Teaching Students How To Troubleshoot, Repair, and Maintain

Technology Report Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Teaching Students How To Troubleshoot, Repair, and Maintain Magnetic Stirring Hot Plates Using Low-Cost Parts or Repurposed Materials Lucas F. de Paula*,† and Reinaldo Ruggiero‡ †

Campus Iturama, Universidade Federal do Triângulo Mineiro, 38280-000 Iturama, Minas Gerais, Brazil Instituto de Química, Universidade Federal de Uberlândia, 38400-000 Uberlândia, Minas Gerais, Brazil

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S Supporting Information *

ABSTRACT: Considering the large number of magnetic stirring hot plates that are damaged or unused in chemical laboratories, this Technology Report presents guidelines for students to be able to diagnose problems, and repair and maintain magnetic stirring hot plates using common and low-cost parts, or even repurposed materials. In addition, this paper presents a control board for stirring or heating with a circuit that uses the triode for alternating current BTA08, as well as various suggestions for refurbishing, maintaining, and replacing parts. Guidelines are provided for diagnosing equipment and conducting a cost analysis to determine the economic viability of proposed maintenance and repair approaches. KEYWORDS: High School/Introductory Chemistry, Public Understanding/Outreach, Hands-On Learning/Manipulatives, First-Year Undergraduate/General, Laboratory Instruction, Laboratory Equipment/Apparatus

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INTRODUCTION Magnetic stirring hot plates are used in numerous routine activities of the chemistry laboratory. In addition to ordinary use, there are reports of modern methods of analysis based on magnetic stirring, such as the detection of parasites in meats or ultrafast spectroscopy.1,2 Stirring hot plates usually require 650 W and have a maximum surface temperature reaching 360 °C. Powerful models are able to promote heating and stirring of 20 L of solution. The cost of new equipment is close to $450.003 or may be higher in brand name models. Chemistry laboratories often present a corrosive atmosphere and high concentration of volatile organic compounds.4 These conditions, combined with hours or even days of continuous use, contribute to the premature wear of magnetic stirrers. Thus, one often sees several inoperative, damaged, or partially destroyed magnetic stirrers stored in the laboratory. Such equipment may present defects in any of its seven main components, as shown in Figure 1. Each part will be discussed below, with suggestions for repair, replacement, or information so that the student can adapt pieces from another source. Many Journal of Chemical Education reports were dedicated to the construction of low-cost magnetic stirrers.5−7 The most recent articles describe building strategies from used computer parts.8,9 However, we present in this article guidelines for restoration of magnetic stirring hot plates. Our strategy is to reuse the strong shaded pole motors commonly found in commercial applications. In addition, there are plenty of © XXXX American Chemical Society and Division of Chemical Education, Inc.

Figure 1. Main components of a magnetic stirrer with hot plate.

damaged magnetic stirrers, which may provide the metal housing and the hot plate, saving time and cost for these parts. Our objective is to present instructions for students to perform the diagnosis, maintenance, and renovation of magnetic stirring hot plates from common and inexpensive Received: April 10, 2018 Revised: September 4, 2018

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DOI: 10.1021/acs.jchemed.8b00267 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Technology Report

protect the motors from liquids, dust, fine particles, or other materials. There is a standardized system of the motor protection class called ingress protection (IP). The first number indicates the ingress resistance of solids and the second the resistance of liquids ingress. In a very simple way, the higher the IP is, the more protected the motor is. Visually, the appearance of the motors depends on their protection type. Motors without a protective enclosure (Figure 2A) are IP00

items found in trade or even the from repurposed materials. These guidelines are valid for nearly all commercially available magnetic stirrers whether analog or digital.

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DETECTING THE PROBLEM Magnetic stirrer repair starts by determining the inoperative parts of the equipment. Students interested in more systematic information on defect analysis may consult the repair flowchart described in the Supporting Information. These devices may have 4 fault situations: • Stirring failure • Heating failure • Panel problems (broken knob, damaged lamps) • Nothing works The student will also be able to make the repair in situations where the equipment’s appearance is compromised. The restoration is also useful for extending the equipment lifetime. These guidelines include housing maintenance (metal frame, electrical cables, and panel items), control circuit board, motor, and heating element. The following topics will detail each equipment part attempting to maintain the same quality as the original equipment. Whenever possible, alternative parts such as repurposed materials or low-cost options will be presented.

Figure 2. Shaded pole motor and ferrite magnet coupled with an aluminum flange: (A) without protection enclosure and (B) with protection enclosure IP44.

type, being more compact than the protected class. They have a low cost and are one of the most used devices in small fans, magnetic stirrers, and air fryers. In the case of shaded pole motors with enclosures, IP10, IP42, and IP44 are common. The last one is able to provide physical protection against water sprays and solid objects greater than 1 mm in diameter (Figure 2B). Figure 2 provides an overview of the shaded pole motors with a coupling flange for magnets. The shaded pole motors, applied in magnetic stirrers, must have sufficient torque for moving a small magnet, so a motor with power of 12 W or higher is typically used. When using magnetic stirring for a long period of time the motor temperature rises, so in order to increase the lifetime, it is coupled to a fan on the rear of the motor shaft, self-cooling the motor as shown in Figure 2A. Magnetic stirrers use a ferrite magnet which can easily be replaced. These ferrite magnets can be found in recyclable materials such as conventional speakers; usually, 3 W automotive speakers have a 40 mm magnet which is suitable for the a large number of magnetic stirrers. A ferrite magnet is used because they have low costs and have sufficient magnetic flux; however, the strong neodymium magnets are also useful and can be obtained from inoperative computer hard drives. In hard drives these magnets are located on the corner of the drive nearest the actuator arm that moves across the disc.12 To attach the magnet to the motor, a flange is required. This would be the only part that may require machining services. An aluminum flange is best as it is not heavy and is not ferromagnetic. It is possible to make a ferrous metal flange, but in this case magnetic insulation is required. The part must also have a low weight since the shaded pole motors do not have higher torque compared to other single-phase induction motor types (e.g., split phase or capacitor start motor). If the motor operates on overload, they will operate at a higher temperature and be less durable due to the premature wear of their parts. These motors can last up to 3000 h of operation with simple maintenance such as dust removal with compressed air and lubrication of their bearing with 3-in-1 multipurpose oil. The motor casing can be cleaned with this type of oil and a clean cloth.

Housing

Usually, the metal housing is made of aluminum or carbon steel and is electrostatically painted with epoxy paint for chemical resistance. It is very common to observe the presence of solvent stains or metal structure corrosion in frequently used equipment, compromising its durability and visual appearance. Prior to restoration, we recommend removal of all items from the panel, such as power switches, indicator lights or light-emitting diodes (LEDs), power cable, fuseholders, etc. Removal of rust and old paint can be accomplished by sanding with 100 grit silicon carbide sandpaper followed by progressive treatment with 220 and 400 grit waterproof sandpaper. After being sanded and dried, the equipment should be painted with epoxy spray paint, which gives greater chemical resistance to metal surfaces. Some galvanized or zinc-plated bolts, nuts, and washers may have rust and should be replaced with stainless steel materials. Even stainless steel can be hit by corrosive substances under heating and over time may present rust, so it is recommended to inspect all of these parts. Some equipment may be functional, but the indicator lights (ac lamps or LEDs) on the panel may not be working. In this situation, only replacement of the indicative light is required. The turning and control part, called the “knob”, is also a commercial product and can be easily replaced. If the panel information is damaged by corrosion or solvents, it is still possible to print a new copy on adhesive paper. This solution is not as durable as factory epoxy printed information but can be reapplied whenever wear occurs. Motor, Flange, and Magnet

Magnetic stirrers typically use an induction motor called a shaded pole motor. This motor is an alternating current (ac), single phase, asynchronous, squirrel cage rotor,10 featuring low cost and high durability; however, they have low efficiency and low torque. This motor class is used in various domestic equipment, mostly fans used in microwave ovens, fryers, grills, dryers, freezers, etc.11 Shaded pole motors are usually marketed in output power ranging between 7.5 and 45 W. There are several types of enclosures that are designed to B



Technology Report

Figure 3. Schematic for speed controller of a shaded pole motor or heating element, using DB3 DIAC and BTA08-600B TRIAC. This has power control capability up to 1000 W.

Hot Plate

maintenance scheme suggests malfunction of the control circuit board, it is possible to detect the damaged components and replace them. A visual inspection of the board should be performed, with an attempt to observe damaged connectors, circuit track breaks, or components that are broken, corroded, or burned. Depending on the component’s physical state, it may not be possible to detect its specifications. If the circuit board is in good condition, the absence of speed/heating control usually is associated with defects in control components, such as the microcontroller, triode for alternating current (TRIAC), and diode for alternating current (DIAC). If the circuit board is based on TRIAC and DIAC, replacement of these components may be sufficient for the equipment to work normally, but these circumstances depend on the type of circuit and the defect. Thus, there is no generic way to solve the problem, and in this case we suggest replacing the original circuit board with the alternative presented in Figure 3. This circuit is simple, inexpensive, and widely known in the literature: a dimmer with TRIAC. This circuit is able to vary the output voltage that feeds the load (in this case it is the motor or the heating element). The dimmer produces voltage variation by cutting out part of the output wave; this cut is performed by fast interruption.14 The interruption period sets the effective output voltage value, as shown in Figure 4. The TRIAC is triggered by sending a pulsed signal to the “gate” pin. This circuit is a simple and reliable dimmer that controls a shaded pole motor or resistive element (that does not exceed 1000 W). A circuit board must be built for each purpose (stirring and heating). In this circuit the pulsed signal is generated by a DIAC to drive the gate pin of the TRIAC. The speed control is performed by adjusting the potentiometer P1, which must support up to 1 W of power, and a fine adjustment is made by the trimpot P2. This circuit type uses an LC filter formed by the L1 inductor and the C3 capacitor required for suppression of radio frequency interference. The inductor construction is quite simple and can be accomplished by wrapping an enameled copper wire (AWG 11), about 40 turns around a small cylindrical ferrite core. The number of turns does not need to be very precise, because there is no need to cut a very specific

The heating plate serves as surface support and conducts heat from the electrical resistance with uniform distribution. Heating plates are made of two different materials: aluminum or glass−ceramic. Aluminum is more economical and may deform, but it will not break if the equipment suffers mechanical shock. The aluminum melting point is near 660 °C; for safety, these boards typically have full heat up to 400 °C, and are heated with tubular heating elements. On the other hand, glass−ceramic plates feature some resistance to concentrated acids or strong bases, being used in devices that support temperatures up to 600 °C, and using a mica heating element. Glass−ceramic has no porosity, allowing easy cleaning and good visibility of the solution under agitation, and has high toughness, but it can break due to mechanical shock. In this case, there are no repair options, with replacement being the only option to consider. Heating is performed by a heating element with a power between 400 and 600 W. There are two typical heating elements: tubular insulated resistance of Ni−Cr or a flat heating element insulated with mica foils. The first type is coated with a carbon steel tube that protects the resistance component from the external environment. Although the carbon steel armor is widespread, it also may be copper or stainless steel.13 The tubular heating elements are commonly used in agitators with aluminum plates in a circular format and are coupled to the hot plate through glass fiber, or direct physical contact. These resistors are also widely used in electric ovens or broilers and can be obtained from repurposed materials; the circular format is not critical. Flat mica heaters are used in compact models and/or heating plates with higher power, are able to heat to about 600 °C, and typically reach the desired temperature faster than the tubulars. They are typically coupled to the plate with glass fiber. Control Circuit Board

There are several strategies to control stirring speed and heating flux. This circuit can vary greatly depending on the model, manufacturer, and manufacture year. Usually reducing the supply voltage of a shaded pole motor reduces the speed, or it reduces the heat flux of the heating element. When the C



Technology Report

Only 3 of 20 restored devices showed a defect in the heating element or in the electric motor. These defects were responsible for a slightly higher maintenance cost due to parts replacement. This fact suggests that in most cases the electronic components, cables, and fuses are the first to degenerate. In situations where the motor is not driven, a routine test can be performed to confirm the source of the problem. We suggest checking for alternating voltage at the terminals that connect the motor or resistive element. This check can be easily carried out using a multimeter set to the alternating voltage function. Normally when the stirring knob is set to the maximum, the observed value will be the root-mean-square (RMS) values of the AC (alternating current) voltage (120 V in North America); when you turn the knob again, the RMS voltage should decrease. If the circuit is sending the correct voltage to the terminals, the motor is likely damaged and must be replaced. Two simple tests can confirm more accurately if there is irreparable motor damage. These tests should be performed with the motor out of the circuit: • With the multimeter in ohmmeter measurement mode, connect the tips to the two motor terminals; if the equipment shows infinite resistance the motor winding is open, unable to operate (as shown in the Supporting Information). • Connect the motor to a variable autotransformer (or a similar variable AC power supply) through a temporary plug and alligator clamps, adjusting the output voltage to 0−60 VAC (as shown in the Supporting Information). The motor must rotate freely and without noises; otherwise, its replacement should be considered. If the motor is damaged, it must be replaced. The motor can be reused from air fryers. These fryers have a motor at their base, responsible for sending warm air that “fries” the food. Small table fans are also an alternative source for obtaining these motors. In some cases, the stirring motor may run but the speed control will not work. Typically the stirrer operates at maximum speed, without adjustment, even after exhaustive attempts to adjust the speed regulation. The same situation can occur with the heater, which may not allow intensity adjustments, and it simply goes to the maximum temperature of the hot plate surface. These cases indicate failure in the control circuit board, and repair or replacement should be considered. To ensure that a heating element is inoperative, the same test sequence used for motor testing is used: • Remove the heating element from the plate and test the resistance. • Connect the component to a variable autotransformer (or a similar variable AC power supply) through a temporary plug and alligator clamps adjusting the output voltage to 0−60 VAC and check if there is any heating (as shown in the Supporting Information). A lack of resistance or nonheating when connected to the variable autotransformer indicates that it is necessary to replace it. Only the resistive element proved necessary to order from specialized companies, because it is difficult to find a grill or electric stove that contains a heating element with similar a model and power as the original equipment. Do not use a water immersion resistor; they are not designed to be operated

Figure 4. Output waveforms of a dimmer circuit with TRIAC from an input of 220 VAC. The output RMS voltage depends of the triggering of the TRIAC.

band; the frequencies of the radiofrequency signals (500,000 Hz in the AM signal case) are very far from the electrical network frequency (60 or 50 Hz), so the construction of the inductors is not critical. In many cases, the inductor can also be reused from the repurposed electronic components from magnetic stirrers or table fans. The circuit has an on−off switch (S1) coupled to the equipment panel. Fuse F1 is rated at 250 V 2 A fuse for the motor circuit, or 250 V and 8 A for the heating control circuit. The circuit also has an indicative LED, and its current limiting resistor R4 must have a power of 0.5 W. All other resistors may be 1/4 W. The configuration of the pin TRIAC is very important. In the case of BTA08, an 8 A TRIAC, be sure not to confuse the terminals.

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HAZARDS

Electrical Safety

The circuit in this design uses alternating current and could be a risk of electrical shock. It is good practice to adhere to appropriate electrical safety precautions. Inspect cords and connections, grounding, and switches before powering on the equipment. No electrical wires or components should be exposed, and electrical tape can be used to insulate any exposures. The magnetic stirrer should be assembled away from flammable materials, and it should be noted that any solutions containing ions will conduct electricity and may cause short circuits.

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DISCUSSION The Supporting Information shows a table that presents an overview of expenses for this type of maintenance. The values in the table refer to new parts and are useful for a feasibility analysis. In general, the repair is economically advantageous even in situations where it is necessary to change the motor and the heating element. D



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under air, have high wattage (>1200 W), and will not be useful for this purpose. A considerable number of companies produce resistive heaters of various formats and wattages, so it is not difficult to find a supplier for this item. The use of a TRIAC from the BTA series from STMicroelectronics is suggested, as they present the insulated body of the other pins. However, other TRIACs such as Texas Instruments TIC226D or TIC236D, whose body is attached to pin 2, may be used, provided that additional care is taken to adequately insulate the component body and to avoid short circuit, since in this application pin 2 is connected to the output voltage. It is important to note that TRIACs can become very hot depending on the load they control (such as resistive elements), and must be coupled to a heat sink. Heat sinks for these components are also marketed in electronics stores, or repurposed from old computer power supplies. In this case a simple heatsink for the TO-220 components is sufficient to maintain thermal stability. The power cable replacement is very useful for preventive maintenance because the old cable connectors can become severely oxidized and no longer conduct electricity. The fuses must be replaced with the same voltage and current values as the originals. These values are marked on the metal part of the fuses or may be obtained in the product manual. All magnetic stirrers repaired according to this proposal are now in daily use; many have been operating for more than two years and are fully operational.

REFERENCES

(1) Mayer-Scholl, A.; Pozio, E.; Gayda, J.; Thaben, N.; Bahn, P.; Nǒckler, K. Magnetic Stirrer Method for the Detection of Trichinella Larvae in Muscle Samples. J. Visualized Exp. 2017, 121, e55354. (2) Dominguez, P. N.; Lehner, F. T.; Michelmann, J.; Himmelstoss, M.; Zinth, W. A magnetic stirring setup for applications in ultrafast spectroscopy of photo-sensitive solutions. Rev. Sci. Instrum. 2015, 86, 033101. (3) Cost of a stirring hot plate on Amazon.com. https://www. amazon.com/Thermo-Fisher-Scientific-SP88857100-Hotplate/dp/ B01HJX9NOY/ref=sr_1_27?s=industrial&ie=UTF8&qid= 1527428079&sr=1-27 (accessed Sep 2018). (4) Valavanidis, A.; Vatista, M. Indoor Air Quality Measurements in the Chemistry Department Building of the University of Athens. Indoor Built Environ. 2006, 15, 595−605. (5) Ellis, J.; Kanamori, S. An inexpensive magnetic stirrer. J. Chem. Educ. 1973, 50, 137. (6) Deckey, G. Regulating the speed of a magnetic stirrer. J. Chem. Educ. 1982, 59, 73. (7) Bennett, C.; Dyer, J. A. A novel and cheap magnetic stirrer. J. Chem. Educ. 1992, 69, 415. (8) Guidote, A. M., Jr.; Pacot, G. M. M.; Cabacungan, P. M. LowCost Magnetic Stirrer from Recycled Computer Parts with Optional Hot Plate. J. Chem. Educ. 2015, 92, 102−105. (9) Mercer, C.; Leech, D. Inexpensive Miniature Programmable Magnetic Stirrer from Reconfigured Computer Parts. J. Chem. Educ. 2017, 94, 816−818. (10) Pessina, G.; Morra, E. Optimization and Design of the Shaded Pole Single Phase Asynchronous Motor. IEEE Power and Energy Society General MeetingConversion and Delivery of Electrical Energy in the 21st Century, Pittsburgh, PA, July 20−24, 2008. DOI: 10.1109/PES.2008.4596022. (11) Newborough, L.; Newborough, M.; Probert, S. D. Electronically commutated direct-current motor for driving tube-axial fans: A cost-effective design. Appl. Energy 1990, 36, 167−190. (12) Salvage Neodymium Magnets from an Old Hard Drive. https://makezine.com/2015/12/10/salvage-neodymium-magnetsold-hard-drive/ (accessed Sep 2018). (13) Selfa Grzejnictwo Elektryczne S.A Product Catalogue. http:// www.selfa.pl/public/www/files/pdf/selfa_ge_katalog_eng.pdf (accessed Sep 2018). (14) Bhattacharya, S. K.; Chatterjée, S. Industrial Electronics and Control, 13th ed.; Tata McGraw-Hill: New Delhi, 2006; pp 297−300.

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CONCLUSION Stirring hot plate restoration is an economical alternative to the purchase of new equipment. These guidelines provide information for a student to use for diagnosis and repair. Magnetic stirrer motors are common to various home appliances and are therefore easy to replace. In situations where the control electronics are inoperative, the circuit board presented in this work is small, simple, and low-cost and provides good performance.

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Technology Report

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00267.

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Repair flowchart, tests and new parts cost list (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lucas F. de Paula: 0000-0001-9428-3926 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Council for Scientific and Technological Development (CNPq) and the Minas Gerais State Agency for Research and Development (FAPEMIG) for financial support. E


Teaching Students How To Troubleshoot, Repair, and Maintain

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