The block diagram of the electric drive is shown in the figure 1.1. The electrical load like fans, pumps, trains, etc., consists of the electrical motor. The requirement of an electrical load is determined regarding speed and torque. The motor which suited the capabilities of the load is chosen for the load drive.
The main parts of the block diagram of electrical drives are as follows:
1) Sources:
Very low power drives are generally fed from single phase sources. Rest of the drives is powered from a 3 phase source. Low and medium power motors are fed from a 400V supply. For higher ratings, motors may be rated at 3.3KV, 6.6KV and 11 KV. Some drives are powered from battery.
2) Power Modulator:
The power modulator regulates the output power of the source. It controls the power from the source to the motor in such a manner that motor transmits the speed torque characteristic required by the load. During the transient operations like starting, braking and speed reversing the excessive current drawn from the source.
This excessive current drawn from the source may overload it or may cause a voltage drop. Hence the power modulator restricts the source and motor current.
The power modulator converts the energy according to the requirement of the motor e.g. if the source is DC and an induction motor is used then power modulator convert DC into AC. It also selects the mode of operation of the motor, i.e., motoring or braking.
Types of Power Modulators
In the electric drive system, the power modulators can be any one of the following:
- Controlled rectifiers (AC to DC converters)
- Inverters (DC to AC converters)
- AC voltage controllers (AC to AC converters)
- DC choppers (DC to DC converters)
- Cycloconverters (frequency conversion)
3) Electrical Machines:
Most commonly used electrical machines for speed control applications are the following:
i) DC Machines: Shunt, series, compound, separately excited DC motors and switched reluctance machines.
ii) AC Machines: Induction, wound rotor, synchronous, PM synchronous and synchronous reluctance machines.
iii) Special Machines: Brush less DC motors, stepper motors, switched reluctance motors are used.
4) Control Unit:
The control unit controls the power modulator which operates at small voltage and power levels. The control unit also operates the power modulator as desired. It also generates the commands for the protection of power modulator and motor. An input command signal which adjusts the operating point of the drive, from an input to the control unit.
5) Sensing Unit:
It senses the certain drive parameter like motor current and speed. It mainly required either for protection or for closed loop operation. The different types of sensing unit are as follows:
i) Speed Sensing: Speed can be sensed by using a tachometer. Wind speed can be sensed by anemometer similarly both speed and velocity can be measured by the speedometer.
ii) Torque Sensing: Magneto elastic torque sensor is used in vehicles applications on race cars, automobile, and aircraft.
iii) Position Sensing: Motion can be sensed through GPS, vibrometer, and rotary encoder.
iv) Current Sensing and Voltage Sensing from lines or from motor terminals.
v) Temperature sensing: Thermistor is a device which is used for temperature measurement.
