Last week a facility maintenance contractor contacted us about repairing a VERY old Johnson Controls HMI panel for a swimming pool at a local country club. This unit’s push buttons were only working intermittently. No replacements were available from the manufacturer or any of its’ distributors around the country. A used, untested, and no guaranteed unit was found on E-bay for $3000.00! The client certainly didn’t want to pay any where near that amount of money for this old of a unit. When asked if we could repair it…..We told the contractor that we would certainly “give it a try”!
He brought the unit over. The case was cracked and broken into many pieces. Upon examination, all of the unit’s push buttons were just worn out and not working. We found that we had some very similar replacement push buttons in stock. Each buttons case took some grinding and filing with a Dremel tool to properly fit, but they worked! All of the units buttons were replaced and then some serious “body” work was required to be done on the exterior case of the unit. With some epoxy and a lot of technician patience, the unit came back together. Having no way to completely test the unit, we contacted the client and informed him about what we had done to his unit. He picked up the unit, reinstalled it and it worked perfectly! This repair was performed in less than one day, and WELL under the used unit cost.
This is a good article from the U.S Department of Energy discussing the effects of VFDs on non-inverter duty rated motors. It discusses the insulation class recommended, in this article, “class H” is suggested. I previously had been recommending at a minimum, “class F” motor insulation. The article further discusses the use of load reactors and dV/dt filters to reduce the damaging effects upon motors, and mentions that these issues are especially high when running multiple motors on one VFD.
Typically when clients ask about running non-inverter duty rated motors with a VFD, they want to know if the motor will fail. The answer is: eventually. I inquire about the criticality of the application. Is this an application that demands constant up-time? If so, then it is wise to replace the motor when installing the VFD. Some VFD/motor manufacturers will extend their warranty if both are replaced simultaneously. If the application lends itself to some down-time, such as a multiple pump application, then run the motor until it fails, to maximize its’ initial ROI. This may be weeks, months or years later, but it will eventually fail. However, the best practice is still to be certain that the motor is rated for inverter duty to insure the longest lifespan. As the above article states, a work-around to this dilemma, rather than replacing the motor, is to install a load reactor or dV/dt filter between the VFD and the motor(s). Especially if there are multiple motors and/or the motor leads (commonly called “T” leads) are longer than about 100 feet. In these two particular applications (multiple motors or long leads), a reactor or filter is a good practice even when the motors are rated as inverter duty. Different VFD manufacturers recommend different maximum cable lengths before requiring reactors or filters. Check with the manufacturers recommendations to be certain.
As with any addressable network, having duplicate addresses is an issue. On Modbus Plus networks, one of three things can occur. Modicon technical support disagrees on this subject but in my 30+ years of working with these networks all over the world, I have proven differently many, many times.
The first, and most likely event to occur, the node that is being added to a network with the same address, will simply not be allowed to join the network.
The second thing that may occur, the node that is being added to the network will displace the existing node with the same address, assuming its’ place upon the network.
The third thing that may occur, is that when connecting the new node to the network with an existing node address is that the two nodes alternate joining and dropping off of the network.
The second and third options are obviously not desirable results. For this reason ALWAYS be certain of what addresses are existing on any given network. Utilizing a network discovery tool such as MBPStat, should eliminate any dual addressing.
Very recently, while performing a quick troubleshooting procedure upon a Modbus Plus network, I found a unique situation. I knew that I had a duplicate address on my computers’ PCMCIA card. however the plant was down, with no equipment running , and I was just doing a quick error check after repairing an illegal splice on the network. I was located at the MB+ repeater between segments 3 and 4 of this network. I monitored segment 3 for errors, all was OK, then I monitored segment 4 for errors, again all was OK. I reconnected the cables back to the repeater and then went to the end of segment 4 to monitor the entire network for errors. I was surprised to see all “U”‘s on every node! Those familiar with MBPStat, know that this indicates the monitor cannot properly identify the nodes. I went back to the repeater and checked both segments individually, again, all was OK. It was then that I remembered that I was utilizing an already assigned node address. I suspected this may be the issue. I again reconnected the cables back to the repeater, went to the end of segment 4, reset the address to a non-assigned node address. Upon checking the network monitor, it now showed all nodes properly with no errors.This further illustrates the unusual results which may occur in the event of a dual node address.
This is even more reason to be aware of existing addresses when adding any nodes to a network!
Inverters, drives, VSDs, variable speed drives, variable frequency drives – the category of products used to control an electric motor’s speed and thus reduce energy usage in a variable speed application has more names than Prince. Although, I suppose when you think about it he only has three really, if you include his decision in 1993 to become known by a symbol instead of a word.
Capacitor wear – Working like a dog
The first reason for inverter failure is electro-mechanical wear on capacitors. Inverters rely on capacitors to provide a smooth power output at varying levels of current; however electrolytic capacitors have a limited lifespan and age faster than dry components. This in itself can be a cause of inverter failure.
Capacitors are also extremely temperature sensitive. Temperatures over the stated operating temperature, often caused by high current, can reduce the life of the component. However, as the electrolytes evaporate faster at higher temperatures, capacitor life increases when they are run at lower than operating temperature.
Happily, keeping a consistent maintenance regime and regularly replacing capacitors avoids most problems that failed capacitors can cause.
Overuse – Too much of a good thing
Using inverters beyond their operating limit, either by choice or due to oversight or lack of knowledge, can contribute to inverter bridge failure. Using any component at a rating higher than its operating limit will decrease its lifespan and lead to failure, so avoiding this issue simply comes down to checking that all inverters are being run correctly.
Over- and under-voltage – The power you’re supplying, it’s electrifying!
The next two issues that can cause inverter failure are over-current and over-voltage. If either current or voltage increases to a level that the inverter is not rated for, it can cause damage to components in the device, most frequently the inverter bridge. Often this damage will be caused by the excess heat generated by the spike in voltage or current.
Over-current can be avoided with fuses or circuit breakers but avoiding over-voltage can be tricky. Sometimes voltage spikes are man-made, but they can also be caused by lightning or solar flares which are difficult to avoid if, like us, you live on planet Earth.
Ultrasonic vibrations – I’m picking up good vibrations
The final problem on the list is one that contributes to the mechanical stress placed on an inverter. Ultrasonic vibrations originating in the cores of inductive components cause friction, adding to the unwanted heat generated by the device and further damaging components in the inverter.
Another item that causes issues are failed fans. Although heat was mentioned in another category, these items fail often enough to warrant being specifically mentioned.
This article was originally published in the January issue of Automated Magazine.
Delta Automation will be hosting a free seminar titled “Introduction to PLCs” on Tuesday, December the 9th at 8:30 A.M. This seminar is targeted at those who desire some very basic knowledge of PLC technology and can be a “bridge” to a more structured fee based training class.
Please reserve a space as soon as possible as seating is limited. This seminar will be held at our 5120 Glen Alden Drive location.
Please contact Joe Sarver or Bob Culley at 804-236-2800 or by email at email@example.com or firstname.lastname@example.org to register for this seminar.
This introduction to PLCs will cover a brief history of the PLC, basic block diagram of PLC operation, how inputs and outputs are utilized, basic logic programming principles, and applications where PLCs can be used.