Introduction
Parameter
x.00
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Menu 5
Commander SK Advanced User Guide 81
Issue Number: 9 www.controltechniques.com
Normally the drive will use space vector modulation to produce the IGBT control signals. High stability space vector modulation offers three
advantages in an open loop drive, but the acoustic noise produced by the motor may increase slightly.
• It is possible for instability to occur around motor rated frequency/2 on light load. The drive uses deadtime compensation to reduce this effect,
however, it is still possible that some machines will be unstable. To prevent this, high stability space vector modulation should be enabled by
setting this parameter.
• As the output voltage approaches the maximum available from the drive, pulse deletion occurs. This can cause unstable operation with a lightly or
fully loaded machine. High stability space vector modulation will reduce this effect.
• High stability space vector modulation will also give a small reduction in drive heat loss.
High stability space vector modulation is not available on Commander SK sizes A, B and C. Available with size D and size 2 to 6.
0: OFF Over modulation disabled
1: On Over modulation enable
The maximum modulation level of the drive is normally limited to unity giving an output voltage equivalent to the drive input voltage minus voltage
drops within the drive. If the motor rated voltage is set at the same level as the supply voltage some pulse deletion will occur as the drive output
voltage approaches the rated voltage level. If Pr 5.20 is set to On(1) the modulator will allow over modulation, so that as the output frequency
increases beyond the rated frequency the voltage continues to increase above the rated voltage. The modulation depth will increase beyond unity
producing trapezoidal waveforms. This can be used for example to get slightly better performance above rated speed. The disadvantage is that the
machine current will be distorted as the modulation depth increases above unity, and will contain a significant amount of low order odd harmonics of
the fundamental output frequency.
Due to various effects in the drive inverter a voltage offset must be produced before any current flows. To obtain good performance at low frequencies
where the machine terminal voltage is small this offset must be taken into account. The value shown in Pr 5.23 is this offset given in line to line rms
volts. It is not possible for the user to measure this voltage easily, and so the automatic measurement procedure should be used (see Pr 5.14 on page
79).
With reference to the diagram below, the transient inductance is defined as
σL
s
= L
1
+ (L
2
.L
m
/ (L
2
+ L
m
))
5.20 Over modulation enable
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
111
Range OFF(0) or On(1)
Default OFF(0)
Update rate Background
5.21 to 5.22 Unused parameters
5.23 Voltage offset
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
1 1 111
Range 0.0 to 25.0 V
Default 0.0
Second motor
parameter
Pr 21.13
Update rate Background
5.24
Transient inductance (σL
s
)
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
21 111
Range 0.00 to 320.00 mH
Default 0.00
Second motor
parameter
Pr 21.14
Update rate Background
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