Patent Description:
Electric drive arrangements are widely used for industrial applications, e.g. for providing and controlling electrical power and energy to various public and industrial applications as well as for driving and controlling various public and industrial applications. Electric drives are used in industry for different applications, such as for driving motors within the transportation industry, for driving different devices within the process and manufacturing industry as well as within the energy industry. There are applications commonly used for electric drives within the transportation industry for example in metro and railway traffic applications as well as in ship propulsion unit applications of the marine industry. Within the process and manufacturing industry, electric drives can be used for example in conveyer applications, in mixer applications or even in paper machine applications. Within the energy industry, electric drives can be used for example as electric drives for wind turbines of the wind power industry.

Electric drives for electric machines, such as for electric motors and electric generators, may be divided into AC drives (AC, alternating current) and DC drives (DC, direct current). AC drives are also commonly referred to as VFDs (VFD, Variable frequency Drive). Variable frequency drives control the speed of an electric machine by changing the voltage and frequency and are used with AC motors and generators.

In a DC motor of a DC drive, a magnetic field is generated by the current through the field winding in the stator. This magnetic field is always maintained at right angles to the field generated by the armature winding. In this way, a DC motor's torque is generated, which torque can then be easily controlled in a DC drive by changing the armature current and keeping the magnetizing current constant. In a DC drive, also the DC motor speed can be controlled directly through armature current.

Within electric drives, the AC drives may further be divided into frequency-controlled AC drives, flux-vector-controlled AC drives and into AC drives utilising direct torque control (DTC, Direct Torque Control). In flux-vector-controlled AC drives and in direct torque control AC drives the torque of the three-phase motor or generator can be controlled, whereas in frequency controlled AC drives the load dictates the torque level.

Controlling the vibration of the electric machine by using VFD control has been a topic of interest in the last years. There are many studies done targeted to reducing the level of electric machine vibrations by using a VFD control device such as a frequency converter. Since even a good structural design cannot avoid resonance frequencies, which produce harmful vibrations when excited.

In electric machines, the foundations of the electric machine may vary significantly. Mechanical properties of the mechanical system such as natural frequencies, stiffness of different parts and damping factors determine the vibration behaviour of the system on site conditions. These natural modes as well as the properties of the mechanical system including the electric machine are closely related to potential excessive vibrations and electric machine faults.

Currently, a typical prior art method determining vibration characteristics of an electric machine is carried out with a kit of hard- and software, i.e. a modal analysis hammer kit. These modal analysis hammer kits are used for impulse testing of the dynamic behavior of mechanical structures by striking the object with the hammer and measuring the resulting response with the accelerometer. The hammer impacts a constant force over a broad frequency range, which depends on the type of tip used. The hammer typically has an integral, constant current quartz force sensor mounted on the striking end of the hammer head. The sensor converts the impact force into electrical signal for display and analysis.

Typically, in a prior art solution for determining vibration characteristics of an electric machine, the vibration excitation is given by a test hammer and the vibrations are measured by transducers. The data is analyzed by a spectrum analyzer. The obtained spectra must be post-processed to reach modal parameters, i.e. the natural modes and frequencies, and damping.

Within solutions for controlling the vibration of the electric machine, it is known that avoiding resonance frequencies is the main design principle related to electric machine vibrations. However, the resonance frequencies of the electric machine will be changed accordingly to the site conditions. Hence it is important to identify all resonance frequencies of the electric machine in site conditions. Moreover, in order to carry our vibration damping, it is necessary to know the different stiffness and damping of the electric machine mechanical system including the foundation. This is why it is needed a set of steps to be carried out by the VFD in order to determine all these parameters that will be filled in the control model and will allow the smart damping of the vibration and noise.

United States Patent Application <CIT> discloses a prior art apparatus and a method of reducing vibrations in a washing machine. The disclosed apparatus and a method involves predicting a resonance frequency before a washing process to prevent a resonance phenomenon and to reduce noise and vibrations in the washing machine. The disclosed apparatus and a method of reducing vibrations is targeted to washing machines and is not suited for controlling vibration of a variable frequency drive controlled electric machine.

<CIT> discloses a prior art apparatus, controller and a method for adaptive control of an electromagnetic actuator. The disclosed apparatus, controller and a method involves auto-tuning a direct drive electromagnetic actuator continuously during a test, so that the test load applied to a specimen during the test is correct.

The foundation of the electric machines changes the natural modes and frequencies of the electric machine compared to the rigid foundation during the Factory Acceptance Test (FAT). The natural modes are closely related to the potential excessive vibrations on site conditions, and these vibrations are connected to the electric machine faults. If the natural modes are known in site conditions, the vibration behaviour can be predicted and the potential risks identified. Identifying potential risks is beneficial in predictive condition monitoring and diagnostics.

Within prior art solutions for controlling the vibration of the electric machine, there are several problems in determining vibration characteristics of an electric machine. First significant drawback with prior art solutions in site conditions is that the modal testing requires a qualified person with a special equipment. Another major problem is that the modal testing is difficult to carry out in operational condition. This is why it is currently only used in serious trouble shooting cases. Currently, the on-line modal testing is mostly not used.

In today's demanding environment, there is a need for a more efficient and easier solution for method and an arrangement for controlling vibration of a variable frequency drive controlled electric machine. There is a demand in the market for a method and an arrangement for controlling vibration of a variable frequency drive controlled electric machine that would be more efficient and easier to carry out in operational condition than the current prior art solutions.

The object of the invention is to introduce a method and an arrangement for controlling vibration of a variable frequency drive controlled electric machine, the condition of which can be more efficiently maintained. Advantageous embodiments are furthermore presented.

It is brought forward a new method for controlling vibration of a variable frequency drive controlled electric machine, in which method: a resonance frequency/frequencies and a stiffness of the electric machine are determined in a ramp up/ ramp down test, which ramp up/ ramp down test is executed with help of said variable frequency drive; an amount of damping is determined in an impulse test; a mechanical transfer function is determined in a steady drive test; a vibration control model is determined utilizing said resonance frequency/frequencies, said stiffness, said amount of damping and said mechanical transfer function; and vibration of the electric machine is controlled utilizing said vibration control model; wherein in the step of determining the resonance frequency/frequencies and the stiffness: a ramp up/ ramp down of the electric machine is executed; vibration is measured continually during said executed ramp up/ ramp down as measured vibration data; the resonance frequency/resonance frequencies is/are determined utilizing said measured vibration data; and the stiffness is calculated utilizing said determined resonance frequency/frequencies; wherein in the step of determining the amount of damping: an impulse is produced in an impulse test; vibration is measured continually during said impulse test as measured vibration data; and the amount of damping is calculated utilizing said measured vibration data.

In a preferred embodiment, in said impulse test, the electric machine is run by a given maximum voltage signal for a short time controlled by said variable frequency drive.

In a preferred embodiment, in the step of determining the mechanical transfer function: the electric machine is driven at a steady load in a steady state test; vibration is measured continually during said steady state test to thereby obtain measured vibration data; torque of the electric machine is recorded continually during said steady state test as recorded torque data; and a mechanical transfer function H(S) is calculated utilizing said measured vibration data and/or said recorded torque data.

In a preferred embodiment, in said steady state test, the electric machine is run at a certain load condition for a certain period controlled by said variable frequency drive.

In a preferred embodiment, in said steady state test, said mechanical transfer function H(S) is calculated in the frequency domain as: <MAT> where.

In a preferred embodiment, in the step of determining the vibration control model, a state space model of the physical system is built as a set of input variables, a set of output variables and a set of state variables related by equations.

Furthermore, it is brought forward a new arrangement for controlling vibration of a variable frequency drive controlled electric machine, said arrangement comprising a frequency converter, an electric machine and one or more vibration sensors, wherein said frequency converter is: arranged for executing a ramp up/ ramp down of the electric machine in a ramp up/ ramp down test and determining a resonance frequency/frequencies and a stiffness; arranged for running the electric machine by a given maximum voltage signal for a short time in an impulse test and determining amount of damping; arranged for running the electric machine at a certain load condition for a certain period in a steady drive test and determining a mechanical transfer function; arranged for determining a vibration control model utilizing said resonance frequency/frequencies, said stiffness, said amount of damping and said mechanical transfer function; and arranged for controlling vibration of the electric machine utilizing said vibration control model; wherein in determining the resonance frequency/frequencies and the stiffness: vibration is measured continually during said executed ramp up/ ramp down as measured vibration data; the resonance frequency/resonance frequencies is/are determined utilizing said measured vibration data; and the stiffness is calculated utilizing said determined resonance frequency/frequencies.

In a preferred embodiment, said electric machine motor is installed on a flexible bench.

In a preferred embodiment, said electric machine motor is installed on a non-flexible foundation.

In a preferred embodiment, said one or more vibration sensors comprise sensors, such as accelerometers, installed on the electric machine.

In a preferred embodiment, said sensors are installed close to the ends of the electric machine.

Furthermore, it is brought forward a new frequency converter of an arrangement for controlling vibration of a variable frequency drive controlled electric machine, said arrangement comprising the electric machine and one or more vibration sensors, wherein said frequency converter is: arranged for executing a ramp up/ ramp down of the electric machine in a ramp up/ ramp down test and determining a resonance frequency/frequencies and the stiffness; arranged for running the electric machine by a given maximum voltage signal for a short time in an impulse test and determining amount of damping; arranged for running the electric machine at a certain load condition for a certain period in a steady drive test and determining a mechanical transfer function; arranged for determining a vibration control model utilizing said resonance frequency/frequencies, said stiffness, said amount of damping and said mechanical transfer function; and arranged for controlling vibration of the electric machine utilizing said vibration control model.

In the following, the present invention will be described in more detail by way of example and with reference to the attached drawings, in which:.

The foregoing aspects, features and advantages of the invention will be apparent from the drawings and the detailed description related thereto.

In the following, the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings of <FIG>.

In the method for controlling vibration of a variable frequency drive controlled electric machine, the resonance frequency/frequencies and the stiffness are determined in a ramp up/ ramp down test; amount of damping is determined in an impulse test; a mechanical transfer function is determined in a steady drive test; a vibration control model is determined utilizing said resonance frequency/frequencies, said stiffness, said amount of damping and said mechanical transfer function; and vibration is controlled utilizing said vibration control model.

<FIG> illustrates a flow diagram of an embodiment of a method according to the present invention. In the method according to the present embodiment, the resonance frequency/frequencies and the stiffness of an electric machine are first determined <NUM>. The determination <NUM> of the resonance frequency/frequencies and the stiffness is done in a ramp up/ ramp down test carried out with the help of a variable frequency drive, e.g. a frequency converter.

Thereafter, in the method according to the present embodiment, the amount of damping is determined <NUM>. The determination <NUM> of the amount of damping is done utilizing an impulse test carried out with the help of a variable frequency drive, e.g. a frequency converter.

Thereafter, in the method according to the present embodiment, a mechanical transfer function is determined <NUM>. The determination <NUM> of the mechanical transfer function is done utilizing a steady drive test carried out with the help of a variable frequency drive, e.g. a frequency converter.

As the next step, a vibration control model is determined <NUM> utilizing said determined resonance frequency/frequencies, said stiffness, said amount of damping and said mechanical transfer function.

In the method according to the present embodiment, the steps of determining resonance frequency/frequencies and stiffness <NUM>, determining amount of damping <NUM>, determining mechanical transfer function <NUM> and determining vibration control model <NUM> may be repeated several times in a continuous loop, as deemed necessary.

In the method according to the present embodiment, an electric machine is driven by a variable frequency drive, The variable frequency drive measures and/or records speed data and torque data of the electric machine. After the vibration control model has been determined <NUM>, vibration is controlled <NUM> utilizing said vibration control model.

<FIG> illustrates a diagram of an embodiment of an arrangement according to the present invention. The presented embodiment of an arrangement comprises a frequency converter <NUM> and an electric machine. In the presented embodiment, said electric machine is a motor <NUM>, which motor <NUM> is installed on a foundation <NUM>. In the presented embodiment, said foundation <NUM> is a flexible bench <NUM>. Said motor <NUM> comprises a motor shaft <NUM> and motor legs <NUM> installed on said flexible bench <NUM>.

In the presented embodiment, said arrangement comprises one or more vibration sensors <NUM>, <NUM>. Said one or more vibration sensors <NUM>, <NUM> may comprise one or more vibration sensors attached to the bearing/bearings of said electric machine. Said one or more vibration sensors <NUM>, <NUM> may comprise sensors <NUM>, <NUM> installed on the motor <NUM>, for example close to the ends of the motor <NUM>, i.e. close to the motor bearings, e.g. in the bearing housing, in the end-shield or in the stator frame depending upon the investigated vibration mode. In the presented embodiment, said one or more vibration sensors <NUM>, <NUM> are installed on the motor <NUM> close to the motor bearings, and are thus in a good position for investigating horizontal vibration mode.

In the presented embodiment of an arrangement according to the present invention, the frequency converter <NUM> is used for driving the motor <NUM> installed on a bench <NUM>. The driving of the motor <NUM> by the frequency converter <NUM> generates vibration in the motor installation. The vibration of is thereafter measured from the motor installation as vibration data by said one or more vibration sensors <NUM>, <NUM>. Whereas in the presented embodiment a motor is shown as an electric machine, in another embodiment of the present invention the motor could be replaced by a generator as electric machine.

<FIG> illustrates a flow diagram of a method step of determining the resonance frequency/frequencies and the stiffness of an embodiment of a method according to the present invention. In said method step of determining the resonance frequency/frequencies and the stiffness, first a ramp up/ ramp down of an electric machine is executed <NUM>. Said ramp up/ ramp down is executed <NUM> with the help of a variable frequency drive, e.g. a frequency converter. During the executed ramp up/ ramp down test, the electric machine is started up and run with a long ramp controlled by a variable frequency drive, e.g. a frequency converter, and the vibration data is measured <NUM> continually. In said ramp up/ ramp down test, said ramp may be a speed ramp. Furthermore, said ramp may be a long speed ramp, up to a speed of <NUM> times the rated speed of the electric machine (i.e. <NUM>% over the rated speed of the electric machine).

During the executed ramp up/ ramp down test, vibration is measured <NUM> from the electric machine as vibration data by said one or more vibration sensors <NUM>, <NUM>. Said one or more vibration sensors <NUM>, <NUM> may comprise accelerometers. The measured vibration data may comprise vibration velocity root mean square, acceleration, velocity and/or displacement.

Said measured vibration data is analysed in order to determine the resonance frequency/frequencies. The measured vibration data may be conditioned in the variable frequency drive, so it is supposed that special provision is in the variable frequency drive hardware for measured vibration data signal processing. In an alternative solution, a commercial vibration amplifier can be used to provide a voltage signal to the variable frequency drive.

As the next step, a resonance frequency / resonance frequencies is/are determined <NUM> utilizing said measured vibration data. After determining the resonance frequency/frequencies, the stiffness of the electric machine is calculated <NUM> utilizing said determined resonance frequency/frequencies.

In the method step of determining the resonance frequency/frequencies and the stiffness according to the present embodiment, the steps of executing a ramp up/ ramp down <NUM>, measuring vibration <NUM>, determining resonance frequency/frequencies <NUM> and calculating the stiffness <NUM> are repeated several times in a continuous loop, as deemed necessary. Thereafter, the resonance frequency/frequencies and stiffness are determined <NUM>.

<FIG> illustrates an axial view of an embodiment of an electric machine of an arrangement according to the present invention. In the presented embodiment, said electric machine is a motor <NUM>, which motor comprises a motor shaft <NUM> and motor legs <NUM>. In the presented arrangement, the distance between the motor legs is indicated as two times b, i.e. 2b. The height from the bottom of the motor legs to the middle of the motor shaft is indicated as h. The height h also indicates the height from the surface of a motor bench to the middle of the motor shaft of a motor installed on said motor bench. Whereas in the presented embodiment a motor is shown as an electric machine, in another embodiment of the present invention the motor could be replaced by a generator as electric machine.

<FIG> illustrates a conceptual view of an embodiment of an electric machine of an arrangement according to the present invention. In the presented conceptual view, a flexible bench is indicated with a reference number <NUM> and a shaft, i.e. a rotor, of an electric machine is indicated with a reference number <NUM> and total torque of the electric machine is indicated with a reference number <NUM>. Said total torque <NUM> is in the direction of about the rotational axis. The height from the surface of a bench <NUM> to the middle of the rotor <NUM> is indicated as h. In the presented conceptual view, the coupling stiffness between the rotor to the feet of the electric machine is indicated as two times the feet stiffness k<NUM>, i.e. <NUM>k<NUM>. In the presented conceptual view, the stiffness of the bench of the electric machine is divided to elements of two times horizontal stiffness k<NUM> and two times vertical stiffness k<NUM>.

In a ramp up/ ramp down test the determination <NUM> of the resonance frequency/frequencies and the stiffness is done in a ramp up/ ramp down test carried out with the help of a variable frequency drive, e.g. a frequency converter. The stiffness of the electric machine is calculated <NUM> utilizing the determined resonance frequency/frequencies. In said ramp up/ ramp down test, the presented conceptual view can be used for determining the stiffness. In determining the stiffness, there is first taken some simplified mechanical model assumptions for a flexible bench:.

According to Newton's second law from the equilibrium of forces, the centre of gravity of the machine structure is moving according to following equations [<NUM>] and [<NUM>]: <MAT> <MAT> where:.

Considering just the horizontal mode, the vibration resonance frequency is calculated as: <MAT> where: <MAT> <MAT> <MAT>.

Stator and rotor masses are usually known as well as the moment of inertia, so from the natural frequencies it is possible by solving the system of equation find the unknown horizontal stiffness k<NUM> and vertical stiffness k<NUM>. When the bench is very soft, the stator feet stiffness k<NUM> can be considered to be large as in a rigid connection. The stiffness of the electric machine is calculated from the now solved elements of two times horizontal stiffness k<NUM> and two times vertical stiffness k<NUM>.

<FIG> illustrates a flow diagram of a method step of determining the amount of damping of an embodiment of a method according to the present invention. In said method step of determining the amount of damping, first an impulse is produced <NUM> in an impulse test.

Said impulse test is executed with the help of a variable frequency drive, e.g. a frequency converter. During the executed impulse test, the electric machine is run by a given maximum voltage signal for a short time controlled by a variable frequency drive, e.g. a frequency converter, and vibration is measured <NUM> continually as measured vibration data. Said a short time controlled by a variable frequency drive may e.g. be only fractions of seconds.

During the executed impulse test, vibration is measured <NUM> from the electric machine as vibration data by said one or more vibration sensors <NUM>, <NUM>. Said one or more vibration sensors <NUM>, <NUM> may comprise accelerometers. The measured vibration data may comprise vibration velocity root mean square, acceleration, velocity and/or displacement.

As the next step, the amount of damping is calculated <NUM> utilizing said measured vibration data. In the executed impulse test, said maximum voltage signal will produce a shock in the mechanical system, the system will vibrate, and the oscillations will damp out by themselves. This type of test will allow the variable frequency drive to calculate the amount of damping in the mechanical system.

Said measured vibration data is analysed in order to determine the amount of damping. The measured vibration data may be conditioned in the variable frequency drive, so it is supposed that special provision is in the variable frequency drive hardware for measured vibration data signal processing. In an alternative solution, a commercial vibration amplifier can be used to provide a voltage signal to the variable frequency drive.

In the method step of determining the amount of damping according to the present embodiment, the steps of producing an impulse <NUM>, measuring vibration <NUM> and calculating the amount of damping <NUM> may be repeated several times in a continuous loop, as deemed necessary. Thereafter, the amount of damping is determined <NUM>.

<FIG> illustrates a flow diagram of a method step of determining the mechanical transfer function of an embodiment of a method according to the present invention. In said method step of determining the mechanical transfer function, first an electric machine is driven at a steady load <NUM> in a steady state test.

Said steady state test is executed <NUM> with the help of a variable frequency drive, e.g. a frequency converter. During the executed steady state test, the electric machine is run at a certain load condition for a certain period controlled by a variable frequency drive, e.g. a frequency converter, and vibration is measured <NUM> continually as measured vibration data.

During the executed steady state test, vibration is measured <NUM> from the electric machine as vibration data by said one or more vibration sensors <NUM>, <NUM>. Said one or more vibration sensors <NUM>, <NUM> may comprise accelerometers. The measured vibration data may comprise vibration velocity root mean square, acceleration, velocity and/or displacement.

During the executed steady state test, torque data of the electric machine is recorded <NUM> from the electric machine as recorded torque data by a variable frequency drive, e.g. a frequency converter.

As the next step, a mechanical transfer function is calculated <NUM> utilizing said measured vibration data and/or said recorded torque data. In the executed steady state test, the coupling stiffness between the electric machine stator to the electric machine feet may also be determined utilizing said measured vibration data and/or said recorded torque data.

In the executed steady state test, said mechanical transfer function H(S) is calculated in the frequency domain as: <MAT>.

In the method step of determining the mechanical transfer function according to the present embodiment, the steps of driving the electric machine at a steady load <NUM>, measuring vibration <NUM>, recording torque data <NUM> and calculating the mechanical transfer function <NUM> may be repeated several times in a continuous loop, as deemed necessary. Thereafter, the mechanical transfer function is determined <NUM>.

In the method according to the present invention, the analysis of a mechanical model and mechanical transfer function may be carried out as described in a conference paper: "<NPL>.

<FIG> illustrates a state space model for determining the vibration control model of an embodiment of a method according to the present invention. In the method step of determining the vibration control model, the equation [<NUM>] is utilized in building a state space model for calculating the actual displacement under different conditions.

In the method step of determining the vibration control model according to the present embodiment, there is built a state space model of the physical system as a set of input variables, a set of output variables and a set of state variables related by equations. In the present embodiment, the built state space model comprises force data as the set of input variables <NUM>, displacement data, velocity data and acceleration data as the set of output variables <NUM>, and a set of state variables related by equations: x' = Ax + Bu and y = Cx + D, here reference numbered as <NUM>. Thereafter, a vibration control model is determined <NUM> utilizing the built state space model. In the method according to the present embodiment, an electric machine is driven by a variable frequency drive, The variable frequency drive measures and/or records speed data and torque data of the electric machine. After the vibration control model has been determined <NUM>, vibration is controlled <NUM> utilizing said vibration control model.

In the method according to the present invention, the analysis of a state space model and data analysis may be carried out as described in a conference paper: "<NPL>.

<FIG> illustrates a diagram of another embodiment of an arrangement according to the present invention. The presented another embodiment of an arrangement comprises a frequency converter <NUM>, electric machine and a load <NUM>. In the presented embodiment, said electric machine is a motor <NUM> installed on a non-flexible motor foundation <NUM>. Said motor <NUM> comprises a motor shaft <NUM> and motor legs <NUM> installed on said non-flexible motor foundation <NUM>. The load <NUM> is coupled to the motor shaft <NUM> via a coupling arrangement <NUM>.

In the presented another embodiment, said arrangement comprises one or more vibration sensors <NUM>, <NUM>. Said one or more vibration sensors <NUM>, <NUM> may comprise sensors <NUM>, <NUM> installed on the motor <NUM>, for example close to the ends of the motor <NUM>, i.e. close to the motor bearings, e.g. in the bearing housing, in the end-shield or in the stator frame depending upon the investigated vibration mode. Whereas in the presented embodiment a motor is shown as an electric machine, in another embodiment of the present invention the motor could be replaced by a generator as electric machine.

With the help of the present invention vibration can be controlled by a variable frequency drive and the level of harmful electric machine vibrations can be substantially reduced.

With the help of the present invention the mechanical properties of the mechanical system such as natural frequencies, stiffness of different parts and damping factors can be identified and included in a mechanical model that allows the control of vibrations excited from different sources such as unbalance, eccentricities, electromagnetic and mechanics problems.

With the help of the method and an arrangement for determining vibration characteristics of an electric machine according to the present invention the determination of the vibration characteristics of an electric machine is easier and more efficient, especially in operational condition.

Furthermore, the method and an arrangement for determining vibration characteristics of an electric machine according to the present invention provides easy and efficient of determining of the vibration characteristics of said electric machine which is also useful in determining the condition of said an electric machine.

Claim 1:
A method for controlling vibration of a variable frequency drive (<NUM>) controlled electric machine (<NUM>), in which method:
- a resonance frequency/frequencies and a stiffness of the electric machine (<NUM>) are determined (<NUM>) in a ramp up/ ramp down test, which ramp up/ ramp down test is executed with help of said variable frequency drive (<NUM>);
- an amount of damping is determined (<NUM>) in an impulse test;
- a mechanical transfer function is determined (<NUM>) in a steady drive test;
- a vibration control model is determined (<NUM>) utilizing said resonance frequency/frequencies, said stiffness, said amount of damping and said mechanical transfer function; and
- vibration of the electric machine (<NUM>) is controlled (<NUM>) utilizing said vibration control model;
- wherein in the step of determining (<NUM>) the resonance frequency/frequencies and the stiffness:
- a ramp up/ ramp down of the electric machine (<NUM>) is executed (<NUM>);
- vibration is measured (<NUM>) continually during said executed ramp up/ ramp down as measured vibration data;
- the resonance frequency/resonance frequencies is/are determined (<NUM>) utilizing said measured vibration data; and
- the stiffness is calculated (<NUM>) utilizing said determined resonance frequency/frequencies;
- wherein in the step of determining (<NUM>) the amount of damping:
- an impulse is produced (<NUM>) in an impulse test;
- vibration is measured (<NUM>) continually during said impulse test as measured vibration data; and
- the amount of damping is calculated (<NUM>) utilizing said measured vibration data.