Using VFDs to save energy.
Variable Frequency Drives as Energy Savers.
A variable frequency drive (VFD) changes the voltage and frequency applied to an induction motor to change it's speed.
The VFD rectifies the applied supply voltage to a DC Bus. This DC voltage is then chopped up to create a pseudo sinusoidal current that is applied to the motor.
The rectifier causes harmmonic currents to be drawn from the supply resulting in a current distortion (THDi) of between 35% and 85% depending on the design of the input circuit.
The output circuit switches at a high frequency to generate a PWM output voltage waveform.
The high frequency switching results in switching losses in the VFD and EMC noise that is injected into the motor and associated equipment.
The operation of the VFD causes increased losses in the supply and in the motor due to the current and voltage waveforms, plus there are additional losses in the VFD itself, so for a fixed speed constant load application, there are additional losses caused by the addition of the VFD.
VFD losses are typically in the order of 3% to 5%.
The addition of a VFD to a motor operating at rated speed continuously will cause an increase in energy losses and thereby increase the energy wasted.
Electrically, the VFD and motor combination will always waste more energy than a motor on its own, but in some cases, the driven load has high losses that can be reduced by slowing the motor down.
Centrifugal applications such as pumps and fans can be flow controlled by throttling the flow or providing some form of flow constriction.
This is commonly done using a valve for a pump, or shutters for a fan.
Throttling a fan or pump, causes a reduction in flow, but causes an increase in pressure or head that the pump or fan is working against.
The constriction causes an energy loss. Energy is equal to the flow times the pressure and the constriction has a pressure loss across it. The energy loss in the constriction is the flow times the presure drop. The actual losses for different flows are dependent on the performance curves of the pump or fan.
In the case where the flow is throttled back to half flow, the pressure drop across the constriction will be greater than half the pressure at the output of the fan or pump resulting in an energy loss greater than the work being done. The efficiency is less than 50%.
Altering the speed of a fan or a pump, is the same as changing the size of the fan or pump impellor and operating them at full speed.
Reducing the impellor size reduces the flow for a given pressure andso reduces the enrgy consumed and the work output. By reducing the impellor size, or reducing the speed, there is no need for a constriction device to reduce the flow. This eliminates the losses in the constriction and increases the operating efficiency.
The use of a VFD to slow the speed of the motor and load can reduce the mechanical, hydraulic or pneumatic losses and thereby save energy.
A VFD can be used to control the speed of the motor and load, and in some cases, where operation at full speed results in excess losses, this can reduce the energy wasted.
Where the operation requires the motor to operate at a fixed speed only, the motor and coupling should be selected for best performance and the VFD will increase the losses.
Examples of applications where energy can be saved.
- Water pump operating under constant pressure with variable flow. i.e. town water supply.
- Irrigation pump with multiple selectable outlets.
- Airconditioning Air Circulation fans where the air flow changes with temperature differential.
- Refrigeration compressors where the refrigeration load changes.
Examples of applications where energy will not be saved.
transferring from one tank to another at constant flow.
- Irrigation pump suppling a single pivot irrigator.
- Granulators, chippers and shredders where the losses are low relative to the work done.
The maximum Energy that can be saved is only a portion of the energy wasted.
The energy wasted in the motor is very small, so the wasted energy must be mechanical (friction), hydraulic or pneumatic.
VFDs are commonly sold as a means of energy savings in applications where the increased losses result in an increase in energy wasted rather than a savings.
Over zealous sales people often claim savings as high as 40% when the load is operating at high efficiency and energy savings are impossible.
Will a VFD save energy in my application?
- Determine what the losses are.
- Can the load / process operate at reduced speed for a portion of the time?
- Will slowing the load reduce the losses more than reducing the work done?
- Should the load be slowed permanently by a mechanical means (fixed speed application)?
Only a portion (15% - 25%) of motor aplications will benefit from the application of a VFD with savings in energy.
For energy saving ideas, see energy saving techniques