DC Motor Control

There are a number of variations of the DC motor and each has different characteristics and requires different means of control.
The DC motor has two major magnetic components, the field and the armature. The DC motor also has a means of converting DC to AC to create the rotating magnetic field. This "comutator" may comprise a set of connections arranged around the shaft of the motor and connected to coils on the armature with brushes to connect the voltage to the armature, or it may comprise an electronic inverter as used in brushless DC motors.

A traditional D.C Motor comprises a wound stator or field, and a wound rotor known as the armature. Voltage is applied to the armature by means of a commutator and a set of brushes. The armature consists of a number of discrete windings which are terminated on the commutator which is in effect a rotary switch.The DC motor can be configured as a series connection, shunt connection or a compound connection.
Brushless DC motors do not have a wound armature, rather they use a permanent magnet rotor and electronics to create a rotating magnetic field in the stator.

Shunt Wound DC Motors

Shunt Wound DC Motors have separate field and armature supplies. Typically, the field voltage is supplied from a fixed supply and the armature is supplied from a controlled supply.
As the armature rotates withing the field, an back emf is generated within the armature circuit which opposes the applied armature voltage. This reduces the armature current and limits the maximum speed. A reduction in the field voltage reduces the back emf and increases the maximum speed of the motor. This is commonly known as field weakening.
The torque produced is directly proportional to the field current multiplied by the armature current. This is a linear relationship.
Shunt Wound Motors

Series Wound DC Motors

Series Wound DC Motors have the field and armature connected in series so the field current and armature current are equal. Increasing armature speed reduces the field current, reducing the induced back emf. This results in there being effectively no upper speed limit. Extreme care must be taken running a series wound motor with no load as it can accelerate to destruction.
The speed is approximately inversely proportional to the current.
Series Wound Motors

Compound Wound DC Motors

Compound Wound DC Motors include both series and shunt field windings.
The torque/speed characteristics are determined by the resistances of the series and shunt fields.
Compound Wound Motors

DC Motor Controllers

Most larger Industrial DC motors are shunt wound motors requiring a fixed field supply and an adjustable armature supply.
The field voltage is fixed to a value to suit the motor being driven. In modern drives, this will commonly be a thyristor based regulator that can be programmed to the motor characteristics and under some installations, can provide field weakening for motor operation at higher speeds.
The armature is supplied by a fully controlled Thyristor bridge which is phase controlled to control the torque produced by the motor.
The main control system adjusts the firing angles of the armature bridge supply in a manner to regulate the speed. There is a speed feedback that is used as a reference. The speed feedback can be eirther an external speed measuring device such as a tachogenerator or encoder, or it can be the back emf of the armature during the Thyristor OFF time.
A second armature bridge is often used to revers the polarity of the Armature supply, reversing the direction of the motor. This can provide a 4 quadrant drive operation.
Two Quadrant operation. A single quadrant controller is able to drive the motor in one direction only. A Two quadrant controller is able to drive the motor forward and in reverse. The two quadrant controller is not able to provide any braking torque.
Four Quadrant operation. A Four quadrant controller is able to drive the motor in forward and in revers, and it is also able to provide braking for the motor in forward and in reverse. Braking can be achieved by dumping the surplus energy into a resistor bank, or by regenerating the surplus energy into the supply. (Regenerative four quadrant drive)

Brushless DC Motors

Brushless DC Motors have an electronic switching "inverter" to create the AC voltage required to establish the rotating magnetic field. Commonly, the "field" is provided by permanent magnets on the rotor and the "armature" is provided by a stator winding. Hall effect sensors are used to determine the position of the rotor and this synchronises the AC voltage applied to the stator. The brushless DC machine does not have brushes and so requires far less maintenance than a traditional DC motor. The speed of the brushless DC motor is determined by the electronics and so better speed accuracy can be achieved than in standard DC motors. The maximum size of the brushles DC motor is limited by the size an strength of the permanent magnets that can be used.


Brushless DC Motors
Shunt wound DC motors
Series Wound DC Motors
Compound DC Motors
Variable Frequency Drives
Space Vector Modulation
Code of practice for connecting VFDs to Pumps