POTTERY PRODUCTION PRACTICES: Get Charged Up

March 1, 2007
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Voltage is an often misunderstood concept that can cause problems with your kiln's condition and performance.



Voltage impacts both kiln operation and performance, and misunderstandings abound. Three important considerations are low voltage, incorrect voltage, and how to mitigate or resolve voltage problems.

First, let's make sure we understand what voltage means. You cannot see or taste little electrons. You could touch them, but that is a highly overrated experience. Simply put, voltage is the amount of pressure that pushes electrons down a wire. Think of an electron as a drop of water and the water pipe as the wire. In your home, increased pressure causes more water to come out of the shower, just as more voltage for your kiln causes more electrons to be sent to the elements that convert electricity into heat. More pressure means more heat.

Voltage at load refers to the actual measured pressure on the electrons when the relays are on and the kiln is consuming electricity. The voltage at load is almost always lower than the voltage at the wall with the kiln off. Most of the time the drop at load is just a few volts, but it can reach as high as 15 volts. The pressure on the electrons drops when a large amount of voltage is being used-just like in the shower when another shower is being used. The voltage at load is what counts, and it must be measured with a multi-tester when the kiln elements are on.

Low Voltage

Low voltage is the most frequently encountered voltage problem. Potters with low voltage experience firing times that are hours longer than normal and may never reach temperature. It is difficult to define how many volts constitute low voltage because it is a relative concept: The impact of low voltage is relative to the amount of power designed into the kiln and the temperature to which it is fired compared to the kiln's maximum rated temperature. A drop in voltage will have little impact on a kiln that is being fired 500øF below its maximum rated temperature or on one that is overpowered by design. The corollary is also true-voltage drops have a greater impact on kilns that are fired close to their maximum temperature and on those that were not overpowered in the design.

Low voltage results from three possible causes, the most common of which is seasonality. During the summer months in hot weather, all of the air conditioners in homes and businesses are operating at such high rates that power providers cannot keep the voltage high enough to meet demand. The result is low voltage or pressure in the electrical system. The voltage or pressure is low during summer days, and it is higher at night and during the winter months when electrical demand is not as high.

The second cause of low voltage is too much distance between the kiln's location and the transformer or power distribution site. Low voltage occurs many times in rural areas at the end of the power grid. The further down the line the kiln is, the lower the pressure. Many times this can be rectified if the power company installs a transformer closer to your location, as this will boost the delivered voltage.

The last cause for low voltage is that the power company does not provide adequate voltage. We do not encounter many situations where this is the sole contributor to firing problems, but combined with one of the other causes, the result can be detrimental to the kiln's firing performance. Power companies are generally required to provide the nominal voltage of 208 or 240, plus or minus a certain percentage (like 10%). This means the voltage for the kiln can be as low as 216 for 240-volt nominal service. If the power company is providing a voltage within the agreed upon range, it is difficult to petition them to increase the delivered voltage.

Incorrect Voltage

Incorrect voltage is not as prevalent as low voltage, but its impact is much greater. The U.S. has two different primary voltages for large electrical appliances-208 and 240 volts. Commercial facilities, schools, warehouses and strip malls predominantly have 208 volts, but 240 and 480 volts can be encountered. Houses almost always have 240-volt service.

Problems arise when a kiln designed for one voltage is installed in a location where another voltage is provided. For example, a kiln designed to operate on 208 volts is installed at a studio where 240 volts are provided. This type of situation can occur when a kiln from a commercial building or school is moved into a residence or another commercial facility. The 240 volts of "pressure" are in excess of what the 208-volt kiln is designed to handle. The excessive voltage forces too many electrons into the kiln, enabling the potential for rapid element failure or tripping of the circuit breaker. Sometimes a 208-volt kiln can be operated on a 240-volt service safely with the only result being a kiln that delivers much greater heat. You should consult the kiln manufacturer before using 240-volt service on a 208-volt kiln.

The opposite, and more common, situation is when a 240-volt kiln is operated on 208 volts. This situation is typically encountered when the kiln is ordered incorrectly or a kiln is moved from a residence to a commercial facility. The pressure provided by the 208-volt service is much lower than the 240-volt kiln is designed to handle. The result is a very significant drop in power and heat generation.

Operating a kiln at an incorrect low voltage has other detrimental effects. Regularly firing a kiln with too low a voltage is insidious because it causes the elements to be on more and therefore decreases their life expectancy. As the elements degrade more quickly, they must be on longer to reach temperature, and the cycle continues.

In these cases, you might not see any difficulties until six months after the kiln is installed. A voltage reading could reveal that the voltage at load is 218, and the kiln was designed for 240 volts. This 22-volt decrease results in a loss of about 18% of power (see Estimating Power Loss sidebar).

If the low-voltage situation was known prior to purchasing the kiln, most kiln companies would have designed the kiln with 220-volt elements installed to prevent problems from developing. If the situation was not known at the time of purchase, most kiln companies can provide a replacement set of elements to compensate. There are other considerations when performing this change, so you should consult the kiln manufacturer prior to replacing elements that compensate for low voltage.

What Can You Do?

To prevent the installation of a kiln with incorrect voltage, you should have the electrical service checked with a multi-tester. If the electrical service is provided by an electrician or landlord, ask them to put the details in writing so you will not be held responsible if a mistake is made. Remember, the voltage cannot be determined by looking at the receptacle in the wall. Make sure your electrician puts his multi-tester in the receptacle to obtain the correct voltage reading.

The U.S. does not have nominal 220-volt service, though you could be experiencing a high 208 or a low 240 volts. 220 volts is a designation that means that the piece of equipment was designed to operate on either 208 or 240 volts. This is correct for commercial lighting and most motors, but not kilns. The lights will illuminate and the motors will turn, but, because of the laws of physics, the effects of voltage are much greater on resistive heating devices like kilns.

If low voltage is a result of seasonal factors, firing the kiln at night may reduce the impact since electrical demand is lower at night than during the day. Many digital controllers include a "Delay" feature that permits users to delay the start of the firing.

If you continue to encounter a situation where the kiln's performance is regularly impacted by low voltage, the kiln manufacturer might be able to assist in resolving the problem with a set of elements specifically designed for the lower voltage.

For additional information on kiln voltage, contact Paragon Industries LP at 2011 S. Town E. Blvd., Mesquite, TX 75149; (972) 288-7557 or (800) 876-4328; fax (972) 222-0646; e-mail paragonind@att.net; or visit www.paragonweb.com.

SIDEBAR: Estimating Power Loss

To estimate how much power is lost through low voltage, divide low voltage squared by the correct voltage squared:

low voltage2/correct voltage2 =

For instance, to estimate the power loss of a 240 volt kiln fired on 208 volts:

2082/2402 =

(208 x 208)/(240 x 240) =

43,264/57,600 = .75

In this case, .75 represents remaining power, so the power loss is .25. This would be like running an eight-cylinder car on six cylinders. The 32-volt decrease in voltage reduces the amount of power provided to the kiln by 25%.

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