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Product category: Industrial Drives/Controls
News Release from: Invertek Drives | Subject: Optidrive
Edited by the Engineeringtalk Editorial Team on 19 February 2008

Variable speed drives for fans and pumps

It is well known that variable-speed drives can make significant savings in the running of pumps and fans: but how exactly are these energy savings made?

Fan and pump applications have been an important part of the variable speed drives industry for many years, where the benefits of energy saving, improved enhanced control and performance are well known However, with ever increasing energy costs, and the worldwide focus on the environment, the market for inverters in these types of applications continues to grow

Centrifugal fans and pumps have a particular characteristic where the torque required to turn them is proportional to the square of the speed.

The power required is then proportional to the cube of the speed.

From this, it can be seen that a reduction in the fan speed gives a much larger reduction in the power consumption, and a 20% reduction in speed can save up to 50% of the energy consumption.

Many fan and pump installations operate at a constant flow rate, even though the actual demand for them varies widely.

The required flow is then adjusted using mechanical valves or dampers, which reduce the overall system efficiency.

Invertek's Optidrive E2, Optidrive Plus and Optidrive VTC all feature built in PI or PID control, designed so that the drive can automatically maintain a constant pressure or flow set point, based on a signal provided from a feedback transducer.

For example, where a pump is used to pump fluid at a constant pressure into a system where the demand may vary, a transducer can be installed to measure the actual pressure, and the Optidrive can be preprogrammed with the desired setpoint.

The Optidrive will then adjust the pump speed to maintain the pressure automatically, providing a simple, reliable means to achieve consistent energy savings, particularly when compared with a pump simply running at full speed continuously, controlled by throttle or recirculation valves.

Automated systems, which remove the need for an operator to manually adjust the speed, almost always provide the optimum energy savings.

Invertek's Optidrive VTC has been specifically for centrifugal fan and pump applications, and as such has features dedicated to this industry which can further assist the energy saving potential.

The PID controller has an additional sleep/wake function, which allows the drive to automatically shut down if the preset feedback level has been maintained over a programmable time period, and restart when demand requires.

Again, this automatic system, once programmed, removes the need for operator intervention, meaning that the maximum savings are achieved with minimal effort, and also simplifies the installation when retrofitting to existing applications.

In practice, the real world results achieved depend largely on how well the overall system is designed and controlled, its typical operating period and power level.

It's important to consider all these factors when considering energy savings, to ensure that the expected end results are realistic, and not based predictions created with unrealistic data or figures.

Pumping applications in particular can be more complex than it may first appear, particularly where the static head of the system is high when compared with the maximum head available, which will limit the amount the pump speed can be reduced before the head can no longer be maintained.

It's also important to remember that not all fans and pumps are centrifugal type, hence some blowers such as Roots type, or pumps such as PCPs (progressive cavity pumps) or displacement pumps may require a constant torque drive (Optidrive E2, Plus 3GV).

Energy savings and system improvements can still be made with constant torque applications, however the overall potential for energy saving is reduced.

Installing an inverter to a fan or pump, and operating constantly at maximum speed is unlikely to be a cost effective, energy saving strategy, and so the first step should always be to look at the existing installation to determine where and how energy can be saved, and then determine how best to reliably and repeatedly achieve those savings.

Invertek Drives has developed a simple to use energy saving calculator, designed to give a fast indication of the energy that could be saved by the installation of an Optidrive on to a fan or pump control system.

Greater savings will always be achieved where larger kilowatt motors are used, and with system which operate for longer periods of time, eg 15kW or more motors operating for 8 hours per day or more.

Installing Optidrives onto these applications will generally provide the shortest payback period - the time taken to recoup the cost of installing the drive based on the energy saved.

When installing drives onto large, high inertia fans, it's often necessary to activate the "spin start" feature of the Optidrive.

With Spin Start enabled, every time the Optidrive starts, it will automatically perform a prestart test to determine whether the motor is already rotating, and if so, find the speed of rotation and direction.

This becomes vital on any fans where the fan can actually be blown around by drafts or air movement whilst the drive is disabled, whereby attempting to start the already rotating motor effectively applies a short circuit directly to the drive output.

Should the motor be turning, the drive will automatically begin control by matching the output frequency to the motor speed, and then controlling the motor to the correct operating direction and speed setpoint.

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