In electric drives, the nature of the load connected to the motor plays an important role in determining the performance, efficiency, speed control, and selection of the motor. Different industrial applications require different load characteristics, and therefore understanding the types of loads in electric drive is essential for electrical engineering students and competitive exams like SSC JE, RRB JE, GATE, and AE/JE exams.
The load in an electric drive refers to the mechanical equipment or machine driven by the motor. Depending upon the operating condition, speed requirement, torque variation, and nature of operation, loads are classified into different categories.
What is Load in Electric Drive?
- A load in an electric drive is the mechanical system that receives power from the electric motor to perform useful work. Examples include pumps, fans, cranes, hoists, conveyors, rolling mills, compressors, and machine tools.
- Each load requires different torque and speed characteristics. Therefore, proper classification of loads helps in selecting a suitable motor for efficient operation.
Classification of Loads in Electric Drive
Loads in electric drives are mainly classified as:
- Based on influence of gravity, compression, or tension
- Based on time of operation
- Based on speed-torque characteristics
1. Types of Loads Based on Influence of Gravity, Compression, or Tension
In this classification, loads are categorized according to the direction and nature of the force acting on the motor.
(a) Active Loads
Active loads are those loads in which gravity acts in such a way that the load tends to move by itself. These loads can continue motion even when motor power is removed.
In active loads, the motor must control the motion of the load against gravitational force.
Characteristics of Active Loads
- Gravity assists the motion
- Load can drive the motor
- Braking is often required
- Motor may operate in generating mode
Examples of Active Loads
- Lifts
- Hoisters
- Elevators
- Downward movement of cranes
For example, when a lift moves downward, gravity helps the motion, making it an active load.
(b) Passive Loads
Passive loads are those loads in which friction, compression, or mechanical resistance opposes the motion. The motor must continuously supply energy to keep the load moving.
Characteristics of Passive Loads
- Load cannot move without motor power
- Friction opposes motion
- Motor always supplies torque
- No self-driving action
Examples of Passive Loads
- Mills
- Conveyors
- Machine tools
- Compressors
In mills, friction and mechanical resistance oppose motion continuously, so the motor must provide continuous driving torque.
2. Types of Loads Based Upon Time of Operation
According to operating duration and variation of load with time, electric drive loads are classified into different categories.
(a) Continuous and Constant Loads
These loads operate continuously for a long duration with nearly constant torque and power demand.
Characteristics
- Load remains almost constant
- Motor runs continuously
- Steady operating condition
- Low fluctuation in torque
Examples
- Centrifugal pumps
- Ventilation fans
- Blowers
Centrifugal pumps generally require continuous operation with almost constant loading conditions.
(b) Continuous and Variable Loads
These loads operate continuously, but the torque and power demand vary with time.
Characteristics
- Continuous operation
- Load changes with time
- Variable torque requirement
- Frequent speed adjustment may be needed
Examples
- Conveyors
- Hoisters
- Escalators
In conveyors, the load may vary depending on the quantity of material being transported.
(c) Pulsating Loads
Pulsating loads are those loads in which torque fluctuates periodically during operation.
Characteristics
- Periodic variation of torque
- Non-uniform motion
- Frequent acceleration and deceleration
Examples
- Reciprocating pumps
- Textile looms
- Reciprocating compressors
In reciprocating pumps, the torque demand changes continuously during each stroke.
(d) Impact Loads
Impact loads are characterized by sudden application of heavy loads or shocks.
Characteristics
- Sudden heavy load application
- High peak torque
- Mechanical shocks and vibrations
- Requires strong motor construction
Examples
- Rolling mills
- Shearing machines
- Punching machines
Rolling mills experience sudden impact loads during metal processing operations.
(e) Intermittent Loads
Intermittent loads operate for short periods followed by rest intervals.
Characteristics
- Frequent starting and stopping
- Motor heating and cooling cycles
- High starting torque requirement
Examples
- Cranes
- Hoisters
- Lifts
Cranes operate intermittently because they work only during lifting and lowering operations.
3. Types of Loads Based Upon Speed-Torque Characteristics
In electric drives, the relation between load torque and speed is extremely important. Different loads require different torque-speed characteristics.
(a) Constant Torque Load
In constant torque loads, the torque required remains constant irrespective of speed.
Mathematically:
Torque = Constant
Characteristics
- Torque remains fixed
- Power varies directly with speed
- High starting torque may be required
Examples
- Hoisters
- Cranes
- Conveyors
- Elevators
Cranes require nearly constant torque at different operating speeds.
(b) Torque Proportional to Speed
In this type of load, torque is directly proportional to speed.
Mathematically:
Torque ∝ Speed
Characteristics
- Torque increases linearly with speed
- Common in fluid friction systems
- Moderate power demand
Examples
- Fluid friction loads
- Viscous friction mechanisms
In fluid friction systems, resistance increases proportionally with speed.
(c) Torque Proportional to Square of Speed
In these loads, torque varies as the square of speed.
Mathematically:
Torque ∝ Speed²
Characteristics
- Torque rises rapidly with speed
- Power requirement increases significantly
- Common in aerodynamic systems
Examples
- Fans
- Blowers
- Centrifugal pumps
Fans require much higher torque at higher speeds because air resistance increases rapidly.
(d) Torque Inversely Proportional to Square of Speed
In this type of load, torque decreases as the square of speed increases.
Mathematically:
Torque ∝ 1 / (Speed)²
Characteristics
- Torque decreases rapidly with speed
- Used in special industrial applications
- Requires careful speed control
Examples
- Grinding machines
- Certain machine tool operations
Grinding operations may require higher torque at lower speeds and reduced torque at higher speeds.
Importance of Load Classification in Electric Drives
Load classification helps engineers in:
- Selecting suitable motors
- Designing proper control systems
- Improving efficiency
- Reducing energy consumption
- Ensuring smooth operation
- Providing proper speed control
Applications of Different Load Types
| Load Type | Examples |
|---|---|
| Active Load | Lifts, hoisters |
| Passive Load | Mills, conveyors |
| Continuous Constant Load | Centrifugal pumps |
| Pulsating Load | Reciprocating pumps |
| Impact Load | Rolling mills |
| Intermittent Load | Cranes, hoisters |
| Torque ∝ Speed² | Fans and blowers |
| Torque ∝ 1/(Speed)² | Grinding machines |
Conclusion
Loads in electric drives are classified based on gravitational effect, operating time, and speed-torque characteristics. Each type of load has unique operating requirements and requires suitable motor characteristics for efficient operation.
Understanding the types of loads in electric drive is very important for motor selection, drive design, industrial automation, and competitive examinations like SSC JE, RRB JE, GATE, and state AE/JE exams.
FAQs on Types of Loads in Electric Drive
What is load in electric drive?
A load in electric drive is the mechanical system driven by the motor to perform useful work.
What are active loads?
Active loads are loads where gravity assists motion, such as lifts and hoisters.
What are passive loads?
Passive loads are loads where friction or resistance opposes motion, such as mills and conveyors.
Which loads are called pulsating loads?
Loads with periodic torque fluctuations, such as reciprocating pumps and textile looms, are called pulsating loads.
Which load has torque proportional to square of speed?
Fans and centrifugal pumps have torque proportional to the square of speed.
Why is load classification important in electric drives?
Load classification helps in selecting suitable motors, improving efficiency, and ensuring smooth drive operation.