Introduction
Have you ever wondered how industrial machines run at different speeds using the same DC motor? The answer lies in speed control methods. In a DC shunt motor, speed can be adjusted smoothly without stopping the motor — making it ideal for applications like lathes, conveyors, and elevators.
In this article, we'll explain the speed equation of a DC motor, then dive deep into the two main methods of speed control — Armature Resistance Control and Field Flux Control. You'll understand not just how they work, but why one method is preferred over the other in practice.
Table of Contents
- Speed Equation of DC Motor
- Methods of Speed Control
- Armature Resistance Control Method
- Field Flux Control Method
- Comparison Table
- Practical Applications
- FAQs
- Conclusion
Speed Equation of DC Motor
Before understanding speed control, you need to know what determines the speed of a DC motor. The voltage equation of a DC motor is:
Where:
- V = Supply voltage
- Eb = Back EMF
- Ia = Armature current
- Ra = Armature resistance
We also know that back EMF is given by:
Since Z, P, and A are constants for a given machine, we can simplify:
Rearranging for speed:
This is the fundamental speed equation of a DC motor. It tells us that speed depends on three factors:
- Supply voltage (V)
- Armature circuit resistance (Ra)
- Field flux (Φ)
By varying any of these, we can control the motor speed.
Methods of Speed Control of DC Shunt Motor
Based on the speed equation, there are two practical methods used for speed control of a DC shunt motor:
- Armature Resistance Control — varies the effective voltage across the armature
- Field Flux Control — varies the magnetic flux produced by the field winding
A third method — varying the supply voltage — requires a separate variable voltage source and is less common in simple installations. We'll focus on the two widely used methods.
Armature Resistance Control Method
In this method, a variable external resistance is connected in series with the armature winding. The field winding remains directly connected across the supply, so the flux stays constant.
How It Works
When external resistance (Rext) is added in series with the armature:
As Rext increases, the voltage drop across the total armature circuit increases. This reduces the back EMF (Eb), and since speed is proportional to Eb, the motor slows down.
Think of it like this: the external resistance "eats up" some of the supply voltage before it reaches the armature, leaving less voltage to drive the motor.
Key Characteristics
- Speed can only be reduced below the rated speed — you cannot increase speed with this method
- The field flux remains constant since the shunt field is connected directly across the supply
- Speed control is achieved below the base (rated) speed
Drawbacks of Armature Resistance Control
- High power loss: The external resistance carries the full armature current, so power dissipated (Ia² × Rext) is significant. This makes the method inefficient.
- Poor speed regulation: Speed changes with load because the voltage drop across Rext depends on armature current, which varies with load.
- Only below-base speed: Cannot increase speed above the normal rated speed.
- Bulky resistors: For large motors, the external resistance must handle high current, requiring physically large and expensive resistors.
When Is It Used?
Despite its drawbacks, this method is used where:
- Speed reduction is needed for short durations (e.g., crane lowering)
- Smooth starting is required along with speed control
- The application doesn't demand high efficiency
Field Flux Control Method
In this method, a variable resistance is connected in series with the shunt field winding. This resistance is called a field rheostat.
How It Works
The field current is given by:
Where Rf is the field winding resistance and Rext is the external field rheostat resistance.
When Rext is increased:
- Field current (If) decreases
- Flux (Φ) decreases (since Φ ∝ If)
- From the speed equation, N ∝ 1/Φ, so speed increases
This is sometimes called field weakening — by weakening the magnetic field, the motor speeds up.
Why Does Weaker Flux Increase Speed?
This seems counterintuitive at first. Here's the intuition: when flux decreases, back EMF drops momentarily. Since V remains constant, the armature current increases (V = Eb + IaRa). The increased current produces more torque, which accelerates the motor until a new equilibrium is reached at a higher speed.
Key Characteristics
- Speed can be increased above the rated speed
- This is the most common and preferred method for DC shunt motor speed control
- Speed control range is typically 2:1 to 3:1 above base speed
Advantages of Field Flux Control
- High efficiency: Field current is small (typically 1–5% of rated current), so power loss in the field rheostat is minimal
- Smooth control: Speed varies smoothly with field resistance adjustment
- Compact: The field rheostat is small since it carries only the small field current
- Economical: Low cost of the control equipment
Limitations
- Excessive field weakening can cause instability and commutation problems
- At very high speeds, the motor may become mechanically unsafe
- Torque capacity reduces at higher speeds (since torque ∝ Φ × Ia, and Φ is reduced)
Comparison: Armature Resistance vs Field Flux Control
Practical Applications
Where Armature Resistance Control Is Used
- Cranes and hoists (for controlled lowering)
- Rolling mills (during threading)
- Applications needing speed control only during starting or braking
Where Field Flux Control Is Used
- Lathes and machine tools (spindle speed variation)
- Paper mills
- Textile machinery
- Any application requiring continuous above-base speed operation
Real-World Analogy
Think of a bicycle with gears. Armature resistance control is like applying a brake — you can only slow down, and you waste energy as heat. Field flux control is like shifting to a higher gear — you go faster with the same pedaling effort, and it's much more efficient.
FAQs
Why can't armature resistance control increase speed above rated value?
Adding resistance in the armature circuit can only increase the voltage drop, which reduces the effective voltage across the armature. Since speed is proportional to (V − IaRa), adding more resistance can only decrease this value, never increase it beyond V.
What happens if the field winding of a DC shunt motor is accidentally opened?
If the field circuit opens, flux drops to nearly zero (only residual magnetism remains). From N ∝ 1/Φ, the motor tries to reach dangerously high speed. This is why DC shunt motors have protective devices to disconnect supply if field current is lost.
Which method is more commonly used in industry?
Field flux control is the preferred method because of its high efficiency, smooth control, and compact equipment. For modern applications, electronic methods like Ward-Leonard system or thyristor-based drives have largely replaced rheostatic methods.
Can both methods be used together?
Yes. In practice, both methods are often combined to get a wide speed range — armature resistance control for below-base speeds and field flux control for above-base speeds. This gives speed control both above and below the rated speed.
What is the Ward-Leonard method of speed control?
The Ward-Leonard system uses a motor-generator set to provide variable voltage to the armature. It offers smooth speed control over a wide range (both above and below base speed) with high efficiency, but requires additional machines, making it expensive.
Conclusion
Speed control of a DC shunt motor is achieved by manipulating the variables in the speed equation N ∝ (V − IaRa) / Φ. The armature resistance method provides below-base speed control but is inefficient. The field flux control method provides above-base speed control efficiently and is the preferred choice in most industrial applications.
Understanding these methods is fundamental for any electrical engineering student, as they form the basis for more advanced drive systems used in modern industry. To build a stronger foundation, make sure you also understand the characteristics of DC shunt motor and the EMF and torque equation of DC machines.