but it does have a DC output that can be configured for PWM.
When you make a graph of velocity on the vertical axis and time on the horizontal axis, the area between the line that represents velocity and the horizontal axis is mathematically equal to the distance traveled. If crying baby can control a motor's velocity and bring it to zero at the proper time, theoretically he can control the position of the mechanism driven by the motor.
It is possible to create an analog signal using a PWM output. The output simply turns on/off/on/off... at some fixed frequency set by the programmer. By varying the percentage of that fixed frequency that the output is on, you can set the average voltage level. At 100 percent on, the voltage level is the same as the DC voltage being switched. At 50 percent on time, the average voltage is 50 percent of the switched voltage.
Crying baby can program the PLC to control the on-time percentage to create the changes needed in the voltage and feed this to the velocity reference input of the VFD. Just be careful not to exceed the maximum voltage. If the VFD wants a maximum of 10 volts, and the PWM output is switching 24 volts, then don't exceed 41.67% on-time.
Without feedback, this approach will not be very accurate or repeatable.
thank u steve,
... at last, some1 understands me .. phew..
no i cant use VFD. only PLC, inverter, and a motor. and maby some safety circuits..
may i recall the outcome again,
outcome= obtain a simple outcome result of speed, position and fast brake control for the induction
