The modern Class II Biosafety Cabinet (BSC) was developed in the early 1960’s as a result of the increased availability of High Eciency Particulate Air (HEPA) lter technology. At the time, the motor of choice to drive the cabinet’s blower was the Per- manent Split Capacitor (PSC) type. The PSC motor, which is an alternating current (AC) motor, oered manufacturers an inexpensive power source whose speed could be electronically controlled to allow for airow adjustment as the HEPA lter(s) loaded.
The PSC motor is known as an induction type. In the motor, stationary windings (known as a stator) surround the rotat- ing part (rotor) composed of iron or steel. As current passes through the stator, it induces a magnetic eld in the rotor, caus- ing it to rotate towards the shifting eld in the stator. Because a magnetic eld must be induced in the rotor, the PSC motor is asynchronous, with the rotor constantly lagging behind the elds being created in the stator. As a result of this asynchro- nous operation, and the induction of a constantly changing magnetic eld in the rotor, the PSC motor is inecient, and generates high amounts of waste heat. Attempting to control the blower speed by reducing its voltage only increases the inef- ciency of the PSC motor.
Direct current (DC) motors are more ecient than their AC counterparts. In a typical DC motor, the stator in an AC motor is replaced with permanent magnets. The rotor then has a series of windings around it. When current is applied to the motor, a
magnetic eld is created in some of the windings of the rotor, causing it to rotate toward the magnetic eld created by the permanent magnets. Brushes in contact with a commutator allow the current, and thus the magnetic eld in the rotor, to progressively shift from winding to winding, forcing the rotor to keep rotating.
The greatest drawbacks of brushed DC motors are the brushes and commutator, which wear each other down, eventually caus- ing motor failure. With the development of greater micropro- cessor power in the 1970s and 80s, the stage was set for an even more ecient type of DC motor — the Electronically Com- mutated Motor (ECM), Figure 1.