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Suggestions regarding PLC

Thread starter #1
Hello.. I am working on industrial automation project. I need to run three Nema 23 stepper motors using PLC (Fatek). I want Fatek FBS 40 MAR or FATEK 40 MCT. I wanted to use FATEK 40 MAR but the seller told me that won't give fast (high frequency) pulses to drive stepper motor since it has relay output . He suggested me to use FBS 40 MCT (transistor output for high frequency pulses for stepper). Can you explain what is the difference between these PLCs just wanted to have an idea.
Attached are the links for FBS 40 MAR and FBS 40 MCT
http://www.fatek.com/en/prod.php?act=view&no=12
http://www.fatek.com/en/prod.php?act=view&no=6
 

MaxHeadRoom78

Well-Known Member
#2
One thing about PLC's they are not much good for 'real time' operations such as closed loop or stepper motion control etc as they have inherent delay as the data table is updated each scan, therefore in order for almost all PLC's to perform these functions, the manuf. of the particular PLC will offer a specialty module specifically aimed to overcome this problem.
If looking at a work-around, then a solid state general output would be preferred.
Max.
 

dknguyen

Well-Known Member
#3
The MAR and MCT only have relay outputs, not solid state outputs. Relays switch slow and wear out a little bit every time they switch so they can only switch a finite number of times. This is because every time a relay turns off and interrupts a current, an arc is formed. Higher inductance loads (like motors) form bigger arcs which wear out the more even more. The advantage is relays are just hunks of metal when conducting so they have a very low resistance (can be small but conduct high current because low heat production) and do not wear while just conducting. So relays are good for applications where switching is infrequent (every few minutes or longer).

On the other hand, solid-state switches (like transistors or thyristors or triacs) do not wear out when they switch. Instead, transistors wear out while they are conducting (due to heating and other wear factors). So transistors are good for applications where switching must happen very frequently (every few seconds up to thousands or millions of times per second).

So applications that require very frequent switching (like motor commutation or PWM) shouldn't use relays since the relay can wear out after a few minutes in some applications. A typical relay might have a lifetime rating of 100,000 switches on average (less if it's abused). If it's switching just once per second, it's going to die after just 27 hours.

You REALLY do not want to use relays to commutate motors.
 
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