Siemens LMV3 series, Технические инструкции

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LMV Series Technical Instructions
Document No. LV3-1000
SCC Inc.
Page 9 Section 5
Brushless DC Blowers (PWM Blowers)
Another common type of variable speed blower used with the LMV3 is the brushless DC blower,
commonly referred to as a PWM blower. These blowers typically have the variable speed drive and DC
brushless motor integrated into one blower mounted unit. These blowers are typically fed single phase
or three phase AC voltage directly, and use some type of AC to DC rectification to produce the DC
voltage pulses necessary to drive the blower motor.
Unlike a brushed DC motor, field windings in the brushless motor are triggered (commutated) via non-
contact Hall Effect sensors. In addition to commutating the motor, these Hall Effect sensors also provide
a pulse output (typically 2 or 3 pulses per revolution) that the LMV3 can use for blower speed feedback.
For this reason, an external speed wheel with external speed sensor is not typically required for PWM
blowers. Wiring of the speed feedback signal is covered in Section 2 - Wiring.
Brushless DC motors usually do not have the same speed limitations as most three phase blower motors
do. While most three phase blower motors are limited to about 3800 RPM, some DC brushless motors
used in blower applications will spin in excess of 10,000 RPM. This high-speed capability is attractive in
a blower application since more air flow at higher pressures can be generated with a smaller blower.
The LMV3 can read blower speeds up to 14,000 RPM via the Hall Effect sensors in the blower, and this is
not a limitation in most applications.
Accurate speed control of a brushless DC motor can be more challenging as compared to a VFD and a
three phase AC motor. The primary reasons behind this are the electromechanical characteristics of the
motors themselves. As previously mentioned, the speed of a three phase AC motor will follow the
frequency of sine waves (AC power) that is being fed with a small amount of variance due to torque
induced slip. An increased torque (power) demand will cause a small amount of additional slip and will
cause greater amperage draw. Variances in voltage, unless these are extreme, will not cause the motor
to change speed. Thus frequency is the primary variable; voltage and torque are secondary variables. In
brushless DC motors, the motor windings are being fed DC pulses of variable duration (hence these
blowers being called pulse-width modulation - PWM). The width of these pulses determines the
blower’s speed for a given torque output and for a given blower input voltage. All three of these
variables – pulse width, torque output, and input voltage have a substantial impact on the blower’s
speed and can be regarded as primary variables.
Some PWM blowers have internal speed controls that compensate for torque output and input voltage
variances. This is done by taking a commanded speed set point (dictated by the LMV3) and adjusting
the width of the pulse to achieve the commanded speed. PWM blowers having fast updating, properly
tuned internal speed controls typically work well with the LMV3.
The ramp rates of a PWM blower carry many of the same considerations as a VFD with a three phase
motor (see above). Increasing the blower speed (ramping up) is typically not an issue; however, ramp
times may need to be increased when decreasing the blower speed (ramping down), especially if the
blower wheel is heavy and / or the air damper is mostly closed.