DC Drives

DC machine drives were the original choice for variable speed drives and precision drives. They are currently becoming less common as they are replaced by AC drives, either vector controlled induction machines or brushless dc machines, depending on the application.

DC Machine Recap

Traditional DC machines have at least two windings, a field winding and an armature winding. In recent years, permanent magnet (PM) DC machines have become more common. In these machines, the field winding is replaced by an arrangement of permanent magnets. PM DC machines effectively operate in the same manner as a separately excited DC machines.

Shunt Excited DC Machine Circuit Diagram

In the figure above, a field winding circuit is shown connected to the same terminal voltage as is applied to the armature circuit. As the field circuit is DC,

and the flux produced by the field winding is given by

In a PMDC machine, the field flux is created by the permanent magnets and may be considered to be constant.

Field Winding Connection

If a field winding exists, it may be connected either

  1. independently from the armature supply (separately excited)
  2. across the terminals of the armature supply (shunt excited, as shown above)
  3. in series with the armature circuit (series excited)

In this review, we will consider the case where the field winding is either shunt or separately excited. These connections give the widest range of control over the machine operation.

Armature Winding Equations

The armature voltage and current are defined by

and he speed and torque equations for a DC machine are given by:

Substituting,

DC Motor Drives

The torque - speed equation shown above highlights the usefulness of DC motor drives. If the flux linkage is held constant, the motor can be controlled simply by varying the terminal voltage and the behaviour of the motor is very simply defined. Alternately, if the terminal voltage cannot be changed, the field winding current may be adjusted. Changing the field winding current will affect the flux linking the armature winding and as a result, control the speed of the motor. These two possibilities for controlling the motor are described as armature voltage control and field control respectively. It should be noted that field control is not possible with PM DC machines. The choice of when to use armature voltage or field control depends, so some extent, on the application.

Armature Voltage Control

When applying techniques to control any type of machine it is important not to exceed rated currents or voltages for any length of time. Exceeding rated currents results in overheating from I2R losses; exceeding rated voltages accelerates ageing of conductor insulation. For this reason, armature voltage control can only be applied at speeds below the rated, or base speed. When bought a all machines are supplied with defined rated speed, torque, terminal voltage and current values. Since speed and torque are functions of flux linkage, voltage and current, operation at rated currents and voltages will result in rated speed and torque. Considering the torque equation, it makes sense to operate at as high a flux linkage value as possible. If flux linkage is reduced, armature current must increase for a given load torque. As a result, the motor I2R losses increase and the efficiency is reduced. If the flux linkage is at its rated value, then from the torque-speed equation, armature voltage control must be applied to control the speed. However, since the terminal voltage cannot exceed rated values, armature voltage control can only be applied at speeds below base speed.

Writing the torque speed equation in terms of base values:

It can be seen that reducing the terminal voltage will allow operation up to rated torque at all speeds below the base speed. Operation in this manner is often called the constant torque region.

Field Control

Field control may be applied for two reasons

  1. Cost.

    The armature winding takes the full power rating of the machine. Therefore any control circuitry and equipment applied to the armature must be rated at the full machine power rating. The field winding power is limited to the windings I2R losses and is therefore quite small. Consequently, circuit components to control the field winding current and flux are much cheaper than those to control the armature circuit.

  2. Higher speed ranges.

    Considering the torque speed equation, the only way to operate above base speed without exceeding current and voltage ratings is to reduce the flux linkage. Unfortunately, if the armature current is to remain within rated values, this means that the motor can no longer produce rated torque. The maximum available torque in the field weakening (φ < φb) region is given by

Operating Regions

The region below base speed where armature voltage control may be applied is called the constant torque region. Above base speed, the flux is reduced and the motor enters the "field weakening region".This region is also sometimes known as the constant power region.

DC Drive Implementation

One of the main reasons for the popularity of DC motor drives is the ease of implementing the control. As outlined above, operation up to rated speed can be obtained by adjusting the terminal voltage. In a traditional large DC motor drive, this is accomplished by means of a phase controlled thyristor rectifier. A three-phase phase controlled thyristor rectifier is capable of converting a 3-phase supply line to a controllable DC voltage with relatively simple control and relatively cheap power electronics. Thyristors are also an old power electronic technology; DC drives could be implemented at a time when the modern control techniques for AC drives were unavailable.

Connecting two thyristor rectifiers with opposing polarity, it is possible to produce a DC voltage source capable of bi-directional current and positive and negative voltages. This is a relatively low cost method to produce a high power, bi-directional DC power supply. With this arrangement, it is possible to operate a DC machine in four quadrants. i.e. forwards and backwards, with both positive and negative torques. Large traditional DC machines are still in use in applications requiring accurate four quadrant control. Example applications are traction (i.e. trains) and mining conveyors. Many of these applications are slowly being phased out in favour of AC drives.

DC Drive Disadvantages

DC machines and DC drives have two significant drawback. The armature winding, the main current carrying winding of the machine, is on the rotor; there must be a brush connection to connect the rotating parts to the stationary terminals of the machine. As a result, DC machines require more maintenance than brushless machines and are prone to sparking, eliminating them from use in hazardous areas. The final drawback is cost. Large DC machines are more expensive than similarly sized AC machines.