How To Make Your Thermal Controls More (or Less)
Sensitive
A major advantage enjoyed by conductive thermal controls is the ability
to easily change them to meet your particular needs. They are extremely
versatile. Start with the bimetallic elements at the heart of these devices.
There are literally hundreds of different types of bimetal currently available.
At Portage Electric Products we have used dozens of these different bimetals
to meet the various sensitivity needs of many different applications.
This versatility helps conductive bimetallic devices enhance safety and
prevent nuisance tripping in a wide range of applications. It even allows
you to increase the ability of a thermal control to work as a circuit
breaker rather than strictly as a temperature control.
The major difference between bimetals lies in the internal resistance of a given type. Each type of bimetal with then exhibit different resistivity when current is passed through. The higher the internal resistance, or resistivity, of a bimetal, the more heat will be generated by current passing through. The more heat generated, the quicker the bimetal will deflect. The self-heating effect of the bimetal provides the reaction needed to function on increases in both temperature or current.
Bimetal resistivity is similar to the way water moves through a small diameter hose as opposed to a large diameter hose. The smaller hose presents greater resistance to the water's movement causing water to spurt out at a greater rate. If you use a larger diameter hose the water will face less resistance and will move through more slowly. The higher the resistivity, the faster the reaction.
The resistivity of any particular bimetal is expressed in ohms per circular mill foot (ohms cm/ft) and determines how quickly a change in temperature will make the bimetal deflect thereby breaking a circuit. The flexivity of a bimetal is the amount it bends at a certain temperature. Resistivity determines the amount of self-heat generation and aids to the speed with which a bimetal will deflect. Flexivity determines how much a bimetal will deflect under the influences of heat generation. This heat can be what is generated through the effects of heat generated by the application or through the self heating effect of current passing throught the bimetal.
The other way to alter the sensitivity of a conductive bimetallic device is to vary the composition of the materials used in any other current carrying parts found in the thermal control. For example, the resistivity of the metal used in the manufacture of a support arm used to support a set of electrical contacts or used to in conjunction with a bimetallic element, can also be increased or decreased, thereby altering the internal resistivity of the device.
The safety consideration
Increasing the current sensitivity of a device will enhance the safety
characteristics. The quicker a device senses a change in current, the
quicker it will shut down the circuit to prevent a catastrophic occurrence.
In cases where you want to alter the sensitivity of a device in order
to improve safety you don't want a device to react normally. You want
it to sense the danger and shut down before a serious problem occurs.
Avoiding nuisance tripping
On the other hand, you don't want a thermal control to be so sensitive
that it activates prematurely. There has to be a balance between too much
resistivity and too little. In every case the needs of the application
will determine the proper balance. A vacuum cleaner motor, for example,
will have higher power needs than the blower motor in a furnace. The right
thermal control might not be the same for these two motors but Pepi products
can be found in both applications. Our experienced Sales Engineers will
help in determining the right thermal control for your application.
The cost-saving advantages of being versatile
By having the ability to alter the operating characteristics of our thermal
controls, we can expand the number of applications for which a particular
thermal control is well-suited. In this manner we can take a limited number
of devices and make them perform better on any application for which a
bimetallic thermal control is the best choice. This leads to lower costs
for thermal controls that are better suited to a particular application.
No two electrical products are exactly alike. Neither should the thermal controls best suited to each. By giving our customers the ability to customize the operating characteristics to the exact needs of each application, you can create products that are safer, more reliable and cheaper to produce. All you have to do is understand how the versatility of these devices can work to your advantage.