How to vary voltage from a PLC

Hello,

I have a motor which requires me to vary the voltage in order to control the speed.

I wish to do this from the PLC using an output.

I.e. Run the motor at 36V until it gets near to the setpoint then switch a PLC output which will drop it to 10V.

Does anyone know a card which will do this. Ideally I would like to be able to tweak the voltage manually also.

Many thanks,

A PLC analog output (variable voltage signal) will not have enough power to drive a motor directly. It is very common to use a PLC analog output connected to a motor drive module which handles the actual motor power and uses the PLC analog signal as a speed reference.

Tell us more about the motor you are controlling and we can make recommendations for what type of a "drive" to use.

Hi OkiePC,

That is currently what we are looking at. A simple card which uses rs232 to control the motor speed. (Not sure why its RS232. Id prefer analogue)
The motor is a very simple lifter in a alluminium case. It is currently being driven at 36V using PLC outputs. I dont have an analogue output on the 1200 so I would need to buy one. Thats why I was hoping I might just be able to switch it simply using digital outputs. There isnt much fine positioning required. A simple slowdown speed would suffice as long as I can get it slow enough

So, you have 36vdc being switched by a "1200" PLC wired directly to the armature of a DC motor? Is it possible you could tell us more about the motor? I am thinking a make/model power rating would be nice. Also, there might be more than one PLC brand you could nickname a "1200". Knowing the facts sure beats guessing games...

Sorry.

Its a 1211C and the lifter is an Alphacolumn. Its just a small device which can sit on a desk. Im not sure of the motor type as its contained within the lifter. I just have an signals to send it up and down

This one?
http://www.industrialdevices.co.uk/pdf/Alpha system 2011.pdf

I believe that you are probably just sending the unit a direction signal and/or run command. I did not find any specifics in the readily available stuff on their site, so you may need to contact them to find out how to vary the speed of the device.

Paul

Hi
Yea thats the one. I contacted them. They said to change the voltage between 0-36volts. I just dont know how to do it electricaly.

Is that a separate 36v control voltage, or the supply voltage to the unit?

I wish to do this from the PLC using an output.

I.e. Run the motor at 36V until it gets near to the setpoint then switch a PLC output which will drop it to 10V.

Click to expand...

Yea thats the one. I contacted them. They said to change the voltage between 0-36 volts. I just dont know how to do it electricaly.

Click to expand...

Cyrus,
Using a PLC analog output to direct-drive your 36 volt DC motor is probably not an option, because the PLC output would not have enough power - it would burn out.

The simplest way, based on driving at 36, then reducing to 10, would be to use two separate DC power supplies, one at 36 volts, and the second at 10 volts (or use a variable-output DC power supply that can be switched with digital inputs). Use a PLC output controlled by an internal PLC timer to first switch on the 36 volts to the Alpha Lifter motor. After some time period, switch off the 36 volts, and at the same time switch on the 10 volts using a second PLC digital output. You might look at a dual-voltage power supply such as the Cosel YAW515, which has both 15 and 30 volt outputs. Then you could use PLC outputs to switch the voltage from 30 to 15 at the right time. You can hook two of the YAW power supplies in series to get different voltage outputs. Two YAW 512E's in series would allow 12, 24 and 36 volt outputs to be selected by PLC outputs.
http://www.trcelectronics.com/Cosel/yaw512e.shtml

An alternative is to buy a small DC Motor Drive that can be controlled with an analog PLC output. This method would allow you to control the motor speed from 0 to 100% (0 to 36 volts). The Dart Controls 65E10 DC Drive would be a typical example that should work. I have used it with a PLC 4-20 mA analog output to control 36 volt DC motor speeds.
http://www.dartcontrols.com/product-guide/battery-pwm-dc-controls/65e10/

If you really must have the RS-232 interface, the Dart MDII DC Drive will provide that.
http://www.dartcontrols.com/product-guide/digital-dc-speed-controls-2/mdii/

If the PLC outputs are transistor, a PWM controlled DC solid-state relay in series with the motor may work.
It must be capable for the motor peak (starting) current.
This has nothing with RS232, of course.