Appendix C How to Select the Right AC Motor Drive|VFD-VL
C-3
2.3 When it is running continuously
The requirement of load capacity should be less than the capacity of AC motor
drive(kVA)
The requirement of load capacity=
)(_____
cos
kVAdrivemotorACofcapacitythe
Pk
M
≤
×
×
ϕη
The motor capacity should be less than the capacity of AC motor drive
)(_____103
3
kVAdrivemotorACofcapacitytheIVk MM ≤××××
−
The current should be less than the rated current of AC motor drive(A)
)(______ AdrivemotorACofcurrentratedtheIk M ≤×
Symbol explanation
M
: Motor shaft output for load (kW)
η : Motor efficiency (normally, approx. 0.85)
cos
: Motor power factor (normally, approx. 0.75)
MV
: Motor rated voltage(V)
M
: Motor rated current(A), for commercial power
k
: Correction factor calculated from current distortion factor (1.05-1.1, depending on
PWM method)
1CP
: Continuous motor capacity (kVA)
Sk
: Starting current/rated current of motor
Tn
: Number of motors in parallel
Sn
: Number of simultaneously started motors
2
GD
: Total inertia (GD
2
) calculated back to motor shaft (kg m
2
)
LT
: Load torque
At
: Motor acceleration time
N : Motor speed
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