SPIM Requirements

SPIM is used for most of the heating, ventilation, air-conditioning (HVAC) applications. Normally, it has two windings: main and auxiliary while auxiliary winding has more turns than main winding has. To do the adjustable speed control, two voltages must be supplied to the main winding and auxiliary winding with variable frequencies and variable amplitudes and they are subjected to the following constraints:

• Voltage ratio Vaux /Vmain is approximately equal to the effective turns ratio a, Naux /Nmain (»1.37 for the motor used for this project).
  
• Current ratio is I aux /I main=1/aVaux leads Vmain by 90º at rated frequency
  
• Constant V/Hz (voltage frequency ratio) for adjustable speed control

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Previously, the phase shift of Vaux leading Vmain by 90º was done by putting a capacitor in series with the auxiliary winding and then paralleling with main winding. The adjustable speed is done by putting some inductor in series with thewindings. With the developments in power electronics, power converters can be used for generating the required voltages for SPIM. The typical configuration is shown in Fig. 1. Typically, the voltage for the auxiliary winding is higher than that for main winding so that the voltage output for the rectifier plus inverter is not high enough. If the voltage doubler is used instead of the rectifier, the DC bus voltage can be double of the dc bus voltage. Therefore, it is enough to generate required voltages. The drive system is composed of and voltage doubler and an inverter as shown in Fig. 2.

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