Emission of ultrafine copper particles by universal motors controlled by phase angle modulation

Abstract

Laboratory experiments were performed to investigate the emission of particulate matter by electric universal motors controlled by phase angle modulation. The motor of a commercial professional vacuum cleaner (PVC) served as a standard source of the particles. Total number concentrations were measured with a condensation particle counter (CNC), number size distributions with a scanning mobility particle sizer (SMPS) and with a portable optical aerosol spectrometer (PAS). Impactor samples were collected to determine the size-segregated concentrations of mass and of copper. The total number concentration produced by the motor operated at full power input ($\sim 0.9\mathrm{kW}$, rotational speed $\sim 23,500{\min }^{-1}$) in a test room (volume $4.2{\mathrm{m}}^3$, air exchange rate $0.021{\min }^{-1}$) saturates rapidly at $3.0\times {10}^{11}\mathrm{particles}{\mathrm{m}}^{-3}$ within 15 min of operation. The concentrations of the total mass and of copper were, at an air exchange rate of $0.009{\min }^{-1}$, 146 and $42\mathrm{\mu }\mathrm{g}{\mathrm{m}}^{-3}$, respectively. With a simple box model these concentrations correspond to emission rates of the motor of $2.7\times {10}^{10}\mathrm{particles}{\min }^{-1}$, $5.4\mathrm{\mu }\mathrm{g}{\min }^{-1}$ of total mass and $1.6\mathrm{\mu }\mathrm{g}{\min }^{-1}$ of copper. When phase angle modulation was employed to reduce the power input of the motor, the number concentration was found to be nearly proportional to the jump in the applied voltage and the total number concentration increased by up to a factor of 5 at a rotational speed of $\sim 21,300{\min }^{-1}$ (power input $\sim 0.5\mathrm{kW}$). The number size distributions indicated that the phase angle modulation increases mainly the emissions of ultrafine particles (UFPs) and the impactor samples suggest that these UFPs consist nearly entirely of copper.

Introduction

Epidemiological studies (Katsouyanni et al., 2001, Peters et al., 2001, Samet et al., 2000, Schwartz, 1999) have associated chronic and acute health effects with airborne particulate matter (PM) and there is evidence that fine $(d<2.5\mathrm{\mu }\mathrm{m})$ and ultrafine particles (UFPs) $(d<100\mathrm{nm})$ are most relevant to this issue (Ibald-Mulli, Wichmann, Kreyling, & Peters, 2002). Hence, it is required to measure and reduce the personal exposure to PM, particularly to fine and ultrafine particles. To assess this personal exposure, it is essential to consider not only ambient air but also indoor air because people spend approximately 85% of their time indoors, and there is clear evidence that a considerable part of the indoor exposure to PM is due to indoor sources such as cooking, smoking and vacuum cleaning (Afshari, Matson, & Ekberg, 2005). As for the vacuum cleaners, it is well known that large amounts of UFPs can be emitted by the motor itself (Lioy, Wainman, & Zhang, 1999), and these particle emissions appear to be related to the frequently used type of electric motor.

This motor type is the electric universal motor, which is essentially a motor having the stator windings in series with the rotor and which can be used alternatively with AC or DC. Universal motors combine the advantages of high starting torque, small size, high number of revolutions (up to more than 30,000 rpm) and low costs. Hence, universal motors are also quite common, not only in vacuum cleaners, but also in other domestic appliances such as toys, hair dryers, blenders and do-it-yourself electric tools such as power drills.

During the operation of a universal motor, brushes made of graphite slide over commutator contact bars made of copper. This movement causes the formation of particles not only by mechanical abrasion, but also by brush sparks. Indeed, spark discharging between two solid electrodes is a suitable and well-known method for the production of UFPs of carbon, metal and metal oxide (Helsper et al., 1993, Roth et al., 2004) and spark generation occurs at voltages as low as 100 V (Horvath & Gangl, 2003).

Continuous speed control of appliances with universal motors is frequently desirable, and hence the electric power applied to the motor must be controlled. As for AC, power control is very frequently accomplished using phase angle modulation, so that the commutator is not just exposed to the smooth sinusoidal alternating voltage but to a rapid jump of the voltage at the ignition angle. Evidently, this causes high induction voltages—unlike the power control by variable transformers for example. Hence, for a universal motor operated with phase angle modulation, a more intense formation of sparks and consequently an enhanced emission of particles can be expected. Having in mind that already the universal motor running on sinusoidal AC might be a major indoor source of particles (Lioy et al., 1999), the particle emission of an universal motor with phase angle modulation deserves a closer investigation, and to the authors’ knowledge, experimental studies referring to this matter have not yet been performed.

An additional aspect is that the particles emitted from universal motors contain—according to the material of the commutator—copper (Lioy et al., 1999) and it may be suspected that these copper emissions are related to the recently reported high indoor/outdoor ratios of copper concentrations (Lai et al., 2004). Copper is of particular concern with regard to potential adverse health effects because—as a transition element—it is suspected to release free radicals in the lung fluid via the Fenton reaction and to cause cellular inflammation (Donaldson et al., 1997). Moreover, UFPs made of copper can induce severe toxicological effects and heavy injuries of kidney, liver and spleen of mice (Chen, Meng, Xing, et al., 2006). Hence, the knowledge of the size distribution of copper particles in indoor air could be quite important.

The first objective of our experiments described here was to examine the emissions of an ordinary commercial universal motor, without power control, both in terms of the produced particle numbers and the particulate mass. The second and main objective was to investigate the effect of power control based on phase angle modulation on the emissions. The third objective was to quantify the copper content of the emissions as a potential cause of toxicological effects.