Need help with the circuit of an AC motor and the PLC

sogoatticus

Member
Join Date
Sep 2007
Location
KL
Posts
29
Im currently doing a project to control a speed of an ac motor..after designing the circuit i noticed that the PLC must me a dc input n output..can anyone help in adjusting the circuit?

circuitdiagram.JPG
 
Why not tell the truth? You have a school assignment of some kind that you have to determine how to do, there is no way you are controlling an AC motor's speed with the circuit you supplied.

Some single phase AC motors can be reversing and/or multispeed but the circuit you have shown could only reverse direction for A DC motor i.e. not enough wiring involved for AC.

There is nothing about the circuit you provided that would allow speed control of either AC or DC, and could be damaging to the motor if used.
 
rsdoran said:
Why not tell the truth? You have a school assignment of some kind that you have to determine how to do, there is no way you are controlling an AC motor's speed with the circuit you supplied.

Some single phase AC motors can be reversing and/or multispeed but the circuit you have shown could only reverse direction for A DC motor i.e. not enough wiring involved for AC.

There is nothing about the circuit you provided that would allow speed control of either AC or DC, and could be damaging to the motor if used.

its not an assignment..its a project..the circuit shown is for DC motor which is my frens project..i just put up for a hint becoz i cant find the circuit on the web..sorry coz din state earlier bout the circuit

i've already know how to control the speed of AC motor using multiple resistor and variable frequency..but the problem is i dont know how to connect it to PLC as the PLC use Dc input. Besides im planning to use a 4 switch relay..anyone can help ?
 
Last edited:
The circuit you posted with your question is not a "hint" as you call it but a complete distraction.

If you "already know how to control the speed of an A/C motor" then it should be a simple matter for you to select a separate power supply for the PLC power, inputs, and outputs and a contactor with the appropriate coil to connect the motor to your speed control device. The PLC will have a separate means of communicating with your speed control device to control speed.

When is your "friend's" assignment due ?

By the way can you see the Petronas Twin Towers from your dorm window?

Stationmaster
 
Last edited:
sogoatticus said:
its not an assignment..its a project

Ok! Either way I will try to help you help yourself.

sogoatticus said:
i've already know how to control the speed of AC motor using multiple resistor and variable frequency

You will need to show us how you intend to do this. We also might need details such as voltage, amperage, frequency of reversals or duty cycle. On the PLC side of the project we will need input specs, output specs, sensor specs, which brand of plc.


sogoatticus said:
..but the problem is i dont know how to connect it to PLC as the PLC use Dc input.

The answers to the details I asked about above will allow us to help you.
 
You are not going to like what I have to say, but I'm saying it anyways.

I really question if you understand what you are doing. The circuit you have drawn, if for a DC motor as you claim, does not control its speed, it controls its direction of rotation. I see other problems with it as well. Running the motor power through output 100.00 and 100.01 is insane. You are about to fry your PLC, or worse, start a fire. The PLC is not meant to handle motor currents. You need another relay.

I suggest you hit the books and learn more about what you are doing, observe the current limits of the PLC IO, and learn some basic motor theory before you damage an expensive piece of equipment.

Now there are lots of us on the forum that will be happy to help mentor you, but clearly you need to begin with the basics. So set this project aside for the moment and really think about what you are doing. Then come back and we'll help point you in the right direction. We won't do the work for you though.
 
I do not mind helping but if it is a real project I believe you are over your head so not sure how much you could understand.

If it is a student assignment, as I believe, then give the details of what you can use or have to work with. As requested state what plc because not all have DC I/O. What motor, 3ph or 1ph, what HP, what speed and range, will it need to reverse? If single phase what kind of motor, not all can have speed control.
 
What you need to do is set up the PLC outputs to control the relay. The relay will switch the power to the motor.

I'm not sure what your switch and sensor are, but you only have one wire to each. This will not result in a compete circuit.

Try re-drawing the circuit with all of the wires shown so that you can see the circuit continuity. Ladder diagrams work well for this.
 
The question is how to control an AC motor with a PLC. The PLC has DC inputs, and NPN outputs. What's missing is a VFD.
I don't see any mention of an analog output. To control speed without analog, he'll need to select different presets on the VFD. Someone want to post an example? That may give a good starting point.

sogoatticus: Your friend's projest has a problem. PLC's aren't designed to directly drive a motor (in most cases). A controller or more relays are needed, unless the motor is very small, and this is only a class demo.
 
1st of all thanks for all the information..
2nd-to Stationmaster i cant see the twin tower from my window..i need to get outside of my house to get the view.

3rd- ok2..i admit..i have only a little basic in circuit design and PLC..this is a project i need to do..'Control the single phas A/C motor using PLC'..the circuit i uploaded is my frens circuit for conveyer belt..its not for speed control but direction control..
i roughly design a circuit but the circuit is kinda for Dc circuit like in the picture..as i stated im also using relay..
im not sure which wire (hot,neutral,ground) to connect to the plc..
for the motor im using 240vac(stated on the motor) which is a washing machine motor..
the PLC is in the univ lab..im not sure what brand..but i remembered it uses the syswin software..

maybe im asking too much here..but please dont scold me bcoz im a noob..i can back off from the forum and close this topic if neccessary.. :(
 
sogoatticus said:
..as i stated im also using relay..
im not sure which wire (hot,neutral,ground) to connect to the plc... :(
Do not connect hot directly to the PLC. Connect a 24DC supply to the PLC to run the relay. Connect the hot to the relay.
 
According to your draiwng it is a DC motor not AC. Why do not use DC speed control module and PWM instruction?
 
Before I make a mistake and presume something that may not be true. In your original post you said
sogoatticus said:
Im currently doing a project to control a speed of an ac motor

Does this mean you want to make the speed variable? If yes then you can read the rest of my post. If all you want to do is start and stop the motor and maybe reverse directions, Then let us know. A washing machine motor will work OK for this.

sogoatticus said:
for the motor im using 240vac(stated on the motor) which is a washing machine motor..

1.Some washing machine motors are reversible some are not.
2.Almost all washing machine motors (in the US) are poor candidates for variable speed control.
3.But some washing machine motors are two speed units.
4.If you have to use a washing machine motor the whole project will cost much more than starting with an acceptable motor.(presuming you want a functional variable speed project)

sogoatticus said:
maybe im asking too much here

If you expect an answer with the details you have given this would be true. If we get some relevant details this would be false.

sogoatticus said:
the PLC is in the univ lab..im not sure what brand..but i remembered it uses the syswin software..

I guess we will have to wait untill you come back from class for details on the PLC and sensors. One thing I forgot to mention in the list of details is. Will you be demonstrating in the class room and what voltage is available?
 
Last edited:
OK....I can see the possibility here for "multi-speed" ac motor control because your washing machine motor probably has "multiple speed windings". The wiring diagram and wire color code for the motor will be required for you to make this work. The PLC will be able to pull different "relays" to connect power to the different "speed" windings. You will need more input info to the PLC for it to automatically choose a speed based on "real world" need. (Otherwise you don't need a PLC at all, just a multi pole rotary switch). When the PLC decides to change the speed of the motor, it will select the appropriate output to pull a PLC compatable "relay" that you will have cleverly wired to supply the correct motor leads with power.

The PLC operating power, the sensor loop power, and the output power to the relay coils will all have to be properly "determined" and "provided for" by you the "engineer".

"Give a man a fish..........

Stationmaster
 

Similar Topics

any one can help me to write program in simatic s7. i have two rotating pulleys with with 4 pulse per turn, i want to detect and give an alarm if...
Replies
3
Views
1,883
i'm having a problem of making a circuit that will retained the last sequence of the program,what could be my circuit that will make the sequence...
Replies
3
Views
2,995
Hello nice to meet you, im new in here, I'm currently trying to convert code written in STL for a S7-400 to SCL for an S7-1500, because when i run...
Replies
5
Views
268
Hello, I am programming in S7-1500, V17. I have some blocks from a drive manufacturer that are not compiling and giving me an error "Invalid...
Replies
2
Views
273
Back
Top Bottom