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Failure of Fractional Horsepower Ventilation Fan Motors

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Abstract

Fractional horsepower motors are used to power exhaust fans such as those found in bathrooms and oven hoods. There was an increase in fires attributed to these motors shortly after the year 2000. Still, many argue that these motors cannot cause fires because they contain a thermal cutoff (TCO) unit that shuts off electrical current when the TCO reaches a specific temperature. In this paper we describe an unsafe failure mode that can occur after extended use. As fans age, the bearings wear out, resulting in heating of the coil wires. Excessive heating results in break down of the wire insulation, which can lead to a short between wires and create an autotransformer configuration. An autotransformer can support high current in only part of the wiring, resulting in resistive heating sufficient for ignition. With the low thermal conductivity of the coil (\(0.0162\,W/(\hbox {cm}\,{^{\circ }}\hbox {C})\)) and the poor performance of the TCO (no cutoff up to \(260{^{\circ }}\hbox {C}\)), a large temperature gradient is possible. The gradient allows for ignition in one part of the motor without activating the TCO. We experimentally validated the proposed mechanism in an isolated motor and a complete fan assembly resulting in ignition after 18 s and 23 s. The assembled fan burned for more than 6 minutes before we extinguished the flames for safety reasons. Our results are consistent with field results from actual fires.

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Funding

Although many of the tests reported herein were funded as part of specific fire investigations, production of this paper was funded by the authors in the interest of public safety.

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Authors and Affiliations

  1. Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA, 02139, USA

    Bryan Haslam, Donald Galler & Thomas W. Eagar

Authors
  1. Bryan Haslam
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  2. Donald Galler
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  3. Thomas W. Eagar
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Corresponding author

Correspondence to Bryan Haslam.

Appendix

Appendix

See Figs. 16, 17, 18, 19, 20 and 21.

Figure 16
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Thermal degradation of outer coil insulation at different temperatures for 10 min

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Figure 17
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Thermal degradation of wire insulation in practice

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Figure 18
figure 18

Diagram of experimental setup to measure temperature gradient. This is a cross-sectional view, so each colored layer is a coil. The voltage and current of each of the three coils was measured

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Figure 19
figure 19

Photograph of experimental setup with IR heaters and motor in the center. Diagram of setup with dimensions and heat flux

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Figure 20
figure 20

Diagram of motor with shorting wire to create autotransformer. The red line is an additional wire which is electrically equivalent to the adjacent wires shorting

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Figure 21
figure 21

Temperature gradient induced by locked rotor test on new fan

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Haslam, B., Galler, D. & Eagar, T.W. Failure of Fractional Horsepower Ventilation Fan Motors. Fire Technol 54, 967–994 (2018). https://doi.org/10.1007/s10694-018-0720-4

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  • DOI: https://doi.org/10.1007/s10694-018-0720-4

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