Learn about speed control of DC shunt motor using field and armature control methods. Includes formulas, diagrams, and speed regulation tips for efficient DC motor speed control.
What is Speed Control of a DC Shunt Motor?
In industrial and automation systems, maintaining the desired speed under varying load conditions is crucial. This is where speed control of DC shunt motor comes into play. A DC shunt motor is popular for its ability to run at nearly constant speed, but sometimes we need to either increase or decrease its speed depending on the application. That’s where specific speed control methods of DC shunt motor are applied.
Let’s explore how we can achieve precise speed control for DC motor, especially the shunt motor, along with relevant diagrams, formulas, and explanations.
Understanding the DC Shunt Motor
A DC shunt motor has its field winding connected in parallel (shunt) with the armature winding. This arrangement helps the motor maintain a relatively constant speed. But despite this natural stability, specific dc motor speed control methods are needed for varying industrial requirements.
Shunt Motor Uses:
- Lathes and drills
- Printing presses
- Textile machinery
- Elevators
Speed Control Methods of DC Shunt Motor
The speed of a DC shunt motor is governed by the equation:
Where:
- N = Speed of the motor
- Eb = Back EMF
- ϕ = Magnetic flux
- V = Supply voltage
- Ia = Armature current
- Ra = Armature resistance
- K = Constant for a given motor
From the formula, it’s clear that the speed of a DC shunt motor can be controlled by either changing the flux (ϕ) or the armature voltage drop (V – IₐRₐ). This leads to three primary methods of speed control:
- Flux Control Method
- Armature Resistance Control Method
- Armature Voltage Control Method
1. Field Control Method (Flux Control)
This method controls the motor’s speed by adjusting the field current, which in turn changes the magnetic flux (ϕ) produced by the shunt field winding.
In this method, a variable resistor—known as a field rheostat—is connected in series with the shunt field winding. The diagram below illustrates the Speed Control of a DC Shunt Motor using the Field Control Method (Flux Control).
When the field current is reduced (by increasing the resistance in the field circuit), the magnetic flux decreases. According to the dc motor speed formula, the speed is inversely proportional to the flux. So, as the flux decreases, the speed increases.
In practical applications, a variable resistance called a field rheostat is connected in series with the shunt field winding. By increasing this resistance:
- The shunt field current (Ish) decreases
- The flux (ϕ) weakens
- As a result, the motor speed increases
This method is only suitable for increasing the speed above the rated speed of the motor.
Advantages of Field Control Method
- Simple and convenient: Easy to implement and operate.
- Cost-effective: Power loss is minimal since the field current is low.
- Load-independent: The speed can be adjusted without being affected by load changes.
- Commonly used in industrial setups requiring speed control above base speed.
Disadvantages of Field Control Method
- Only higher speeds possible: This method cannot reduce the speed below rated, as the total field circuit resistance can’t be lower than the winding resistance.
- Limited maximum speed: Excessive weakening of the field flux can lead to poor commutation and possible motor instability.
2. Armature Resistance Control Method
One of the most basic speed control methods of a DC shunt motor is the armature resistance control method. This method is primarily used to reduce the motor’s speed below the rated value.
In this technique, a variable resistor (controller resistance) is inserted in series with the armature winding. This external resistance causes a voltage drop in the armature circuit, effectively reducing the back EMF (Eb) and thus slowing down the motor.
The diagram below shows the Speed Control of a DC Shunt Motor using the Armature Resistance Control Method.
The working principle is based on the speed equation of a DC motor:
As the controller resistance (Rc) increases, back EMF Eb decreases, which in turn reduces the motor speed N.
Speed control of DC shunt motor in armature circuit is therefore effective only for reducing speed below the rated value.
Key Features of Armature Control Method
- Simple to implement with basic components
- Effective for applications where speed reduction is needed
Disadvantages of Armature Resistance Control Method
- High Power Loss: The added resistor carries the full armature current, leading to substantial heat loss.
- Reduced Efficiency: Power is wasted in the external resistor, lowering the overall efficiency and output of the motor.
- Poor Speed Regulation: The speed varies with the load, since the voltage drop across the controller resistance depends on load current.
- Limited Speed Range: The maximum speed achievable is the rated speed (when Rc = 0). No speeds above rated are possible with this method.
3. Armature Voltage Control Method using Thyristor DC Drive
This is a modern and efficient method for DC motor speed control using power electronic converters (thyristors).
- Principle: The armature voltage is directly controlled by a thyristor-based DC drive, usually through phase-controlled rectifiers. Varying the average armature voltage changes the back EMF, and thus, controls the speed.
- Applicable Range: This method can control speed both above and below rated values (with field weakening for higher speeds).
Advantages:
- Precise and smooth control over a wide speed range.
- High efficiency compared to resistance methods.
- Suitable for automatic control systems and remote operation.
- Can handle dynamic load conditions effectively.
Disadvantages:
- Higher initial cost and system complexity.
- Requires proper filtering to reduce harmonics.
Summary Table: Comparison of DC Shunt Motor Speed Control Methods
| Method | Speed Range | Efficiency | Control Type | Key Application |
|---|---|---|---|---|
| Flux Control Method | Above rated speed | High | Manual | Fans, blowers |
| Armature Resistance Control Method | Below rated speed | Low | Manual | Hoists, cranes |
| Armature Voltage Control (Thyristor) | Above & below rated | High | Electronic | Modern industrial uses |
Speed Regulation vs Speed Control
While speed control involves externally changing the speed of a motor, speed regulation refers to the ability of the motor to maintain speed when load varies.
Speed Regulation DC Motor (%) is given by:
A lower percentage means better speed regulation.
Numerical Example – Speed Control of DC Shunt Motor
A 300 V DC shunt motor has the following specifications:
- Armature resistance Ra=0.2 Ω
- Armature current Ia=50 A
- Initial speed N1=750 RPM
The flux is reduced by 10% using the field rheostat.
Assume that the load torque remains constant.
We are to calculate the new speed of the motor.
Step-by-Step Solution
Step 1: Calculate Back EMF at Initial Condition
Back EMF (Eb1) is given by:
Step 2: Determine Effect of Flux Reduction
Flux is reduced by 10%, so new flux:
Step 3: Use Speed-EMF Relation
Since torque is constant, Ia remains the same.
Speed is proportional to back EMF and inversely proportional to flux:
Now calculate
So,
New speed of the motor N2= 833 RPM.
Conclusion
The speed control of DC shunt motor is vital for adapting to various operational requirements. Whether you need speed above or below rated values, you can achieve it using either the field control method or armature control method.
These dc motor speed control methods ensure performance flexibility, longer lifespan of equipment, and energy efficiency. Understanding the speed of DC motor formula, associated diagrams, and control techniques makes it easier for engineers and technicians to choose the right setup.
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