Electrical Drives

FACTORS GOVERNING SELECTION OF ELECTRICAL MOTORS NATURE Of ELECTRICAL SUPPLY



TYPE

OF DRIVE

ELECTR ELECTRICA ICAL L

CHARACT CHA RACTERISTI ERISTICS CS

(a) Starting Characteristics (b) Running Characteristics (c) Speed Control (d) Braking



NATURE OF LOAD

MECHANICAL

CONSIDERATION

(a) Ty Type of Enclo Enclosur sure e and and Bearin Bearing g (b) Arrangement Arrangement for transmission of power  (c) Noise (d) Cooling SIZE

AND RATING OF MOTOR

(a) Requirement for Continuous ,Intermittent or  Variable Load Cycle (b) Overload Capacity COST

(a) Capital Cost (b) Running Cost

ELECTRICAL CHARACTERISTICS  Starting

Characteristics

(i) Torque Required to Overcome Static Friction. (ii) Torque required to accelerate motor and its load to rated speed.

Starting Torque Of DC Motor  T = 0.159  Z Ia P/A N-m T =  Ia Torque Depend upon the product of flux and armature current and independent of speed.

MOTOR CHARACTERSTICS  Torque

±Armature Current (T/Ia) : Electrical Characteristics

 Speed

- Armature Current (N/Ia)

 Speed

± Torque (N/T): Characteristics

Mechanical

Following Important Relations T   Ia

N  Eb /   V- IaRa

SERIES MOTOR Torque ±Armature Current (T/Ia) Rse

V

In

Ia

Eb

series motor   Ia T  Ia

T  (Ia)2

Ra

Ta

Ia

Speed - Armature Current (N/Ia)

N

Ia

Speed ± Torque (N/T): Mechanical Characteristics

N

Ta

SHUNT MOTOR

Field Winding

V

Ia

Eb

Ra

Rsh

Torque ±Armature Current (T/I a) T  Ia  Ia

Ta

Ia

Speed - Armature Current (N/Ia) Ideal  Actual

N

Ia

Speed - Armature Current (N/Ia)

N

Ta

ELECTRICAL BRAKING A Good braking system should satisfy following Conditions It

should be quick & reliable.

It

must be provided suitable means for  dissipating stored energy of the rotating parts.

A

failure of any part in the braking system should result in the application of brakes.

TYPES OF BRAKING Two types of braking system are possible  Mechanical or Friction braking  Electrical Braking

ELECTRICAL BRAKING ADVANTAGES OF ELECTRICAL BRAKING OVER MECHANICAL BRAKING 1.Since Mechanical braking due to excessive wear  on the brake blocks or brake lining ,needs frequent and costly replacement but no such replacement is required in electrical braking ,hence there is saving in cost.

2.Frequent adjustment is required in mechanical braking hence more maintenance cost as compared to electrical braking 3.By employing electrical braking the capacity of the system can be increased by way of higher speed and haulage of heavy load. 4.In electrical braking a part of energy is returned to the supply there by saving in energy.This is not possible in case of Mechanical braking. 5.In electrical braking the heat produced during barking is not hramful

METHODS OF BRAKING 1.PLUGGING 2.RHEOSTATIC BRAKING 3.REGENERATIVE BRAKING

PLUGGING OR COUNTER CURRENT BRAKING PLUGGING WITH DC MOTOR

DC SHUNT MOTOR

SERIES MOTOR ELECTRIC BRAKING TORQUE = EBT  I = Tb EBT = Tb = K1 I But I = (V+ Eb) /R

 This

type of braking is employed in the following cases

(i) For quick stopping of the motor. (ii) For reversing drives requiring a short time for reversal. (iii) In cranes and hoists

TORQUE SPEED CHARACTRISTICS Speed

B

1 A

2 C Ia

PLUGGING WITH INDUCTION MOTOR R  Y B NORMAL

PLUGGING

RHEOSTATIC OR DYNAMIC BRAKING

Normal Running

Braking DC MOTOR

DC SERIES MOTOR

RHEOSTATIC OR DYNAMIC BRAKING WITH INDUCTION MOTOR +

+

-

Connection of stator Wdg of 3 Ph IM For Dynamic Braking

+

-

Connection of stator Wdg of 3 Ph IM For Dynamic Braking

REGENERATIVE BRAKING + I SUPPLY VOLTAGE

E

(a) E is greater than V, I being returned to the supply

+ I

If 

E

SUPPLY VOLTAGE

(b) Regeneration at constant speed

Regenerating Braking with DC Shunt Motor 

Regenerative Braking With DC Series Motor  French Method + ve

+ ve

E E