Wiring AC Drive to PLC with digital

Merganzer

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Join Date
Sep 2005
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St. John's
Posts
23
Hi,

I'm using an Allen~Bradley PLC, Micrologix 1500, 1764-24AWA Base Unit and the Allen~Bradley 1769-OB32 DC Output Expansion Card.

The AC drive is a Siemens Micromaster Vector.

I want to use digital connections to control the drive from the PLC. The problem here is that the DC output card requires a common return(-DC) but I cant find the common return from the drive. I have the control connection diagram for the drive, and it doesnt specify a common return.

Suggestions?
 
I still always use interposing relays for this task.
You don't have to, but I find it cuts the potential problems.
 
My instructor tells me its probably better to use analog control for the drive. Maybe thats what I will have to use.

Oh Im in Electrical Engineering Technology program. This is for my technical thesis.
 
With most Drives, you have three choices for speed control from a PLC:

1) Use an analog output from the PLC to set the VFD output speed (most common)

2) Use a communications link from the PLC to the VFD to set the output speed (increasing in popularity)

3) Use several discrete outputs from the PLC to select from several pre-set speed inputs on the VFD. The speeds associated with each input must be set in the VFD configuration, and typically three or four different pre-sets are available. (not used very often)
 
To add to the comments already mentioned, you should always use an interposing relay or an isolated relay output card in this situation. Once you use the common from the VFD control circuitry you CANNOT use the card for any other control. The VFD will have a reference that must be applied to the specified digital input. This will be done with an interposing relay or isolated card.

Usually, in my experience, we use a digital input to start the VFD and one input to reference the analog input. The associated HAND-OFF-AUTO will apply the input to reference the analog input. In hand the only input active will be "RUN" (references the locally entered speed aka HIM module in AB drives)and in auto the VFD will be commanded from the PLC digital ouput to "RUN" and activate the "SPEED REFERENCE" input which selects the analog input from the PLC.
 
I guess ill have to learn how to use all this interposing relay stuff. Yes, I know hopw a relay works. But i'll have to do my research and see how it works for the drive
 
Merganzer said:
I guess ill have to learn how to use all this interposing relay stuff. Yes, I know hopw a relay works. But i'll have to do my research and see how it works for the drive

Most drives look for a "dry contact" on their discrete inputs. An interposing relay is just a relay operated by the PLC output and providing dry contacts to the VFD. Dry contact just means that the power for the control circuit is provided by the controlled device, in your case the VFD.
 
I would suggest you also take feedback from the drive if you are using it on something like an elevator, when you get close to the floor you need to slow the motor, this can only be determined by RPM feedback. It doesn't matter what HZ your throwing at it, you will never get it to stop consistently for different loads.

When the elevator is 12" below floor, slow motor to 100 rpm and keep going until the floor limit is reached, something like that i guess
 
I concur with using interposing relays, and in addition, I prefer relays with gold plated contacts. Most modern drive control circuits use very little current, and just a little corrosion on a cheap relay contact can stop the signal. I work in the rubber industry, and our air is full of sulphur which corrodes silver conductors.

We just received a panel full of cheap AD relays for drive logic and within a week, we had to replace half of them.

There may be an isolated 24vdc input module available for the drive that would allow you to connect directly and safely. AB has one for their 160 series drives and I have used it with no problems.

Using analog speed control does give the system superior flexibility. Now the programmer has total control of the drive behavior. Additionally, any (correctly sized) common off the shelf AC drive can be substituted in the future.

This is not true if you use a proprietary comms approach. I reserve the latter method to save money on projects with lots of drives. When the cost of the analog modules is greater than the cost of the comms hardware, or if you need good diagnostic information for a complex system, then it's the way to go.

I like to monitor a fault/ready contact and a single analog signal from the drive. I usually choose to monitor current or velocity depending on the application. That is usually plenty of information for the control system to adequately deal with about anything that could go wrong.
 
I'm not sure if I remember correctly, but I think the AWA base offers contact outputs. You can wire these directly to the drive.
 
figured out what you guys were saying with the isolated relays.

got a handful of omron solid state relays. worked like a charm.

thanks for the help!

J
 
Just a note, I have never used a solid state relay to do this. This is similar to a triac output card. These devices can provide leakage current and with the high input impedance of newer drives on there inputs you may be able to activate. Not talking from experience, may never have troubles with it, but I know triac output cards cannot be used on devices like buzzers or sonalerts as they will constantly emit a small amount of sound. The relays we always use are omron G2K I believe.
 
If you are referring to controlling the Micromaster by starting, stopping, and reversing, its fairly straight forward. Its not well documented, but it can be done without external relays. On the left side of the following pic, it shows how to wire directly to a PLC with 24VDC outputs. The "Q"'s are the outputs of the PLC. The DC COM is the common of the external power supply. Your output card must be a sourcing card, in other words it must supply the +24VDC to the terminals 5,6, and 7 of the drive.



MM440.JPG
 
I am trying to connect a VSD to an hydraulic pump. The PLC talking to VSD has 110V I/Os but the VSD has 24V I/Os. Someone suggested I connect 24Vrelays between my VSD outputs and the PLC. Your description above tells me it is not required. Kindly advise. Cheers.
 

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