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Variable Frequency Drives

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Analyse the System as a Whole

Since the process of converting incoming power from one frequency to another will result in some losses, energy savings must always come from optimizing the performance of the complete system. The first step in determining the energy savings potential of a system is to thoroughly analyse the operation of the entire system. Detailed knowledge of the equipment operation and process requirements are required in order to ensure energy savings.

VARIABLE FREQUENCY DRIVES Energy Efficiency Reference Guide

The most promising candidates for realizing energy savings are systems for centrifugal fans, blowers and pumps that have been designed to meet peak loads but actually operate at reduced load for extended periods. The following discussion pertains to centrifugal systems, but the same principles can be applied to other systems. Evaluating the energy-saving potential of a centrifugal system should include the following steps:

  1. Develop a complete understanding of the process requirements and the equipment to which the centrifugal machine supplies fluid.
  2. Obtain complete engineering specifications and performance curves for the centrifugal machine.
  3. Obtain specifications for all components of the system, including dampers, valves, ducts or pipes, and heating or cooling coils.
  4. Develop a load/time profile for the system. This step is critical for calculating accurate energy savings. For each component, list annual hours, all flow conditions and the input power to the centrifugal machine.
  5. For each point in the above step, calculate the input power required by a VFD-driven motor and centrifugal machine delivering the same flow. Input shaft power can be determined from the centrifugal machine performance curves.
  6. For each operating point, calculate the difference in power resulting from adding a VFD, and multiply each by the number of hours of annual operation. The difference is the energy savings resulting from installing a VFD.

The above process requires qualified staff with sufficient expertise with all components of the system, from VFD through to the process equipment.

Pump and fan affinity laws govern the relationship among speed, flow and input power. The laws state:

  • flow is proportional to speed
  • pressure increases with the square of the speed
  • power increases with the cube of the speed

When speed is reduced to 75 percent of design speed, flow decreases to 75 percent, outlet pressure decreases to 56 percent, and the input power requirement drops to 42 percent of full-speed values. For a similar flow reduction with a throttling valve or damper, the input power drops to only 80 percent. Note that pressure falls off more rapidly than flow.

VFD and Motor Selection

Once you have verified the energy savings achievable using a VFD in your application, the following topics should be considered when selecting a motor and drive combination.

Is software available to help choose a VFD and estimate savings?

Most VFD manufacturers can provide such software.

Free software is also available. ABB offers Energy Saving Tools: PumpSave for comparing AC drive control against throttling, on/off and hydraulic coupling control in pumps; and FanSave, for comparing AC drive control with traditional flow-control methods in fans.

Will the motor withstand the repetitive voltage stresses from use of a VFD?

General-purpose induction motors are not designed for repetitive voltage overshoots that exceed line voltage plus 1000 volts. With a 230 VAC system, overshoots may not exceed this limit, but with a 575 VAC system, overshoots are likely. Repeated voltage stresses may lead to insulation breakdown and premature motor failure.

To use a VFD with an existing general-purpose motor, additional filtering and transient protection may be required. NEMA definite-purpose motors rated "Inverter Duty" are recommended for use with VFDs. These motors can withstand repetitive voltage spikes that are 3.1 times the rated RMS voltage.

What NEMA design type is best suited for a VFD?

VFD control algorithms are usually optimized to the speed torque characteristics of NEMA design B motors. Use of other design types should be discussed with the VFD manufacturer.

Should I use a motor that has a different pole count (i.e., six-pole instead of four-pole)?

A VFD may be able to provide the required speeds with better performance using a motor that has a different number of poles. This can reduce the inventory of spares. The use of a bypass, however, may limit the number of choices of pole count. Be sure to discuss your options with the VFD manufacturer.

Is motor cooling adequate for extended operation at very low speeds?

Cooling often depends on motor speed, such as with totally enclosed fan-cooled (TEFC) motors. To meet constant torque loads, therefore, a motor should not be operated at less than 30 percent speed without additional cooling. Consider a larger motor, constant speed cooling or a totally enclosed non-ventilated (TENV) motor for these conditions. Motor thermal protection devices will prevent high-temperature damage when motors operate continuously at very low speeds. With variable torque loads such as centrifugal machines, the rapidly decreasing power at low speed reduces cooling problems.

Will harmonics affect nearby sensitive equipment?

Additional line filtering is often required to reduce the propagation of harmonics and radio frequency interference (RFI) to other equipment. Short leads between the motor and the VFD help minimize RFI propagation. When leads are longer than 15 metres (50 feet), reactive filters are recommended. Motor leads should also be enclosed in a rigid conduit to reduce RFI.

Is the VFD starting torque and acceleration/deceleration adequate for the load?

The VFD breakaway torque is less than the motor locked rotor torque and is limited by the VFD maximum current rating. This current rating also limits the rate of load acceleration. Acceleration, deceleration and maximum current are user-programmable.

Can a VFD be used for all types of loads?

Yes. VFDs for use with constant torque loads should be rated for operation at 150 percent load for a period of one minute. Variable torque loads such as fans and pumps are easier to start, and therefore the VFD overload rating is lower. The drive should be matched to the load.

Does the application have a high static pressure or head?

Applications in which a minimum pressure must be maintained may not be suitable candidates for a VFD. For example, if high pressure is required even at low flow, it may not be possible to significantly reduce pump speed. When speed and flow reduce, so does pressure. For this application, other energy-saving strategies such as parallel pumps may offer more energy savings. Check the pressure limitations in your system. More information on VFD pumping applications is in Variable Speed Pumping - A Guide to Successful Applications, available from the U.S. Department of Energy's Industrial Technology Program.

What type of enclosure is required?

Check the ratings of both the drive and its enclosure to make sure that they are suitable for the climate to which they will be exposed (i.e., outdoor weather protection).

Is speed control accuracy important for my application?

Most VFDs incorporate a user-programmable, constant volts-to-frequency ratio over the operating frequency range of the drive. For more accurate speed control, a flux vector control strategy with either direct or indirect measurement of rotor flux may be required.

Is direction of rotation affected?

The phase sequence of the supply connection to the VFD does not affect the rotation direction of the drive. Changing the phase sequence between the drive and the motor will change the direction of rotation of the motor. Be sure to verify rotation before connecting the drive to the equipment. Some equipment may be damaged if rotation is reversed. If a bypass contactor is used with a VFD, be sure that the rotation direction is correct during bypass operation

Does the application require dynamic braking?

Load braking is usually accomplished by switching in a power load resistor across the DC bus to dump excess energy. This is usually an optional feature that is available only with some drives.

How many motors can be operated on a drive?

More than one motor on a drive is common. All receive the same frequency, so they change speed in unison. Each motor must have its own overload protection.

Do I require remote monitoring or flexible control and set-up software?

You may need drive monitoring and set-up software with RS-485 multi-drop communication. RS-485 allows monitoring and control of several drives from a remote location.

Do I need bypass switching?

Bypass switches are sometimes used so that a motor can operate when connected directly to the utility power supply. This enables operation of the motor when the drive is out of service.

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