Selecting Compressor Motors

The first step in selecting motors for large compressors (1,500 hp and over) is to determine the motor voltage, speed, and enclosure type.

Motor voltage selection is determined by economics and by the availability of adequate system capacity to permit motor starting without excessive voltage drop. Restrictions by the utility or by the size of the plant's generating capacity may limit the maximum drop to less than 20%. In some cases, system dips as small as 5% or less may be the maxi mum tolerable. For example, many utilities insist on limiting dips to this magnitude at the customer interface, and some motors will be installed at a system location that can influence large amounts of critical lighting or other similar loads. Applications of this type will often require some type of soft start. However, in the usual refinery or chemical plant process unit, large blowers and compressors are very seldom started once the plant has been on stream for some time. The undesirable effects normally associated with local drops as high as 20% or more have been tolerable when occurring infrequently.

The motor cost is but one facet of any cost study for selecting voltage level. The study must compare installed cost of motor, starting equipment, transformers, and power and control cables at the various levels under consideration, as well as plant standards.

Higher voltage levels such as 13,200 volts are sometimes more economical overall, although the cost of the motor itself is higher than it would be at 2,300 or 4,000 volts. For example, if the plant has a 13.8 kV distribution system, it can be more economical to install a 13,200-volt motor than to provide the primary switchgear required for lower voltage motors. Operating experience at levels as high as 13,200 volts has been good, but it is not extensive when compared to lower voltages. The number of manufacturers with experience at these levels is more limited. Also, there are motor design considerations that limit the minimum size at which these high voltage levels can be applied. The motor manufacturer probably would not recommend a 3,000-hp, 13,200-volt motor as a first choice.

When the plant distribution voltage is above utilization levels, for instance 23 kV or higher, economics will usually favor the 2,300- or 4,000-volt motor. However, each application has its own peculiarities. An examination of the relative cost of alternate schemes sometimes favors 4,160 volts. This is true particularly with motors in the 4,000 hp and larger sizes, and short circuit levels are above 150 MVA.

Where speed-increasing gears are a consideration, the 3,600-rpm motor is eliminated for all practical purposes because of higher cost, less favorable torque characteristics, and mechanical design considerations.

The motor and gear combination must provide the proper input speed to the compressor. Therefore, speed selection should consider whether the 1,800 rpm, or the 1,200 rpm motor and the corresponding gear provides the more economical combination. Before selecting motor speed, motor characteristics should be obtained for both speeds. The speed-torque and speed-current curves, power factor and efficiency values should be compared and evaluated. The most important considerations in matching the motor to a compressor are:

Some engineers specify motors for across-the-line starting. Motor output torque varies approximately as the square of applied voltage. A 10% voltage drop means a 20% drop in torque, which is enough to keep some drives from ever reaching full speed. This relationship should be borne in mind for operation as well as starting. A lower than required voltage while operating may appear as an overloaded compressor.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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