Description

Induction motors, low cost and robust as they are, are found in many industrial applications - from a few kilowatts to the megawatt range. There has been a trend away from mains operation towards inverter operation in the last 10-20 years as a result of efficiency demand and economical power electronics.

It is also state of the art to mount motors directly with the load machine on a common foundation frame. This can be offered to customers as a cost-effective complete product. The structure varies because of product-specific properties, such as type and operating environment, and the elasticity of the foundation selected by the respective customer and speed ranges must be blocked as a consequence of the vibration behaviour.

The Campbell diagram (Figure 1) shows the problem of the standardised design of electrical motors for a rigid foundation compared to an installation on an elastic steel frame foundation.

Demonstration - what can the actuator system do?

The video shows a brief portion of a vibration measurement on the test bench with the motor running with increased feather key unbalance at a stationary operating point of approx. 1630 rpm. At this rotor speed, the rotor frequency meets a natural frequency of the system (consisting of an elastic steel frame foundation, actuator system, and motor). At this natural frequency, the motor performs mainly translational motion in vertical direction.

Without active vibration control:

When the motor is operated without active control, the vibration of the bearing shield at the drive end has a very high vibration velocity of approx. 21 mm/s (approx. 15 mm/s rms). These high vibration amplitudes are too high for stationary operation and would damage the motor if operated for prolonged periods.

With active vibration control:

By activating the control of the actuator system, the vibration amplitudes are reduced to approx. 5 mm/s (approx. 3.5 mm/s rms) within less than half a second, which is a permissible stationary operating point.

Conclusion:

The demonstration shows that reducing vibration amplitudes with active vibration control is possible. Moreover (not shown in video), it is possible to influence all vibration modes within the speed setting range of the motor and reduce the vibration amplitudes to an extent that enables steady-state operation at all motor speeds.

Thanks to the developed actuator system, the motor no longer has a natural mode shape where it rotates purely on its vertical axis, which means that only vertically acting active elements can be used. This results in a low-cost system that requires little mounting space.

In addition, the vibrations of both the motor and the foundation are reduced. The cause of the vibration (e.g, due to unbalance or electromagnetic forces in the air gap of the electrical machine) is irrelevant.

Thinking in other dimensions (ADAM-2)

Given the positive experience in the ADAM-1 project, it was decided to continue the project. ADAM-2 comprises implementing investigations that are necessary for scaling the system for vibration-optimised operation. A view into the project is available in this online seminar.

Continued success in ADAM-2

The positive results on the test bench led to the easy decision to continue the project with a two-pole induction motor in the 2 MW class at Innomotics GmbH (formerly Siemens AG). Now it can be shown that the basic concept of active vibration control of electric machines on elastic steel frame foundations can be scaled up to a completely different dimension.

Initial measurements were taken using various mechanical setups, both with and without an actuator system in the uncoupled state. These measurements provide a broad database ranging from experimental modal analyses and vibration measurements during run-up to FE calculations. After these in-depth investigations, the test object was coupled with a 2 MW induction motor and measurements were taken at nominal torque (see Figure 4). The test motor is initially placed on solid steel blocks (positioned between the motor feet and the steel frame) in order to demonstrate the problem of the restricted speed ranges and the resulting need for adjustment. In the next step, the steel blocks are replaced by the actuator system and the vibration measurements are repeated during the run-up.

Vibration measurement results - no more restricted speed ranges ...

A common method for investigating the vibration behaviour in variable-speed drives is to take the measurements during a slow run-up. Figure 5 shows horizontal vibrations on the bearing shields during run-up (vibration velocity (rms) over rotor speed) in three variants.

• Variant 1: Steel blocks between elastic steel frame and motor (basically the standard case in industrial systems, blue curve)
• Variant 2: Actuator system instead of steel blocks - control off (orange curve)
• Variant 3: Actuator system instead of steel blocks - control on (green curve)

As Figures 1 (Campbell diagram) and 5 illustrate, restricted speed ranges must be designated with steel blocks because of the very pronounced resonances. Continuous operation at these speeds (or close to them) should be avoided. Variant 2 shows the vibration behaviour during run-up with an actuator system with the control turned off. Comparing variants 1 and 2 shows that resonances are shifted and, because of the special mechanical design of the actuator system, all vibration modes are more strongly coupled and have vertical motor foot movements.

The vibration measurements in the run-up of variant 3 (control on) show the vibration amplitudes of all vibration modes are greatly reduced in the operating speed range although only vertical forces are introduced into the system. And because of the low vibration values and the very good damping, no restricted speed ranges must be defined.