Consider these factors to achieve a rigid shaft design while maintaining the proper electrical characteristics.
by Ryan Maynus
August 16, 2016

Flexibility in an industrial application can allow for improved reliability and less downtime. At the same time, too much flexibility in a pump system design can have negative impacts related to initial costs and system performance concessions. Finding the right balance can be a challenge.

Recently, a motor provider was approached by a major pipeline company about addressing the need to introduce more flow control into its pipelines. This company wanted to improve the flexibility of the process control and reduce its initial investment while maintaining a reliable system. The motors connected to its pipeline pumps used a variable frequency drive (VFD) and were designed around a 3,600-rpm base design motor that contained a flexible shaft design.

A flexible shaft design is a motor with a critical speed below synchronous speed. The user wanted to operate the motor below synchronous speed to optimize the flow in the pipeline. However, it is a challenge for many motor suppliers to move this critical speed away from the pump operating speed range, along with some margin, to avoid any system vibration issues caused by operating the motor on or too close to a critical speed. The other challenge was eliminating the need for the motor to have an oil flood lubrication system to provide adequate cooling for the motor bearings.

To provide a solution for the flexible shaft problem, a motor had to be provided where the critical speed was removed from the operating range. There are many factors to consider related to the mechanical and electrical properties of a motor to develop the right environment to achieve a rigid shaft design while also maintaining the proper electrical characteristics.

Flexible vs. Rigid Shaft Design

Before getting too far into the motor solution, we must first understand a little more about the differences between a flexible and a rigid shaft motor design. Motors can reach a natural resonance speed that can greatly raise the vibration levels and cause damage not only to the motor but also to the entire system connected to the motor. Of course, it is best to avoid these natural resonance speeds.

In simple terms, a flexible shaft is a shaft that bends when rotating. This type of shaft can also vibrate at a critical speed, which is especially concerning if the pump and motor need to operate through a speed range where the motor critical speed exists. Operating near this point could damage the entire pump and motor drive train, potentially impacting the pump’s uptime. Blocking out the speed range on the VFD is possible, but blocking too much can hinder application efficiency from a cost and material flow standpoint.

On the other hand, a rigid shaft design does not bend when rotating. Another key characteristic of this style of shaft design is that the shaft does not vibrate at a critical speed. Having a motor with a rigid shaft will keep the vibrations down in the motor to allow for a safe and reliable operating level.

A fluted shaft design with a higher degree of stiffness along with a modified rotor lamination design will get the motor shaft and rotor assembly moving in the right direction to remove the critical speed concerns.
The next area to address is the bearing arrangement. Considering the bearing span along with the overall bearing design can lead to an optimized design that will allow the shaft and rotor assembly to operate successfully. Modifying the bearing design to allow for an improved cooling scheme eliminates the need for supplying a separate oil-cooled flood lubrication system for the motor bearings.

User Solutions

The result for this user was a motor that overcame previous motor challenges and allowed the user to operate the application with a higher degree of flexibility in the process. The application is up and running without any problems related to vibration because the motor has a highly damped rigid shaft design. By adding this shaft design, the process is now flexible.

Upfront project costs related to the motor decreased by as much as 20 percent. This reduction was achieved by giving the user the option of a smaller framed motor. In this example, the user would have used an 8010 frame to meet the power and rigid shaft requirements. Now, they can use a smaller 6813 frame and achieve power levels up to 5,000 horsepower on a 3,600-rpm design.

In addition, the motor lead time, which is often the longest lead time item for the motor and pump assembly, has been reduced by as much as 15 percent. Reducing the project lead times also helps keep the project on time and decrease overall costs.

One of the initial challenges was trying to retain reliability while reducing the need for flood lubrication. By modifying the bearing design, the bearing temperatures allowed a safer operating oil temperature, which in turn permitted the removal of a flood lubrication system for the motor bearings. The motors are operating at a safe temperature that improves the overall reliability of the system and reduces the user’s cost to supply a flood lubrication system. Overall, the user was able to address the need to create a more flexible process by introducing a more rigid motor design into the pump system.