13-05-2014, 02:53 PM
Course Notes on Electric Drives
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Control of DC Drives
The Direct Current machine have been dominating the field of adjustable speed drives for
over a century; they are still the most common choice if a controlled electric drive operating
over a wide speed range is specified. This is due to there excellent operational properties and
control characteristics; the only essential disadvantage is the mechanical commutator which
restricts the power and the speed of the motor, increases the inertia and the axial length of
and requires the periodic maintenance.
In the Figure ? a schematic cross section of a DC machine is shown, containing the fixed
stator S and the cylindrical rotor, called armature A. While rotor and pole shoes are al-
ways laminated to reduce the iron losses caused by the varying magnetic field, the rest of
the stator is laminated only in larger machines, when the motor is required to operate with
rapidly varying torque and speed are when a static power converter with highly distorted
voltages and currents is employed as the power supply. The main poles M P are fitted with
the field windings, carrying the field current ie which drives the main flux φe through the
stator and rotor. A closed armature winding is placed in the axial slots of the rotor and
connected to the commutator bars; it is supplied through the brushes and the commutator
with the armature current ia. This creates a distributed ampere-turns (m.m.f.)wave, fixed
in space and oriented in the direction of the quadrature axis, orthogonal to the main field
axis, so that maximum torque for a given armature current is produced.
GATE DRIVE
Typical gate drive to turn ON a GTO is shown in fig. 2.7. The gate trigger pulse has to
be long enough to establish anode current to atleast the level of latching current. Below the
latching current, GTO behaves like a BJT. The recommended practice is a high (4-8 times)
gate current with a short rise time (1 μs) followed by normal gate current during ON time.
After turning ON, the GTO must remain ON for atleast TON (min), required for complete
discharge of RC snubber across GTO.
FORWARD CHARACTERISTICS
The forward characteristic of GTO is shown in Fig. 2.8. When the anode current is less than
IL (latching current), the GTO behaves like BJT. In this mode of operation, the current gain
is IA /IG (similar to β of BJT). If the gate current is zero, the GTO blocks the anode voltage
with low leakage current (C on the characteristics). If the gate current is high, then GTO
conducts with the low forward voltage drop (A on the characteristics). As long as the anode
current is below the latching level (B on the characteristics), the GTO will return to the
blocking state on the removal of gate drive.
The MOSFET
Like the BJT, the MOSFET is a three terminal device. The main terminals are source (S)
and the drain (D). The control terminal is the gate (G). The gate is insulated from the rest
of the device by an oxide layer and therefore draws no current in steady state. When the
gate is placed at a suitable potential with respect to the source, a conducting path known
as the channel is established between the source and the drain and current flow becomes
possible. MOSFETs used for power switching applications are of the enhancement type, i.e.,
the channel comes in to existence only when a suitable gate to source voltage is applied.
Otherwise the drain and source are isolated from one another, with ideally infinite drain to
source resistance. Depending on the nature of the semiconductor material consisting the
drain, the source and the channel, MOSFETs are classified as N-channel or P-channel types.
N-channel power MOSFETs are more commonly used, although P-channel types are also
commercially available.