QUOTE:

Hello,
I am currently looking for a PLC unit with the following
characteristics and was wondering if you had any ideas on which would
be
most suitable for my needs,

Plc power supply, 240v, 50 Hz
N0. of Relay outputs, 254 max rating 1mA each
No. of transistor, triac or thyristor outputs, 1
No. of contact closure inputs (unit switches), 84
No. of three line proximity devices, none, unknown rating
No. of analogue inputs, 1, 0-10v dc
No. of analogue outputs, none
Type of computer interface, RS 232C or similar
Required memory size 4k
Required scan time better than 100m sec

The unit must be protected to IP57 standard
External ambient temperature, 0 degrees Celsius

Availability per unit value of 0.98
MTBF value of 1000 hrs
MTTR value of 1 hr
Therefore maintenance time = 19.4 hrs

The purpose is,
One output drives a contactor to switch a
motor of 240v, 20A rating.
I was also wondering what type of contactor would be most suitable
and would the PLC be able to control the contactor coil directly?
One problem I have is, the PLC will be housed close to a high-powered
radio transmitter. What problems may this cause and how severe would
they
be? Hope you can help.

Regards,

Gavin Currie,
Instrument Technician.

It's not big deal to get PLC with this number of I/O.
Temperature also doesn't look bad. I don't have my
IP reference handy but this should be no problem either,
just take enclosure with higher rating and make sure
all knockouts are sealed properly.

Most I/O cards are available in configurations of 4, 8, 16 and 32
points. Relay output cards are normally available in up to 16
points. This means that you would need at least 16 of them.
(add one more for spare...)

1mA rating is definitelly strange at least for relay output as
typical relay output is rated 0.5-2 Amp. Also, I don't remember
seeing thyristor outputs on PLC. Besides you have to be sure
which type you need as transistors are used for DC and low voltage
while for AC you would use triacs. They are NOT interchangable.

Since you need only single Analog Input, chances are you would have
to go for 4point card. You can put 16 point but that would be waste of money.

Serial interface, 4k memory and 100ms scan time are not a problem
(those are very low demands).

Type of contactors would depend on few factors. Normally contactors
with AC coils are cheaper and smaller in size. Since you are looking
for relay outputs, it's not a problem to drive contactors directly.

Regarding transmitter, noise filter on PLC power line would probably
be sufficient since the radio frequency is very high. If case of any problems
you can still consider TVSS and UPS.

Since this question was posted in AB section, I assume you are
looking for something like SLC503 with 13 slot rack and another expansion
rack (10 to 13 slot). You could use AB 100C23xxxx contactors and to be safe
you could oversize them to next bigger.

For all of this, you should normally give a call to your distributor and
let them size it for you.

regards,

panic mode