The first three articles in this series focused on the Insulated Box, the Conveyor System and the Heating Mechanism. My final article looks at the Control Panel, which has evolved dramatically over the years as electronic equipment has improved.
The earliest conveyor ovens merely had an on/off switch, an analog temperature controller and a belt-speed control switch. To this day, these are the basic functions needed to perform an adequate heat-treatment process. However, the control panels of today offer a huge array of functions. Some are merely useful to make an operator’s task easier, but others are necessary new functions that help meet regulations and other requirements for precision spring manufacturing.
Belt Controls
|
|
|
JN Conveyor oven built in 1982.
|
|
|
|
Three types of belt-speed controllers.
|
|
|
|
Oven control panel.
|
|
|
|
Circuit breaker with power tag.
|
|
|
|
Analog (left) and digital (right) timers.
|
|
|
|
The temperature controller connected to a contactor allows electricity to feed the heating elements.
|
|
|
|
A temperature chart recorder can be connected to the control panel and mounted to the oven.
|
One of the first new functions added to conveyor ovens was a belt-reversing switch. The main purpose of this switch is to reverse out of a jam-up. (Note: The conveyor belts are not designed to operate in the reverse direction for a long period of time.) There are two types of belt reverse switches, and when replacing a switch, it is important to know which one you need. Until the last five years, most ovens were controlled by DC motors. In order to avoid damaging the motor, a hesitation switch should be used. A hesitation switch forces the motor to come to a stop before it goes in the reverse direction. The switch itself stops momentarily at the stop position before allowing it to be set in reverse. It is also useful to protect against accidentally hitting the switch. The other kind of switch will allow you to instantly go from forward to reverse. Although this type of switch is cheaper, it is dangerous to use on ovens with DC motors.
The vast majority of belt-speed controllers are three-wire potentiometers connected to a DC motor controller. The potentiometers are usually one-turn, slow-to-fast type dials. While these controllers can serve most needs, it is very hard, if not impossible, to repeat a speed. The slightest touch to the dial can cause a 30-second change in belt speed. Also, once you have painstakingly determined that “dial position No. 2” yields a four-
minute cycle, the next time you set the dial to “No. 2,” you may have a 3:45 cycle. These types of dials do not have much repeatability. A one-turn dial can be replaced by a 10-turn dial. Ten-turn dials have 10 times the precision, and they usually have a lock-down to prevent accidental changes in speed by touching the dial. The best part is the dial settings are repeatable. Recent ovens can come equipped with digital speed controllers. The advantage of such a controller is the display shows the exact cycle time in minutes and seconds, which greatly speeds up setup time and meets ISO 9000 compliance. Some digital speed controllers display the Hertz output of the motor and provide a table to convert Hertz to minutes. These are not as accurate as a true digital speed controller because they are measuring voltage and not measuring actual shaft rotation.
Circuit Breakers
Circuit breakers on some oven makers’ models act as an On/Off switch. Their specific purpose is to protect the other electronic components from power surges. If broken or damaged, they will have to be replaced. Some circuit breakers can operate on various voltages, such as 220V, 480V and any voltage between. The important factor in selecting the correct breaker is to make sure the amperage rating is at least 10 percent over the maximum amperage draw of your oven. Almost every oven maker will affix a power tag near the circuit breaker to let you know the recommended amperage for that oven. Also, every breaker will have its rated amperage clearly marked on the face or side of the breaker itself.
Oven Timers
A fairly new function that has just about become standard on most ovens is a timer. Timers can be analog or digital, and you can swap one for another if you want. Analog timers usually give you control in two-hour increments, and they are useful only if the current time has been accurately set. Also, analog timers usually only give you one day’s worth of programming. On the other hand, a digital timer gives you control down to the minute, and there are almost unlimited programming capabilities. For example, you can set the oven to turn itself on every morning, Monday-Friday, at 7:30 a.m. so it can be hot and ready at 8:00 a.m. when a coiler starts running. Likewise, you can program a midday shutdown. For instance, if a plant meeting is scheduled right after lunch, you can have the oven off while you are away, and up and running when you return.
A similar function seen on many ovens is a delay shutdown. Although you can set a timer to turn an oven off, the delay shutdown has the long-term life of the oven in mind. Ovens with a delay shutdown allow you to flip the oven control switch to “off” and walk away from the oven. The oven will instantly cut power to the heating elements, but it lets the fan motor and belt continue for a set period of time. (There is a dial that lets users specify the amount of time). After the delay time has elapsed, the oven will shut down and remain in standby mode.
The purpose of the delay shutdown function is to allow the heat chamber to cool before stopping. If you were to suddenly stop the whole oven, the portion of the belt that remains inside the chamber will essentially “cook,” and you will degrade the belt. When an oven is in standby mode, it draws only enough power to light up the temperature controller display (and the digital speed controller display, if there is one). It’s akin to a VCR or TV light remaining on for the remote control.
Temperature Controls
The temperature controller is the main player in the control panel. The temperature controllers available today are a far cry from the large analog controllers that existed on early conveyor ovens. However, they function the same as they always have. As long as the thermocouple sends a signal to the temperature controller that the oven is below temperature, the temperature controller connected to a contactor will allow electricity to feed the heating elements.
The good news if you’re an oven owner with an old temperature controller is that you can upgrade to a new model. You may need a conversion faceplate to cover the difference in controller size, however.
Today’s temperature controllers have so many functions that you would never use them all (especially in the way conveyor oven operators in springmaking companies need to use them). It is not worth the extra money to buy a fancy temperature controller if you aren’t going to use the advanced functions. If you have a specific application or need a particular function, your oven supplier should be able to assist you with retrofitting a different temperature controller on your oven.
A very simple item often found on control panels is a green or red light to indicate whether the oven is operating in the temperature range you have selected. These lights obtain signals from the temperature controller, and most temperature controllers can be programmed to provide the signal either way (red when out of control, or green when in control). If your temperature controller is new enough, you can even add a red or green light to your oven if you don’t have one at all.
Heating-Element Wiring
Another part of a control panel that we haven’t covered yet is the wiring for the heating elements. One end of the wires is connected to the elements themselves, but the other end terminates in the control panel. (In some ovens, the control panel is separate from the electrical panel, in which case the element wires usually terminate inside the electrical panel). The vast majority of conveyor ovens used in North America are wired for 240VAC operation. If you buy a used oven or move an oven to another plant that operates at 480VAC, it is easy to convert the wiring if the number of elements is an even number and divisible by three (six, 12, 18, 24, etc…elements). The 240V elements can be wired in pairs for 480V operation. The net change in wiring from 240V to 480V is a subtraction of wires; usually half the wires can merely be pulled out. Unfortunately, it is not so easy to convert a 480V oven to 240V. In this case you have to add wires, and it is not easy to snake new wires back through an oven’s shell. Often, you will have to temporarily remove the top of the oven as well as some insulation. Although it is possible to change element wires yourself, you have to make sure other components in the oven are also changed.
Here is a list of the components often needing a change when converting voltage:
1. Transformers: These usually require only a jumper, and the tops of most transformers show a schematic on which wires to move.
2. Fan Motors: If these run at 110V, then no change is required. If they run at 230V, the electrical box on the fan motor will also have a schematic showing which wires need to be changed.
3. Fan Relays (if present in the first place): These may need to be replaced with relays that have the correct trip amperage.
4. Elements: These usually do not need to be changed, but they will have to be wired differently.
5. Circuit Breaker: Increasing the power from 240V to 480V means using a breaker with approximately half the amps.
6. Power Tags: Once an oven has been converted to a new voltage, a new tag should be affixed to let other users know the new power requirements for the oven.
Most oven suppliers have supplied their ovens in all the known voltages, so they can assist you with a power conversion, too.
New Control Panel Functions
What is on the horizon for control panels? With the advance of Programmable Logic Controllers (PLCs) and the somewhat lower costs for them, the choices for functions are virtually unlimited. The majority of new functions on conveyor ovens come from the individual needs requested by springmakers. For example, a lot of springs need proof that they reached a certain temperature during heat-treatment. Control panels can be fitted with temperature chart recorders for this purpose. Some temperature controllers can directly save temperature data, too, allowing users to connect a computer to the ovens and manage the data that way. It is also possible to get an oven to talk to a coiler to know when to shut down. So, instead of programming a shut-down time based on your guess of when the coiler will be finished, the coiler can directly signal the oven that no more springs are coming, and the oven will enter its cool-down, shut-down mode. Another area where conveyor ovens are evolving is assisting with automatic product handling. Conveyor belts do not have to move in a slow constant speed; they can be programmed to start and stop in quick succession so objects can be placed onto or picked off of the belts.
This concludes my series of four articles on preventive maintenance of inline conveyor ovens. The vast majority of old, beat-up ovens can often be restored with upgrades to components or, at times, replacement of part of the oven itself. Giving your oven a proper health check every few years will ensure that you get maximum life out of it. Some of the very first conveyor ovens put on the market in the U.S. are still in operation, and their longevity of service is specifically due to the great maintenance that has been performed on them by their owners.
If you have any questions on this or the previous three articles, please contact me or anyone at JN Machinery Corp.
