# Connecting PLC to motor

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#### Damien Manbodh

I'm trying to connect a D.C. Motor to a PLC, the PLC produces a 4-20mA output and the motor accepts the change in current and varies accordingly. My problem is that when i hook up the motor to the PLC and change the current nothing happens. The motor just makes a noise as if it is strainig to operate. How do I make the motor turn, any suggestions?

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#### Al Boake

P=(I^2)R. 4 to 20 mA best spec is to drive 1000 ohms loop impedance. So, (0.020)(0.020)(1000)=0.4Watts. That is all the power available, best case. If the motor requires >=0.4 Watts there will be a power amplifier required. Al Boake P.E. [email protected]

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#### Kirk S. Hegwood

It sounds like you need a signal isolator. Sometimes the connections between two devices "bump" and won't communicate. Very frustrating when it happens. Kirk S. Hegwood President Signing for Hegwood Electric Service, Inc. [email protected]

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#### Yuzna, Ray

If its a new installation and programmable, check current limit, drives are shipped from the factory with I limit set to 0%.

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#### Burda, Jason M.

Your best bet would be to contact one of your local distributors, I'd start with the same person you bought the motor from and have them recommend a DC drive/Motor Controller that has an input for a 4-20 signal. All the big three GE, Allen-Bradley, Siemens all make comparable products for this application that are relatively easy to install/implement. JB

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#### Johan Bengtsson

True, but even if the motor requires less power than availiable it wouldn't work. The current thru a DC-motor gives you a torque, not a speed. This effectively means that you are controlling the torque and I suppose that wasn't your intention. You have to use an amplifier taking the 4-20mA and provide a DC voltage in the correct range and with the needed power. I suppose such circuits can be bought of the shelf (others have to answer that for certain), but if you would like to build it yourself write back and I give you some suggestions. Regardless of if you want to build it yourself or buy an off the shelf product you have to give more information about the motor such as voltage and power needed. /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: http://www.pol.se/ ----------------------------------------