Medium-Voltage Motors Ease Limits on Current Capacity and Power
Most electric motors run on supply voltages under 600 V, yet operating at these so-called ”low voltages” limits current-carrying capacity and output power needed for large industrial loads. Current limits pose a special problem during motor start-up. Medium-voltage (MV) ac motors overcome such limits. (See sidebar ”What actually is ‘medium voltage’?”)
MV motors work across a gamut of industries from utilities and raw materials processing to pulp and paper production. User plants tend to be large and sited far from the power source. Applications center on driving large compressors, conveyors, crushers, fans, pumps, and related machines at fixed or variable speeds.
Medium-voltage motors carry an image of giant size, and most do fall into that category. However, models as small as 75 kW (100 hp) have been built. In the MV motor arena much depends on the application. Only in the lower power ranges can MV and low-voltage (LV) motors be fairly compared. MV induction motors are somewhat less efficient and more costly than their comparable LV counterparts. However, MV’s advantages of smaller input cable sizes and better overall power conversion economy from the power source offset the above negative factors.
ABB Inc. (New Berlin, WI) views MV motors from a special ”made-to-order” perspective, because they’re manufactured in far fewer numbers than low-voltage motors. As a result, MV motors can be more readily ”engineered to meet the customer’s voltage supply network and application requirements, like starting of high-inertia loads at low inrush current,” says Carl-Gustav Michelsson, application engineering manager for Electrical Machines at ABB.
Other reasons for selecting MV versus LV motors go beyond the ability to customize them for an application. For example, Mr. Michelsson mentions limiting power losses in long supply cables-where I2R represents the magnitude of loss. MV motors also are the choice if medium-voltage switchgear already exists, or if short-circuit power has to be minimized in the user’s supply network.
Current draw to operate motors above a certain size (power) reaches a limit when running on low-voltage supply. The situation is exacerbated for induction motors due to the large inrush current multiplier at start-up-to the point where it ”can place a heavy demand on the local power distribution system,” according to Rockwell Automation Power Systems (Greenville, SC).
Tim Rahill, product manager for Rockwell Reliance large ac motors, notes the greater physical size and cost of cables and equipment needed to feed large LV motors. At some point it becomes more practical to operate motors on MV. ”While MV motors are more expensive, they operate at lower current values, resulting in more practical power system costs,” he says. ”The break point depends on the industry, factory, and/or local utility, but is typically in the 250-500 hp range.”
At Baldor Electric Co. (Fort Smith, AR), the attraction of MV motors includes lower amperage draw, and smaller transformers and transmission lines. John Malinowski, Baldor product manager for ac and dc motors, points out that MV motors also are somewhat larger than similar LV ratings due to more active materials needed in the motor, and thicker insulation required to contain those higher voltages. Still, the benefits of medium voltage are overriding.
While MV motors tend to be large, units at the lower end of output power also have their place. Medium voltage models in the 200-500 hp range can help standardize voltage in plants, explains Mr. Malinowski. ”A machine using a number of large MV motors, can add smaller motors of the same voltage in the same control center,” he says.
Siemens Energy & Automation (Alpharetta, GA) considers application of medium-voltage motors ”more practical” at higher power levels. A transition point is reached around 600 hp (450 kW), but it’s not a hard-and-fast rule. Below 600 hp, MV motors still maintain an advantage if they’re compatible with a plant’s MV distribution system or control equipment, explains Herb Ashby, Siemens E&A’s supervisor for motor sales. At lower power, MV motors also offer higher tolerance for voltage surges resulting from line disturbances, starting, and reclosing surges, explains Mr. Ashby.
”Above 600 horsepower, cost of the control becomes greater for low voltage. From the motor’s design and manufacturing aspects, higher horsepower with low voltage requires large conductors, which can be difficult or impossible to wind; while low horsepower with medium voltage requires small conductors, which are difficult and more costly to wind,” he states.
According to John Mallon, vp of engineering at Emerson Motors (St. Louis, MO), higher cost and lower efficiency become non-factors for medium-voltage motors around 500 hp and up. Above that point is where MV benefits blossom. Among the benefits, Mr. Mallon includes: reduced transformer costs, since a smaller turns ratio suffices to step-down transmission voltage; lower voltage drop to the motor due to lower current draw by using MV; and form-wound coil effects. Form-wound coils have a cost premium, ”but can also mean sturdier windings. Form-wound motors also can be more easily repaired in case of winding failure in only one part of the motor,” he says.
At higher power levels, MV motors provide the only choice; but even at the low end of power, they present opportunities. For example, Mr. Mallon cites the case of an industrial facility where available LV power may be near its maximum capacity, but the medium voltage source is not. Another scenario for smaller MV motors is where the production system and controls are in place for medium voltage, ”but not sufficient for additional low voltage.” Finally, overall system costs at a given plant, including power distribution, may be favorable to switch to medium voltage, explains Mr. Mallon.
Ken Polcyn, senior application engineer at GE Industrial Systems (Fort Wayne, IN) lists among main reasons for using medium-voltage motors: smaller power cables, less voltage drop from the utility to the motor, and more power output with less current. ”Ability to specify upstream circuit breakers and starters of smaller size offers a further advantage,” adds Mr. Polcyn.
He agrees that lower power MV motors have a place in the plant if suitable voltage is available. The specific low end of motor size depends on the actual application.
As a practical point, motors cannot be manufactured for all voltage and power ratings. The National Electrical Manufacturers Association (NEMA, Rosslyn, VA) designates ”preferred” motor voltage ratings in its MG-1 standard, ”Motors and Generators.” Applicable in North America, MG-1, Part 20 defines the following. [Note that the first row corresponds to LV motors.]
|460 or 575||110-600||75-500|
|4,000 or 4,600||200-10,000||150-7,500|
|13,200||3,500 and up||2,500 and up|
The International Electrotechnical Commission (IEC, Geneva, Switzerland) standard IEC 34-1, paragraph 4.7, ‘Coordination of voltages and outputs,’ does the same for the rest of the world.
|Minimum rated||Rated Voltage, kV output, kW (or kVA)|
ABB and Siemens make special reference to these preferred voltage ratings, but note exceptions to the guidelines due to specific user needs and diverse applications.
Differences in construction affect cost and efficiency of MV and LV motors. Medium voltage motors are built with form-wound stator coils, while LV coils are mostly random wound.
Inherent in the manufacturing process, a random-wound motor will be less costly than a form-wound motor for the same horsepower, explains Mr. Ashby of Siemens E&A.
Baldor’s Mr. Malinowski adds, ”Typically, most manufacturers wouldn’t random wind a motor over 1,000 volts. Form-wound coils are used above that level.”
Says Mr. Ashby, ”The advantage of medium voltage (and/or form-wound at low voltage) is that the dielectric strength of the form-wound coil is greater with a resulting capability to withstand higher voltage surges.”
Installing coils in the slots requires special attention, particularly for larger MV motors, states Mr. Polcyn of GE Industrial Systems. The end turns in particular are subjected to large forces when the motor starts. Therefore, the end turns are typically laced to a rope or steel ring for support prior to encapsulation in epoxy resin.
From a construction viewpoint, the practical limit for a LV motor is approximately 1,000 hp in the experience of Rockwell
Automation’s Mr. Rahill. Practical limits likewise exist for MV as indicated in the table of preferred voltages.
The efficiency of low-voltage motors is somewhat higher than MV models because of how the stator slots are filled. Form-wound coils for MV require more insulation, including thicker tape and wrapper materials. As a result, less MV copper can fit into the same-sized slot. Longer or larger slots are needed to compensate when using a MV design, compared to LV.
Mr. Rahill concurs about reduced MV space available for copper. ”More copper equals lower losses equals more efficiency,” is the way he puts it. ”Generally, the cost of the distribution system would outweigh the small difference in efficiency.” He further notes that larger MV motors (>1,000 hp) have quite high efficiencies in their own right. ”Values are higher than a typical low-voltage motor at a lower hp.” Premium efficient MV motors are also available.
What about still larger capacity ac motors-whether we call them medium or high voltage? Are they viable for today’s industry? The short answer is yes. One notable example is ABB’s high-voltage 40 kV synchronous motor at work driving a compressor at an air separation plant in Sweden. Trademarked as Motorformer, the machine outputs 6.5 MW of active power, connects directly to a 42-kV bus ”without an intervening transformer,” and dramatically cuts the plant’s energy losses. Such large, high-voltage motors offer the potential of even newer applications. But that’s a story for another time.
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What actually is ”medium voltage?”
What could be simpler than defining one range of medium voltages? Well, don’t bet on that. Medium voltage (MV) is not uniformly defined. It varies by industry, application, standards-making bodies, technical associations, etc.
The National Electric Code defines MV as occupying the 601-6,000 V range, with 6,001 V and up designated as ”high voltage!” Institution of Electrical and Electronic Engineers’ (IEEE) Std. 100 defines MV for system ratings as ‘greater than 1,000 V and less than 100,000 V, followed by two more classes: high voltage and extra-high voltage. For power cables, yet other ranges are specified in IEEE Std. 100. In keeping with metric practice, 1 kV is considered the MV threshold in Europe. Some manufacturers designate medium voltage for wire and power cable products with a 5-69 kV range. And, of course, some servo motor folks regard their 460 V units as ”high voltage.”
For motors and variable-speed drives, a practical nominal MV range might run from 600 V up to 15 kV. Available products have a narrower range. In part, this is determined by existing ”common” voltages that exist globally. In the 60-Hz world, these are 2.3, 4.16, 6.6, and 13.2 kV, while 3.3, 6.0, and 10 kV are common to the 50-Hz metric world.
How these voltages are related to motor size is shown the main text tables. Still other voltages exist, depending on the local power grid or user preferences.
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