Speed Control of DC Motor (Shunt, Series, and Compound)

A drive may need to set the speed of a DC motor to match its process or load. This commanded adjustment is called speed control of a DC motor.

Speed control can be manual or part of a closed-loop drive. It differs from inherent speed regulation, which describes how much steady speed changes as load changes under stated operating conditions.

In steady state, the approximate speed of a DC motor (N) follows:

The equation shows why shunt, series and compound DC motors can be controlled through armature voltage, armature-circuit voltage drop or field flux. The usable method depends on how each motor’s armature and field are connected.

The controllable quantities are:

1. The armature terminal voltage, V.
2. The total armature-circuit resistance, including armature resistance R_a and any added series resistance.
3. The field flux per pole, φ.

Armature voltage and series resistance act through the armature circuit. Field control changes the motor’s magnetic field. The traditional methods of speed control of DC motor are therefore grouped as:

1. Armature Control Methods
2. Field Control Methods

The circuit behavior differs between DC series motors and DC shunt motors, so each method needs its own limits and protection.

Speed Control of DC Series Motor

Speed control methods for a DC series motor fall into two traditional groups:

1. Armature Control Methods
2. Field Control Methods

Armature Controlled DC Series Motor

A DC series motor can use the following armature control methods:

1. Armature Resistance Control Method
2. Shunted Armature Control Method
3. Armature Terminal Voltage Control

Armature Resistance Control Method

This historical method connects a variable resistance in series with the motor, as shown in the figure. The resistor reduces motor terminal voltage by an amount that changes with armature current.

The control resistor carries full motor current and dissipates I²R power, so its loss cannot be neglected. Speed also changes with load because the resistor voltage drop follows current. Resistance control was used for short-duration or stepped low-speed operation of a DC series motor, but a correctly rated electronic drive usually gives better efficiency and regulation. Resistors, contactors and motors must be selected for the actual duty cycle and heat load.

Shunted Armature Control

This circuit combines a rheostat across the armature with another rheostat in series with the supply. Changing series rheostat R₁ changes the voltage available to the parallel branches. Changing shunt rheostat R₂ redistributes current between the armature and series field path. The method can provide speeds below the normal operating point, but both resistors waste power and make speed dependent on load.

Armature Terminal Voltage Control

A variable-voltage converter can control a DC series motor without inserting a continuously dissipative series resistor. Modern DC-DC choppers or controlled rectifiers make armature terminal-voltage control practical, subject to current limiting, commutation, supply and motor ratings.

Field Controlled DC Series Motor

A DC series motor can use these traditional field control methods:

1. Field Diverter Method
2. Tapped Field Control

Field Diverter Method

A field diverter sends part of the motor current around the series field winding. Reducing diverter resistance lowers field current and flux, which raises speed for a given back emf. Field weakening also reduces torque per ampere and can cause overspeed if the load falls. An electric drives application must enforce the motor’s minimum field, maximum speed, armature current and commutation limits.

Tapped Field Control

Taps on the series field select how many turns carry armature current. Selecting fewer turns reduces flux and raises speed, while also reducing torque per ampere. Older traction systems used stepped field control, with contactors and protection arranged to prevent open-field operation and unsafe transitions.

Speed Control of DC Shunt Motor

The traditional speed control methods for a DC shunt motor use the same two broad control variables:

1. Armature Control Methods
2. Field Control Methods

Armature Controlled DC Shunt Motor

Armature control for a DC shunt motor can be implemented in two ways:

1. Armature Resistance Control
2. Armature Voltage Control
   

   
   

Armature Resistance Control

Armature resistance control adds a variable resistor in series with the armature while the shunt field remains across its supply. The field flux is therefore approximately constant, but the added I²R drop lowers armature voltage and speed. This inefficient method gives poor load regulation and is limited to short-duration, stepped or legacy duties below base speed.

Armature Voltage Control

Armature-voltage control uses a variable DC source for the armature while maintaining the required field current. With closed-loop current and speed control, it provides efficient operation from standstill to base speed and avoids the continuous loss of an armature series resistor.

The historical Ward Leonard System supplies the DC motor armature from an adjustable-voltage DC generator driven by a separate prime mover. This motor-generator (M-G) set provided smooth low-speed control, reversal and, with the correct connections, regenerative operation for duties such as rolling mills, elevators and mine hoists. Electronic converters now perform the same control in most new installations.

Advantages of Armature Controlled DC Shunt Motor

1. Smooth armature-voltage control over a wide speed range and in either direction
2. Controlled acceleration through gradual voltage and current adjustment
3. Good speed regulation when feedback control is included
4. Regenerative braking when the generator and supply arrangement support reverse power flow

Disadvantages of Armature Controlled DC Shunt Motor

1. Multiple rotating machines increase cost, floor space and installation work
2. Rotating losses reduce efficiency, especially at light load
3. Brushes, bearings and machines add noise and maintenance

Field Controlled DC Shunt Motor

Traditional field control varies shunt-field current with a field rheostat. It is mainly a field-weakening method for speeds above the base-speed point.

Field Rheostat Controlled DC Shunt Motor

Adding resistance in series with the shunt field reduces field current and flux, so speed rises until back emf again balances the armature circuit. Load still affects speed through armature voltage drop, armature reaction and the torque required. The field circuit carries less current than the armature, but its resistor still dissipates power. Related field control can be used in a suitable DC compound motor.

Disadvantages of Field Rheostat Controlled DC Shunt Motor

• Field weakening does not provide low-speed operation below base speed.
• Available torque falls as flux is weakened and speed rises.
• Minimum field is limited by commutation, armature current, mechanical speed and loss-of-field safety requirements.

Solid State Speed Control

Solid-state DC drives replace rotating M-G sets in most new systems. A DC source can feed a motor through PWM choppers, while an AC source can use a controlled rectifier or active converter. The converter topology determines whether the drive supports one, two or four-quadrant operation and regenerative braking. Suitability for intermittent or continuous load depends on the drive’s current, thermal and duty ratings.

DC Motor Speed Control Theory

The speed relation begins with the DC motor’s generated electromotive force, or back EMF. EMF means electromotive force, not electromagnetic force:

Rearranging the equation gives:

• N = 60A E / PZØ

Defining the machine constant k = PZ/60A gives:

• N = E / kØ

Using E = V – I_aR_a for the simplified steady-state armature circuit gives the DC motor speed N below. A practical calculation may also include brush voltage drop, converter drop, armature reaction and magnetic saturation.