Introduction
Wave soldering using a linear electromagnetic motor is a known principle for decades. These machines make use of a hollow parabolic wave over which boards fixed in carriers or pallets are soldered. The wave should be covered with either a thin oil film or nitrogen to optimize the process and to reduce dross formation. New electromagnetic motor systems make it possible to create the common 'flat' solder waves up to a wave height of about 8-mm. The advantage of this way of creating solder waves is that no mechanical pumps are necessary.
This means that there are no moving parts at the solderpot and one has better access to the solderpot. On the other hand the setting of the wave height is not as direct as with an impeller pump. Also the pump efficiency is very poor which limits the capacity for height massive waves. Induction pumps have due to numerous types of losses a restricted efficiency to values drastically below mechanical pumps.
Principle of electromagnetic pumps in soldering equipment
A liquid metal electromagnetic pump operates by passing an electric current through the liquid metal in a direction transverse to an applied magnetic field and the longitudinal axis of the liquid metal flow pipe.
The pump should be given time to get its equilibrium temperature during operation. One should start with a low power setting (10%) and slowly rise this to the demanded level. If the necessary power would directly be applied, parts of the pump coils will get a too high temperature and might be damaged. Also one should take into account that a temperature drift in the coils will also affect the ohmic resistance and so the pump stability and effectivity. The input current through the coils should be limited in order to limit the coil temperature.
There is a response time delay to changes in the supply current. This means that the wave height will not immediately react if one changes the pump settings like it does with a mechanical driven pump. Plugging oxides can restrict pump capacity. Since the active part of the electromagnetic pump is a relatively small gap this might be a point of concern.
Instability arising in electromagnetic pumps
Electromagnetic pumps are sensitive for instability as the slip increases. The instability is characterized by low frequency pressure pulsation, non-uniform magnetic field along the azimuth, wide-frequency vibration of the pump and pipe, fluctuation of winding voltage and current. In addition, the non-uniformity causes a decrease of the developed pressure and sometimes a dip on the P-Q curve.
The performance of the electromagnetic pump clearly show in investigations that the end effects give rise to the obstructive force to the liquid metal flow at inlet and outlet ends of the pump. In addition, the efficiency is found to be lowered due to end effects, especially when the speed of the flow is high.
Maintenance
Although the mechanical construction of an electromagnetic pump is quite simple, due to the fact that the motor consists mainly of a narrow gap, clogging by dross particles might create problems. The access to this type of motors for the necessary cleaning might not be as easy as the pump motors that are impeller driven.
Conclusions
Electromagnetic pumps were in principle developed for the nuclear industries but are also used in soldering machines. They have the advantage that no mechanical parts are necessary to create a solder wave. Depending on the necessary capacity they need a start procedure to prevent overheating of the pump windings.
The result of a change in the settings for the pump capacity react not as direct as with a mechanical impeller pump. Changes in the pump setting might create instability. With a good control the effect of these potential drawbacks can be minimized. Although the pump has a simple mechanic design, access of the pump gap for maintenance might not be easy.
References
Magnetohydrodynamic instability in annular linear induction pump. Part I. Experiment and numerical analysis, ARASEKI Hideo et al. Nuclear engineering and design (Nucl. eng. des.) ISSN 0029-5493 CODEN NEDEAU. Source: 2004, vol.227, no1, pp. 29-50 [22 page(s) (article)] (11 ref.).
The magnetic field and performance calculations for an electromagnetic pump of a liquid metal, Suwon Cho et al 1998