BASIC ELEMENTS OF ELECTRICAL DRIVE

 

Basic components or elements of electric drives:

1.      Electric motors

2.      Power modulators

3.      Sources

4.      Sensing unit

5.      Control unit

6.    Load

1. Electrical motors:

Generally used motors in electrical drives are,

  1. DC motors
    1. DC series motors
    2. DC shunt motors
    3. DC compound motors 
  2. AC motors or induction motors:
    1. Squirrel cage induction motors
    2. Slip ring induction motors 
  3. Synchronous motors

                       In earlier stage, induction and synchronous motors were adopted mainly for constant speed drives application. Since obtain variable speed in above motors creates either high expensive or very poor efficiency. But now a day with the help of semiconductor converters like Thyristors, IGBTs, and GTOs are now employing the variable speed in the same AC motors.

                        DC motors also employed, but due to the presence of commutator and brushes in DC motors have a lot of drawbacks, such as high costs, weight volume and inertia for the same rating, necessity of maintenance, restriction on minimum voltage, speed and power ratings.

2. Power modulators:

Power modulators are doing the following works.

Ø  It modulated the flow of power from source to load through motor.

Ø  During transient operations, such as starting, braking, speed reversal it restricts source and motor current with in permissible level.

Ø  It is also used for selecting the operating mode such as whether the operation is motoring, braking or reversing.

                        For the control of DC motor one requires variable DC voltage where as for AC motors one requires either fixed frequency variable voltage AC. Since the drives need for a converter arises where nature of the available electrical power is different than what is required for the motor like fixed voltage and fixed frequency AC and fixed voltage DC. Therefore the following power modulating circuits needs for various applications. 

  1. AC to DC converter (Rectifier):

             The process of converting AC into DC is called rectification. The rectifiers can be designed by using semi conducting devices like Diodes, Thyristors. 

ii. AC voltage controllers (AC voltage regulators):

                     These are employed to get variable AC voltage of the same frequency from a fixed voltage source input. This variable voltage can be obtained from auto transformer, transformer with taps, thyristors controllers, magnetic amplifiers are used as a AC voltage regulator.

iii. DC to DC converters (Choppers):

                 Choppers are nothing but switch, which is used to get a variable DC output voltage from the fixed DC input by using power semi conductor devices like power transistors, thyristors, IGBTs, GTOs, power MOSFETs based on the duty ratio. Dutyratio is nothing but the ratio of on time to total time.

iv. DC to AC converters (Inverters):

                     Inverters are nothing but static power converters for converting DC to AC with pulse width modulation (PWM) techniques or by controlling the conducting periods of thyristors it is possible to obtain variable frequency at the output terminals of the inverters.

                        Variation in output voltage or current can be obtained by varying the input DC voltage. This is achieved either by introducing a chopper in between fixed voltage DC source and the inverter or the inverter may be fed from any one of the AC to DC converter.

                         Similarly variable voltage or frequency can be obtained from fixed DC voltage when the inverter is controlled by pulse width modulation (PWM) technique. This PWM technique reduces harmonics available in it. These inverters are designed by using semiconductor devices like power transistors, power MOSFETs, GTOs, IGBTs by controlling the firing pulses of each.

v. Cyclo converters:

                         Cyclo converters are nothing but fixed voltage/frequency to variable voltage/frequency converters by using semiconductor devices like thyristors, IGBTs, GTOs, and etc. Basically cyclo converters are at two types, namely

Step - down cyclo converters (f0<fs)

Step - up cyclo converters (f0>fs)

Where

             f0=output frequency

             fs=supply frequency

The above classification purely depends on output frequency.

3. Sources:

                        The choice of a motor depends on the type of supply. The source adopted for drives are AC or DC.

Ø  AC supply for induction motor drives single phase AC supply and three phase AC supply

Ø  DC supply for DC motor drives

Ø  Some drives are powered from a battery source. Depending on size of battery voltage may have typical values of 6V, 12V, 24V, 48V and 110V DC.

4. Sensing unit:

                        Used to sensing the certain drives parameters like speed, current, voltage and etc. Normally speed is sensed by using tachometer and it requires for implementation of closed loop speed control schemes.

                         Similarly the current sensor employing Hall effect and it involves the use of a non-inductive resistance parallel with isolation amplifier. Anyhow this sensed signal is sent to control unit.

5. Control unit:

                        It controls the system motion without any damages according to the sensing unit along with input command. Nature of the control unit for a particular drive depends on the power modulator is used. This consists of firing circuits if the control unit is made up of semiconductor devices also it having equipped control is required when the control circuit is made up of linear and digital integrated circuits and transistors or micro processors. Also control may be provided with the help of PLCs.

6. Load:

                        Normally loads are designed or employed for accomplishing the given task. For example fans, pumps, Robots, Washing machine. A load requirement depends on speed-torque characteristics and capabilities. 


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