Page 132 - Offshore Electrical Engineering Manual

Alternating Current Variable-Frequency Drives 119

DIRECT CURRENT VARIABLE-SPEED DRIVES

Thyristors or Silicon controlled rectifiers (SCRs) have been used for some years
for the simple speed control of DC motors, particularly for offshore drilling equip-
ment. SCRs known commonly as thyristors are three-junction PNPN semiconduc-
tor devices which can be regarded as two inter-connected transistors that can be
used in the switching of heavy electrical loads. They can be latched-“ON” by a
single pulse of positive current applied to their gate terminal and will remain “ON”
indefinitely until the anode to cathode current falls below their minimum latching
level SCRs have largely been replaced by water-cooled insulated gate bipolar tran-
sistors (IGBTs).

ALTERNATING CURRENT VARIABLE-FREQUENCY DRIVES
A pulse width modulation drive consists of four stages:

1. A rectifier which converts the fixed (mains) frequency power into DC.
2. A DC link circuit which provides inputs to the inverter and includes a filter
which reduces harmonics reflected back into the fixed frequency system.
The filter unit may be passive or active. Passive filters consist of a
smoothing network of inductors and capacitors which is switched in and
out by the control circuitry, whereas active filters consist of a full inverter
circuit capable of injecting a variable corrective current to cancel out the
major harmonics.
3. The inverter.
4. The induction motor designed to operate with the variable-speed drive package
above.

The rectifier circuit of a pulse width modulated drive normally consists of a three-
phase diode bridge rectifier and capacitor filter. The rectifier converts the three-phase
AC voltage into DC voltage with a slight ripple. This ripple is removed by using a
capacitor filter. (Note: the average DC voltage is higher than the root mean square
(rms) value of incoming voltage by AC (rms) × 1.35 = VDC).
The control section of the adjustable-frequency drive accepts external inputs
which are used to determine the inverter output. The inputs are used in conjunction
with the installed software package and a microprocessor. The control board sends
signals to the driver circuit which is used to fire the inverter.
The driver circuit sends low-level signals to the base of the transistors to tell
them when to turn on. The output signal is a series of pulses (Fig. 2.11.3), in both the
positive and negative directions, that vary in duration. However, the amplitude of the
pulses is the same. The sign wave is created as the average voltage of each pulse, and
the duration of each set of pulses dictates the frequency.