Labconco PURICARE Procedure Station Models 31260 - Manual (Page 42)

Product Service 1-800-522-7658 42
ECM Motor
The modern Class II Biosafety Cabinet was developed in the early 1960’s as
a result of the increased availability of High Efficiency Particulate Air (HEPA)
filter technology. The motor of choice at the time to drive the cabinet’s
blower was the Permanent Split Capacitor (PSC) type. At the time, the PSC
motor offered manufacturers an inexpensive power source whose speed
could be electronically controlled to allow for airflow adjustment as the HEPA
filter(s) loaded.
The PSC motor is known as an induction type, for stationary windings
(stator) surround a rotating part (rotor) composed of iron or steel. As
current passes through the stator windings, it induces a magnetic field in the
rotor, causing it to rotate towards the shifting field in the stator. Because a
magnetic field must be induced in the rotor, the PSC motor is asynchronous,
with the rotor constantly lagging behind the fields being created in the
stator. As a result of this asynchronous operation, the PSC motor is
inefficient, and generates high amounts of waste heat. Attempting to control
the blower speed by reducing its voltage only increases the inefficiency of
the PSC motor.
DC motors are more efficient 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 field is created in some of the windings of the rotor,
causing it to rotate toward the magnetic field created by the permanent
magnets. Brushes in contact with a commutator allow the current, and thus
the magnetic field in the rotor to progressively shift from winding to winding,
forcing the rotor to keep rotating.
The greatest drawback of brushed DC motors is the brushes-they wear
themselves and the commutator down, eventually causing motor failure.
With the development of greater microprocessor power in the 1970s and
80s, the stage was set for an even more efficient type of DC motor – the
Electronically Commutated Motor (ECM).
Figure 1-10
In the ECM, the magnets and windings switch position –
the permanent magnet is on the rotor, and the series of
windings are placed around the rotor. The
microprocessor precisely controls the creation of
magnetic fields in the stator, so that the rotor is always
synchronous with the magnetic fields being created in
the stator. As a result, the ECM will always run more
efficiently and cooler than a comparable PSC motor.
Because of the simple, robust construction of the ECM, it
offers far greater reliability and operational service life
than the PSC motor.