Facility Managers are continually being challenged to be environmentally conscious and seek ways to optimize our systems within our businesses. The boiler room is a great place to uncover opportunities to increase efficiencies and reduce the impact on our environment. Variable Speed Drives (VSDs), also known as Variable Frequency Drives, are an example of support devices that can be applied to boiler feed pump motors, and combustion air blower motors to increase efficiency.
The traditional method for energizing 3-phase AC squirrel-cage induction motors has been with a full-voltage motor starter. A control voltage is used to energize the mo
tor stater, which in turn applies higher voltage 3-phase power to the motor. When power is applied, it takes a short period of time to run the motor up to a full synchronous, steady-state, operating speed.
When the motor first starts, most of the power is converted to thermal energy and mechanical stress on the motor until full synchronous speed is reached, where most of the power is converted to rotational energy. This is shown on a typical induction motor starting current curve:
The variable speed drive (or VSD) was introduced as a method of controlling the speed of a motor. The variable frequency power supply uses solid-state components to produce a pulse-width modulated current that varies the power and frequency supplied to the motor. In turn, varies the speed of the motor.
The Laws of Affinity for centrifugal pumps and fans states:
- Air or water flow is proportional to the (pump or fan) motor speed,
- Air or water pressure is proportional to the square of the (pump or fan) motor speed, and
- Motor power is proportional to the cube of the (pump or fan) motor speed.
Traditional fan applications use a constant speed motor, along with variable inlet vanes or modulating outlet damper to control the pressure or flow of air. Similarly, traditional pump applications use a constant speed motor, along with a modulating control valve to control the pressure or flow of water.
VSDs are used in HVAC pump and fan applications to vary the pump or fan speed according to demand where the pump or fan may not be required to operate at 100% capacity for 100% of the time. In these cases, the VFD can ramp down the frequency and voltage to the motor to meet the load requirements of the process. As the load requirements of the process change, the VFD can simply ramp the motor speed up or down to meet demand.
VSDs save energy by enabling electric motors to operate at less than full speed.
Reducing motor speed by 25% decreases energy consumption by nearly 60%, while reducing motor speed by 50% decreases energy consumption by nearly 90%.
Process efficiency can be improved when a fan or pump use closed-loop constant-pressure or flow control which can be achieved with feedback from a pressure or flow transmitter. Also, due to the inherent soft-start function of a VSD, motor starting current can be reduced to decrease thermal and mechanical stresses. This will increase the service life of the motor.
VSDs should be used on fan and pump applications where process load demands are variable, and not steady-state. Using VSDs in fan and pump applications with motors 5 HP and above, will generally have a payback (return on investment) in under (5) five years. As a rule-of-thumb, VSDs should be used on fans or pumps with motors that are 10 HP and above. VSDs can save a substantial amount of energy and can also extend the service life of 3-phase AC squirrel-cage induction motors.
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