Usually, a unit of solid state power converts AC power from the plant to an adjustable DC voltage. This voltage feeds the motor armature. DC motor speed varies in proportion to the armature voltage assuming:
The engine is big enough to feed the load.
The motor field current is constant.
DC motors have two main components: armor and field. The interaction of magnetic fields of both components cause the rotation of the rotor.
The stage can be implemented using:
- Thyristors
- Transistors.
DRIVE thyristor
Silicon Controlled Rectifiers (SCR) commonly called "thyristors", used in the power unit to convert AC voltage DC voltage control. The SCR conducts current when a small voltage pulse is applied to your terminal "gate", see Figure 2
Figure 2
Most thyristor inverters designed to operate with single-phase AC power, have 4 thyristors. The units operate on three phase are often built with six thyristors, see Figure 3.
A variant of this design include the replacement of the bottom row of diode rectifiers and thyristors adding a switching diode across the DC armature output.
Figure 3
The source of the field shown in Figure 3 is implemented by a diode bridge, so the country receives a fixed DC voltage equal to its nominal value. The armor receives variable DC voltage, in order that the motor in constant torque region as seen in Figure 16 of the first part.
To physically disconnect the motor from the power unit, solid-state contactor is used (M), see Figure 4. When energizing the contactor coil M control, the thyristor rectifier bridge fed to the armature causing the machine to work as an engine driving the load coupled to its axis.
When you want to have control of the engine braking, you can stop quickly by a dynamic braking resistor (Dynamic Brake: DB) through the motor armature, see picture. 4. The contactor control coil must be energized M to allow the DB resistor acts as a load of armor, which by action of the inertia of its load, has become a generator. This dynamic braking is only effective while the armature is in motion.
Adding another set of thyristors (called reverse section) connected with reversed polarity, see Figure 5, the drive gets regenerative capabilities and can operate in four quadrants, see Figure 6.
This configuration provides two-way operation without the use of contactors controlled investment and regenerative braking.
Figure 5
Figure 6
Regenerative braking is defined as the return of power from the engine (during braking moment acts as a generator) to the AC power source. This energy must be absorbed somehow by the source. Figure 6 shows that during braking, the polarity of the armor does not change but if the current direction. This means that regenerative braking to supply voltage to the armature to do less than the counter electromotive voltage. See Equation (1-1) in the first half.
Thyristor drives are typically used in industry because they can control DC motors power fractions to tens of MW.
