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126
Unidrive SP Advanced User Guide
www.controltechniques.com Issue Number: 10
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 dead-time 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 also gives a small reduction in drive heat loss.
Open loop
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 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; first
producing trapezoidal and then quasi-square waveforms. This can be used for example to obtain high output frequencies with a low switching
frequency which would not be possible with space vector modulation limited to unity modulation depth. 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.
As the rated voltage parameter is increased for a given d.c. link voltage the modulation depth is also increased. Therefore if the rated voltage is set to
a level higher than the supply voltage the point at which pulse dropping, over-modulation and quasi-square operation each begin may occur at a
frequencies below the rated frequency.
A suitable field controller gain is automatically set by the drive from the motor parameters. However it is possible by setting this parameter to a 1 to
reduce this gain by a factor of 2 if instability problems occur above base speed.
5.19
High stability space vector modulation
Drive modes
Open-loop
Coding
Bit
SP
FI
DE Txt VM DP ND RA NC NV
PT
US RW BU
PS
1
1
1
Default
Open-loop,
0
Update rate
Background read
5.20
Quasi-square enable
Drive modes
Open-loop
Coding
Bit
SP
FI
DE Txt VM DP
ND
RA NC NV
PT
US RW BU
PS
1
1
1
Default
Open-loop
0
Update rate
Background read
5.21
Field gain reduction
Drive modes
Closed-loop vector, Servo
Coding
Bit
SP
FI
DE Txt VM DP
ND
RA NC NV
PT
US RW BU
PS
1
1
1
Default
Closed-loop vector, Servo
0
Update rate
Background read
Summary of Contents for unidrive sp
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