Series-Connected Multi-Phase Multi-Motor Drives

Prof. Emil LeviLiverpool John Moores University

School of Engineering

Standard Multi-Motor Drives with Independent Control

• Three-phase configuration of k motors with independent vector control: 3k inverter legs.• Motor/inverter sets connected in parallel, with common dc link.

• Independent control of two or more three-phase

motors supplied from one inverter is not possible.

Vector Feedback control common DC link 3-phase PWM Machine VSI 1 3-phase Machine PWM 2 VSI Vector control Feedback

Multi-phase machines versus three-phase machines

• Higher torque density: since torque production can be enhanced using injection of higher stator

current harmonics.• Greater fault tolerance, since the machine can continue to operate in the event of failure of one

(or more) inverter legs.• Reduction in the required rating per inverter leg.

• The first two advantages will not exist in the drive systems discussed here.

Modelling of an n-Phase Induction Machine• Sinusoidal distribution of MMF in the air-gap.

• Torque production: fundamental stator current harmonic only.• All the other standard assumptions apply.

• The concept is independent of the ac machine type.• Model transformation using decoupling transformation (n = odd).

−

−

−

−

−

−

−

−

−

−

−

−

−

−

−

−−−

−−−

−−−

−−−

=

21

21

21...

21

21

21

21

21sin

212sin

213sin...

213sin

212sin

21sin0

21cos

212cos

213cos...

213cos

212cos

21cos1

......................................4sin8sin12sin...12sin8sin4sin0

4cos8cos12cos...12cos8cos4cos13sin6sin9sin...9sin6sin3sin0

3cos6cos9cos...9cos6cos3cos12sin4sin6sin...6sin4sin2sin0

2cos4cos6cos...6cos4cos2cos1sin2sin3sin...3sin2sinsin0

cos2cos3cos...3cos2coscos1

2

nnnnnn

nnnnnn

nC

α

αααααα

αααααααααααααααααααααααααααααααααααααααααααααααα

Modelling of an n-Phase Induction Machine (cont.)

• For even system phase numbers decoupling transformation matrix is:

−−−−

−

−

−

−

−

−

−

−

−

−

−

−

−

−

−

−−−

−−−

−−−

=

−

+

−

−

212121.....21212121212121.....212121212

2sin2

22sin2

23sin.....2

23sin2

22sin2

2sin0

22cos

222cos

223cos.....

223cos

222cos

22cos1

........................................3sin6sin9sin.....9sin6sin3sin0

3cos6cos9cos.....9cos6cos3cos12sin4sin6sin.....6sin4sin2sin0

2cos4cos6cos.....6cos4cos2cos1sin2sin3sin.....3sin2sinsin0

cos2cos3cos.....3cos2coscos1

00

...2

24

24

2

2

1

1

αααααα

αααααα

αααααααααααααααααααααααααααααααααααα

βα

nnnnnn

nnnnnn

y

x

yxyx

n

n

n

C

Machine Model after Transformation( ) ( )

( ) ( )

dtdi

LiRdt

diRv

dtdi

LiRdt

diRv

dtdi

LiRdt

diRv

iidtdL

dtdi

LLiRdt

diRv

iidtdL

dtdi

LLiRdt

diRv

slsss

ssss

sylssys

sysyssy

sxlssxs

sxsxssx

rrms

mlssss

sss

rrms

mlssss

sss

00

000

11

111

11

111

...........................................................................

...........................................................................

cossin

sincos

+=+=

+=+=

+=+=

++++=+=

−+++=+=

ψ

ψ

ψ

θθψ

θθψ

βαβ

ββ

ββ

βαα

αα

αα

( ) ( )

( ) ( )

dtdi

LiRdt

diRv

dtdi

LiRdt

diRv

dtdi

LiRdt

diRv

iidtdL

dtdi

LLiRdt

diRv

iidtdL

dtdi

LLiRdt

diRv

rlrrr

rrrr

rylrryr

ryryrry

rxlrrxr

rxrxrrx

ssmr

mlrrrr

rrr

ssmr

mlrrrr

rrr

00

000

11

111

11

111

0

...........................................................................

...........................................................................

0

0

cossin0

sincos0

+=+==

+=+==

+=+==

+−+++=+==

++++=+==

ψ

ψ

ψ

θθψ

θθψ

βαβ

ββ

ββ

βαα

αα

αα

( ) ( )[ ]srsrsrsrme iiiiiiiiPLT ββαααββα θθ +−−= sincos

Model Properties• The model contains one (two) zero sequence component equations, one pair of α-β component equations and (n – 3)/2 [or (n – 4)/2 for even phase numbers] pairs of x-y component

equations.• Stator to rotor coupling appears only in α-β component

equations; hence the torque is entirely governed by α-βcurrent components.

• A mechanism that would induce x-y or zero sequence components in rotor does not exist.

• CONCLUSION: Independent flux and torque control of a multi-phase machine asks for only two stator currents,

regardless of the number of phases.

The Idea• Only two currents required for the control of one machine – why

not use the remaining currents for control of other machines?• At most (n – 1)/2 [or (n – 2)/2 for even phase numbers] machine

can be controlled independently.• Why? To save in the number of required legs.

• How? By using a series connection. n-phase CC stator of stator of stator of voltage machine machine machine source 1 2 (n-1)/2 +

A 1 1 1 + B 2 Phase 2 Phase 2 + C 3 transpo- 3 transpo- 3 sition sition + N n n n

What is Phase Transposition?

• Flux/torque producing (α-β) currents of one machine must be non-flux/torque producing (x-y) currents for all the other machines, and vice versa.• Simple series connection cannot achieve this. Hence a transposition is required when connected

in series phases of stator windings.• How to establish the required rules for connection

of stator windings in series? Well, it follows directly from the decoupling (Clark’s)

transformation matrix.

Transposition Rules

−

−

−

−

−

−

−

−

−

−

−

−

−

−

−

−−−

−−−

−−−

−−−

=

21

21

21...

21

21

21

21

21sin

212sin

213sin...

213sin

212sin

21sin0

21cos

212cos

213cos...

213cos

212cos

21cos1

......................................4sin8sin12sin...12sin8sin4sin0

4cos8cos12cos...12cos8cos4cos13sin6sin9sin...9sin6sin3sin0

3cos6cos9cos...9cos6cos3cos12sin4sin6sin...6sin4sin2sin0

2cos4cos6cos...6cos4cos2cos1sin2sin3sin...3sin2sinsin0

cos2cos3cos...3cos2coscos1

2

nnnnnn

nnnnnn

nC

α

αααααα

αααααααααααααααααααααααααααααααααααααααααααααααα

• Column 1: connect phases ‘a’ of all machines directly in series.• Column 2: connect phase ‘b’ of M1 to phase ‘c’ of M2 to phase ‘d’ of M3 etc.• Column 3: connect phase ‘c’ of M1 to phase ‘e’ of M2 to phase ‘g’ of M3 etc.• Column 4: connect phase ‘d’ of M1 to phase ‘g’ of M2 to phase ‘j’ of M3 etc.• And so on.

General Connectivity Matrix A B C D E F G H I J K L M N O

M1 a b c d e f g h I j k l m n …

M2 a b+1 c+2 d+3 e+4 f+5 g+6 h+7 i+8 j+9 k+10 l+11 m+12 n+13 … M3 a b+2 c+4 d+6 e+8 f+10 g+12 h+14 i+16 j+18 k+20 l+22 m+24 n+26 … M4 a b+3 c+6 d+9 e+12 f+15 g+18 h+21 i+24 j+27 k+30 l+33 m+36 n+39 … M5 a b+4 c+8 d+12 e+16 f+20 g+24 h+28 i+32 j+36 k+40 l+44 m+48 n+52 … M6 a b+5 c+10 d+15 e+20 f+25 g+30 h+35 i+40 j+45 k+50 l+55 m+60 n+65 … M7 a b+6 c+12 d+18 e+24 f+30 g+36 h+42 i+48 j+54 k+60 l+66 m+72 n+78 … …. … … … … … … … … … … …. … … … …

• Upper case letters: source phases.• Lower case letters: machine phases, according to the

spatial distribution around the air-gap.• Boxed: five-phase, seven-phase, eleven-phase and thirteen-

phase systems.

Two-Motor Five-Phase Drive

A B C D E

as1bs1cs1ds1es1

as2 bs2 cs2 ds2 es2

Stator ofmachine 1

Stator ofmachine 2

A B C D E M1 1 2 3 4 5 M2 1 3 5 2 4

21

21

21

21

21

dsesE

bsdsD

escsC

csbsB

asasA

vvvvvvvvvvvvvvv

+=+=+=+=+=

21

21

21

21

21

dsesE

bsdsD

escsC

csbsB

asasA

iiiiiiiiiiiiiii

==========

Three-Motor Seven-Phase Drive

7654321M1

5263741M36427531M2

GFEDCBA

A

B

C

D

E

F

G

Source

a1

b1

c1

d1

e1

f1

g1

a2

b2

c2

d2

e2

f2

g2

Machine 1 Machine 2

a3

b3

c3

d3

e3

f3

g3

Machine 3

Four-Motor Nine-Phase Drive

2

4

6

8

H IGFEDCBA

5

4

3

2

1

1

1

1

M4

M3

M2

M1

717417

842975

7

7

63849

96543

2

4

6

8

H IGFEDCBA

5

4

3

2

1

1

1

1

M4

M3

M2

M1

717417

842975

7

7

63849

96543

A

B

C

D

E

F

G

H

I

Source

a1

b1

c1

d1

e1

f1

g1

h1

i1

a2

b2

c2

d2

e2

f2

g2

h2

i2

Machine 1 Machine 2

a3

b3

c3

d3

e3

f3

g3

h3

i3

Machine 3

a4

b4

c4

Machine 4

Two-Motor Six-Phase Drive

A B C D E F M1 1 2 3 4 5 6 M2 1 3 5 1 3 5

A

B

C

D

E

F

Source

a1

b1

c1

d1

e1

f1

a2

b2

c2

Machine 1 Machine 2

Four-Motor Ten-Phase Drive A B C D E F G H I J M1 1 2 3 4 5 6 7 8 9 10 M2 1 3 5 7 9 1 3 5 7 9 M3 1 4 7 10 3 6 9 2 5 8 M4 1 5 9 3 7 1 5 9 3 7

A

B

C

D

E

F

G

H

I

J

Source

a1

b1

c1

d1

e1

f1

g1

h1

i1

j1

a2

b2

c2

d2

e2

f2

g2

h2

i2

j2

Machine 1 Machine 2

a3

b3

c3

d3

e3

Machine 3

a4

b4

c4

d4

e4

Machine 4

The Number of Connectable Machines

• All the cases illustrated so far have enabled series connection of the maximum number of machines

k = (n – 1)/2 [or k = (n – 2)/2 for even system phase numbers].

• An odd system phase number and the subsequent even number enable connection of, at best, the same number of machines. Hence odd numbers

save more inverter legs.• Machine phase numbers are all the same only if

the system phase number is a prime number.

The Number of Connectable Machines (n = odd)

n = an odd number, ≥ 5 Number of

connectable machines

Number of phases of machines in the multi-drive system

n = a prime

number

n = 5,7,11,13….. 2

1−=

nk n

n ≠ a prime

number

.....4,3,2 , == mln m

21−

=nk 12

......., , , ,−ml

nln

lnn

njnnnn ⋅⋅⋅⋅= .....3212

1−<

nk njnnnnnnn

,or ...... 3 ,or 2 ,or 1 ,

,...4,3,2

,.....21=

⋅⋅⋅⋅=m

lnjnnn m

2

1−<

nk 12

.....,or

,or ..... 2,or 1 ,

−mln

ln

ln

njnnnnn

The Number of Connectable Machines (n = even)

n = an even number, ≥ 6 Number of connectable machines Number of phases of

machines n/2 = prime

number

22−

=nk

k/2 are n-phase and k/2 are n/2-phase

n/2 ≠ prime number

.....5,4,3 , 2 == mn m 2

2−=

nk 22 2 ......., ,

2 ,

2 ,

−mnnnn

all other even numbers 2

2−<

nk n, ....4/,3/,2/ nnn

as appropriate

Vector Control of a Series-Connected Multi-Motor Drive

PI

jφr

e

2

n

i1*i2*i3*

in*

ids* = idsn

iqs*ω*

s P

θ

φr

ω

K1

1/s

• The same vector control schemes are applicable as for a three-phase machine

of the same type.• Current control in the stationary reference frame

is assumed throughout.• Figures illustrate indirect

rotor flux oriented controller for induction and

synchronous (permanent magnet and reluctance)

machines.

PI

jφr

e

2 n

i1*

i2*

i3*

in*

ids*

iqs* ω*

s P

θ

φr

ω

Creation of Phase Current References

( ) ( )

( ) ( )])1(sin)1(cos[2

------------------------------------------

]sincos[2

]sincos[2

)()(*)()(*)(*

)()(*)()(*)(*2

)()(*)()(*)(*1

αφαφ

αφαφ

φφ

−−−−−=

−−−=

−=

ninin

i

iin

i

iin

i

Mjr

Mjqs

Mjr

Mjds

Mjn

Mjr

Mjqs

Mjr

Mjds

Mj

Mjr

Mjqs

Mjr

Mjds

Mj

• Phase current references are created at first individually for each machine in the group.

• Inverter phase current references are built next, by summing the individual machine phase current references

according to the connection diagram.

Inverter Phase Current References

Three-motor seven-phase drive Two-motor six-phase drive

*3

*2

*1

*

*3

*2

*1

**3

*2

*1

*

*3

*2

*1

**3

*2

*1

*

*3

*2

*1

**3

*2

*1

*

efgG

bdfFfbeE

cgdDgecC

dcbBaaaA

iiii

iiiiiiii

iiiiiiii

iiiiiiii

++=

++=++=

++=++=

++=++=

*2

*1

**2

*1

*

*2

*1

**2

*1

*

*2

*1

**2

*1

*

5.0 5.0

5.0 5.0

5.0 5.0

cfFbeE

adDccC

bbBaaA

iiiiii

iiiiii

iiiiii

+=+=

+=+=

+=+=

Two-motor five-phase drive Four-motor nine-phase drive

*4

*3

*2

*1

*

*4

*3

*2

*1

**4

*3

*2

*1

*

*4

*3

*2

*1

**4

*3

*2

*1

*

*4

*3

*2

*1

**4

*3

*2

*1

*

*4

*3

*2

*1

**4

*3

*2

*1

*

)3/1(

)3/1( )3/1(

)3/1( )3/1(

)3/1( )3/1(

)3/1( )3/1(

cfhiI

bbfhHagdgG

ccbfFbhieE

adgdDciecC

becbBaaaaA

iiiii

iiiiiiiiii

iiiiiiiiii

iiiiiiiiii

iiiiiiiiii

+++=

+++=+++=

+++=+++=

+++=+++=

+++=+++=

*2

*1

**2

*1

*

*2

*1

*

*2

*1

**2

*1

*

dsesEbsdsD

escsC

csbsBasasA

iiiiii

iii

iiiiii

+=+=

+=

+=+=

Modelling Example: Two-Motor Five-Phase Drive

++−+

=

+++++

=

=

021

21

21

21

21

21

21

21

21

0

sys

sxs

yss

xss

de

bd

ec

cb

aa

E

D

C

B

A

INV

INVy

INVx

INV

INV

vvvvvvvv

vvvvvvvvvv

C

vvvvv

C

vvvvv

β

α

β

α

β

α

21

21

21

21

sysINVy

sxsINVx

yssINV

xssINV

iii

iii

iii

iii

β

α

ββ

αα

==

==

−==

==

Model in the stationary common reference frameInverter equations Rotor and torque equations

dtdi

Ldt

diLLiR

dtdi

LiRv

dtdi

Ldt

diLLiR

dtdi

LiRv

dtdi

LiRdt

diL

dtdi

LLiRv

dtdi

LiRdt

diL

dtdi

LLiRv

qrm

INVy

mlsINVys

INVy

lsINVys

INVy

drm

INVx

mlsINVxs

INVx

lsINVxs

INVx

INVq

lsINVqs

qrm

INVq

mlsINVqs

INVq

INVd

lsINVds

drm

INVd

mlsINVds

INVd

2222211

2222211

221

1111

221

1111

)(

)(

)(

)(

+++++=

+++++=

+++++=

+++++= ( ) ( )( )

( ) ( )( )111111

11111

111111

11111

0

0

drmlrINVdm

qrmlr

INVq

mqrr

qrmlrINVqm

drmlr

INVd

mdrr

iLLiLdt

diLL

dtdi

LiR

iLLiLdt

diLLdt

diLiR

++−+++=

++++++=

ω

ω

( ) ( )( )

( ) ( )( )222222

22222

222222

22222

0

0

drmlrINVxm

qrmlr

INVy

mqrr

qrmlrINVym

drmlr

INVx

mdrr

iLLiLdt

diLL

dtdi

LiR

iLLiLdt

diLLdt

diLiR

++−+++=

++++++=

ω

ω

[ ][ ]22222

11111

qrINVx

INVydrme

qrINVd

INVqdrme

iiiiLPT

iiiiLPT

−=

−=

Simulation Studies: Three-Motor Seven-Phase Drive

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-200

0

200

400

600

800

1000

1200

1400

1600

IM1

IM2

IM3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

0.5

1

1.5

2

2.5

3

3.5

Rotor flux refe rence

Rotor flux space ve ctor ma gnitude in the ma chine

IM1,IM2 & IM3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-5

0

5

10

15

20

25

IM1

IM2

IM3

0.3 sRated speed (1428 rpm)IM1

0.5 s1/3 of rated (476 rpm)IM3

0.4 s2/3 of rated (952 rpm)IM2

Instant of application

Speed command

0.3 sRated speed (1428 rpm)IM1

0.5 s1/3 of rated (476 rpm)IM3

0.4 s2/3 of rated (952 rpm)IM2

Instant of application

Speed command

Simulation Studies: Seven-Phase Drive (cont.)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-5

0

5

10

15

Stator phase "a" current reference (A) I

M1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-15

-10

-5

0

5

Time (s )

Stator phase "a" current reference (A) I

M2

IM2

IM1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-400

-200

0

200

400

600

800

Stator phase "a" voltage (V) I

M1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1200

-1000

-800

-600

-400

-200

0

200

400

Time (s )

Stator pase "a" voltage (V) I

M2

Stator phase current references Stator phase ‘a’ voltages

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-5

-4

-3

-2

-1

0

1

2

3

4

5

Time (s )

Stator phase "a" current reference (A) I

M3

IM3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-150

-100

-50

0

50

100

150

200

Time (s )

Stator phase "a" voltage (V) I

M3 IM3

Simulation Studies: Seven-Phase Drive (cont.)

Inverter phase voltages

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-800

-400

0

400

800

1200

1600

2000

Source phase "a" voltage (V)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-2200

-1700

-1200

-700

-200

300

Time (s )

Source phase "b" voltage (V)pha se "b"

pha se "a "

Inverter output currents

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-10

-5

0

5

10

15

20

25

30

Source phase "a" current (A)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-30

-25

-20

-15

-10

-5

0

5

10

Time (s )

Source phase "b" current (A)

Pha se "b"

Pha se "a "

Simulation Studies: Two-Motor Five-Phase Drive

(inverter included; hysteresis current control)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

0.5

1

1.5

2

2.5

3

Time (s )

Rot

or fl

ux (W

b)

Re fe rence rotor flux for IM1 & IM2

Rotor flux in IM1 & IM2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-10

0

10

20

Time (s )

Torq

ue IM

1 (N

m)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-1000

-500

0

500

1000

1500

2000Commanded & actual torque

S peed

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-5

0

5

10

15

20

Time (s )

Torq

ue IM

2 (N

m)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-200

0

200

400

600

800

Spe

ed IM

2 (rp

m)

Commanded & actual torque Speed

Spe

ed IM

1 (rp

m)

Currents Voltages

Simulation Studies: Five-Phase Drive (cont.)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-15

-10

-5

0

5

Time (s )

Sta

tor p

hase

'a' c

urre

nt re

fere

nce

IM1

(A)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-5

0

5

10

15

Sta

tor p

hase

'a' c

urre

nt re

fere

nce

IM2

(A)

IM1

IM2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-2000

-1500

-1000

-500

0

500

Time (s )

Sta

tor p

hase

'a' v

olta

ge IM

1 (V

)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-600

-100

400

900

1400

1900

Sta

tor p

hase

'a' v

olta

ge IM

2 (V

)

IM1

IM2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-1000

-800

-600

-400

-200

0

200

400

600

800

1000

Time (s )

Inve

rter p

hase

'a' v

olta

ge (V

)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-10

-8

-6

-4

-2

0

2

4

6

8

10

Time (s )

Inve

rter p

hase

'a' c

urre

nt (A

)

Reference current

Actual current

Simulation Studies: Five-Phase Drive (cont.)

(disturbance rejection)Machine 1

0.9 0.95 1 1.05 1.1 1.15 1.2-5

0

5

10

15

Time (s )

Torq

ue IM

1(N

m)

0.9 0.95 1 1.05 1.1 1.15 1.21200

1500

2000

Spe

ed IM

1 (rp

m)

Commanded & actua l torque Applied loadtorque

S peed

Machine 2

0.9 0.95 1 1.05 1.1 1.15 1.2-10

-5

0

5

10

Time (s )

Torq

ue IM

2 (N

m)

0.9 0.95 1 1.05 1.1 1.15 1.2650

750

800

Spe

ed IM

2 (rp

m)

Commanded & actual torque

Applied load torque

S peed

Simulation Studies: Five-Phase Drive (cont.)(reversing transients)Torque and speed responses Currents

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2-20

-10

0

10

20

Time (s )

Torq

ue IM

1 (N

m)

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2-2000

-1000

0

1000

2000

Spe

ed IM

1 (rp

m)

Commanded & actual torque

S peed

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2-15

-10

-5

0

5

Time (s )

Sta

tor p

hase

'a' c

urre

nt re

fere

nce

IM1

(A)

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2-5

0

5

10

15

Sta

tor p

hase

'a' c

urre

nt re

fere

nce

IM2

(A)

IM1

IM2

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2-20

-15

-10

-5

0

5

10

15

20

Time (s )

Torq

ue IM

2 (N

m)

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2-1000

-750

750

1000

Commanded & ac tua l torque

S peed

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2-10

-8

-6

-4

-2

0

2

4

6

8

10

Time (s )

Inve

rter p

hase

'a' c

urre

nt (A

)

Reference current

Actual current

Spe

ed IM

2 (rp

m)

Properties of Series-Connected Multi-Motor Drives• Advantages of the concept:

– Saving in the number of inverter legs (except in 6-phase case).– Simplicity of the control realisation within a single DSP –

execution of vector control algorithms in parallel with subsequent inverter phase current generation by summation.

– Braking energy can be directly used and does not have to circulate through the inverter.

• Disadvantages:– An increase in the stator winding losses and a much smaller

increase in the core losses.– Consequently, a somewhat worsened efficiency of the

complete drive system.

Experimental systemUtilised number of the inverter phases (maximum = 12)

5 6* 7 8* 9 10* 11**

*

12**

Number of connectable

machines

2 2 3 3 4 4 5*** 4**

** The twelve-phase supply can control at most four series-connected motors, rather than five.

***At least one machine must operate in speed sensorless mode.

• Four three-phase inverters with paralleled DC links. Each inverter with a DSP for current control and an encoder/resolver input..

• Vector control algorithm executed within a PC (C code). Communication between inverters and the PC through a dedicated FPGA board.

Experimental system (cont.)

• Six-phase two-motor drive system is in the commissioning stage.

Preliminary Experimental Results (steady states)

-6

-4

-2

0

2

4

Inve

rter p

hase

cur

rent

(A)

0 0.025 0.05 0.075 0.1 0.125 Time (seconds)

n (6phase) = 800rpm; n (3phase) = 600 rpm-4

-3

-2

-1

0

1

2

3

4

3-ph

ase

mot

or c

urre

nt (A

)

0 0.025 0.05 0.075 0.1 0.125 Time (seconds)

n (6phase) = 800rpm; n (3phase) = 600 rpm

0

0.5

1

1.5

2

Inve

rter c

urre

nt sp

ectru

m (A

rms)

0 25 50 75 100 Frequency (Hz)

n (6phase) = 800 rpmn (3phase) = 600 rpm

0

0.5

1

1.5

2

3-ph

ase

mot

or c

urre

nt sp

ectru

m (A

rms)

0 25 50 75 100 Frequency (Hz)

n (6phase) = 800 rpmn (3phase) = 600 rpm

Preliminary Experimental Results (cont.)

-6

-4

-2

0

2

4

6

Inve

rter p

hase

cur

rent

(A)

0 0.025 0.05 0.075 0.1 0.125 Time (seconds)

f (IM1) = 50 Hz; f (IM2) = 16.67

0

0.5

1

1.5

2

2.5

Inve

rter c

urre

nt sp

ectru

m (A

rms)

0 25 50 75 100 Frequency (Hz)

f (IM1) = 50 Hzf (IM2) = 16.67

Further Work• Experimental:

– Testing of the six-phase two-motor drive (transients and steady states).

– Testing of the five-phase two-motor drive (transients and steady states).

• Theoretical:– applicability of current control in the rotating reference

frame to multi-phase multi-motor drive systems.– detailed modelling (transient and steady state) for the

six-phase two-motor drive.– evaluation of the efficiency loss due to series

connection.

Thank You for Your Attention!