Page 214 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

VARIABLE-SPEED DRIVES

VARIABLE-SPEED DRIVES 10.13

● The rating of the diode bridge may also require an increase due to the additional dissipa-
tion in the semiconductors. The consequences of the unbalance can be very significant
because the voltage unbalance in typical power systems can be 2 to 3 percent. This prob-
lem will become more significant if the ac feeder has single-phase loads because these
loads will increase the already existing voltage unbalances.

AC POWER FACTOR

The increase in harmonics in a power circuit (Fig. 10.9) will decrease the total power fac-
tor. It can drop to 60 percent for an inverter without a dc link reactor. Obviously, this is not
acceptable. The problem is solved by adding a dc link reactor in series with the capacitor
bank. If the reactor is chosen large enough, the diode bridge will always see an inductive
load. The total power factor will increase to 0.95.

AC Input Power Changes with AC Input Voltage
The output inverter of the drive is a separate entity from the input source converter. They
operate independently from each other. Assume that the drive is feeding a constant kilowatt
load near its full rating. If the ac line voltage drops by 10 percent, the line alternating cur-
rent increases by 10 percent. This will overload the drive.
The current increase due to the reduction of ac line voltage will be added to any current
unbalance caused by a line voltage unbalance.

IGBT SWITCHING TRANSIENTS

Voltage transients are generated due to the switching in the PWM inverter. They prop-
agate down the power cables to the motor. If the motor cables are not terminated prop-
erly, the switching waves will be reflected when they reach the motor. They will be
transiently increased or decreased depending on the relative impedance between the line
and the load. An alternative solution for this problem would be to slow down the rate of
voltage rise.
If the connecting cable is long, the mismatch of impedances will generate a voltage
reflection at the point where the line impedance changes (at the motor terminals). A tran-
sient rise in voltage of the wave will occur at the motor terminals due to the voltage
reflection.
For a given switching rise time, the voltage rise at the terminals will increase with the
length of the connecting cable. The wave front is reflected back to the inverter. If no mit-
igating actions are taken, the transient voltage at the motor terminals can double that of
the inverter in the worst case. In most cases, the reflection problem increases gradually
with the length of the motor cable. The rise in transient voltage is important for the fol-
lowing reasons.

Insulation Voltage Stress

The increase in voltage stress in the motor connecting cables and the motor insulation will
shorten the lifetime of the insulation. If the peaks of any of the voltage transients exceed

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