Specifying large HP motors for compressors

For large centrifugal compressor applications, turbines will continue to be the prime mover of choice, but electrics have their place. When permits are not available or noise is a problem, an electric prime mover is required. So what are the cost drivers for a medium-voltage motors, and are we over specifying them?

(A TMEIC motor)

The amount of horse power (HP) to start and run the load determines the frame size of the motor. Like most mechanical equipment, the frame is the main driver of cost. What is not always recognized, however, is that some specifications increase frame size.

Accurately determining the frame size begins with the power required to drive the load. But the next factor to consider is the enclosure, the second major cost driver. The industry typically uses three types of enclosures:

Weather Protected II (WPII) is an open motor with baffling and screening for protection against contaminants. It offers good cooling, the smallest frame and lowest cost. However, noise is an issue. Further, WPII is the least reliable motor when reviewed against performance during 20 years of service.

The next type of enclosure is Totally Enclosed Air to Air Cooled (TEAAC). It is cooled using a large air-to-air radiator on top of the motor. The outside air does not circulate through the motor, which is more reliable than WPII. The disadvantage is the lower efficiency of an air-to-air heat exchanger. It typically has to be specified one frame size larger than a WPII.

The Totally Enclosed Water to Air Cooled (TEWAC) has a water-to-air heat exchanger in the motor which makes it more efficient than a TEAAC. It has shown to be the most reliable enclosure, but requires plant water. TEWAC is the favorite of motor designers with its low noise and good cooling. It is in the same frame as a similar-rated WPII. In terms of costs, WPII is the least expensive, and requires the most maintenance. A TEWAC costs 10% to 15% more for the same power rating, while a TEAAC costs about 30% more. Exceptions are 2 pole or 3,600 RPM@60 HZ motors which are rarely used. WPII is not an option for these larger, high-speed motors due to high noise levels.

Service Factor is the percentage of overloading a motor can handle for short periods when operating normally within the correct voltage tolerances. Medium Voltage (MV) motors must be capable of continuous operation at the service factor and still meet all specifications. Therefore, a 1,000 HP motor with a 1.15 service factor is actually a 1,150HP motor. API motor specifications for driven equipment no longer includes Service Factor. Reason: A 15% service factor generally increases the motor to the next largest frame size. Therefore, adding the typical 10% for the driven equipment and 15% for the motor would create a motor that is oversized by 26%.

If the driven equipment has a service factor, users should consider eliminating the motor service factor. The least understood specification is usually listed under starting requirements as 3C/2H or 2C/1H. This is the reference to how many times a motor can be started safely. The “C” refers to the number of times a motor can be started cold (motor temperature at ambient temperature) and the “H” the number of times it can be started hot (motor temperature above ambient but not exceeding its rated load temperature) per NEMA MG1 20.12. API calls out 3C/2H, which means 3 cold (C) and 2 hot (H) starts.

Having the ability to start 3 times in quick succession may be important for certain applications but it comes at a cost. The motor supplier will generally have to increase the frame size to handle the heat generated from each start. This is why some specifiers take exception to the API 3C/2H and only require 2C/1H.

Over the last 30 years, motor operating voltage has gone from 2,300 V to 4,160 V and now it is common for 13kV to be requested. As voltage rises for an electric motor, the insulation requirements for the winding also increase. At 10 kV, it is usually necessary to add corona shielding to insulation systems. That necessitates a larger frame to accommodate thicker insulation and enough copper to meet HP requirements. Even if the incoming power is 13 kV, this may not be the best choice if costs are to be controlled. As well as the price of a second transformer for a 6 kV operation, cabling costs can add up. Similarly, switchgear is less expensive at 6 kV than 13 kV and has fewer safety requirements. For motors under 10,000 HP, therefore, operating at a lower voltage should be considered as a way to reduce CAPEX.

When users give careful consideration to the above factors, however, they can still be tripped up by a long list of options and accessories. While these are generally not cost drivers for motors over 2,000 HP, they are important in smaller motors. Most options have the same cost at 500 HP as they do at 20,000 HP.

Read more in January-February 2016 issue of Turbomachinery International magazine

 (James Nanney is technical sales manager for oil and gas at TMEIC Corporation.)