Patent Description:
Induction generators may be used in a power distribution system, such as in aircrafts and other vehicles, to provide an alternating current (AC) power signal for use by various AC loads. The AC loads typically have power frequency requirements that limit the AC power signal to a constant frequency or to a frequency band that is narrower than a typical operational frequency band of a prime mover, such as an engine.

In a typical power distribution system, in order to achieve a constant generator output frequency, a constant speed drive (or some other type of variable speed transmission) may be positioned between the prime mover and a shaft of the generator. Constant speed drives may be complex, heavy, and/or bulky. As such, they may be inappropriate for some applications, particularly with respect to aircraft applications. Other disadvantages may exist.

Further background information is provided in the following documents.

<CIT> states a system which may include a first bus transfer switch configured to open and close connections between a generator control unit, a pilot permanent magnet generator stage of an independent speed variable frequency (ISVF) generator, and an external power source. The system may further include an inverter configured to set a main field winding of the ISVF generator into a motor state. The system may also include a second bus transfer switch configured to open and close a connection between a main armature winding of the ISVF generator, a power distribution bus, and a motor start driver configured to send a current through the main armature winding to generate a magnetic field pattern that causes the rotor to turn, enabling startup of an engine.

<CIT> states a synchronous brushless machine having a single exciter field stator winding. The single exciter field stator winding is energized by a high frequency alternating current to provide a single excitation field to magnetically couple with the exciter field armature winding in both the starter mode and the generator mode. With a higher excitation frequency relative to the main armature current frequency, a steady main field voltage can be achieved which improves stability control while in the starter mode. In one or more configurations, the single exciter field stator winding is driven by a H-bridge converter.

There is provided a system as set out in claim <NUM>. There is further provided a method as set out in claim <NUM>.

Disclosed herein are systems and methods that may overcome one or more of the disadvantages of typical power distribution systems. A system according to embodiments is set out in Claim <NUM>.

In some embodiments, the generator control unit is configured to generate an excitation signal to control the ISVF generator, where an equivalent excitation frequency equals an excitation frequency of the excitation signal multiplied by the number of pole pairs. In such an embodiment, setting the generator output frequency equal to the lower frequency limit includes setting the equivalent excitation frequency to a difference of the lower frequency limit and the equivalent shaft frequency, setting the generator output frequency equal to the upper frequency limit includes setting the equivalent excitation frequency of the excitation signal equal to a difference of the upper frequency limit and the equivalent shaft frequency, and setting the generator output frequency equal to the equivalent shaft frequency includes setting the equivalent excitation frequency of the excitation signal to zero.

In some embodiments, the system includes a coupler positioned between the prime mover and the shaft and configured to convert a second torque associated with the prime mover to the torque associated with the shaft. In some embodiments, the coupler includes a fixed ratio gear coupling, a belt, or a combination thereof. In some embodiments, the prime mover is configured to rotate the shaft without any constant speed drive device coupled therebetween. In some embodiments, the system includes a set of AC loads electrically connected to the AC bus, where the lower frequency limit and the upper frequency limit are determined at least partially based on operational requirements of the set of AC loads. In some embodiments, the system may include an alternating-current-direct-current (AC/DC) converter electrically connected to the AC bus and a direct current (DC) bus electrically connected to the AC/DC converter, where the AC/DC converter is configured to convert the AC power signal on the AC bus to a DC power signal on the DC bus. In some embodiments, the prime mover is an aircraft engine.

In an embodiment, a method includes rotating a shaft using a prime mover. The method further includes converting torque from the shaft to an AC power signal using an ISVF generator, where the ISVF generator has one or more pole pairs, and where an equivalent shaft frequency equals a shaft frequency of the shaft multiplied by a number of the pole pairs. The method further includes applying the AC power signal to an AC bus having a lower frequency limit and an upper frequency limit. The method also includes setting a generator output frequency of the ISVF generator equal to the lower frequency limit when the equivalent shaft frequency is less than the lower frequency limit. The method includes setting the generator output frequency of the ISVF generator equal to the upper frequency limit when the equivalent shaft frequency is greater than the upper frequency limit. The method further includes setting the generator output frequency of the ISVF generator equal to the equivalent shaft frequency when the equivalent shaft frequency is between the lower frequency limit and the upper frequency limit.

In some embodiments, the method includes generating an excitation signal to control the ISVF generator, where an equivalent excitation frequency equals an excitation frequency of the excitation signal multiplied by the number of pole pairs, where setting the generator output frequency equal to the lower frequency limit comprises setting the equivalent excitation frequency to a difference of the lower frequency limit and the equivalent shaft frequency, where setting the generator output frequency equal to the upper frequency limit comprises setting the equivalent excitation frequency of the excitation signal equal to a difference of the upper frequency limit and the equivalent shaft frequency, and where setting the generator output frequency equal to the equivalent shaft frequency comprises setting the equivalent excitation frequency of the excitation signal to zero.

In some embodiments, the method includes converting second torque associated with the prime mover to the torque associated with the shaft using a coupler. In some embodiments, the coupler includes a fixed ratio gear coupling, a belt, or a combination thereof. In some embodiments, rotating the shaft is performed without any constant speed drive device coupled between the shaft and the prime mover. In some embodiments, an AC/DC converter is electrically connected to the AC bus, and where a DC bus is electrically connected to the AC/DC converter. In these embodiments, the method may include converting, at the AC/DC converter, the AC power signal on the AC bus to a DC power signal on the DC bus.

In an embodiment, a system includes a prime mover configured to rotate a shaft. The system further includes an ISVF generator configured to convert torque from the shaft to an AC power signal, where the ISVF generator has one or more pole pairs, and where an equivalent shaft frequency equals a shaft frequency of the shaft multiplied by a number of the pole pairs. The system further includes an AC bus and a generator control unit configured to generate an excitation signal to control the ISVF generator, where an equivalent excitation frequency equals an excitation frequency of the excitation signal multiplied by the number of pole pairs, and where the generator control unit maintains a constant generator output frequency by setting the equivalent excitation frequency equal to a difference between the constant generator output frequency and the equivalent shaft frequency.

In some embodiments, the system includes a coupler positioned between the prime mover and the shaft and configured to convert second torque associated with the prime mover to the torque associated with the shaft, where the coupler includes a fixed ratio gear coupling, a belt, or a combination thereof. In some embodiments, the prime mover is configured to rotate the shaft without any constant speed drive device coupled therebetween. In some embodiments, the system includes a set of AC loads electrically connected to the AC bus, where the constant generator output frequency is determined at least partially based on operational requirements of the set of AC loads. In some embodiments, the system includes an AC/DC converter electrically connected to the AC bus, and a DC bus electrically connected to the AC/DC converter, where the AC/DC converter is configured to convert the AC power signal on the AC bus to a DC power signal on the DC bus.

Referring to <FIG>, an independent speed variable frequency (ISVF)-generator-based power system <NUM> is depicted. As described herein, the system <NUM> may be configured to generate and distribute power within a frequency band that, for a given range of shaft speeds, is narrower than would be generated using a typical induction generator without a constant speed drive device or other type of transmission device.

The system <NUM> includes a prime mover <NUM>. For example, the prime mover <NUM> may be an aircraft engine or another type of vehicle engine. The prime mover <NUM> may be attached to a coupler <NUM>. The coupler <NUM> may include a fixed gear ratio coupler, a belt coupler, or a combination thereof, and may be configured to convert torque <NUM> from the prime mover <NUM> to torque <NUM> on a shaft <NUM>. The coupler <NUM> may differ from a constant speed device in that the torque transfer ratios of the coupler <NUM> may be fixed, rather than variable. The shaft <NUM> may be attached to an ISVF generator <NUM>. By using an ISVF generator <NUM> instead of a typical induction generator, the system <NUM> may omit any constant speed drive device between the prime mover <NUM> and the shaft <NUM>.

The ISVF generator <NUM> is configured to convert torque <NUM> from the shaft <NUM> to an AC power signal <NUM>. The ISVF generator <NUM> generates the AC power signal <NUM> such that a frequency of the AC power signal <NUM> is independent of a shaft speed of the shaft <NUM>. An example of an ISVF generator <NUM> usable with the system <NUM> is described further in <CIT>, published as <CIT>, and entitled "Independent Speed Variable Frequency Alternating Current Generator".

The ISVF generator <NUM> has one or more pole pairs <NUM>. <FIG> depicts the ISVF generator <NUM> as having two pole pairs <NUM> (i.e., four poles). However, more or fewer than two pole pairs are possible and consistent with the disclosure. The number of pole pairs <NUM> may act as a multiplier between a frequency of the shaft <NUM> and a frequency of the AC power signal <NUM>. As such, the shaft <NUM> is associated with an equivalent shaft frequency that equals a shaft frequency of the shaft <NUM> multiplied by the number of pole pairs <NUM>.

The system <NUM> includes a generator control unit (GCU) <NUM>. The generator control unit <NUM> may be configured to generate an excitation signal <NUM> to control the ISVF generator <NUM>. The excitation signal <NUM> may be used by the ISVF generator <NUM> to generate a rotating magnetic flux at a rotor of the ISVF generator <NUM>, resulting in a frequency of the AC power signal <NUM> being an algebraic sum of the frequency of the shaft <NUM> and a frequency of the excitation signal <NUM>. The number of pole pairs <NUM> of the ISVF generator <NUM> may also affect the contribution of the excitation signal <NUM> to the AC power signal <NUM>. Thus, an equivalent excitation frequency of the excitation signal <NUM> may equal an excitation frequency of the excitation signal <NUM> multiplied by the number of pole pairs <NUM>.

The ISVF generator <NUM> may be coupled to an AC bus <NUM>. A set of AC loads <NUM> may be coupled to and configured to receive power from the AC bus <NUM>. The set of AC loads <NUM> may have operational requirements such that the set of AC loads <NUM> is adapted to operate within an operational frequency band having a lower frequency limit and an upper frequency limit. The operational frequency band may be narrower than an operating frequency band associated with the prime mover <NUM>.

An AC/DC converter <NUM> may be coupled to the AC bus <NUM>. The AC/DC converter <NUM> may be configured to convert the AC power signal <NUM> into a DC power signal <NUM> and to power a DC bus <NUM> using the DC power signal <NUM>. A set of DC loads <NUM> may be coupled to the DC bus <NUM>.

A direct-current to alternating-current (DC/AC) converter <NUM> may be coupled to the DC bus <NUM>. The DC/AC converter <NUM> may be configured to convert the DC power signal <NUM> to a second AC power signal <NUM> to power a second AC bus <NUM>. A set of AC loads <NUM> may be coupled to the second AC bus <NUM>. The second set of AC loads <NUM> may have different operational frequency and voltage requirements than the set of AC loads <NUM>. In some examples, the second set of AC loads <NUM> corresponds to motor loads, such as for actuating flight surfaces, etc..

During operation, the generator control unit <NUM> is configured to control the ISVF generator <NUM> to generate the AC power signal <NUM> to fall within a frequency band, having a lower frequency limit and an upper frequency limit, that, for a given range of shaft speeds, is narrower than would be generated using a typical induction generator. In a first configuration, in order to achieve the frequency band, the generator control unit <NUM> is configured to set a generator output frequency of the ISVF generator <NUM> equal to the lower frequency limit in response to the equivalent shaft frequency (e.g., the shaft frequency multiplied by the number of pole pairs <NUM>) being less than the lower frequency limit. The generator control unit <NUM> is configured to set the generator output frequency of the ISVF generator <NUM> equal to the upper frequency limit in response to the equivalent shaft frequency being greater than the upper frequency limit. In response to the equivalent shaft frequency being greater than or equal to the lower frequency limit and less than or equal to the upper frequency limit, the generator control unit <NUM> is configured to set the generator output frequency of the ISVF generator <NUM> equal to the equivalent shaft frequency. In a second configuration, the generator control unit <NUM> may simply maintain a constant generator output frequency rather than narrowing the frequency band.

A benefit of the system <NUM> is that by using the ISVF generator <NUM> to narrow a frequency range of the AC power signal relative to a rotational frequency range of the shaft, the system <NUM> may omit complex, heavy, and/or bulky equipment, such as constant speed drive, between the shaft and the ISVF generator <NUM>. A further benefit is that the generator control unit <NUM> may limit a generator output frequency to a constant generator output frequency that corresponds to operational requirements of the set of AC loads <NUM>. Another benefit is that, in cases where the AC bus <NUM> may support a range of frequencies, by narrowing the generator output frequency <NUM> (shown in <FIG>) to a range having a lower frequency limit and an upper frequency limit, the load equipment may be lighter and less complex due to a favorable operation condition of a narrow frequency band as described herein. Other benefits and advantages may exist.

<FIG> describe the concept of an equivalent shaft frequency <NUM> and an equivalent excitation frequency <NUM>. These frequencies depend on the number of pole pairs <NUM> associated with the ISVF generator <NUM>. In cases where there is only one pole pair, the equivalent shaft frequency <NUM> and the equivalent excitation frequency <NUM> are equal to a shaft frequency <NUM> and an excitation frequency <NUM>, respectively.

Referring to <FIG>, a graph is depicted that relates a shaft frequency band <NUM> to an equivalent shaft frequency band <NUM> and to an AC output signal frequency band <NUM>. As shown in <FIG>, a shaft frequency <NUM> may fall within the shaft frequency band <NUM>. The shaft frequency <NUM> represents the frequency at which a prime mover (e.g., the prime mover <NUM>) rotates a shaft (e.g., the shaft <NUM>). The shaft frequency band <NUM> may be bounded between a lower shaft frequency limit <NUM>, representing the lowest operational frequency at which the prime mover will rotate the shaft, and an upper shaft frequency limit <NUM>, representing the highest operational frequency at which the prime mover will rotate the shaft. For illustrative purposes, <FIG> depicts the lower shaft frequency limit <NUM> as being <NUM> and the upper shaft frequency limit <NUM> as being <NUM>.

The equivalent shaft frequency band <NUM> may be equal to the number of pole pairs of an ISVF generator (e.g., the ISVF generator <NUM>) multiplied by the shaft frequency band <NUM>. In the example of <FIG>, there may be two pole pairs such that if the shaft frequency <NUM> is about <NUM>, then an equivalent shaft frequency <NUM> is about <NUM>. Likewise, a lower equivalent shaft frequency limit <NUM> may be about <NUM> and an upper equivalent shaft frequency limit <NUM> may be <NUM>. Although, the example of <FIG> contemplates two pole pairs, any number of pole pairs may be used.

While a typical induction motor would ordinarily generate an AC power signal having a frequency that falls within the equivalent shaft frequency band <NUM>, an ISVF generator (e.g., the ISVF generator <NUM>) may be controlled through an excitation signal (e.g., the excitation signal <NUM>) to generate the narrower AC output signal frequency band <NUM>. For example, the AC output signal frequency band <NUM> may have a lower frequency limit <NUM> that is greater than the lower equivalent shaft frequency limit <NUM>. Likewise, the AC output signal frequency band <NUM> may have an upper frequency limit <NUM> that is less than the upper equivalent shaft frequency limit <NUM>. Thus, the AC output signal frequency band <NUM> may be limited to meet operational requirements of AC loads.

Referring to <FIG>, a graph is depicted and relates an excitation frequency band <NUM> to an equivalent excitation frequency band <NUM>. An excitation signal (e.g., the excitation signal <NUM>) may be applied to the field windings on a rotor of an ISVF generator (e.g., the ISVF generator <NUM>) to effectively increase or decrease a frequency of an output power signal. The excitation signal may have an excitation frequency <NUM> that falls within the excitation frequency band <NUM>. Because the number of pole pairs associated with the ISVF generator affects the output frequency, the equivalent excitation frequency band <NUM> may be equal to the excitation frequency band <NUM> multiplied by the number of pole pairs. Likewise, an equivalent excitation frequency <NUM> may be equal to the excitation frequency <NUM> multiplied by the number of pole pairs.

In a single pole pair system, the generator output frequency may be the algebraic sum of the shaft frequency and the excitation frequency: <MAT>.

For a system with multiple pole pairs, the shaft frequency and the excitation frequency may both be multiplied by the number of pole pairs (PP): <MAT> where fShaft*PP is the equivalent shaft frequency and fExcit*PP is the equivalent excitation frequency.

Referring to <FIG>, a graph <NUM> depicts a functional relationship <NUM> between power capacity requirement SExcit of an excitation signal and a frequency fExcit of the excitation signal. While the description in <FIG> may apply to a single pole pair system, the concepts may be expanded to multiple pole pairs as would be understood by persons of ordinary skill in the art, having the benefit of this application. As shown in the graph <NUM>, as the frequency fExcit moves away from zero, the excitation signal may need more power SExcit in order to maintain a constant power output from an ISVF generator. Analysis shows that this relationship can be roughly represented by a conic section curve as shown. When an excitation frequency fExcit is zero, which means that the shaft speed is equal to the generator output frequency, the excitation signal need only provide sufficient power to compensate for power loss at rotor windings (e.g., y<NUM>). The excitation frequency fExcit is positive when the generator shaft speed is lower than the generator output frequency. In this case, the excitation source provides an apparent power to the stator windings. The excitation frequency fExcit is negative when the generator shaft speed is higher than the generator output frequency. In this case, the excitation source sinks an apparent power from the generator shaft. The further the deviation (between excitation frequency fExcit and a generator rated output frequency) is, the higher the power capacity SExcit of excitation signal may be.

Referring to <FIG>, a first graph <NUM> depicts power requirements associated with a first configuration of the system <NUM> and is compared to a second graph <NUM> depicting power requirements associated with a second configuration of the system <NUM>. The first configuration may correspond to a constant frequency output and the second configuration may correspond to a narrowed frequency band output.

As shown in the first graph <NUM>, a generator control unit (e.g., the generator control unit <NUM>) may maintain a constant generator output frequency <NUM>. This may be performed by setting the equivalent excitation frequency <NUM> equal to a difference between the constant generator output frequency <NUM> and the equivalent shaft frequency <NUM>. In other words, the generator output frequency <NUM> may equal the algebraic sum of the shaft frequency <NUM> and the excitation frequency <NUM>: <MAT>.

In order to maintain a constant generator output frequency <NUM>, the excitation frequency <NUM> may be set to zero when the shaft frequency <NUM> is equal to the generator output frequency <NUM>, may be set to a positive value when the shaft frequency <NUM> is less than the generator output frequency <NUM>, and may be set to a negative value when the shaft frequency <NUM> is greater than the generator output frequency <NUM>: <MAT> <MAT> <MAT>.

When factoring in the pole pairs associated with an ISVF generator, the generator output frequency <NUM> may be determined as: <MAT>.

In order to maintain a constant generator output frequency <NUM>, the equivalent excitation frequency <NUM> may be set to zero when the equivalent shaft frequency <NUM> is equal to the generator output frequency <NUM>, may be set to a positive value when the equivalent shaft frequency <NUM> is less than the generator output frequency <NUM>, and may be set to a negative value when the equivalent shaft frequency <NUM> is greater than the generator output frequency <NUM>: <MAT> <MAT> <MAT>.

As shown in the first graph <NUM>, as the shaft frequency shifts away from the generator output frequency <NUM>, more power is allocated to the excitation signal. In order to provide power for a range of frequencies between the lower equivalent shaft frequency limit <NUM> and the upper equivalent shaft frequency limit <NUM>, a relatively high power <NUM> may be used. Thus, the constant generator output frequency configuration depicted in the first graph <NUM> may be appropriate when the range of shaft frequencies is relatively narrow. For engines that utilize a wider range of shaft frequencies, the configuration depicted in the second graph <NUM> may be more appropriate.

As shown in the second graph <NUM>, a generator control unit (e.g., the generator control unit <NUM>) may maintain a generator output frequency <NUM>. Instead of being constant, the generator output frequency <NUM> may be held between a lower frequency limit <NUM> and an upper frequency limit <NUM>: <MAT>.

If the shaft frequency <NUM> is between the lower frequency limit <NUM> and the upper frequency limit, then the generator output frequency <NUM> may be set equal to the shaft frequency <NUM> by setting the excitation frequency <NUM> of the excitation signal to zero. If the shaft frequency <NUM> is less than the lower frequency limit <NUM>, then the generator output frequency <NUM> may be set equal to the lower frequency limit <NUM> by setting the excitation frequency <NUM> to a difference of the lower frequency limit <NUM> and the shaft frequency <NUM>. If the shaft frequency <NUM> is greater than the upper frequency limit <NUM>, then the generator output frequency <NUM> may be set equal to the upper frequency limit <NUM> by setting the excitation frequency <NUM> of the excitation signal equal to a difference of the upper frequency limit <NUM> and the shaft frequency <NUM>: <MAT> <MAT> <MAT>.

When factoring in the pole pairs associated with an ISVF generator, if the equivalent shaft frequency <NUM> is between the lower frequency limit <NUM> and the upper frequency limit, then the generator output frequency <NUM> may be set equal to the equivalent shaft frequency <NUM> by setting the equivalent excitation frequency <NUM> of the excitation signal to zero. If the equivalent shaft frequency <NUM> is less than the lower frequency limit <NUM>, then the generator output frequency <NUM> may be set equal to the lower frequency limit <NUM> by setting the equivalent excitation frequency <NUM> to a difference of the lower frequency limit <NUM> and the equivalent shaft frequency <NUM>. If the equivalent shaft frequency <NUM> is greater than the upper frequency limit <NUM>, then the generator output frequency <NUM> may be set equal to the upper frequency limit <NUM> by setting the equivalent excitation frequency <NUM> of the excitation signal equal to a difference of the upper frequency limit <NUM> and the equivalent shaft frequency <NUM>: <MAT> <MAT> <MAT>.

As shown in the second graph <NUM>, as the shaft frequency shifts away from the generator output frequency <NUM>, more a constant level of power is allocated to the excitation signal while the shaft frequency is between the lower frequency limit <NUM> and the upper frequency limit <NUM>. Thus, a relatively low power <NUM> may be used for the same range of shaft frequencies. The narrow band generator output frequency configuration depicted in the second graph <NUM> may be appropriate when the range of shaft frequencies is relatively broad and AC loads are able to operate with frequencies between the lower frequency limit <NUM> and the upper frequency limit <NUM>.

Referring to <FIG>, an example of a method <NUM> for ISVF-generator-based power distribution is depicted. The method <NUM> includes rotating a shaft using a prime mover, at <NUM>. For example, the shaft <NUM> may be rotated using the prime mover <NUM>.

The method <NUM> further includes converting torque from the shaft to an AC power signal using an ISVF generator, where the ISVF generator has one or more pole pairs, and where an equivalent shaft frequency equals a shaft frequency of the shaft multiplied by a number of the pole pairs, at <NUM>. For example, the torque <NUM> from the shaft <NUM> may be converted to the AC power signal <NUM> using the ISVF generator <NUM>.

The method <NUM> may also include converting a second torque associated with the prime mover to the torque associated with the shaft using a coupler, at <NUM>. For example, the second torque <NUM> may be converted to the torque <NUM> using the coupler <NUM>.

The method <NUM> may further include generating an excitation signal to control the ISVF generator, where an equivalent excitation frequency equals an excitation frequency of the excitation signal multiplied by the number of pole pairs, at <NUM>. For example, the generator control unit <NUM> may generate the excitation signal <NUM>.

The method <NUM> also includes applying the AC power signal to an AC bus having a lower frequency limit and an upper frequency limit, at <NUM>. For example, the AC power signal <NUM> may be applied to the AC bus <NUM>.

The method <NUM> includes setting a generator output frequency of the ISVF generator equal to the lower frequency limit when the equivalent shaft frequency is less than the lower frequency limit, at <NUM>. Setting the generator output frequency of the ISVF generator equal to the lower frequency limit may include setting the equivalent excitation frequency to a difference of the lower frequency limit and the equivalent shaft frequency, at <NUM>.

The method <NUM> further includes setting the generator output frequency of the ISVF generator equal to the upper frequency limit when the equivalent shaft frequency is greater than the upper frequency limit, at <NUM>. Setting the generator output frequency equal to the upper frequency limit may include setting the equivalent excitation frequency of the excitation signal equal to a difference of the upper frequency limit and the equivalent shaft frequency, at <NUM>.

The method <NUM> includes setting the generator output frequency of the ISVF generator equal to the equivalent shaft frequency when the equivalent shaft frequency is between the lower frequency limit and the upper frequency limit, at <NUM>. Setting the generator output frequency equal to the equivalent shaft frequency comprises setting the equivalent excitation frequency of the excitation signal to zero, at <NUM>.

Claim 1:
A system (<NUM>) comprising:
a prime mover (<NUM>) configured to rotate a shaft (<NUM>);
an independent speed variable frequency, ISVF, generator (<NUM>) configured to convert torque (<NUM>) from the shaft (<NUM>) to an alternating current, AC, power signal (<NUM>), such that a frequency of the AC power signal (<NUM>) is independent of a shaft speed of the shaft (<NUM>), wherein the ISVF generator (<NUM>) has one or more pole pairs (<NUM>), and wherein an equivalent shaft frequency (<NUM>) equals a shaft frequency (<NUM>) of the shaft (<NUM>) multiplied by a number of the pole pairs (<NUM>),
characterized by further comprising:
an AC bus (<NUM>) having a lower frequency limit (<NUM>) and an upper frequency limit (<NUM>); and
a generator control unit (<NUM>) configured to:
set a generator output frequency (<NUM>) of the ISVF generator (<NUM>) equal to the lower frequency limit (<NUM>) when the equivalent shaft frequency (<NUM>) is less than the lower frequency limit (<NUM>);
set the generator output frequency (<NUM>) of the ISVF generator (<NUM>) equal to the upper frequency limit (<NUM>) when the equivalent shaft frequency (<NUM>) is greater than the upper frequency limit (<NUM>); and
set the generator output frequency (<NUM>) of the ISVF generator (<NUM>) equal to the equivalent shaft frequency (<NUM>) when the equivalent shaft frequency (<NUM>) is greater than or equal to the lower frequency limit (<NUM>) and less than or equal to the upper frequency limit (<NUM>).