Types Of Vfds: Vector And Volt/Hz; Centrifugal Blower Fundamentals - Siemens LMV5 Series Technical Instructions

LMV Series
Types of VFDs: Vector and Volt/Hz
Although there are over a hundred different manufacturers of VFDs, two main types of VFDs are
produced by these manufacturers for use on blower motors. These two types are Vector and Volt/Hz.
Vector VFDs can usually be run in either Vector mode or Volt/Hz mode. Vector VFDs are also typically
slightly more expensive than Volt/Hz VFDs for a given size.
The advantage of Vector VFDs is that they provide more accurate torque control of the motor. This
accurate torque control enables much more accurate speed control of the motor, especially at lower
motor speeds. More accurate speed control of the motor enables more accurate, repeatable control of
the airflow.
As mentioned earlier, the LMV52 employs a safety‐related speed feedback on the blower shaft, thus
continuously checking and adjusting (if necessary) the signal to the VFD to achieve the desired blower
speed within a certain band. The LMV52 can lockout and shut down the burner if blower speed
deviations are large and persist for too long. Due to their increased accuracy, Vector VFDs provide
trouble‐free operation on almost all LMV52 VFD blower applications. Volt/Hz VFDs can work
satisfactorily in some applications, but are not preferred due to their decreased accuracy.
Vector VFDs are typically run in Open Loop Vector (OLV) mode. In this mode, the VFD uses a
mathematical model of the motor combined with extremely accurate, fast scanning of the current and
other data taken from the rotating motor. In reality, Open Loop Vector mode does have feedback, but
the Vector VFD itself does not require a separate encoder to achieve this.
Since Vector VFDs use a mathematic model of the motor, and the design of motors differs somewhat
between motor OEMs, a static or dynamic auto‐tune is sometimes required so that the Vector VFD
"learns" key aspects of the motor it is connected to. A static auto‐tune (motor is not spun) does not
require that the load (blower wheel) be de‐coupled from the motor. A dynamic auto‐tune (motor is
spun) typically requires that the load (blower wheel) be de‐coupled from the motor, which is not
possible or practical in many situations. A dynamic auto‐tune typically generates the best "learning" of
the motor properties. A static auto‐tune is typically all that is necessary if speed control issues are
encountered on a Vector VFD.
Centrifugal Blower Fundamentals
Since a centrifugal blower is the piece of machinery being controlled by the LMV52 and the VFD, a brief
mention of its basic characteristics is warranted. Specifically, there are three fundamental "fan laws"
that a person working on such equipment should be aware of. These are:
1. Air flow varies linearly with the speed of the blower. In other words, the CFM of the blower is
directly proportional to the RPM of the blower.
SCC Inc. Page 7
HOME
Technical Instructions
Document No. LV5‐1000
Section 5