Analog output and Voltage Control

khanz

Member
Join Date
Mar 2008
Location
Sharjah
Posts
36
Hi all,
I want to control the speed of a three phase motor through a VFD. For the input to the VFD I am using a PLC. Now at present I am using only the digital inputs but I need the variations according to the music. So for the variations I have designed a new circuit which is input to V1 and COM of the VFD. Since I am using a music sensor. After which I use an AC to DC converter to be input to V1 and COM. Now my question is this can I use an analog input or output or input and output module of a PLC instead of this circuit. Becuase I want to get rid of this circuit designing, and since I never use an analog input or output till yet in any of my projects. So curious that how it will work. What about controlling the voltage through the PLC specially in programming? Can anybody explain this to me.
 
The VFD doesn't care where the 0-10VDC signal comes from. So, anything that can make variable voltage from 0 to 10VDC can be used. And, it doesn't matter what makes the voltage vary----music, the position of the moon, or the temperature in the room. The VFD simply follows the voltage signal and tells the motor to run at the corresponding speed.
 
my advise is, first disconnect the terminal to the motor, if you have a scope connect those wires to scope lead else use a multimeter, and then powered up the entire unit, observe the performance of your controller on the scope or the multimeter which ever you uses. if you have a variation in your output that tells you that your unit is functioning properly and that its a problem with your servo. however if the controller comes with a driver, you might need to first of all load the driver on a pc and configure appropriately. try those and let see how it goes.
 
thanks for your suggestions. But one thing that needs clarification is that what about analog outputs of the PLC. will it work instead of applying variable dc voltage to the VFD. Please explain it that how it will control.

Thanks
 
khanz said:
thanks for your suggestions. But one thing that needs clarification is that what about analog outputs of the PLC. will it work instead of applying variable dc voltage to the VFD. Please explain it that how it will control.

Thanks
http://en.wikipedia.org/wiki/Image:VFD_System.png

http://en.wikipedia.org/wiki/Image:VFD_System.png



A variable frequency drive system generally consists of an AC motor, a controller and an operator interface

Variable frequency drive controllers are solid state electronic power conversion devices. The usual design first converts AC input power to DC intermediate power using a rectifier bridge. The DC intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching circuit. The rectifier is usually a three-phase diode bridge, but controlled rectifier circuits are also used. Since incoming power is converted to DC, many units will accept single-phase as well as three-phase input power (acting as a phase converter as well as a speed controller); however the unit must be derated when using single phase input as only part of the rectifier bridge is carrying the connected load.

http://en.wikipedia.org/wiki/Image:PWM_VFD_Diagram.png

http://en.wikipedia.org/wiki/Image:PWM_VFD_Diagram.pngVFD operator interface

The operator interface provides a means for an operator to start and stop the motor and adjust the operating speed. Additional operator control functions might include reversing and switching between manual speed adjustment and automatic control from an external process control signal. The operator interface often includes an alphanumeric display and/or indication lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit is often provided on the front of the VFD controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the VFD controller. Most are also provided with input and output (I/O) terminals for connecting pushbuttons, switches and other operator interface devices or control signals. A serial communications port is also often available to allow the VFD to be configured, adjusted, monitored and controlled using a computer or {PLC}, it now depends entire on you operational sequence, but do not try to connect an out from PLC to the VFD directly, you might damage some things, honestly most output from PLC either goes through an interface or relay to limit the magnitude of the current that could be demanded by the equipment being operated



VFD Operation

When a VFD starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Starting at such a low frequency avoids the high inrush current that occurs when a motor is started by simply applying the utility (mains) voltage by turning on a switch. When a VFD starts, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop 150% of its rated torque while drawing only 50% of its rated current. When a motor is simply switched on at full voltage, it initially draws at least 300% of its rated current while producing less than 50% of its rated torque. As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed while drawing only 150% current.

With a VFD, the stopping sequence is just the opposite as the starting sequence. The frequency and voltage applied to the motor are ramped down at a controlled rate. When the frequency approaches zero, the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can be obtained by adding a braking circuit to dissipate the braking energy or return it to the power source

VFD power ratings

Variable frequency drives are available with voltage and current ratings to match the majority of 3-phase motors that are manufactured for operation from utility (mains) power. VFD controllers designed to operate at 110 volts to 690 volts are often classified as low voltage units. Low voltage units are typically designed for use with motors rated to deliver 0.2kW or 1/4 horsepower (Hp) up to at least 750kW or 1000Hp. Medium voltage VFD controllers are designed to operate at 2400/4160 volts(60Hz), 3000 volts(50Hz) or up to 10kV. In some applications a step up Transformer is placed between a low voltage drive and a medium voltage load. Medium voltage units are typically designed for use with motors rated to deliver 375kW or 500Hp and above. Medium voltage drives rated above 7kV and 5000 or 10,000Hp should probably be considered to be one-of-a-kind (one-off) designs.

 
Peter Nachtwey said:
For what? A position or velocity reference? Most servo systems will only provide a very low sub sonic response.

DickDv, do you really think a VFD can follow a music sensors for a reference? Even if following just the amplitude that would still be tough to do.

I don't think the OP is trying to simulate an instrument. I am picturing a school project where a servo is moving to positions along a traditional stringed instrument or a piano, based on a sound track.

Which I think would be a pretty cool project.

At least he is not trying to use a single phase motor lol!
 
hi all,
thanks for the comments. But still it's not clear to me what about analog output of a PLC. Will it work instead of DC power supply circuit which I am using to operate the speed of the motor through v1 and com terminals. and how it will work means the analog output of the PLC.
 
Peter, as I'm sure you know, it depends on the response time of the drive and what component of the music it is trying to follow. For example, if the input is a 0-10V signal that varies based upon the pitch, I could see the motor following it. On the other hand, expecting the motor to reproduce mechanically the accoustical waves in a 20-2khz system would be asking too much.

So, as usual, it depends.
 
khanz said:
hi all,
thanks for the comments. But still it's not clear to me what about analog output of a PLC. Will it work instead of DC power supply circuit which I am using to operate the speed of the motor through v1 and com terminals. and how it will work means the analog output of the PLC.
Yes, it will work. The analog output is very easy to program.
The range can be 0-4096, 0-8192, or 0-16384.

For a 0-10 volt 8 bit analog output, the range is 0-4096. Write 4096 to the port, and the output will be 10 volts. Write 2048 to the port, and the output will be 5 volts.

Most programming packages will allow you to scale the output to whatever range you want. Otherwise, you do the math in your program. For example, if you want to rescale 0-1000, multiply your input number by 4096/1000, and send the result to the port.
 

Similar Topics

Pretty simple question, but I was wondering how I can set the Analog Output 1 of an S7-1200 (DC/DC/Relay) to be proportional to a data tag. I...
Replies
1
Views
834
Hi; I need to monitor and log the 3phase AC input voltage for which I need a device to convert AC input voltage (say 350 - 440V) to analog (0-10v...
Replies
2
Views
1,354
I'm experimenting with Automation-Direct "Terminator" I/O analog modules and wondering if anyone has experience in configuring the T1F-16DA...
Replies
8
Views
4,479
Hey guys. Im working on a PLC project. A voltage output from VFD is connected to a PLC analog input card. I will need the same information in my...
Replies
1
Views
1,599
Hi I am getting a cpu 314c-2dp. I need to control hydraulic servo valves that need +/- 5V for proportional speed/direction. I am looking into...
Replies
2
Views
1,585
Back
Top Bottom