Due to the decoupled nature of the homopolar machine, two independent loops for sensorless control have been designed as shown in Figure 3.16 (Tsaoet al., 2002).
One closed loop is a proportional–integral (PI) module for controlling the quadrant- axis current iq for providing the desired response for tracking the commanded active current, which finally feeds into the armature winding. The other closed loop is an integral module for controlling the direct-axis current id, which finally feeds into the field winding to ensure a unity power factor.
Unlike the field-oriented control schemes, an observer is not required to resolve the reference frame. The inverter voltage can determine the reference frame angle q. By activating the control loop from the inverter switching, the angular orientation of the reference frame can be obtained. Therefore, rotor position information is not required and sensorless control is achieved.
3.4.2 FESS control
As shown in Figure 3.17, the main power source, the FESS and the propulsion motors in an EV are coupled together by a DC-link via power electronic circuits.
The main power source provides the necessary power for the FESS and the vehicle
+ PI
θ
PI
HM ABC
/DQ
+ I +
–
– –
iq*
id*
Figure 3.16 Sensorless control of homopolar machine for FESS
Ultrahigh-speed flywheel energy storage for electric vehicles 89
propulsion. The FESS assists the main power source for providing power aug- mentation and recuperates the regenerative power from the traction motor during the braking. Therefore, the FESS mainly have three operation modes: charge, partial charge and discharge (Kennyet al., 2005). In the charge mode, the main power source produces enough current to both charge the flywheel at its set point and provides the required current for driving the traction motor. The power con- verter of the main power source regulates the DC-link voltage in the charge operation. Therefore, the currents in the DC-link satisfy the following relationship and the main power source regulates the DC-link voltage:
isẳif ỵimi (3.10)
if ẳic (3.11)
whereisis the current of the power converter connected to the main power source, ifis the DC-link current, imi is the current of the power inverter connected to the traction motor andicis the charge current reference.
In the partial charge mode, the recovery power returns to the FESS from regenerative braking. In this circumstance, the currents in the DC-link satisfy the following relationship and the DC-link voltage is regulated by the FESS:
imi ẳif (3.12)
if <ic (3.13)
In the discharge mode, the FESS provides power to assist the main power source to drive the traction motor for starting-up or accelerating. In this circum- stance, the currents in the DC-link satisfy the following relationship and the DC- link voltage is regulated by the FESS:
imi ẳisỵif (3.14)
if <0 (3.15)
To provide a steady power supply, the DC-link voltage must be kept unchanged. The FESS has the capability of regulating the DC-link voltage during
Main power source
+ –
M
M/G
Traction motor Flywheel motor/generator is
imi
Vdc ic
if ifi ifm
Figure 3.17 Block diagram model for EV energy system with FESS
both the charge and discharge modes. The relationship between the power inverter current ifi and the motor/generator current ifm can be obtained based on the condition of power balance where the power flows into the power inverter should be equal to the power flows into the motor (Kenny et al., 2005). By neglecting the power inverter losses, the relationship of the power flow is governed by the following equation:
Vdcifiẳmvfmifm (3.16)
whereVdcis the DC-link voltage,mis the number of phases andvfmis the motor/
generator voltage.
The motor/generator voltagevfmcan be expressed as:
vfm ẳRfmifmỵLfmdifm
dt þwmlf (3.17)
whereRfm andLfmare the motor/generator internal resistance and the inductance, respectively, wmis the motor/generator speed andlfis the no-load flux linkage.
When the steady-state conditions are assumed, the derivative term in (3.17) can be omitted. By combining (3.16) and (3.17), the following equation can be obtained:
Vdcifiẳmvfmifm ẳmRfmi2fmỵmwmlfifm (3.18) The first term in (3.17) is very small compared to the back-EMF voltage, especially for ultrahigh-speed operation. By neglecting this term and rearranging (3.18) between the motor/generator current ifm and the average power inverter current ifi, the following equation can be obtained:
ifm Vdcifi mwmlf
(3.19) This relationship between the motor/generator current ifm and the average power inverter currentifihas become the basis for controlling the motor/generator in all modes of operation. In the charge mode, it is used as an FF term in the controller, and in the partial charge mode and the discharge mode it is used as a DD term and the FESS regulates the DC-link voltage.
3.4.3 Charge and discharge control
In the charge mode, the flywheel is charged at a constant power via the constant DC current from the main power source. The charge control scheme regulates the acceleration of the flywheel motor so that the DC current is maintained at a reference value. The block diagram of the charge control is shown in Figure 3.18.
As shown in Figure 3.18, two modules are included in the charge controller, namely the PI module and the FF module (Kennyet al., 2005). The FF module uses the DC charging current referenceicas the input and converts it into a motor current reference based on (3.19), with the measured DC-link voltage and the estimated Ultrahigh-speed flywheel energy storage for electric vehicles 91
motor speed. The PI module aims to compensate for any inaccuracies and ensures zero steady-state error. Therefore, fast and accurate performance can be achieved.
In the partial charge and discharge modes, the flywheel motor/generator must decelerate at an appropriate rate to maintain a constant DC-link voltage. For the discharge mode, it must also supply the necessary current to the traction motor. The corresponding control block diagram is shown in Figure 3.19.
There are two modules in the controller, namely the PI module and a DD module (Kenny et al., 2005). In the DD module, the DC flywheel current if is measured and adopted as an early indicator to the controller to check for any increase or decrease that has happened in the traction motor. If there is an abrupt increase of current in the traction motor, such as vehicle acceleration or climbing a hill, the capacitor will initially maintain the DC-link voltage and there will be an increase in the DC current to supply the new load. This increase is measured and used to calculate the corresponding flywheel motor currentifm. The flywheel motor responds by decelerating more quickly based on the requirement, avoiding a drop in the DC-link voltage to trigger the PI module.
In the PI module, the measured DC-link voltageVdcis compared to the DC- link voltage reference Vf and the relevant error signal is processed. The average power inverter current needs to maintain the DC-link voltage by adjusting the current to an appropriate value. The PI module aims to compensate for any inac- curacies in the DD module and maintains the DC-link voltage.
PI
FF
+ +
+ –
ic*
ifi*
ifm*
Vdcifi
Vdc
if
mωmλf
ωm
Figure 3.18 Control block diagram for the charge mode
Vf* ifi*
if
Vdc
Vdc
Vdcifi
mωmλf
+
PI
DD +
+ –
ifm*
ωm
Figure 3.19 Control block diagram for the partial charge and discharge mode
As previously mentioned, the control strategy for the charge mode is a current regulation scheme. The main power source provides sufficient current to meet the requirements for both vehicle propulsion and FESS charging. The control strategy for the partial charge and discharge mode is a voltage regulation scheme. The FESS regulates the DC-link voltage for stabilization.
The control schemes shift between the current regulation and the voltage regulation based on different operation modes (Kenny et al., 2005). When the current isis sufficient to provide both the load and the charging current, the main power source will regulate the DC-link voltage to a set value and slightly higher than the flywheel regulation set value. When the EV needs to accelerate or climb a hill, the DC-link voltage decreases due to the large current drawn by the traction motor. This condition is detected in the controller by comparing the difference between the actual DC-link voltage and the FESS set value. The FESS begins to operate in the discharge mode and regulates the DC-link voltage. The other con- dition is that when the EV decelerates and the regenerative braking is activated, the current imi reverses its direction and the DC-link voltage increases. The FESS regulates the DC-link voltage and meanwhile absorbs the power from the regen- erative braking.
Similarly, as the EV resumes cruising, the main power source supplies suffi- cient current. When the current exceeds the demand from the traction motor, the integrator in the current regulator module is reset and the FESS shifts back to the charge mode where the FESS regulates the current into the flywheel motor and the main power source regulates the DC-link voltage.