Keeping your world up and running.®

The control system is busted

The amazing new CNX™ 3000 Wireless System to the rescue!

August 2013

CNX 3000 wireless multimeter

Using the CNX 3000 wireless multimeter to check controller output to VFD. Controller output signal 0v-10vdc.

By Ron Auvil

Just a few days ago I got a service call from one of our major clients, a community college. The call was a fairly common one: “The control system is busted.”

Because my current position is senior technician for a major controls manufacturer, I hear that a lot. In the HVAC industry, the mechanical reps often say “It’s a controls problem” and the controls techs often say “It’s a mechanical problem.” I don’t get involved in this because sometimes it is one and sometimes the other. Only careful analysis of the particular situation will determine the exact problem.

I called the building manager on my drive to the college. He told me they had just replaced an old variable speed drive with a brand-new one, and now the air handler supply fan was not working properly. Hmmm, I had seen this one before, but bit my tongue and told him I would be there after a while. To make a long story short, many older variable speed drives (VFDs) are being replaced by newer models. Unfortunately, sometimes the installing contractor does not match the parameters of the new drive to the parameters of the old drive. Also, there may be wiring termination differences between the old and new drives, which lead to faulty wiring and improper operation.

Checking out the new variable speed drive

tools on worktable

For the sake of convenience, we put some of the tools on a movable work table.

We used a smartphone to access the customer BAS and to override the drive output anywhere between 0-100%, so we could check drive output and operation at different points - for example, 0%, 25%, 50%, 75%, 100%.

We took amp, volt, and hertz measurement from the drive, to verify changed BAS control and drive setup parameters.

Using CNX Hz to compare with drive settings after parameters were changed.

Upon arrival, I checked in with the building owner and we went out to the building with the new variable speed drive. Sure enough, the drive was in the hand position so that they could get the fan speed up close to what it should be. As soon as we put it in the “auto” mode the fan immediately stopped. I asked the owner whether the old drive had worked properly, and he indicated it had. This told me that the control system had not changed and the problems originated after the new drive was installed.

Together we called the contractor who installed the drive. He indicated there was no way the drive or installation was bad and that obviously it had to be a control system problem. Having been in this scenario many times before, I immediately suspected that the new drive had not been installed or started up properly. But now the question was how do I avoid the finger-pointing and give absolute proof that this was indeed the case.

On to the controller

What I needed was a way to show that my controller was generating the proper voltage output and the drive parameters or installation was improper. Hopefully I could show these simultaneously. It would be even better if this could take place in front of the customer, so they could see that I was doing a thorough job.

Fortunately I had just purchased my Fluke CNX Industrial kit. I used my laptop to log into the customer building automation system and override the output signal to 100 %, which should be 10 vdc.

I had connected the CNX 3000 multimeter to measure the input voltage at the drive, and simultaneously used three i3000 modules to indicate the amperage and hertz in each motor phase. The customer was incredulous when they saw my CNX 3000 setup! After setup and linking all the modules, I read the voltage at the drive input terminals at 15 vdc. I overrode the BAS web based system front end to 25 %, 50 %, and then 100 %, respectively. The input voltage should have changed to 2.5 vdc, 5 vdc, and then 7.5vdc. But the voltage never changed; maybe it WAS a control problem.

I disconnected the wires from the BAS controller baseplate and repeated the series of overrides. The controller output worked perfectly, as measured at the CNX 3000 digital multimeter (DMM). It was at that time I remembered that the digital inputs supplied a constant 15 vdc at the controller. A quick visual check of the wiring indicated that the installing contractor had miswired the input terminal block at the variable speed drive.

Making the customer happy

They had wired the alarm digital input to the analog input terminals, and vice-versa. After a quick rewiring job, I again did an override at the BAS. This time the voltage indicated at the drive input changed perfectly.

During the entire process the customer was looking over my shoulder in amazement at what I was able to do. (Also, they were really unhappy with the quality of their installation.) I decided to make sure the customer was happy, and checked out the drive to see if anything else had been missed. Sure enough, I started to find problems. The i3000 indicated an amperage reading inconsistent with the nameplate on the motor. Sure enough, the drive had not been setup properly on installation.

I used the drive keypad macro to change the drive parameters to match the motor voltage and amperage. After these changes I put the drive back in auto for a final time to perform a full checkout of all parameters with the customer present. I put the drive through its paces, with my laptop indicating both the BAS controller output as well as the hertz and amps of each leg of the fan motor.

The bottom line

The system is maintaining its exact duct static pressure setpoint and the drive is working great. Another satisfied customer that, due in large part to the CNX 3000 system, we will be doing a lot more business with in the future. In fact, in the CNX 3000 will pay for itself in just one or two additional service calls.

Oh, by the way, after shaking hands with this very satisfied customer, on my way out the door, I noticed that he was on the phone with the contractor who had installed the drive and blamed the control system. It wasn’t pretty.