Electric Motors

Modern technology has reduced the size of motors, increased their expected life and improved their resistance to dirt and corrosion. Other important developments of the last 30 years are brushless excitation for synchronous motors and new two-speed, single-winding, induction motors.

Long-term cost of ownership is the prime factor in the selection of any equipment. Selection should be based on the least expensive, most efficient motor that will meet the requirements. Improper selection increases operating costs. Oversized motors are commonly purchased either because the actual load requirements are not known or because of anticipated load growth. It is important that the motor be sized for all known design and offdesign operating conditions. Gross oversizing is wasteful because motors perform best and cost less to operate (maximum power factor and efficiency) at the manufacturer's rating. The best checks against improper size are careful review of drive requirements prior to purchase and periodic checks of the individual motors in operation.

Serious consideration should also be given to enclosure selection. Many improvements have been made in recent years in both enclosures and insulation. Therefore, it is important to review purchasing practices to make sure they are based on today's technology.

Review the motor requirements and specifications to make sure that all the unnecessary, nonstandard, special features have been eliminated. Each special requirement such as nonstandard mounting dimensions and nonstandard bearings should be eliminated unless it can be demonstrated the feature is cost effective. In actual practice, many special features are specified because of an isolated case of trouble that occurred years ago. Likewise, some special features may become obsolete through changes in refinery or chemical plant practice or through improved manufacturing techniques.

Synchronous and induction motors cannot always be compared on an equal speed basis. In geared applications such as high-speed centrifugal compressor drives (above 3,600 rpm), the most economical induction motor speed is usually 1,800 rpm. The most economical synchronous motor speed for the same application might be 900 or 1,200 rpm, depend ing on the horsepower required. For compressor drives above 3,600 rpm, motor prices must be studied within the total evaluation concept.

For 3,600-rpm compressor drives below 5,000 hp, simplicity of installation almost dictates using the two-pole induction motor. No gear is required, and the overall electrical and mechanical installation is the simplest possible.

Electric motors above 5,000 hp are custom-designed for the specific application, taking into consideration compressor characteristics and the power system parameter limitations.

While logistics favor the use of the two-pole motors, there is a break point in mechanical design between the four-pole and two-pole motors. Because opinions vary concerning the desirability of two-pole motors, those contemplating their use should first investigate the "track record" of the motor supplier in the required size range.

A 5,000 hp break point has been used rather arbitrarily, as larger motors are built, ranging in size to 30,000 hp. Generally as the size increases, the synchronous motor becomes more competitive. However, final selection is not only dictated by the driver economics above, but includes the power system as well.

Voltage

The least expensive motors, typically below 200 hp, are low voltage (less than 600 volts). Above 500 hp and up to 5,000 hp, the preferred voltage is 4,000 volts for use on a 4,160 volt system. It should be noted that motor nameplate voltages are commonly about 5% less than the nominal voltage of the system they are used on. However, synchronous motors that may operate at a leading power factor, in general, have a nameplate voltage equal to the nominal system voltage. As the motors become larger, coil capacity becomes more of an issue and increased space is available for insulation, so it becomes economically practical to use higher voltage motors and controls. It is apparent that, as the size of a refinery or chemical plant increases, the distribution system increases, and it is necessary to go to higher system voltages. For distribution system voltages above 5,000 volts, the usual practice is to transform down to 2,400 or 4,160 volts for large motors. Cable and transformer costs frequently make it economical to select motors with higher voltages such as 13.8 kilovolts. Numerous 11 to 14 kV motors are now in service, even outdoors.

Lightning and switching surges that damage motors are related to high-voltage motors. The insulation level of motors is below that of many other types of apparatus such as transformers, switchgear, cables, etc. Because of the low insulation level, the system lightning arresters will not protect the motors adequately. Special surge protection equipment is often needed, particularly for large motors with long cores. Surge protection consists of special low-discharge voltage arresters in parallel with surge capacitors. The capacitor slopes off the wave front to reduce the motor winding turn-to-turn voltage, and the arrester limits the voltage rise to a safe value. Surge capacitors are used without the arrester in some applications To give maximum protection, the arrester should be mounted at the motor terminals.

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