Name: Emerson unidrive sp
Brand: Emerson
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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