Introduction; Vfd And Ac Induction Motor Fundamentals - Siemens LMV5 Series Technical Instructions

Technical Instructions LMV Series
Document No. LV5‐1000
Introduction
The LMV52 features an integrated, closed‐loop Variable Speed Drive (VSD) control that is typically used
to ramp the speed of the combustion air blower with firing rate. This is accomplished by transmitting a
4‐20 mA signal from the LMV52 to the VSD, and then reading the speed of the motor via a motor‐
mounted, safety‐rated encoder wheel (speed wheel) and speed sensor. By using the speed wheel and
sensor, the LMV52 can positively verify both the speed and direction of the blower, thus ensuring
proper VSD operation.
Blower speed and direction of rotation have a large impact on the airflow delivered to the burner, and
thus the fuel‐air ratio. The most common type of VSD, a Variable Frequency Drive (VFD), is typically not
safety‐rated and will typically not fail in a safe manner (a VFD failure will typically cause the combustion
air blower to slow down or stop, causing the burner to go rich). The combustion air pressure switch
offers only a small amount of protection in a VFD application, since the switch must be set to allow low
fire operation when the blower is spinning slowly and the blower output pressure is low. The motor or
blower shaft‐mounted, safety‐rated speed wheel and sensor ensure that a VFD failure will be quickly
detected and the burner will shut down in a safe way.
VFD and AC Induction Motor Fundamentals
VFDs are typically connected to a three‐phase alternating current (AC) induction motor that is used to
power the combustion air blower. Modern VFDs operate by taking single‐ or three‐phase AC and
rectifying this power to high voltage direct current (DC) for the DC bus. The AC power is typically
rectified to DC with banks of diodes. The DC bus feeds a bank of Insulated Gate Bipolar Transistors
(IGBTs), and a microprocessor is used to fire the IGBTs in a way that the voltage and frequency of the
modified sine waves can be controlled. This is done for each of the three phases on the VFD output.
The microprocessor varies the voltage and frequency of the modified sine waves in response to a signal;
in this case, the 4‐20mA input.
By design, a three‐phase AC induction motor will attempt to approximately synchronize its speed with
the frequency of three‐phase power that it is being fed. Thus, if the frequency can be adjusted, so can
the speed of the motor. As their name suggests, three‐phase induction motors generate magnetic fields
in the rotor of the motor by using induction rather than by using slip rings or brushes. The advantage of
this type of construction is very low maintenance, and a small disadvantage is a phenomenon called slip.
Slip is defined as the difference between the theoretical speed at a given AC frequency and the actual
speed at a given AC frequency. Slip increases as the load on the motor (torque output) increases.
Three‐phase AC motors that do not have slip are referred to as synchronous motors, since these motors
exactly synchronize their speed to the frequency of the incoming AC power. This type of motor is not
typically used on blowers, but is mentioned as a comparison to the AC induction motor. A truly
synchronous 2‐pole motor will spin at exactly 3600 RPM if it is fed exactly 60 Hz. A truly synchronous 4‐
pole motor will spin at exactly 1800 RPM if it is fed exactly 60 Hz. In contrast, a 2‐pole, three‐phase AC
induction motor fed 60 Hz will spin less than 3600 RPM, and how much less is determined by how
heavily the motor is loaded and how much slip that loading causes.
Section 5 Page 2 SCC Inc.
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