Labconco PURICARE Procedure Station Models 34812xx - Manual (Page 33)

Page 33 of 197
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 2-3
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.
Controlling the motor’s operation with a microprocessor adds numerous advantages.
Motor speed can be efficiently controlled across a speed range that would damage or