Electric Drives - DC Motors

(Description and Applications)

 

Brushed (Field Energised) Motors (Motors using wound rotors)

The traditional DC motor needs two current supplies, one through the stator windings to provide the magnetic field and the other through the rotor windings to interact with the magnetic field to generate the motive force. There are three ways of accomplishing this, each one resulting in unique characteristic motor performance. Because they all use wound rotors, they all need a commutator to feed the current into the rotor windings.

Speed is controlled by varying the rotor voltage and hence the rotor current, or by varying the magnetic flux in the air gap by changing the current in the field windings.

With access to both the field and rotor windings, all DC motors offer the facility of simple speed and torque control.

Series Wound

The series wound motor has only one voltage supply to the motor and the field winding is connected in series with the rotor winding.

o Characteristics

The series motor has poor speed regulation. It delivers increasing torque with increased motor current but this is at the expense of speed which falls with increasing torque demands.

This motor has a very high starting torque because there is zero back EMF at zero speed however as the speed builds up so does the back EMF causing a reduction in torque.Increasing the load on the motor tends to slow it down, but this in turn lowers back EMF and increases the torque to accommodate the load.

Speed control is possible by varying the supply voltage. Under no load conditions the speed will accelerate to dangerous levels possibly causing destruction of the motor. The motor can be reversed by reversing the connections on either the field or the rotor windings but not both.

Regenerative braking is not possible since the field current needs to be maintained but it collapses when the rotor current passes through zero and reverses.

o Applications

The series DC motor is an industry workhorse for both high and low power, fixed and variable speed electric drives.Applications range from cheap toys to automotive applications.

They are inexpensive to manufacture and are used in variable speed household appliances such as sewing machines and power tools.

Its high starting torque makes it particularly suitable for a wide range of traction applications.

Shunt Wound

The shunt wound motor also has only one voltage supply to the motor but in this case the field winding is connected in parallel with the rotor winding.

o Field Weakening

The speed of a shunt wound motor can be controlled to a limited extent without affecting the supply voltage, by "field weakening". A rheostat in series with the field winding can be used to reduce the field current. This in turn reduces the flux in the air gap and since the speed is inversely proportional to the flux, the motor will speed up. However the torque is directly proportional to the flux in the air gap so that the speed increase will be accompanied by a reduction in torque.

o Characteristics

The shunt wound motor turns at almost constant speed if the voltage is fixed. The motor can deliver increasing torque, without an appreciable reduction in speed, by increasing the motor current.As with the series wound motor, the shunt wound motor can be reversed by reversing the connections on either the field or the rotor windings.

Regenerative braking is possible. Self excitation maintains the field when the rotor current reverses.

o Applications

Fixed speed applications such as automotive windscreen wipers and fans.

Separately Excited

The separately excited motor has independent voltage supplies to the field and rotor windings allowing more control over the motor performance.

o Characteristics

The voltage on either the field or the rotor windings can be used to control the speed and torque of a separately excited motor.

o Applications

Train and automotive traction applications.

 

Permanent Magnet Motors

As the name implies, these motors use permanent magnets rather than electromagnets to provide either the rotor or the stator field. They are used extensively in small DC motors and to an increasing extent in traction applications.

Rotor Magnets

These are by far the most common types of permanent magnet motors. They have no rotor windings but use permanent magnets to supply the rotor field and they behave like shunt wound DC motors with a fixed shunt current.

Their major advantage is the elimination of the commutator.

Field Magnets

These motors have no field winding but use permanent magnets to provide the magnetic field. Current is still supplied to the rotor via a commutator as in other brushed motors and the speed can be controlled by varying the voltage on the rotor windings. In this way their behaviour is similar to a series wound DC motor.

 

Permanent magnet motors are explored in more depth in the section on Brushless DC Motors

Brushless DC (BLDC) Motors

The advent of inexpensive high power switching semiconductors has enabled radical new solutions to the commutation problem and much simpler mechanical designs. Permanent magnet and switched reluctance motors depend on electronic drive systems which produce rotating magnetic fields to pull the rotors around.

Synchronous Operation

Brushless DC motors are not strictly DC motors. They use a pulsed DC fed to the stator field windings to create a rotating magnetic field and they operate at synchronous speed. Although they don't use mechanical commutators they do however need electronic commutation to provide the rotating field which adds somewhat to their complexity.

 

Rotating FieldIn the diagram below, pole pair A is first fed with a DC pulse which magnetises pole A1 as a south pole and A2 as a north pole holding the magnet in its initial position. The other poles are not energised. Then the current to pole pair A is switched off and pole pair B is fed with a DC pulse causing pole B1 to be magnetised as a south pole and B2 to be a north pole. The magnet will then rotate clockwise to align itself with pole pair B. By pulsing the stator pole pairs in sequence the magnet will continue to rotate clockwise to keep itself aligned with the energised pole pair. In practice the poles are fed with a polyphase stepped waveform to create the smooth rotating field.

 

 

 

The speed of rotation is controlled by the pulse frequency and the torque by the pulse current.

Mechanical Construction

No current is supplied to, nor induced, in the rotors which are constructed from permanent magnets or iron and which are dragged around by the rotating field. With no currents in the rotors these machines have no rotor I2R losses.

Without the mechanical commutator and rotor windings, the motors have low rotor inertia allowing much higher speeds to be achieved and with the elimination of this high current mechanical switch, the source of sparking and RFI is also eliminated.

The stator windings are, easy to manufacture and install, bobbin windings.

Since all the heat generating circuits are in the stator, heat dissipation is easier to control and higher currents and motor powers can also be achieved.

 

 Universal Motors

In a series wound DC motor, reversing either the field winding leads or the rotor winding leads will reverse the direction of the motor. However, simply reversing the leads from the power supply will have no effect on the direction of rotation since it is equivalent to reversing the current through both the individual windings - in effect a double reversal. In other words the motor will turn in the same direction even though the current through the series windings is reversed. This means that the motor can run on alternating current as well as direct current since the direction of rotation is independent of the direction of the current through the series windings.

Hysteresis Motor

The Hysteresis Synchronous motor consists of a wound stator producing a rotating field and a rotor in the form of a cylindrical shell with crossbars all made from hard steel with relatively high magnetic hysteresis.

 

 

At start up, the combined effects of eddy currents in the steel causing induction motor action and remanent magnetism in the steel causing the magnetic poles to follow the rotating field, together cause the motor speed to build up. As the motor approaches synchronous speed the magnetic effect of the crossbars behaving like a permanent magnet causes the motor to lock on to synchronous speed. The net result is that the torque is roughly constant at all speeds.

 

Characteristics

Simple design

Starts as an induction motor and locks in as a synchronous motor.

Having a smooth rotor of homogenous material, the noise and vibration produced is inherently low. Since there are no pole faces or saliencies, the magnetic path is of constant permeability, thus eliminating the magnetic pulsations which are the major cause of noise in the salient pole type.

Applications

Their efficiency is low, and applications are restricted to small power ratings.

Used extensively in tape recorders and clocks.

Now mostly replaced by permanent magnet motors.