Systems employed for motion control are called “Drives” and may employ and of the prime movers such as, diesel or petrol engines, gas or steam turbines, steam engines, hydraulic motors and electric motors for supplying mechanical energy for motion control. Drives employing electric motors are known as “Electrical Drives”.
Block diagram of an electrical drive is shown in below fig. Load is usually a machinery designed to accomplish a given task, example fans, pumps, robots, washing machines, machine tools, trains and drills.
Power modulator performs following functions,
i. Modulates flow of power from the source to the motor in such a manner that motor is imparted speed-torque characteristics required by the load.
ii. During transient operations, such as starting, braking and speed reversal, it restricts source and motor currents within permissible values.
iii. Converts electrical energy of the source in the form of suitable to the motor.
iv. Selects the mode of operation of the motor is motoring or braking.
From the figure, the major elements of an electrical system are follows:
1. Electrical Motor and load
2. Power Modulator
4. Control Unit
5. Sensing Unit
1. Electrical Motor
Most commonly used electrical drives are,
DC Motors: Shunt, Series, Compound and Permanent magnet motors.
Induction Motors: Squirrel Cage, Wound Rotor and Linear motors.
Synchronous Motors: Wound field and permanent magnet motors. Brush less direct current (BLDC) motors, stepper motors and switched reluctance motors (SRM) can be used.
2. Power Modulators
Some drives may employ more than one of these modulators. It can be classified into power modulators.
Rectifiers (AC to DC Convertors)
Inverters (DC to AC)
AC Voltage controllers (AC to AC)
DC Choppers (DC to DC)
Cyclo converters (Frequency Conversion)
3. Electrical sources
In India single phase and three phase 50 Hz, AC supplies are readily available in most locations. Very low power drives are generally fed from single phase source. Rest of the drives are powered from 3 phase source. Low and medium power motors, are fed from 400 V supply. For higher ratings, motors may be rated at 3.3 KV, 6.6 KV and 11 KV. Some drives are powered from a battery, battery voltage may have 24 V, 28 V and 110 V DC.
4. Control unit
Controls for a power modulator are provided in the control unit. Nature of the control unit for a particular drive depends on the power modulator that is used.
When semiconductor converter are used, the control unit will consists of firing circuits, which employ linear and digital integrated circuits and transistors and a microprocessor when sophisticated control is required.
5. Sensing unit
Speed sensing is required for implementation of closed loop speed control schemes. Speed is usually sensed by using tachometers. When very high speed accuracies required, as in computer peripherals and paper mills etc., digital tachometers are used.
Two commonly used methods of sensing the current.
i. Use of current sensor employing Hall Effect.
ii. It involves the use of a non-inductive resistance shunt in conjunction with an isolated amplifier which has an arrangement for an amplification and isolation between power and control circuits.
ADVANTAGES OF AN ELECTRIC DRIVE
The various advantages of an electric drives are
1. The control characteristics can be manipulated as per requirements.
2 Availability of simple and easy speed control methods.
3. Electric breaking can be employed in easy manner,
4. The operation is pollution free.
5. The variety of electric drives with wide range of speed. Power and torque ratings are available.
6. The efficiency is higher as no load losses are less.
7. They have short time overload capacity.
8. They can be operated in different working conditions such as explosive, radioactive and also can be submerged into liquids.
9. Most of the motors used in electric drives are self starting without requirement of any starting equipment.
10. Instantaneous starting of motors is possible and can drive full load immediately.
11. Compared to other prime movers the operation is cleaner, noise is lesser and lower maintenance.
See More: Four and Two Point Starter
See More: Three Point Starter
See More: Working Principle of D.C Motor