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Permanent Magnet Motors in Direct-Drive Applications

Permanent Magnet Motors in Direct-Drive Applications

To achieve systems design goals, where motors are involved, of increased efficiency and improved power density, along with lower noise and variable speed operating capability, technologies beyond induction motors should be considered. Permanent magnet (PM) motors have long been recognized as providing higher efficiencies than comparable induction motors. However, limitations in terms of motor control, as well as magnet material performance and cost, have severely restricted their use. However, dramatic improvements in magnetic and thermal properties of PM materials over the past 20 years have led to the development of synchronous PM motors that are now viable alternatives. Figures 1 & 2 show typical efficiencies and power factors for various motor types [1].

Another innovation to consider in your systems design project is laminated frame motor technology. Laminated frame motors consist of a stack of laminations permanently riveted under controlled pressure. The cast iron outer frame is eliminated, allowing more room for active (torque-producing) magnetic material.

Permanent Magnet Motors in Direct-Drive Applications

A particular advantage of this construction is that the air used to cool the motor is in direct contact with the electrical steel. There is no thermal resistance path as that which exists in a traditional cast iron frame with contact to the stator lams. The heat transfer mechanism in a cast iron frame motor is highly dependent upon the stator to frame fit. Laminated frame construction eliminates this issue.

In recent years, industry drivers have forced the development of an optimized, finned, laminated motor design. To improve the cooling and increase power density, fins have been added to the exterior of the stator laminations. The addition of the optimized cooling fins increases the surface area available for heat dissipation. The result is improved heat transfer and a power increase of 20-25 percent is typical for a given lamination diameter and core length.

It is this improved cooling method, along with the higher efficiency and power factor achieved with the PM technology that allows for increased power density in these motor designs. Power density is the key for being able to match the height restriction of the existing gearbox.

Retrofit Case Study

This case study involves the retrofit of an existing cooling tower constructed in 1986 at Clemson University in South Carolina. The existing tower had:

  • Fan Diameter: 18 ft
  • Flow Rates: 4,250 gal per minute per cell; 8,500 gpm total
  • Motor Information: Frame - 326T, hp - 50/12.5; speed - 1765/885 rpm
  • Gearbox: Size - 155, Ratio - 8.5:1

The tower is comprised of two identical cells. For this study, one cell was retrofitted with the new slow speed PM motor and VFD while the other was left intact as originally constructed. This allows for a direct comparison of the two fan drive solutions. PM Motor image shows the PM motor installed in place of the gearbox.
Permanent Magnet Motors in Direct-Drive Applications

Prior to the installation, the current being drawn by the two original induction motors was measured with the fans running at full speed. An ammeter was used and the current was measured to be 47A, rms on both induction motors. As the induction motors are identical, this is a good indication that both cells were operating under the same load conditions. After the PM motor and VFD installation was complete, the current was again re-checked and found to be only 41A for the PM motor. The induction motor on the original, identical, tower was still drawing 40 47A.

From this data, it was determined that both cells were running at less than full load and that the load should be increased on each cell. To this end, the pitch of the blades on each fan was increased to 12 degrees. This change of pitch caused the fans to draw more air, thus increasing the load on each motor. Further, the increased air flow improved the effectiveness of the overall tower performance. Again, power measurements were made and a third party testing service was engaged to verify the manufacturer's results.

For the final blade pitch, 4.5 kW less power consumption was observed on the cell with the PM motor installed. The PM motor solution requires less input power for each load point (blade pitch).

Electrical Considerations

In addition to the PM motor design features already detailed, another challenge of this application was that the PM motor had to be run sensorless. There was no room to install a speed feedback device, such as an encoder or resolver, and still meet the height restriction of the existing gearbox. In this harsh environment, a feedback device would be a liability as far as reliability is concerned. Therefore, a sensorless PM control scheme was developed to satisfy the requirements of this application.

Permanent Magnet Motors in Direct-Drive Applications

Several things had to be considered when forming this algorithm. One challenge was the inertia of the fan. This was taken into account to prevent the motor from falling out of synchronism when starting and changing speeds. Figure 3 shows a portion of a typical start from rest. Note the smooth acceleration and low starting current required. A typical 480-V induction motor started across the line would draw 347A [2], compared to 12A for this PM design started on the VFD.

The use of a VFD also provides the opportunity to offer some additional features that across the line systems do not. The drive may be configured to apply a trickle current to the motor windings to act as a brake during down time. This prevents the fan from free wheeling due to nominal winds or adjacent cooling tower turbulence. However, a mechanical locking mechanism should be used during any maintenance procedures. This trickle current also acts as an internal space heater by raising the winding temperature, preventing condensation when the motor is not running.

Inside the fan stack is an extremely humid environment. Therefore, the insulation system on the stator windings must be robust and highly moisture resistant. To this end, an insulation system derived from a system originally developed for use by the U.S. Navy was employed. This system utilizes an epoxy compound applied via a vacuum pressure impregnation system.

Mechanical Considerations

Due to the harsh environment inherent with a cooling tower application, the motor's drive end is protected by a metallic, non-contacting, non-wearing, permanent compound labyrinth shaft seal that incorporates a vapor blocking ring to prevent an ingress of moisture. This seal has been proven to exclude all types of bearing contamination and meets the requirements of the IEEE-841 motor specification for severe duty applications.

Permanent Magnet Motors in Direct-Drive Applications

Another consideration on this project was overall system maintenance. For motor/gearbox combination drives, the lubrication interval is determined by the high-speed gear set. The recommended lubrication interval for this type of gear is typically 2,500 hours or six months, whichever comes first. In addition, gear manufacturers recommend a daily visual inspection for oil leaks, unusual noises or vibrations. With the elimination of the high speed input due to application of the slow speed PM motor design, the lubrication cycle can now be extended up to two years. The PM motor need not be inspected daily for oil leaks, as the motor contains no oil.

With the elimination of the high speed input to the gearbox, the system dynamics from a vibration standpoint have been simplified. There are no longer any resonance issues with the driveshaft. The maximum rotational excitation is now limited to the rotational speed of the fan. The number of bearings in the drive system has been reduced from six to two for a single reduction gearbox and from eight to two for a double reduction gearbox. This reduces the number of forcing frequencies present in the system.

Many cooling towers are in locations where airborne noise can be an issue, such as hospitals and universities. To this end, a third-party testing company was engaged to conduct comparative sound tests between the two cells. Data was taken at both high speed and low speed for both cells. At high speed, the PM motor cell was 4.6 dBA lower than the induction motor cell. For low-speed operation, the PM motor cell was 5.4 dBA lower. The removal of the high-speed induction motor from the outside of the fan stack appears to have the biggest influence on the noise level of the tower itself.

References

[1] Steve Evon, Robbie McElveen and Michael J. Melfi, "Permanent Magnet Motors for Power Density and Energy Savings in Industrial Applications," PPIC 2008
[2] NEMA MG 1-2006, Motors and Generators

Robbie McElveen, Bill Martin and Ryan Smith are senior development engineers at Baldor. The authors extend their thanks to Clemson University and Tower Engineering Inc. for their contributions and participation in the project.

Permanent Magnet Motors in Direct-Drive Applications


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