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The use of Bypass with Frequency converter
Practice in some frequency converter applications has been to specify and install a bypass that allows the motor to operate on line power in the event of a failure of the frequency converter. This paper will explore the history of the bypass solution, as well as the advantages and disadvantages of the practice of installing a bypass.
Bypass details
A bypass can be supplied in different forms: 2-contactor, 3-contactor and a wide range of input devises and control relay logic. See figure 1, for a simplified single line power diagram of the two different types of bypasses. In each case the purpose of the bypass is to operate the motor via a motor starter in case of a frequency converter failure. In the case of a 3-contactor bypass the frequency converter can also be isolated so that the frequency converter can be serviced while the motor is running in bypass mode. The motor starter, or bypass, includes a contactor, overload and some type of power supply, usually a control transformer, that powers the bypass operation and relay logic.
History of bypass
Bypass packages came into being during the first generations of frequency converter development during the 1970's and early 1980's. Though customers could be shown the energy savings potential that frequency converters could supply, high frequency converter failure rates limited their practical installation. To combat the risk associated with higher failure rates frequency converter manufactures began offering bypass packages as part of their solution. During this period a bypass was considered a relatively inexpensive method to ensure motor operation if the frequency converter failed.
Through the years, frequency converters like other electronic devises, have become much more reliable, smaller and economical. In China today the use of bypasses in the commercial industry is virtually unheard of.
Advantages a bypass installation
It is understandable how the custom of installing bypasses developed when frequency converters had failure rates exceeding 100% in the 1980's. Today, frequency converters have reliability rates similar to motors and pumps. In addition, with advancements in control technology and frequency converter capabilities, there are more disadvantages when bypasses are used today. Higher energy costs and higher first cost (in some cases) are examples of these disadvantages.
Bypass details
A bypass can be supplied in different forms: 2-contactor, 3-contactor and a wide range of input devises and control relay logic. See figure 1, for a simplified single line power diagram of the two different types of bypasses. In each case the purpose of the bypass is to operate the motor via a motor starter in case of a frequency converter failure. In the case of a 3-contactor bypass the frequency converter can also be isolated so that the frequency converter can be serviced while the motor is running in bypass mode. The motor starter, or bypass, includes a contactor, overload and some type of power supply, usually a control transformer, that powers the bypass operation and relay logic.
History of bypass
Bypass packages came into being during the first generations of frequency converter development during the 1970's and early 1980's. Though customers could be shown the energy savings potential that frequency converters could supply, high frequency converter failure rates limited their practical installation. To combat the risk associated with higher failure rates frequency converter manufactures began offering bypass packages as part of their solution. During this period a bypass was considered a relatively inexpensive method to ensure motor operation if the frequency converter failed.
Through the years, frequency converters like other electronic devises, have become much more reliable, smaller and economical. In China today the use of bypasses in the commercial industry is virtually unheard of.
Advantages a bypass installation
- Allows the motor to operate on line power if a frequency converter fails.
- Bypass packages allow extra space to provide main or motor short circuit protection, motor disconnect means and other installation options.
- Critical applications design in redundancy. Bypass packages add another layer of redundancy.
- Modern frequency converters are much more reliable than in the past. In the 1980's frequency converter failure rates were over 100%! Today rates are in the neighborhood of 0.25%. In many cases it has been found that bypass components fail at a higher rate than frequency converters.
- Frequency converters operate over a wider voltage range than motor starter.
- Redundancy needs must be carefully considered. Many HVAC application include a redundant pump such as 2 - 100%, 3 - 50%, 4 - 33%, etc. pumping systems. These systems have redundancy built in. By adding a bypass to each of these pumps the costs are increased substantially. Additional complete pumps for redundancy can be added economically with the use of either twin or dualArm pump configurations.
- The integration of two methods of operating the motor complicates control logic.
- Frequency converters have software protection against voltage issues that motor starters do not possess.
- Many frequency converter manufacturers offer a solution for short circuit protection and motor disconnect without the need of a bypass.
- Motors started in bypass mode require up to 6 times the starting current resulting in increased power demand charges and high stresses on motor windings and bearing. High starting current is the major cause of broken rotor bars. NEMA standard mg1 limits starts to 1 per hour from hot and 2 per hour from cold. This limitation is not an issue with frequency converter starting.
- Bypass packages will add substantially to the cost, extending the payback.
- Bypass packages increase the frequency converter solutions RFI because of the added exposed wiring and higher number of connections.
- Bypass packages increase the size of the solution and decrease the possible mounting locations. A bypass package makes it impractical to integrate the controls within the pumping unit and the benefits of such are lost.
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Pumping systems that are optimized for variable loads cannot operate at full speed continuously. To accommodate the bypass the system must operate in a sub optimized mode for the life of the product. Optimizations which can be compromised include:
- Efficiency and capacity optimization by the use electronic load limiting
- Utilization of speeds between induction and synchronous (ie 57 vs 60 Hz) or even higher than synchronous (ie 63 Hz) as allowed by applicable motor standard such as NEMA.
- When an operator changes a frequency converter to bypass there is the risk that the system will not be returned to frequency converter mode and the tremendous energy savings is not realized.
It is understandable how the custom of installing bypasses developed when frequency converters had failure rates exceeding 100% in the 1980's. Today, frequency converters have reliability rates similar to motors and pumps. In addition, with advancements in control technology and frequency converter capabilities, there are more disadvantages when bypasses are used today. Higher energy costs and higher first cost (in some cases) are examples of these disadvantages.
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