GE FANUC control speed of AC POLIPHASE MOTOR

[crying_baby 0]

actual posting ndescriptions using LM90 Edited 5 Mar 2005 by crying_baby

[Steve Bailey 118]

If I understand your question correctly, you want to know whether it is possible to control position using an AC motor and drive without any position feedback device. The answer is yes as long as you're not particularly concerned with accuracy, repeatability or speed of response and as long as you've reached the target position you don't need much torque to maintain it.

[crying_baby 0]

ok. using open loop, configuration, would it be possible not to fry the PLC OR the motor if i m to use the control scheme of PLC --->PWM----> 3phase induction machine with less than 1hp without any other supporting devices such as starters . the PLC has a build in feature of PWM for pulse outputs. thanks again

[Steve Bailey 118]

NO. The PLC PWM output is a low-voltage DC signal. A 3-phase AC motor requires an AC signal. The rated current of the PLC output at 24 VDC is 0.75 amp. The speed of a 3-phase AC is a function of the frequency of the applied voltage. Even if you argue that the PWM waveform looks like an AC waveform, the PWM output does nothing about frequency. How were you planning to wire this up? A 3-phase AC motor needs three wires to it. There is only one solid-state output on your PLC (IC200UDR005). Do you need any more reasons? You might be able to do crude motion control with a PWM output fed to a VFD's analog speed reference input. Make sure the RMS voltage from the PWM output doesn't exceed the maximum value that the VFD can accept. The basic strategy would be to have the PLC create a trajectory, and assuming that the motor follows the reference. The area under the plot of velocity vs time is the distance traveled. Without feedback, the emphasis is on the word crude. Your accuracy and repeatability will be lousy.

[crying_baby 0]

thanks steve, i m getting a vfd to aid me on it. does the vfd normally, bundled wif its communication board? the board that allows the inverter to communicate with PLC> so if i get to use ge fanuc lm90, to comuncate wif teh inverter by omron, there is an interface it need to work out rite>> i got this optional mvodule that inside the manual rs-422/485 for the communicating part , is this module necessary since, i m using ge fanuc plc to communicate wif teh inverter instead of omron? i came from reading the manual please enlighten me sir, thank u for your time and attention :) haf a nice day! Edited 13 May 2005 by crying_baby

[Steve Bailey 118]

Once you have properly connected the VFD to the main power supply and to the motor, there are two control signals required to make the motor run. The first is a command to tell the motor how fast to turn. This can come from several different sources. You can set it from the keypad that comes with the VFD. You can wire a potentiometer to terminals on the VFD. You can connect a PLC analog output signal to the VFD. You can connect a contact closure between two terminals on the VFD that command a preset speed. The other is a command to start. In the simplest possible configuration, this is a contact closure between two terminals on the VFD. You have to decide how you want the motor to act and then how to provide those basic signals to make it do what you want. You do not need to establish serial communications between the PLC and the VFD to do the simple things that you have been talking about. You are making this much more complicated than it needs to be. Read the manual. Talk to someone who understands these things and who also speaks the same language as you. Many of the same people who have been following your questions over at PLCs.net are also regulars here.