Siemens LMV5 Quick Start Manual page 185

Section 7 VSD Control
Overview of VSD / VFD / AC Induction motors 7-1
The LMV52 features an integrated, closed loop VSD (Variable Speed Drive) controller that is
typically used to vary the speed of the combustion air blower.
Since the LMV52 has a VSD control, it is capable of controlling many different types of
drives and Motors. Both DC drives and motors as well as AC drives and motors can be
controlled with the LMV52.
By far the most common type of drive and motor that is used with the LMV52 VSD control is
the VFD (Variable Frequency Drive) driving an AC induction motor.
This section will focus on the unique characteristics of a VFD driving an AC induction motor
and will also explain the effects of a centrifugal blower on this arrangement.
Throughout this section the terminology VSD control is used since it is a more generic term,
and is also the terminology used on the AZL52.
A VFD is basically a specific type of VSD.
Most modern VFD's operate in basically the same way. They take 3 phase AC power,
rectify this power to DC, and then they output pulsed DC to an AC motor.
Thus, pulsed "square wave" DC is used to approximate a sine wave on each of the motors
three phases. This approximation of a sine wave is adequate to run the motor.
As one might expect, the frequency and the amplitude of the square waves is controlled by
the VFD microprocessor, and can be regulated to control the speed of a synchronous motor.
The motors that are used on most burner blowers are AC induction motors. These motors
are called induction motors since they do not have slip rings or brushes to power the
electromagnets in the rotor.
Instead, AC induction motors use principles of induction to power the electromagnets in the
rotor. Induction is very attractive since this means that there are no slip rings or brushes to
wear out, and the only part of the AC induction motor that really wears are the rotor (shaft)
bearings.
The slight disadvantage of using induction motors is a phenomenon called slip.
In the absence of slip, an AC motor supplied with 60 Hz (Hertz) three phase power having:
two poles per phase would turn at exactly 3600 RPM
four poles per phase would turn at exactly 1800 RPM.
These two hypothetical "non slip" motors are called synchronous motors since they exactly
synchronize themselves with the frequency of power they are supplied with.
In contrast to no-slip synchronous motors, AC induction motors have slip, so they are only
semi-synchronous.
Furthermore, the amount of slip depends upon how heavily the motor is loaded. Slip will
increase as the motor's load increases.
LMV5 QSG Rev 4 Sec 7 Pg 1 www.scccombustion.com