Reliability, controllability and capital cost have been the primary considerations in selecting power supplies for use in industrial heat-treating and other thermal-processing applications rather than power factor and operating cost. Reliability is important, as process cycle times are measured not just in hours but often in days, weeks and even months. A process interruption resulting from a power supply failure can result in tens of thousands of dollars of lost product and could result in equipment damage. Industrial power supplies are expected to have a life expectancy of 20 years or more. Controllability is important because the temperature within the furnace must be precisely controlled over a wide operating range. Temperature regulation requirements of I1 degree are common. Capital cost, or return on investment, also is an important consideration.
Two traditional power-supply designs are saturable-magnetic and silicon controlled rectifier (SCR) power supplies, both of which operate at low power factor, which is not cost efficient. An increasing awareness of the cost penalties associated with operating low power factor loads has led to the development of a new cost-effective industrial power supply design, which can operate at a near-unity power factor. The performance of the power supply is illustrated in a vacuum furnace heat-treating plant case study.
In its simplest form, a power supply can be represented as a box having an input and an output, and containing a controlled switch and a step down transformer (Fig. 1). The transformer is used to match the impedance of the load, typically a resistive heating element, to the utility voltage. The switch controls the amount of power delivered to the workload over time.
Normally, a process controller external to the power supply compares a feedback signal (most often the temperature of the workload) to a set point and then provides a command signal to the switch control in the power supply.
For simplicity, Fig. 1 and all other figures and analyses discussed here depict a single-phase power supply. The methods of control are equally applicable to three-phase, three-to-two phase, and single- and three-phase ac-to-dc power supplies.
Various types of switches and their associated controls have been used in power supplies for decades. Some of these are discussed below. Representative voltage and current waveforms, as viewed at the power supply input, are shown in Fig. 2.
An electromechanical switch, or contactor, controls voltage and current to the load by closing and opening a set of mechanical contacts. The switch represents the mechanical contacts in the contactor (refer to Fig. 1). The switch control represents the contactor coil and associated circuitry. The contactor is either open or closed. Therefore, there is either full voltage or no voltage at the load. Current either is flowing or not flowing to the load, and its magnitude is a function of the workload impedance. Because the load is resistive (and assuming the transformer reactance is negligible), the input voltage and current are in phase (Fig. 2a); that is, unity power factor.
The capital cost of a contactor is relatively low compared with other switch types. Its control is relatively simple; turn it on when more heat in the furnace is needed and turn it off when the heat is no longer needed, which is known as time proportioning control.
The level of temperature regulation that can be achieved is limited using a contactor, and its reliability is poor. A typical contactor has a mechanical life expectancy of 5 to 10-million operations, but electrical life expectancy under its rated thermal capabilities is 0.5 to 1-million operations. This translates to a life expectancy of one to two years if the contactor is cycled once per minute to maintain temperature. The life can be extended significantly by derating the contactor, wherein the cost advantages are diminished.
On/off cycling of a contactor also can cause thermal and mechanical fatigue of heating elements that make up the load. Additional reliability concerns arise due to switching transients and arcing of the contacts causing noise in electrical circuits, which can disturb electronic controls and result in high maintenance.
The contactor discussed in this article for its simplicity and reference typically is not used in vacuum furnace control applications due to its poor reliability and limited control capabilities and is not further considered in this article.
A saturable reactor is an electromagnetic "switch" that controls current flow to the load by switching in and out of saturation during each half cycle. The resulting current waveform is rich in harmonics. The switch represents the ac coils and core of the saturable-core reactor (refer to Fig. 1). The switch control represents the dc coil and dc drive circuitry used to saturate the core. The amount of current to the load is controlled by controlling the amount of dc current applied to the dc coil, which, in turn, controls the amount of saturation in the core.
Typical input voltage and current waveforms for a saturable-core reactor power supply (with the core partially saturated) is shown in the oscilloscope plot in Fig. 2b. When the core is fully saturated, the current waveform is near sinusoidal. The inherent series reactance of a saturable reactor causes a phase shift between the voltage and current. Fig. 2b shows the voltage leading the current. Due to the series reactance, the best power factor that can be achieved when the core of the saturable-core reactor is fully saturated is a power factor of about 0.95.
The advantage of a saturable-core reactor power supply is its reliability because of no moving parts. Disadvantages are its poor power factor, higher losses, larger size, and higher cost. A typical saturable reactor costs more than a contactor or a silicon controlled rectifier (SCR) controller. A variation of saturable reactor control is the variable-reactance transformer, which combines the saturable reactor and transformer functions onto a common core structure. The term saturable magnetics is used later in the article to refer to both saturable reactors and variable-reactance transformers.
Silicon controlled rectifier
A silicon controlled rectifier (SCR) is capable of controlling voltage to the load similar to a contactor. However, there are no moving parts to wear out. An SCR controls voltage cycle by cycle in response to a signal from the switch control circuitry applied to its cathode and gate terminals. The time that the SCR is turned on can be controlled so it turns on and conducts for only a portion of the sine wave. The SCR turns off at the zero crossing.
Two SCRs connected in antiparallel are used to make up the SCR switch with one of the SCRs conducting the positive half cycle and the other conducting the negative half cycle. Representative input voltage and current waveforms are depicted in Fig. 2c. This type of control is referred to as phase-angle control; that is, controlling the phase angle at which the SCR is fired and, in turn, controlling the voltage applied and the current flow to the load. The magnitude of current to the load is a function of the load impedance.
This type of power supply offers fine control and excellent regulation of the temperature in a furnace. In terms of reliability, the use of SCR control 25 years ago was very limited due to its poor reliability with respect to traditional saturable-magnetic control methods. However, during the past 25 years, there have been significant advancements in the design and manufacture of SCRs and the control circuitry used with them. Now, the reliability of SCR control approaches that of saturable-magnetic control and, in many instances, has replaced saturable-magnetic control due to its lower cost. Like saturable-magnetic control, SCR phase-angle control offers good power factor when operating near full output and the SCRs are phased fully on. However, at reduced power, when the SCRs are phased back and only conducting part of the sine wave, the power factor is poor-comparable to that of saturable-magnetic control.
New power-supply design
A new power-supply design (FCS-2000 Smart Power Supply) developed by Magnetic Specialties Inc. (MSi), Trenton, N.J., and manufactured in partnership with Warner Power, combines the benefit of a contactor's near unity power factor control with the controllability and reliability of an SCR-controlled power supply (Fig. 3). At the heart of the power supply is a custom transformer and a new switch configuration. The transformer has multiple taps, which enable impedance matching for different load requirements between 25% and 100% of the power supply rating. The switch configuration provides automatic switching between the taps. The switch incorporates four SCR switches, one switch connected to each tap.
The SCRs are controlled by a microprocessor, which determines proper tap selection based on the power requirement to meet a given temperature requirement. The 25% tap is used for the best impedance match from 0% to 25% and then phase-angle control is used for fine control. As power level requirements are increased the microprocessor control transfers from phase-angle control to time proportioning control (similar to a contactor), so only full sine waves are passed to the load, thereby operating at near unity power factor. The microprocessor monitors the power delivered to the load. As the power requirements change, the microprocessor selects which switch to operate.
The power factor of SCR phase angle-controlled power supplies, saturable magnetic-controlled power supplies and the smart power supply design are compared in Fig. 4. As mentioned previously, SCR control and saturable-magnetic control schemes have similar power factors, shown in Fig. 4 as a single curve. The curves are theoretical in the sense that they do not include any transformer or system losses. However, if adjustments were made for losses, they would account for a minimal reduction in the power factor.
The power factor curves are plotted against delivered power as a percentage of full rated power of the power supply. A 100% power represents the power supply delivering its full output to the furnace load. The curves show that when operating at 100% power, all three types of power supplies operate at near unity power factor. At lower power requirements (typically required to maintain heat in a furnace after initial heating is completed), the power factor drops off for the SCR and saturable-magnetic power supplies, while the smart power supply maintains a near-unity power factor until the power requirements are below 25%.
On the basis of relative capital cost, the smart power supply is more expensive than traditional saturable-magnetic power supplies and SCR-type power supplies. Fig. 5 compares the capital cost for a basic power supply including only the switch element, transformer and an enclosure. All power supplies are assumed to be air cooled and are compared over a range of kVA sizes.
Compared with a saturable-magnetic power supply (the oldest of the three designs) as a base, the SCR type power supply is about 75% the cost of a saturable-magnetic supply and the smart power supply is about 140% of a saturable-magnetic supply-the curves are relatively flat over a range of kVA sizes. The differences in cost diminish when you take into account the cost of power supply associated equipment such as circuit breakers, contactors and other accessories, as well as water cooling requirements of some saturable magnetic supplies (the smart power supply is air cooled). A typical installed cost for the smart power supply is estimated at about 125% of the saturable-magnetic power supply cost, while the SCR power supply cost might be closer to 80-85% of the cost of the saturable-magnetic supply depending on features.
To compare operating costs, it is necessary to consider both the load and power required by the load to accomplish the task. In this example, the task is to heat a load weighing 15,000 lb (6.8 metric ton) inside a vacuum furnace to a temperature over 1200F (650C).
A typical production run is illustrated in Fig. 6a showing furnace and load temperatures versus time. The furnace temperature increases from ambient to over 1200F in about two to three hours and the furnace is held at temperature for about 25 hours while the load temperature increases over the 28 hour period. When the load reaches the required temperature, the power is turned off and the furnace is back-filled with helium to facilitate rapid cooling of the workload.
The power required to achieve the temperature profile shown in Fig. 6a is shown in Fig. 6b, the output delivered by the power supply indicated by the heavy solid line. The power supply is quickly ramped to about 40% of its rating and held at this level for a short period of time to bring the furnace to temperature. Then the power requirement is backed off to 20% to maintain the temperature of the furnace over a long time period. The dashed line shows the input kVA required to deliver the desired output for either a saturable-magnetic or SCR power supply as a percentage of the power supply rating. The curve is derived using the power factor versus power curve shown in Fig. 4, where for a given percentage output power, the input kVA is equal to the output kilowatts divided by the power factor.
The light solid line in Fig. 6b shows the input kVA required to deliver the desired output for the smart power supply as a percentage of the power supply rating; power supply and system losses are not included. The input kVA for the smart power supply equals the output kilowatt requirements for power levels of 25% and more. This is due to the fact that the smart power supply operates at near unity power factor above 25% power.
The power requirements shown in Fig. 6b can be used to determine operating costs. It is necessary to work with the local utility to project operating costs. Projected kWh, anticipated power factor, and kVA are needed to estimate operating cost. If local utility charges are based on kVAH, Fig. 6b shows that the smart power supply design is less expensive to operate.
An expansion at Solar Atmospheres Inc.'s Hermitage, Pa. vacuum furnace heat treating facility in 2001 included the installation of what is said to be the world's largest vacuum furnace. One of the goals established in planning the expansion was to operate the plant at a power factor of 0.9 or better. Solar and MSi reviewed capital costs- the power supply for a vacuum heat-treating furnace is one of the most expensive pieces of equipment-and utility costs (local utility charges for electricity use were based on kVAH) associated with operating a saturable-magnetic, SCR-type and smart power supplies.
The company selected MSi's smart power supply for the furnace based on a projected fast return on investment. Although the capital cost for the smart power supply was greater than that of either a saturable-magnetic or SCR-type power supply, the projected savings in utility costs offered an 18-month or less payback for the higher capital cost. The furnace is powered by a 1360-kVA smart power supply, consisting of 6 single-phase zones rated between 155 and 300 kVA. Since it was commissioned in the fall of 2001, the furnace is operating at 0.9 or better power factor.
To achieve the goal of operating the entire facility at a power factor of 0.9 or better, other equipment needs were reviewed and the necessary changes made. For example, two smaller vacuum furnaces also are equipped with the smart power supply design. In addition, three 300-hp blowers, one 200-hp blower and numerous vacuum pumps and water pumps ranging in size from 5 to 50 hp are powered using variable-speed ac drives having power-factor correction.