Appendix C How to Select the Right AC Motor Drive
VFD-EL-W
C-2
C.1 Capacity Formulas
1. One AC motor drive operates one motor
The starting capacity should be less than 1.5x the rated capacity of the AC motor drive.
The starting capacity equals:
)(_____5.1
375cos973
2
kVAdrivemotorACofcapacitythe
t
NGD
T
Nk
A
L
×≤
×+
××
×
ϕη
2. One AC motor drive operates more than one motor
2.1 The starting capacity should be less than the rated capacity of the AC motor drive
.
Acceleration time ≤ 60 seconds
The starting capacity equals:
( )
[ ]
( )
)(_____5.11
cos
111
kVAdrivemotorACofcapacitythek
n
n
Pknn
Nk
sCss
T
s
T
×≤+=+
×
×
−−
ϕη
Acceleration time ≥ 60 seconds
The starting capacity equals:
2.2 The current should be less than the rated current of the AC motor drive (A).
Acceleration time ≤ 60 seconds
)
(_
_____5.111 AdrivemotorACofcurrentratedthek
n
n
In
SM
T
S
T
×≤++
−
Acceleration time ≥ 60 seconds
)
(______11 AdrivemotorACofcurrentratedthek
n
n
In
SM
T
S
T
≤++
−
2.3 When running continuously
The load capacity requirement should be less than the capacity of the AC motor drive (kVA).
The load capacity requirement equals:
)(_____
cos
kVAdrivemotorACofcapacitythe
Pk
M
≤
×
×
ϕη
The motor capacity should be less than the capacity of the AC motor drive.
)
(_____103
3
kVAdrivemotorACofcapacitytheIVk
MM ≤××××
−
The current should be less than the rated current of the AC motor drive (A).
)(______ AdrivemotorACofcurrentratedtheIk
M
≤×
( )
[ ]
( )
)(_____1
cos
111
kVAdrivemotorACofcapacitythek
n
n
Pknn
Nk
sCss
T
s
T
≤+=+
×
×
−−
ϕη
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