i have vfd ABB ACS800- Drive  i want to control it through RTD 250 ohm  directly  on water  temp (min 20 c to max 70 c)  20c ----speed  20 hzon temp 70 ----- 60 hz

RTD is a analog device.. and you should check whether ABB ACS800 ac drive has analog input or not! And again fixed voltage applied against RTD and we get variable analog current.

yes it has both analog I/O ... do i need .... pid card b/w them

Yes You need PID controller.

I think u have to use PID Controller. First attach the RTD with PID controller and program it then give the output of this controller (4 mA to 20 mA ) to VFD.

I think u have to use PID Controller. First attach the RTD with PID controller and program it then give the output of this controller (4 mA to 20 mA ) to VFD.

I think in ABB VFD drives having built in function of PID blocks along with AI & AO channels... which can easily configured .... consult ACS800 manual or contact local ABB representative at Lahore or karachi.....

It is beeter way for going for PID controller . which you can control the speed by setting appropriate P I & D values. In market lot of PID controllers available with easy programming steps.(e.g. ABB,OMRON,YOKOGAWA,HONYWELL,EUROTHERM ETC.)More over in PID you can got the temp high low alram which useful for the generating the alrams of Temp..

i think you should use a seperate PID controller for RTD feedback and connect output of PID i.e 4-20 MA to VFD for better control.You could use Eurotherm's 3216 controller for this purpose.