Question about PLC Input Module


Thread Starter


I am a student of Electrical Power Engineering, and i am doing a project of Induction motor protection through PLC. Now i am in the studying stage of this project. I have 9 analog input and one digital output. Help me about analog input module that how many analog input cards are needed and which PLC is best to achieve these analog inputs.

Bob Peterson

You need enough analog input cards to cover how many analog inputs you have. Some PLCs have modules with as few as one analog input per module and some 4, 8, 16, or 32.

The key to deciding what PLC and what analog card to use is to determine how fast the analog input has to be captured and processed, and what the nature of the analog signal is (4-20 mA, 01-0 VDC, etc.). That will lead you to what PLC and/or module to use.

Most PLCs have adequate speed to handle routine analog work. But your application may be different. For instance, if you are trying to look at an AC waveform at 60 Hz to do some kind of Fourier analysis realtime, there are no PLCs that are fast enough to handle that type of application. There are other types of hardware that can handle that level of speed requirement though.

If the sensors are just RTds and all you care about is looking at the temperature as it goes up just about any PLC would handle that fairly easily.
You need to google the various major PLC manufacturers; Rockwell, Modicon, Siemens, etc; and review their online documentation. They will have all the info available; CPU scan time, I/O capacity, I/O scan time, I/O voltage ranges, etc.

You will probably find that the CPU and I/O scan times are not fast enough for electrical protection; but you need to verify that for yourself.

Good luck;

D. Patterson, BScEE, P.Eng
I am a student and i just start this project. But i don't have a information about which plc manufactures are best and also confused about analog input modules.

As u asked why the 9 analog input. 3 of them are sensors (temp sensor, ball bearing temp senor of motor and speed sensors). 6 of these analog inputs are voltage and current measurements means CTS and PTS, because of three phase induction motor 3 CT's and 3 PT's will bw used. so if u have any idea then plz help me.

Cost is a major factor. As i am a student so i can't afford expensive project. so if anyone of you have cost effective idea then tell me.
As I noticed, you need at least three Analog input for measuring temperatures and speed. Usually, there is RTD or Thermocouple PLC cards for measuring temperature and it depends on your sensor. For speed also it depends on your sensor, for example, if the speed sensor is a kind of encoder you should use a suitable card for measuring encoder sensors or maybe you need a fast counter card to measure the speed. For measuring voltage and current usually there is power measurement card in different brands (In BECKHOFF products it is KL3403 or EL3403), which measure the Voltage, Current, Power Factor, Frequency and other power elements. Usually, with this card You don't need a fast PLC because the card do the calculation.

Curt Wuollet

For one of the lowest costs per analog channel look at Automation Direct Click.
I've used several and they seem to work just fine.


Curt Wuollet

You may be in the envelope of a PC and data acquisition card. They offer the channel counts and sampling rates needed for detailed analysis. The signal conditioning needed for any generic analog gear is going to be the pinch point.

Isolation becomes a necessity for general application.