Speed Control of DC Series Motor — Methods, Formulas & Applications
A DC series motor is widely used in electric traction, cranes, and hoists because of its high starting torque. However, many applications require precise speed regulation. Understanding the speed control methods of a DC series motor is essential for electrical engineers working with industrial drives and electric vehicles.
Speed Equation of DC Motor
The speed of any DC motor depends on the back EMF and field flux. The fundamental speed equation is:
Where:
- N = Speed of the motor (RPM)
- V = Supply voltage
- Ia = Armature current
- Ra = Armature resistance
- Φ = Field flux per pole
This equation reveals three variables that can be manipulated to control speed: supply voltage V, armature circuit resistance Ra, and field flux Φ. In a series motor, since the field winding carries the full armature current, Φ is directly proportional to Ia (until saturation).
Armature Resistance Control
In this method, an external variable resistance (Re) is connected in series with the armature circuit. Increasing Re increases the total voltage drop across the resistance, reducing the effective voltage available to the armature.
Key characteristics:
- Speed can only be reduced below the normal (rated) speed
- Provides smooth speed control below base speed
- Large power loss (I²Re) in the external resistance — poor efficiency
- Speed regulation is poor — speed changes significantly with load variation
- Used in cranes and hoists where low-speed operation under heavy load is needed
Field Flux Control (Field Diverter Method)
A variable resistance called a diverter (Rd) is connected in parallel across the series field winding. This diverts a portion of the armature current away from the field winding, reducing the field flux Φ.
Key characteristics:
- Speed can only be increased above the normal speed
- Decreasing Rd diverts more current → less flux → higher speed
- More efficient than armature resistance control (less I²R loss)
- Risk of dangerously high speed if field flux is reduced too much
- Commonly used in electric traction for high-speed operation
Tapped Field Winding Method
Instead of using a diverter resistance, the series field winding is provided with multiple taps. By selecting different taps, the number of active field turns is changed, directly controlling the field flux.
- Fewer active turns → less flux → higher speed
- Provides discrete speed steps (not continuous control)
- Most commonly used in electric traction (trains, trams, metro systems)
- Simple, robust, and efficient — no external resistance losses
- Combined with series-parallel control for wider speed range
Series-Parallel Control
When two or more series motors are used (as in electric traction), they can be connected in series for low speed and in parallel for high speed. This method is extensively used in railway traction systems.
- Series connection: Each motor gets V/2 voltage → lower speed, higher torque (starting/acceleration)
- Parallel connection: Each motor gets full V voltage → higher speed (cruising)
- Transition from series to parallel is done through resistive bridging to avoid current surges
- Provides two distinct speed ratios (1:2) without any resistive losses at steady state
Supply Voltage Control
Modern drives use power electronic converters (choppers or controlled rectifiers) to vary the supply voltage to the motor. This is the most efficient method available today.
- DC chopper: Varies average voltage using PWM — smooth, stepless control
- Controlled rectifier: Varies DC output from AC supply by adjusting firing angle
- Speed control in both directions (above and below base speed) is possible
- High efficiency — minimal resistive losses
- Used in modern electric vehicles, metro trains, and industrial drives
Comparison of Speed Control Methods
Applications of DC Series Motor Speed Control
- Electric traction: Trains, trams, and metro systems use tapped field + series-parallel control
- Electric vehicles: Modern EVs with DC series motors use chopper-based voltage control
- Cranes and hoists: Armature resistance control for precise low-speed lifting
- Rolling mills: Field flux control for above-base-speed operation
- Conveyor systems: Variable speed drives with chopper control
Frequently Asked Questions
1. Why is armature resistance control inefficient?
Because the external resistance dissipates power as heat (P = I²Re). At low speeds, a significant portion of the supply power is wasted in the resistance rather than being converted to mechanical work.
2. Can we increase the speed of a DC series motor using armature resistance control?
No. Adding resistance in the armature circuit only reduces the effective voltage across the armature, which can only decrease the speed below the rated value. To increase speed, field flux control or voltage control methods are used.
3. What happens if the field winding of a DC series motor is open-circuited?
The flux drops to nearly zero (only residual magnetism remains). Since N ∝ 1/Φ, the motor speed increases dangerously and can lead to mechanical failure. This is why DC series motors should never be run without load.
4. Which speed control method is used in Indian Railways?
Indian Railways traditionally uses a combination of series-parallel control and tapped field winding for DC traction motors. Modern locomotives (like WAP-7) use three-phase AC induction motors with VVVF drives, but older EMUs still use DC series motor control.
5. What is the advantage of chopper control over resistance control?
Chopper control varies the average voltage using high-frequency switching (PWM) with minimal power loss. It provides smooth, stepless speed control across the full range with efficiency above 95%, compared to 50-70% for resistance control at reduced speeds.