| ??? 08/28/09 09:50 Read: times |
#168586 - Replies to Erik and Daniel Responding to: ???'s previous message |
Hi all,
Thanks Erik and Daniel for your replies. I was out for a social visit, so I am replying late, Sorry. I have seen AC Drives from Danfoss, and many others, they don't take any feedback. You just set the output frequency and they control the motor. They also show the RPM of the Motor without any feedback from the motor. Erik Malund said:
I suggest you buy an inverter (you do not state amps, but from the components I guess not a lot) which, for the size I guess, should be available for about $200. This controller board is damaged, but my friend wants to design a new one. There are many units that come for repairs to his office from the textile companies. Most of them have a problem in these boards. He repairs them, only if the peripheral devices such as MOSFETS/SCRs/IGBT, are damaged. But if it is a CPU problem, he cannot repair it. The number of boards having CPU problem is increasing day by day. Hence he wants a permanent solution. So if he has a replacement board for it, then he can use his own board, and sell it to his customers. The motors used in the textile mills are having almost the same specifications, he might have to change the capacity of MOSFET/SCR/IGBT. Erik Malund said: Well, we both are ready for that risk; of course it will be tested in our office. The old controller board's CPU is already damaged, but its peripheral devices (MOSFETS,etc) are working OK. So we might as well develop a new board with same peripheral components but use an MCS51 CPU.
Attempting a repair of the old unit will, most likely, cost you more and if you are not successful in repairing the old unit and connect it you may have the added expense of a motor. Erik Malund said: PWM signal is given to the MOSFET. The duty cycle of the PWM signal is changed after every PWM pulse. The duty cycle is given in such a way that, if you put a resistor and a capacitor at the output of this PWM signal, then you will see the sine wave. So I will call the frequency of the PWM signal as the "carrier frequency" and the final output frequency (after the capacitor).
2) yes but how can you have half frequency and "the PWM frequency remains the same". I don't know the basic working principle of an AC Drive, but I am assuming that this is how an AC Drive is working. I might be mistaken. Daniel Contarino said: Well, we are not trying to control the RPM of the motor. We just want the motor to run at either FullSpeed or HalfSpeed or Stop. I think by changing the output frequency, the AC drives are controlling the speed of the AC Motor. And this is what "AN984 from Microchip" is saying.
- how is the feedback? I mean, How the controller knows the necessry motor speed? I have just read the AN984 and it states on page 1: "The frequency and amplitude of the drive voltage must be varied to change motor speed". It also says that previously SCRs were used for AC motor control, but now MOSFETS and IGBTS are used to reduce the high dissipation due to harmonic content. |
