by Adam Jacobson & Daniel Pierre III, JN Machinery
JN Machinery receives a lot of phone calls related to the technical aspects of stress relief or whether an oven can handle a specific task. On occasion, we get calls because an older oven is not acting like it should, or an oven has been ignored way too long. This article will focus on those calls, and what an operator or maintenance person can do for themselves to troubleshoot. When we categorize the themes of these calls, there are two that stand out: the (non) heating function and the (non-moving) conveyor function.
Technical Issue #1: “My oven is not heating.”
The heating system in an oven consists of five components or parts. Elements, thermocouples, thermocouple leadwire, element contactor and temperature controller. (There is a sixth: a high-limit controller, and we will address this at the end). Any one, or a combination of these issues, can be the cause of heating issues. Troubleshooting should start from inside the oven and then work toward the control cabinet.
The elements either work or fail and there is no in-between with them. Make sure to routinely check whether your wire is securely tightened to the element via the element wire nut. They can loosen up as the oven heats and cools over time. The loose connection can lead to early element failure. When everything is functioning correctly, all the elements receive power at the same time to maintain the desired operating temperature.
Checking for bad elements is accomplished using an amp reading multi-meter in the location shown in figure 1. If the reading comes back with an unbalance for one high voltage wire to another, it is likely related to the elements. While there is always a slight imbalance from wire to wire, I am referring to a significant change in amperage from one wire to the next. Simply remove the necessary covers to gain access to the elements and do an inspection of the wiring. If the wiring is OK, use the amp meter again to test every wire on the elements until you find the bad one. A bad element will be displayed on the amp meter as zero amps. This entire scenario can be avoided if your oven is equipped with burnout detectors. Burnout detectors have lights that will flash on and off as the elements have power. If one element on a pair of elements goes out, the indicator light will no longer display. This saves time and makes it easy to replace the bad element.
The thermocouple is the most important thing that keeps an oven heating properly. We always recommend you annually change your thermocouples, depending on use. Think of their function as being like your car’s thermostat. If they are gooped up with carbon residue from oily springs, it can’t provide your oven with accurate readings. Thermocouples are an inexpensive part to replace to keep your ovens functioning at a high level.
If the amperage is balanced across all three high voltage wires of the element contactor and the oven’s temperature isn’t going up, or goes up very, very slowly, you have a thermocouple problem. Either the thermocouple is damaged in some way, or it has a loose connection to the thermocouple holder. There is a 1 percent chance of this scenario in that somehow the oven lost an element on each of the three phases of power. The only time I see this happen is on larger ovens, when, over time, you may not notice one or two elements being out. Again, if element burnout detectors were installed, this scenario could be avoided. See figure 2 for an example of working burnout detectors.
Thermocouple Lead Wire
A thermocouple lead wire is the wire that connects the thermocouple to the temperature controller. It is an easy thing to overlook. The wire itself is signal stranded, which means after a long period of time the wire can become brittle and inaccurate. If the thermocouple has been replaced and the temperature controller is working properly, it may be time to replace the thermocouple lead wire.
The element contactor lets 3 phase power go on and off to the elements, based on the temperature control signal. The element contactor works about as hard as any component on the oven. It is constantly switching on and off to maintain the desired oven temperature. The element contactor should be upgraded if it is using mercury. Mercury contactors are actually banned now in some states. The direct replacement is a solid-state relay, which will last longer as well.
Testing for a bad element contactor isn’t difficult. Test for high voltage going across all three legs and high voltage going out. If the oven is on and calling for heat and there are no burned out elements, the voltage will equal. If you see that the voltage is significantly different, it is time to replace the element contactor with a new solid-state relay.
When you turn an oven on and select your temperature, there is a lot going on to make this simple for our customers to operate. The thermocouple is communicating its temperature reading to the temperature control. The temperature controller checks that reading against your desired operating temperature. If the temperature inside the oven is too low or too high, the temperature controller will send a signal to the element contactor accordingly.
The temperature controller, like anything else, can fail after years of use and abuse. We have our own program on our temperature controllers that is designed specifically for our ovens. Buying a replacement temperature controller from anywhere other than JN Machinery will not have this program installed.
You can investigate the temperature controller by testing for voltage on the control power terminals of the element contactor. If there is no voltage, turn the controller on and off as it could be related to the temperature. Once the faulty temperature controller has been discovered, it can only be replaced. There are no replaceable components inside the temperature controller.
High Limit Controller
The high limit controller is the last component to check. It is simply a safety device with its own thermocouple and thermocouple lead wire. The high limit controller is set for a very high temperature, say 1000 degrees Fahrenheit. The control voltage to the element contactor runs through the high limit controller. If the element contactor is damaged in such a way that it holds the elements on the high limit, the controller will kick out the power to the element contactor. Doing this will prevent a “runaway oven.” The high limit controller rarely if ever fails and only should be looked at if all other parts are functioning properly.
Technical Issue #2: “My belt is not moving.”
Of course, heating an oven is only one part of a conveyor oven. There is also the conveyor function side of the oven. Conveyor related or “belt is not moving” problems can be more complicated. The complications are due to the constant and continuous improvements JN makes to the oven to keep up with current technology and custom feature requests from customers. In fact, right now we are in the process of switching our current single-phase motor to three-phase motors. The change is due to our dwindling supply of the obsolete SCP-11L controllers. If you have this type of controller, we advise that you stock up on spares.
Part jams are a common problem that are usually related to something mechanical on the conveyor. The two major mechanical parts of the conveyor are the drums and bearings. Like any bearing, it is important to keep it greased with the proper amount. The right kind of high temperature grease is equally important. Failure to do this can cause bearings to freeze. With a frozen bearing, stainless-steel conveyor belts will grind through the carbon drums. See figure 3 for an example of this failure. JN can quickly build most JN replacement drums within a day, and all competitors drums with a little more time. However, what is that down time costing you, thatdown time that could have been avoided with simple maintenance?
Part Jams can also be related to, you guessed it: part jams. . I have seen quite a number of JN oven rebuilds go through our shop that have had a mass amount of products lodged between the underside of the belt on the conveyor bed. It is important to keep product out of this area, as the movement of the belt can grind down the conveyor bed over time and shorten belt life. When enough product gets in this area. it can bring the oven to a standstill.
Belt tension is a subject for more information that we will cover in a follow-up article. Improperly set belt tension is currently the No. 1 issue we see that causes premature part failure to conveyor ovens. Over the coming months, we will be working on a study to help our customers set the belt tension for our most popular ovens. The study will focus on the correct amount of torque in relationship to proper belt tension, and how to set that proper tension through the takeups.
The trickiest part of troubleshooting a nonmoving conveyor belt is on the electrical or control side. As I mentioned earlier, our controls have improved over the years, which has led to many different variations of wiring diagrams, motors and motor controllers. To keep this easy, check the simple things first. Check to see if there is any wiring that is damaged. Next, go through each wire terminal connection and tighten them. Some screws will vibrate loose over time.
Ovens will have either a potentiometer (pot) or a digital speed controller that are independent of the motor controller. The pot or digital speed controller sends a signal to the motor controller to set the oven to a desired speed. These items too can fail overtime. Digital speed controllers will always be easier to use as opposed to a pot.
Some motors are DC voltage, some are AC voltage, some have sprockets and chain, while others are a right-angle style. The point is, there are many different motors and it can be tricky when trying to troubleshoot them. Always check to make sure the oven is switched to the forward position. Check the voltage at the motor, be it AC or DC for power. If you don’t have any voltage, investigate the wiring to the motor further; whether a circuit breaker or motor capacitor, the motor doesn’t have power. If the motor has good voltage, it could be that the motor has burned out. Don’t be afraid to give that motor a smell, because we all know that burned electrical smell.
We have only scratched the surface of many different troubleshooting scenarios. We would like to remind you that if you are attempting to do any of these repairs, be safe and know your limits. All conveyor ovens operate with high voltage, so make sure you are comfortable working with it. At JN, the most import thing to us is your safety. If these procedures are more than what you’re capable of doing, you can always contact us to arrange an on-site visit or to have the oven sent back to us for a rebuild. Be safe and remember that electricity flows like a garden hose.