METHOD OF OPERATING A MOTOR COUPLED TO A MECHANICAL LOAD

Abstract

The present disclosure relates to a method of operating a motor having a rotary component coupled to a mechanical load. The method includes determining a fluctuating speed demand corresponding to a temporally varying component of a speed of the rotary component. The method also includes determining a rotary component speed reference value based on the fluctuating speed demand. Also, the method includes controlling the motor in accordance with the rotary component speed reference value. The fluctuating speed demand is determined so as to reduce variations in a total power drawn or provided by the motor in use.

Description

FIELD OF THE INVENTION

The present disclosure relates to a method of operating a motor coupled to a mechanical load. The present disclosure relates further to an electric drive apparatus comprising a motor coupled to a mechanical load and a controller adapted to perform such a method and to a transport refrigeration system comprising such an electric drive apparatus.

BACKGROUND OF THE INVENTION

It is known to provide an electric drive apparatus comprising a DC bus, an inverter and an electric motor having a rotor which is coupled to a mechanical load. The electric drive apparatus may generally be configured to drive the mechanical load. The mechanical load may be, for instance, a compressor or a fan associated with a refrigeration circuit of a transport refrigeration system. The DC bus may be configured to receive power from an external power supply.

In use, an electrical power drawn from the DC bus by the electric motor may vary, e.g. as a result of variations in the mechanical load driven by the electric motor. A voltage of the DC bus may be subject to undesirable fluctuations as a result.

It is thus desirable to provide a method of controlling an electric motor and/or to provide an electric drive apparatus which is able to effectively and efficiently prevent large fluctuations in the voltage of the DC bus.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, there is provided a method of operating a motor having a rotary component coupled to a mechanical load, the method comprising:

• • determining a fluctuating speed demand corresponding to a temporally varying component of a speed of the rotary component;
  • determining a rotary component speed reference value based on the fluctuating speed demand; and
  • controlling the motor in accordance with the rotary component speed reference value, wherein the fluctuating speed demand is determined so as to reduce variations in a total power drawn or provided by the motor in use.

The fluctuating speed demand may be determined so as to reduce variations in a total electrical power drawn by the motor in use.

The fluctuating speed demand may correspond to a cyclically varying component of the speed of the rotary component. The speed of the rotary component may be monitored using a rotary component sensing arrangement.

It may be that controlling the motor in accordance with the rotary component speed reference value includes controlling the motor to target maintaining the speed of the rotary component at the rotary component speed reference value.

It may be that the rotary component speed reference value varies with respect to time in accordance with the fluctuating speed demand.

It may be that the method comprises:

• • calculating a torque associated with driving of the mechanical load based on a predetermined inertia value, the speed of the rotary component and a total torque applied to the rotary component; and determining the fluctuating speed demand based on the predetermined inertia value and the torque associated with driving of the mechanical load.
  • The predetermined inertia value may correspond to a combination of an inertia of the rotary component and an inertia the mechanical load.

It may be that the method comprises calculating the torque associated with driving of the mechanical load based on the predetermined inertia value, a rate of change of the speed of the rotary component and the total torque applied to the rotary component.

It may be that the method comprises:

• • calculating an inertial torque by multiplying the predetermined inertia value by the rate of change of the speed of the rotary component; and
  • calculating the torque associated with driving of the mechanical load by subtracting the inertial torque from the total torque applied to the rotary component.

Calculating the torque associated with driving of the mechanical load (240) may include setting the torque associated with driving of the mechanical load as being equal to a subtraction of the inertial torque from the total torque applied to the rotary component.

It may be that the method comprises determining the fluctuating speed demand by dividing a time-integral of a fluctuating component of the torque associated with driving of the mechanical load by the predetermined inertia value.

It may be that the method comprises:

• • determining or receiving an average speed demand corresponding to a constant component of the speed of the rotary component; and
  • determining the rotary component speed reference value based on the average speed demand and the fluctuating speed demand.

According to a second aspect, there is provided an electric drive apparatus comprising:

• • a controller adapted to perform a method in accordance with the first aspect; and
  • a motor having a rotary component coupled to a mechanical load.

It may be that the electric drive apparatus further comprises a power converter, wherein the motor is configured to draw an AC electrical power or a DC electrical power from the power converter in use.

It may be that the electric drive apparatus further comprises a DC bus. The power converter may be configured to receive an input DC input voltage from a power supply via the DC bus.

According to a third aspect, there is provided a transport refrigeration system comprising an electric drive apparatus in accordance with the second aspect and a vapor-compression refrigeration circuit including a compressor. The mechanical load may include the compressor.

According to a fourth aspect, there is provided a computer program comprising instructions which, when executed by the controller of an electric drive apparatus in accordance with the second aspect, cause the controller to execute a method in accordance with the first aspect.

According to a fifth aspect, there is provided a machine-readable medium having stored thereon a computer program in accordance with the fourth aspect.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a vehicle comprising a transport refrigeration system;

FIG. 2 is a schematic diagram of example transport refrigeration unit suitable for use with the vehicle of FIG. 1, the example transport refrigeration unit comprising a vapour-compression refrigeration circuit having a compressor;

FIG. 3 is a schematic diagram of an example electric drive apparatus suitable for use with the transport refrigeration unit of FIG. 2;

FIG. 4 is a flowchart which shows an example method of operating an electric motor of the electric drive apparatus of FIG. 3;

FIG. 5 is a block diagram which shows part of the example method of FIG. 4 in detail; and

FIG. 6 is a highly schematic diagram of a machine-readable medium having stored thereon a computer program which, when executed by a controller, causes the controller to perform the example method of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a vehicle 10 comprising a transport refrigeration system 20. In the example of FIG. 1, the transport refrigeration system 20 forms a part of an over-the-road refrigerated semi-trailer having a structure 22 supporting (or forming) at least one climate-controlled compartment 24 which is configured to be cooled and/or heated by a TRU 110. The climate-controlled compartment 24 can take the form of multiple compartments or have multiple zones. The structure 22 includes a chassis. The vehicle 10 comprises an electrical apparatus 100 which includes various components disposed within an under-chassis box 105. In some examples, one or more components of the electrical apparatus 100 may be integrated or incorporated into the TRU 110. The structure 22 supports the TRU 110 and the under-chassis box 105. The vehicle 10 further comprises a tractor unit 14 removably couplable to the trailer.

FIG. 2 schematically shows a diagram of an example TRU 110 suitable for use within the vehicle 10 and the transport refrigeration system 20 of FIG. 1. The TRU 110 comprises a vapour-compression refrigeration circuit 400. The vapour-compression refrigeration circuit 400 includes an evaporator 408 which is configured to receive heat from the climate-controlled compartment 24 of the transport refrigeration system 20 and a condenser 404 which is configured to reject heat to a thermal sink 44 (e.g., ambient air outside of the climate-controlled compartment 24). For these purposes, the vapour-compression refrigeration circuit 400 also includes a compressor 402 and an expansion valve 406. Accordingly, the vapour-compression refrigeration circuit 400 may be controlled to cause heat to be removed from the climate-controlled compartment 24. The TRU 110 also comprises a plurality of fans 462, 464. In the example of FIG. 2, a first fan 462 is associated with (e.g. located in proximity to) the condenser 404 for improving heat transfer at one or more surfaces of the condenser 404, and a second fan 464 is associated with (e.g. located in proximity to) the evaporator 408 for improving heat transfer at one or more surfaces of the evaporator 408.

The compressor 402 is mechanically coupled to, and is therefore configured to be driven by, an electric drive apparatus 200. The electric drive apparatus 200 may be operated to drive the compressor 402 and thereby control the vapour-compression refrigeration circuit 400. The compressor 402 may be any suitable type of compressor, as will be apparent to those skilled in the art. However, in particular examples, the compressor 402 may be a reciprocating compressor (e.g., a piston compressor) or a scroll compressor.

FIG. 3 is a schematic diagram of an example electric drive apparatus 200 which is particularly, but not exclusively, suitable for use with the vapour-compression refrigeration circuit 400 of FIG. 2 and the vehicle 10/transport refrigeration system 20 of FIG. 1. The example electric drive apparatus 200 comprises a DC bus 210, an inverter system 220, an electric motor 230 (e.g., an AC electric motor 230), a plurality of current sensors 282A-282C, a rotor sensing arrangement 283 and a controller 290.

The electric motor 230 includes a rotor 234, a stator 232 and a plurality of phase windings 221-223. In the example of FIG. 3, the plurality of phase windings comprises a first phase winding 221, a second phase winding 222 and a third phase winding 223. Consequently, the electric motor 230 shown by FIG. 3 may be described as a three-phase AC electric motor 230. Also in the example of FIG. 3, each of the plurality of phase windings 221-223 is associated with (e.g., disposed on or within) the stator 232. Nevertheless, it will be appreciated that in other examples in accordance with the present disclosure, the plurality of phase windings may be comprised of any suitable number of phase windings and/or each of the plurality of phase windings may be associated with (e.g., disposed on or within) the rotor 234. In addition, in the example of FIG. 3, the phase windings 221-223 are shown as being connected in a delta configuration. However, it will be appreciated that the phase windings 221-223 may be otherwise connected, for example in a star configuration. Each phase winding 221-223 may comprise one or more coils. The rotor 234 may be referred to as a rotary component 234.

The rotor 234 is mechanically coupled to a load 240 (e.g., a mechanical load 240) and is therefore configured to drive the load 240. The load 240 is directly or indirectly driven by rotation of the rotor 234. By way of example, the load 240 may be directly mechanically coupled to the load 240 via a driveshaft. By way of further example, the load 240 may be indirectly coupled to the load 240 via a transmission comprising a gearbox. In the example electric drive apparatus 200 of FIG. 3, the load 240 is (e.g., includes) a compressor 402 (e.g., the compressor 402 of the vapour-compression refrigeration circuit 400 of FIG. 2).

The inverter system 220 is generally configured to receive an input DC voltage from the DC bus 210 and to provide a plurality of output AC voltages to the plurality of phase windings 221-223. The input DC voltage corresponds to an operating voltage of the DC bus 210. Accordingly, the electric motor 230 draws AC electrical power from the inverter system 220 in use. The inverter system 220 may comprise a plurality of switching devices, at least one inductive element and/or at least one capacitive element arranged in a suitable inverter system topology, as will be known to those skilled in the art. The inverter system 220 may also be referred to as a power converter 220. In other examples in accordance with the present disclosure, the electric motor 230 may be configured to draw DC electrical power from a DC-DC converter (e.g., a buck converter, a boost converter, a buck-boost converter or the like) which is coupled to the DC bus 210.

The DC bus 210 is configured to receive a DC supply voltage from a DC power supply 205. The DC power supply 205 may be, or be derived from, an internal power supply (e.g., a fuel-cell pack, a battery or a rectifier coupled to a generator). If so, the DC power supply 205 may be disposed within the electrical apparatus 100 of the vehicle 10 shown by FIG. 1 (e.g., within the under-chassis box 105). Otherwise, the DC power supply 205 may be, or be derived from, an external power supply (e.g. a shore power supply) which is couplable to the electrical apparatus 100 via an external connection port and appropriate cabling. In particular, the DC power supply 205 may be a rectifier which is configured to receive an AC voltage received from the shore power supply. Such a rectifier may be referred to as a grid converter.

The controller 290 is generally configured to control operation of the electric drive apparatus 200. More specifically, the controller 290 is configured to operate the electric motor 230 according to a method in accordance with the example method described below with reference to FIGS. 4 and 5.

Although it has been described that the example electric drive apparatus 200 comprises an AC electric motor 230 and/or an inverter system 220, this need not necessarily be the case. By way of example, the electric motor 230 may be a DC electric motor comprising one or more windings and the electric motor 230 may be configured to receive an input DC voltage directly from the DC bus 210 for supply to the winding(s) of the electric motor 230. By way of further example, the electric motor 230 may be a DC electric motor comprising one or more windings and the electric drive apparatus 200 may also comprise a DC-DC converter. The DC-DC converter may be configured to receive an input DC voltage from the DC bus 210 and to provide one or more output DC voltage to the winding(s) of the electric motor 230.

FIG. 4 is a flowchart which shows a method 300 of operating a motor 230 having a rotor 234 coupled to a mechanical load 240. The motor 230 may be the example motors 230 described above with reference to FIG. 3. The method 300 includes an action of determining, at block 310, a fluctuating speed demand ω_d,f(t) as well as an action of determining or receiving, at block 320, an average speed demand ω_d,a. The fluctuating speed demand ω_d,f(t) corresponds to a temporally (e.g., cyclically) varying component of a speed of the rotor 234 whereas the average speed demand ω_d,a corresponds to a temporally constant component of the speed of the rotor 234. Consequently, the fluctuating speed demand ω_d,f(t) varies with respect to time while the average speed demand ω_d,a does not vary with respect to time.

The action of determining, at block 310, the fluctuating speed demand ω_d,f (t) is described in further detail below with reference to FIG. 5.

The average speed demand ω_d,a may be determined or received, at block 320, as a steady-speed setpoint for the rotor 234. If the mechanical load 240 is, or comprises, a compressor 402 as shown by FIG. 3, the average speed demand ω_d,a may be determined or received as a steady-speed setpoint for the rotor 234 for driving the compressor 402 at a speed which is appropriate for controlled operation of a vapour-compression circuit 400 of which the compressor 402 forms a part (e.g., the vapour-compression circuit 400 of FIG. 2). In an example, the controller 290 may determine the average speed demand ω_d,a based on a signal received from a dedicated controller of the TRU 110. In another example, the controller 290 may receive the average speed demand ω_d,a based on a signal received from a dedicated controller of the TRU 110.

The method 300 also includes an action of determining (e.g., calculating), at block 330, a rotor speed reference value ω_d,r (t) based on the fluctuating speed demand ω_d,f(t) and the average speed demand ω_d,a as determined or received at blocks 310 and 320, respectively. The rotor speed reference value ω_d,r (t) may also be referred to as a rotary component speed reference value ω_d,r(t). In particular, the rotor speed reference value ω_d,r(t) may be determined as a sum of the fluctuating speed demand ω_d,f(t) and the average speed demand ω_d,a, as shown by Equation (1). It follows that the rotor speed reference value ω_d,r(t) varies with respect to time in accordance with the fluctuating speed demand ω_d,f(t).

$\begin{array}{cc}{\omega }_{d,r}\left(t\right)={\omega }_{d,f}\left(t\right)+{\omega }_{d,a}& \left(1\right)\end{array}$

The method 300 further comprises an action of controlling, at block 340, the motor 230 in accordance with the determined rotor speed reference value ω_d,r (t). Controlling, at block 340, the motor 230 in accordance with the rotor speed reference value ω_d,r (t) may generally include controlling the motor 230 to target maintaining the speed of the rotor 234 at the rotor speed reference value ω_d,r(t). Controlling the motor 230 to target maintaining the speed of the rotor 234 at the rotor speed reference value may be implemented by means of, for example, a proportional-integral (PI) logic, a proportional-integral-derivative (PID) logic or a feedforward logic. If so, the rotor speed reference value ω_d,r(t) may serve as the demand signal for such a logic, as will be appreciated by those skilled in the art.

FIG. 5 is a block diagram which shows the action of determining the fluctuating speed demand ω_d,f(t) represented by block 310 in FIG. 4 in detail. Although the processes shown in FIG. 5 are expressed in the time domain, those skilled in the art will understand that various processes shown in FIG. 5 may generally be carried out in the Laplace domain (that is, the s-domain).

For the purpose of the processes shown in detail by FIG. 5, a total torque T_m(t) applied to the rotor 234 is assumed to be substantially only a function of (e.g., composed of) a torque T_l(t) associated with (e.g., provided for) driving of the mechanical load 240 and an inertial torque T_j(t).

The total torque T_m(t) applied to the rotor 234 relates to a torque applied to the rotor 234 by means of a magnetic field induced by an electric current supplied to the winding(s) 221-223 of the motor 230. Accordingly, the total torque T_m(t) may be controlled by, for instance, controlling the electric current supplied to the winding(s) 221-223 of the motor 230. For the same reason, the total torque T_m(t) may be determined based on the electric current supplied to the winding(s) 221-223 of the motor 230 and an appropriate motor torque coefficient (e.g., a motor torque constant). The electric current supplied to the windings(s) 221-223 of the motor 230 may be monitored using suitable current transducers, such as the current sensors 282A-282C described above with reference to FIG. 3.

The inertial torque T_j(t) relates to an angular acceleration of the rotor 234. If the inertial torque has a positive sign, the rotor 234 accelerating and therefore energy is being stored in the form or rotational kinetic energy of both the rotor 234 and the mechanical load 240. Otherwise, if the inertial torque has a negative sign, the rotor 234 is decelerating and energy is being converted from the form of rotational kinetic energy of both the rotor 234 and the mechanical load 240 into the form of mechanical work (e.g., to drive of the mechanical load 240).

The inertial torque T_j(t) may be calculated using Equation (2), in which J_m+1 is a predetermined inertia value and ω(t) is an actual angular speed of the rotor 234. The actual angular speed of the rotor 234 may be monitored using a suitable rotor speed transducer, such as the rotor sensing arrangement 283 described above with respect to FIG. 3. The rotor sensing arrangement 283 may be configured to directly monitor an angular position of the rotor 234 and to calculate the angular speed of the rotor as a rate of change of the angular position or the rotor 234 with respect to time t. The predetermined inertia value J_m+1 corresponds to a combination of (e.g., a sum of) an inertia J_m of the motor 230 (that is, an inertia of the rotor 234) and an inertia J_l of the mechanical load 240.

$\begin{array}{cc}{T}_J\left(t\right)=J_{m+l}\times \frac{d\omega \left(t\right)}{\mathrm{dt}}& \left(2\right)\end{array}$

On the basis of this assumption, a torque T_l(t) associated with driving of the mechanical load 240 may be calculated using Equation (3), in which the torque T_l(t) associated with driving of the mechanical load 240 is set as being equal to a difference between the total torque T_l(t) applied to the rotor 234 and the inertial torque T_j(t). The torque T_l(t) associated with driving of the mechanical load 240 may be referred to as the overall load torque T_l(t).

$\begin{array}{cc}{T}_l\left(t\right)=T_m\left(t\right)-T_J\left(t\right)\therefore T_l\left(t\right)=T_m\left(t\right)-J_{m+l}\times \frac{d\omega \left(t\right)}{\mathrm{dt}}& \left(3\right)\end{array}$

Equations (2) and (3) may each be evaluated using the processes shown by FIG. 5. Namely, the actual angular speed ω(t) of the rotor 234 is accepted, at block 512, as a first input, the predetermined inertia value J_m+1 is accepted, at block 514, as a second input and the total torque T_m(t) applied to the rotor 234 is accepted, at block 516, as a third input. The actual angular speed ω(t) is differentiated, at block 522, to give a rate of change dω(t)/dt of the actual angular speed of the rotor 234. The rate of change dω(t)/dt of the actual angular speed or the rotor 234 is multiplied, at block 524, by the predetermined inertia value J_m+1 to give the inertial torque T_j(t) as defined by Equation (2). The inertial torque T_j(t) is then subtracted, at block 526, from the total torque T_m(t) applied to the rotor 234 to give the overall load torque T_l(t) as specified by Equation (3). Calculation of the overall load torque T_l(t) represents a first part of the action of determining the fluctuating speed demand ω_d,f(t) represented by block 310 in FIGS. 4 and 5. The calculated overall load torque T_l(t) may be provided, at block 544, as an output for use in other parts of a method 300 in accordance with the present disclosure and/or for other monitoring and/or control purposes in respect of the electric motor 230 and/or the electric drive apparatus 200.

A second part of the action of determining the fluctuating speed demand ω_d,f(t), represented by block 310, is now described with reference to each of blocks 532-538 in FIG. 5. An underlying assumption of the second part of the action of determining the fluctuating speed demand ω_d,f(t) is that, at any given point in time, a total electrical power P_m drawn by the motor 230 is given by Equation (4), in which P_j(t) is an inertial power and P_l(t) is a power associated with (e.g., provided for) driving of the mechanical load 240. The total electrical power P_m drawn by the motor 230 is substantially equal to a total mechanical power provided by the motor 230 (neglecting electrical losses in the motor 230), and the two may be referred to interchangeably herein. The power associated with (e.g., provided for) driving of the mechanical load 240 may be referred to as the load power P_l(t), whereas the total power P_m drawn or provided by the motor 230 may be referred to as the motor power P_m.

$\begin{array}{cc}{P}_m=P_J\left(t\right)+P_l\left(t\right)& \left(4\right)\end{array}$

In Equation (4), the load power P_l(t) is denoted as being a function of time t. In particular, the load power P_l(t) may be highly variable with respect to time due to the nature of the mechanical load 240. For instance, if the mechanical load 240 is, or comprises, a compressor 402 as shown in the example of FIG. 3, the power associated with driving of the compressor 402 (i.e., load power P_l(t)) may vary throughout a compression cycle of the compressor 402 due to variations in the rate of change of mechanical work done on the working fluid provided to the compression during the compression cycle. If the compressor 402 is a reciprocating compressor, the power associated with driving of the compressor 402 may vary significantly throughout the compression cycle of the compressor 402. Those skilled in the art will be aware of other types of mechanical loads which may also result in the load power Pt (t) varying with respect to time.

Also, for the purposes of the second part of the action of determining the fluctuating speed demand ω_d,f(t), the load power P_l(t) is considered to be composed of a fluctuating power component P_l,f(t) and an average (e.g., constant) power component P_l,a, per Equation (5). Of these two components, only the fluctuating power component P_l,f(t) is a function of time t.

$\begin{array}{cc}{P}_l\left(t\right)=P_{l,f}\left(t\right)+P_{l,a}& \left(5\right)\end{array}$

Likewise, the overall load torque T_l is considered to be composed of a fluctuating load torque component T_l,f(t) and an average (e.g., constant) load torque component T_l,a, per Equation (6). Again, of these components, only the fluctuating load torque component T_l,f (t) is a function of time t.

$\begin{array}{cc}{T}_l\left(t\right)=T_{l,f}\left(t\right)+T_{l,a}& \left(6\right)\end{array}$

In order to enable determination of a rotor speed reference value ω_d,r (t) including a fluctuating speed demand ω_d,f(t) which at least partially eliminates (e.g., reduces) variations in total power P_m drawn by the motor 230, the inertial power P_j(t) is set as being equal to the fluctuating power component P_l,f (t) per Equation (7a). That is to say that the fluctuating power component P_l,f (t) is considered to be entirely provided by the inertial power P_j (t). This means that the combined inertia of the rotor 234 and the mechanical load 240 is used as an energy store for the purpose of smoothing variations in the total power P_m drawn by the motor 230.

$\begin{array}{cc}{P}_J\left(t\right)=P_{l,f}\left(t\right)& \left(7a\right)\end{array}$

By noting that rotational power is by definition equal to a product of torque and rotational speed, Equation (7a) may be written in the form of Equation (7b).

$\begin{array}{cc}{T}_J\left(t\right)\times \omega \left(t\right)=T_{l,f}\left(t\right)\times \omega \left(t\right)\therefore T_J\left(t\right)=T_{l,f}\left(t\right)& \left(7b\right)\end{array}$

Further, substituting Equation (2) into Equation (7b) yields Equation (7c) where ω_d,r(t) is the fluctuating speed demand to be appropriately determined as discussed above. In turn, Equation (7c) rearranges to give an expression for the fluctuating speed component ω_f (t), as shown by Equation (7d).

$\begin{array}{cc}{J}_{m+l}\times \frac{d{\omega }_{d,r}\left(t\right)}{\mathrm{dt}}=J_{m+l}\times \frac{d\left({\omega }_{d,f}\left(t\right)+{\omega }_a\right)}{\mathrm{dt}}=J_{m+l}\times \frac{d{\omega }_{d,f}\left(t\right)}{\mathrm{dt}}=T_{l,f}\left(t\right)& \left(7c\right)\end{array}$ $\begin{array}{cc}{\omega }_{d,f}\left(t\right)=\frac{1}{J_{m+l}}\times \int \left(T_{l,f}\left(t\right)\right)\mathrm{dt}& \left(7d\right)\end{array}$

Since the fluctuating load torque component T_l,f (t) is equal to a difference between the overall load torque T_l(t) and the constant load torque component T_l,a, Rearrangement and reformulation of Equation (5) gives Equation (8), which may in turn be substituted into Equation (7d) to give Equation (9).

$\begin{array}{cc}{T}_{l,f}\left(t\right)=T_l\left(t\right)-T_{l,a}\left(t\right)=T_l\left(t\right)-\stackrel{\_}{T_l\left(t\right)}& \left(8\right)\end{array}$

Equations (7d) and (8) are each evaluated by the additional processes shown by FIG. 5. That is, the overall load torque T_l(t) is averaged, at block 532, to give the constant load torque component T_l,a. The averaging of the overall load torque T_l(t) at block 532 may be carried out by calculating an arithmetic mean of a bounded time-series of values of the overall load torque T_l(t) received during a suitable time period preceding the current time. Other techniques for carrying out the averaging of the overall load torque T_l(t) at block 532 will be apparent to those skilled in the art and may be used in accordance with the method 300. The constant load torque component T_l,a is then subtracted, at block 534, from the overall load torque T_l(t) to give the fluctuating load torque component T_l,f (t) as defined by Equation (8). Subsequently, the fluctuating load torque component T_l,f(t) is integrated, at block 536 to give a time-integral ∫(T_l,f (t))dt of the fluctuating load torque component T_l,f (t). The time-integral ∫(T_l,f (t))dt of the fluctuating load torque component T_l,f (t) is divided, at block 538, by the predetermined inertia value J_m+1 to produce a fluctuating speed demand ω_d,f(t) for at least partially reducing variations in the motor power P_m (that is, the total power drawn or supplied by the motor 230, as the case may be). The fluctuating speed demand ω_d,f(t) is then provided, at block 542, as an output for use in the following step(s) of the method 300 (that is, in block 330).

In accordance with the method 300, energy is selectively converted into, and from, rotational kinetic energy of the rotor 234 and the mechanical load 240 to allow the motor power P_m to remain substantially constant whilst the load power P_l(t) varies with respect to time. The rate at which energy is to be converted into, and from, rotational kinetic energy of the rotor 234 and the mechanical load 240 therefore corresponds to the inertial power P_j(t). In a previously-considered electric drive apparatus, the operating voltage of the DC bus 210 was prone to significant variations with respect to time due to variations in the motor power P_m and a lack of an ability of the power supply (e.g., a grid converter) to provide a correspond variable quantity of electrical power to the DC bus. To address this issue, a relatively large electrical energy storage device (i.e., a large capacitor) was functionally provided to the DC bus 210 for the purpose of smoothing variations in the operating voltage of the DC bus 210. An electric drive apparatus 200 controlled according to the example method 300 described herein may not exhibit substantial variations in the motor power P_m and therefore the operating voltage of the DC bus 210 may be generally stable without a need for a relatively large electrical energy storage device to be provided to the DC bus 210. For this reason, an electric drive apparatus 200 in accordance with the present disclosure may be lighter, less costly, and/or have a lower part count than a previously-considered electric drive apparatus.

FIG. 6 shows, highly schematically, a machine-readable medium 600 having stored thereon a computer program 60 comprising instructions which, when executed by the controller 290 provided to an electric drive apparatus in accordance with the example electric drive apparatus 200 as described above with reference to FIG. 3, cause the controller 290 to execute the method 300 described above with reference to FIGS. 4 and 5.

Except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore, except where mutually exclusive, any feature described herein may be applied to any aspect and/or combined with any other feature described herein. Moreover, while the present disclosure is made with in the context of transport refrigeration systems and/or vapour-compression circuits, it will be appreciated that the present disclosure has other possible applications in other technical areas.

Further, although the method 300 has been described herein with reference to an electric motor 230 which forms a part of an electric drive apparatus 200, it should be appreciated that the principles described herein have applications which go beyond this technical application. By way of example, many of the principles described herein may be utilized for control of a motor in the form of a heat engine, such as a spark ignition (e.g., an Otto-cycle engine) engine or a compression ignition (e.g., a Diesel-cycle) engine using any necessary adaptations which will be apparent to those skilled in the art. If so, the rotor 234 may form part of, or be, a rotary component 234 of the engine (i.e., a crankshaft) and the mechanical load 240 may form part of, or be, a piston 240 of the engine. If so, a fluctuating speed demand as described herein may be determined so as to reduce variations in a total mechanical power (e.g., a brake power or a shaft power) provided by the motor 230 (i.e., the engine) via the crankshaft in use. The motor 230 may be controlled in accordance with the rotary component speed reference value ω_r,d (t) by means of various actuators provided to the engine 230, such as valve(s) and/or crankshaft drive means. Implementation of the method 300 in such a manner may reduce or eliminate the need for a large and/or heavy flywheel to be coupled to the crankshaft to reduce variations in a mechanical power (e.g., a brake power or a shaft power) provided by the motor 230 (i.e., the engine) in use.

Claims

1. A method of operating a motor having a rotary component coupled to a mechanical load, the method comprising: determining a fluctuating speed demand ωf,d(t) corresponding to a temporally varying component of a speed ω(t) of the rotary component;determining a rotary component speed reference value ωr,d(t) based on the fluctuating speed demand; andcontrolling the motor in accordance with the rotary component speed reference value, whereinthe fluctuating speed demand is determined so as to reduce variations in a total power drawn or provided by the motor in use.

2. The method of claim 1, wherein the rotary component speed reference value ωr,d(t) varies with respect to time in accordance with the fluctuating speed demand ωf,d (t).

3. The method of claim 1, comprising: calculating a torque (Tl(t)) associated with driving of the mechanical load based on a predetermined inertia value Jm+1, the speed ω(t) of the rotary component and a total torque Tm(t) applied to the rotary component; anddetermining the fluctuating speed demand ωf,d(t) based on the predetermined inertia value and the torque associated with driving of the mechanical load.

4. The method of claim 3, wherein the predetermined inertia value Jm+1 corresponds to a combination of an inertia of the rotary component and an inertia the mechanical load.

5. The method of claim 3, comprising: calculating the torque Tl(t) associated with driving of the mechanical load based on the predetermined inertia value Jm+1, a rate of change dω(t)/dt of the speed of the rotary component and the total torque Tm(t) applied to the rotary component.

6. The method of claim 5, comprising: calculating an inertial torque Tj(t) by multiplying the predetermined inertia value Jm+1 by the rate of change dω(t)/dt of the speed of the rotary component; andcalculating the torque Tl(t) associated with driving of the mechanical load by subtracting the inertial torque from the total torque applied to the rotary component.

7. The method of claim 6, wherein calculating the torque Tl(t) associated with driving of the mechanical load includes setting the torque associated with driving of the mechanical load as being equal to a subtraction of the inertial torque Tj(t) from the total torque Tm(t) applied to the rotary component.

8. The method of claim 3, comprising: determining the fluctuating speed demand by dividing a time-integral of a fluctuating component Tl,f (t) of the torque Tl(t) associated with driving of the mechanical load by the predetermined inertia value Jm+1.

9. The method of claim 1, comprising: determining or receiving an average speed demand ωa,d (t) corresponding to a constant component of the speed ω(t) of the rotary component; anddetermining (330) the rotary component speed reference value ωr,d(t) based on the average speed demand and the fluctuating speed demand ωf,d (t).

10. An electric drive apparatus comprising: a motor having a rotary component coupled to a mechanical load; anda controller adapted to: determine a fluctuating speed demand ωf,d(t) corresponding to a temporally varying component of a speed ω(t) of the rotary component;determine a rotary component speed reference value ωr,d(t) based on the fluctuating speed demand; andcontrol the motor in accordance with the rotary component speed reference value,wherein the fluctuating speed demand is determined so as to reduce variations in a total power drawn or provided by the motor in use.

11. The electric drive apparatus of claim 10, further comprising a power converter, wherein the motor is configured to draw an AC electrical power or a DC electrical power from the power converter in use.

12. The electric drive apparatus of claim 11, further comprising a DC bus, wherein the power converter is configured to receive an input DC input voltage from a power supply via the DC bus.

13. A transport refrigeration system comprising the electric drive apparatus of claim 10 and a vapor-compression refrigeration circuit including a compressor, wherein the mechanical load includes the compressor.

14. A computer program comprising instructions which, when executed by the controller of the electric drive apparatus of claim 10, cause the controller to: determine a fluctuating speed demand ωf,d (t) corresponding to a temporally varying component of a speed ω(t) of the rotary component;determine a rotary component speed reference value ωr,d(t) based on the fluctuating speed demand; andcontrol the motor in accordance with the rotary component speed reference value,wherein the fluctuating speed demand is determined so as to reduce variations in a total power drawn or provided by the motor in use.

15. A machine-readable medium having stored thereon the computer program of claim 14.