Electronic Control of Switched Reluctance Machines 10 - The switched reluctance generator

The switched reluctance

generator

Tadashi Sawata

The switched reluctance machine can operate as a generator as well as a motor by

simply changing the firing angles. Figure 10.1 shows generating and motoring currents

controlled by current chopping, together with the phase inductance.

In generating operation the firing angles are chosen so that the current flows when dL/dO < 0; while in motoring operation they are chosen so that the current flows

when dL/dO > 0. The circuit equation for a phase of the switched reluctance machine

is that:

v=Ri+ dt

di dO dL : Ri +L-~ + idt- ~

di = Ri + L-~ + e (10.1)

where v is the applied voltage, i is the phase current, R is the phase resistance, L is

the phase inductance and 0 is the rotor position. The back-EMF e is defined as

.dL e = eot~ (10.2) dO

where o~ is the rotor speed o~ = dO/dt. As unipolar currents are normally employed the

sign of i is always positive. Therefore, the sign of e is determined by dL/dO. When dL/dO > 0 the back-EMF is positive and it tends to force the current to decrease, being

against the applied voltage and changing the electrical power supplied into mechan-

ical output (motoring). When dL/dO < 0 the back-EMF is negative and it tends to

increase the current and convert the mechanical power into electrical power (gener-

ating). However, the amplitude of the back-EMF varies with the rotor speed o~ and the

behaviour of the current is determined by the relationship between e and v, so that the

actual operation is more complicated as described later.

228 The switched reluctance generator

Idealized phase inductance

Generating current

Motoring current I

Ou Oa L a

i i i i Lu , i I I I I i I I I I i i I ~- i

Fig. 10.1 Phase inductance and generating current.

Figure 10.2 shows the circuit diagram of a generator with one phaseleg and

Figure 10.3 shows the phase currents, flux-linkage and idealized inductance. Both the

converter and load are connected to the same d.c.-bus. The bus can be separated

into two: one for the excitation and the other for the load for higher fault-tolerance (Radun, 1994).

A simplified circuit diagram showing the energy flow is shown in Figure 10.4. The

integral of the currents in Figure 10.4 can be defined by referring to Figures 10.2 and Figure 10.3 as:

f O off lin -- iph dO JOo,,

"4 "i~~ l i out VDC Load

Fig. 10.2 Generator circuit for one phase. (Courtesy of Dr P.C. Kjaer).

La l ......... h ;4,ou,

' 01u Oon Oa Ooff Old Oext

Fig. 10.3 Generator phase currents, flux-linkage and idealized inductance. (Courtesy of Dr P.C. Kjaer).

Electronic control of switched reluctance machines 229

Io= lout- lin IL

Io ( VDo e on, eo ci-

Load

Fig. 10.4 Simplified circuit diagram showing energy flow. (Courtesy of Dr P.C. Kjaer).

f Oext lout -- iph dO J Oo#

Io -- lout - Iin

where Io is the net generated current. The excitation penalty e is defined as follows"

Iin Iin e - = . (10.3) lout Io + Iin

An example of idealized current waveforms with single-pulse control is shown in

Figure 10.5. The angles are defined in Table 10.1. The peak current occurs either at

0off or 0~d. Figure lO.5(a) shows the case where the current increases after turning off

the switches at 0oy, when the back-EMF in the coil is larger than the d.c.-bus voltage VDC. In (b), the back-EMF and VDC balance and the current stays constant until the

pole overlap ends at 01d. In (c) the back-EMF is smaller than VDC and the current

decreases after 0o~.

From 0o, to 0off excitation power is supplied from the d.c. power source through the

power electronic converter to the machine, and it is stored in the airgap as magnetic

energy. After the power electronic switches are turned off at Ooff regenerated current

keeps flowing through the freewheeling diodes returning the generated power into

Oon Oa Ooff Old Oext (a)

(b)

(c)

Fig. 10.5 Idealized currentwaveforms.

230 The switched reluctance generator

Table 10.1 Definition of the angles

0on Turn-on angle 0 a Aligned position Ooff Turn-off angle Old Angle at which pole overlaps ends O ex t Angle at which the flux reaches zero

the d.c. power supply until the current vanishes at Oex t . If the generated power Pgen is larger than the excitation power supplied from the d.c. supply Pexc, the system

has generated the net power by converting the mechanical power into electrical

power.

As generated power is returned to the d.c. power supply and/or filter capacitor the

switched reluctance machine is a d.c. power generator. If the power is supplied to loads

which require a.c. power, the d.c. power has to be converted into a.c. power by means

of an inverter. In many applications all or part of the electric power generated has to