Know about the Important Ways for DC Motor Speed Control

In the period of the 18^th century itself, there was the evolution of DC motors. The development of DC motors has widely enhanced and they are significantly applied in multiple industries. In the early period of the 1800s and with the enhancements made in the year 1832, DC motors were initially developed by the British researcher Sturgeon. He invented the initial commutator type of DC motor where it has the capability to simulate machinery too. But one might wonder what the functionality of the DC motor is and why it is important to know about DC motor speed control. So, this article clearly explains its operation and various speed controlling techniques.

What is DC Motor?

A Dc motor is operated by using direct current where it transforms the received electrical energy into mechanical energy. This triggers a rotational change in the device itself thus delivering power to operate various applications in multiple domains.

DC motor speed control is one of the most useful features of the motor. By controlling the speed of the motor, you can vary the speed of the motor according to the requirements and can get the required operation.

The speed control mechanism is applicable in many cases like controlling the movement of robotic vehicles, movement of motors in paper mills, and the movement of motors in elevators where different types of DC motors are used.

DC Motor’s Working Principle

A simple DC motor works on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force. In a practical DC motor, the armature is the current-carrying the conductor and the field provides a magnetic field.

When the conductor (armature) is supplied with a current, it produces its own magnetic flux. The magnetic flux either adds up to the magnetic flux due to the field windings in one direction or cancels the magnetic flux due to field windings. The accumulation of magnetic flux in one direction compared to the other exerts a force on the conductor, and therefore, it starts rotating.

According to Faraday’s law of electromagnetic induction, the rotating action of the conductor produces an EMF. This EMF, according to Lenz’s law, tends to oppose the cause, i.e., the supplied voltage. Thus, a DC motor has a very special characteristic of adjusting its torque in case of varying load due to the back EMF.

Why DC Motor Speed Control is Important?

Speed control in the machine shows an impact on the speed of rotation of the motor where this direct influence on the machine functionality and is so important for the performance and outcome of the performance. At the time of drilling, every kind of material has its own rotational speed and it changes based on drill size too.

In the scenario of pump installations, there will be a change in the throughput rate and so a conveyor belt needs to be in sync with the functional speed of the device. These factors come are either directly or indirectly dependent on the speed of the motor. Because of this, one should consider DC motor speed and observe various types of speed control methods.

DC Motor speed control is done either done manually by the worker or by using any automatic controlling tool. This seems to be in contrast to speed limitation where there has to be speed regulation opposing the natural variation in the speed because of the variation in the shaft load.

The Principle of Speed Control

From the above figure, the voltage equation of a simple DC motor is

V = Eb + IaRa

V is the supplied voltage, Eb is the back EMF, Ia is the armature current, and Ra is the armature resistance.

We already know that

Eb = (PøNZ)/60A.

P – number of poles,

A – constant

Z – number of conductors

N- the speed of the motor

Substituting the value of Eb in the voltage equation, we get

V = ((PøNZ)/60A) + IaRa

Or, V – IaRa = (PøNZ)/60A

i.e., N = (PZ/60A) (V – IaRa)/ ø

The above equation can also be written as:

N = K (V – IaRa)/ ø, K is a constant

This implies three things:

1. The speed of the motor is directly proportional to supply voltage.
2. The speed of the motor is inversely proportional to the armature voltage drop.
3. Speed of the motor is inversely proportional to the flux due to the field findings

Thus, the speed of a DC motor can be controlled in three ways:

• By varying the supply voltage
• By varying the flux, and by varying the current through the field winding
• By varying the armature voltage, and by varying the armature resistance

Multiple Techniques of DC Motor Speed Control

As there are two types of DC motors, here we will clearly discuss the speed controlling methods of both DC series and shunt motors.

DC Motor Speed Control in Series Types

It can be categorized into two types and those are:

• Armature controlled technique
• Field controlled technique

The armature controlled technique is further classified into three types

• Armature controlled resistance
• Shunted armature control
• Armature terminal voltage

Armature Controlled Resistance

This technique is most widely employed where the regulating resistance has a series connection with that of the motor supply. The below picture explains this.

The power loss that happens in the DC series motor’s controlling resistance can be ignored because this regulating technique is mostly used for a long period in order to decrease the speed at the time of light loading scenarios. It is a cost-effective technique for persistent torque and mainly implemented in driving cranes, trains, and other vehicles.

Shunted Armature Control

Here, the rheostat will be in both series and shunting connection with the armature. There will be a change in the voltage level which is applied to the armature and this varies by changing the series rheostat. Whereas the change in excitation current takes place by changing the shunt rheostat. This technique of controlling speed in DC motor is not so costly because of significant power losses in speed regulation resistances. The speed can be regulated to some extent but not above the normal level of speed.

Armature Terminal Voltage

The speed of a DC series motor can also be done through power supply to the motor using an individual varied supply voltage, but this approach is costly and not extensively implemented.

The field-controlled technique is further classified into two types:

• Field Diverter
• Controlling of tapped field (Tapped field control)

Field Diverter Technique

This technique makes use of a diverter. The flux rate which is across the field can be decreased by shunting some part of the motor current across the series field. The lesser is the resistance of the diverter, the field current is less. This technique is utilized for more than the normal range of speeds and is implemented across electric drives where the speed increases when there is a decrease in load.

Controlling of Tapped Field

Here also, with the reduction of flux, the speed will be increased and it is accomplished by reducing the field winding turns from where the flow of current takes place. Here, the number of tapping’s in the field winding is taken out and this technique is used in electric tractions.

Speed Control of DC Shunt Motor

It can be categorized into two types and those are:

• Field controlled technique
• Armature controlled technique

Field Control Method for DC Shunt Motor

In this method, the magnetic flux due to the field windings is varied in order to vary the speed of the motor.

As the magnetic flux depends on the current flowing through the field winding, it can be varied by varying the current through the field winding. This can be achieved by using a variable resistor in a series with the field winding resistor.

Initially, when the variable resistor is kept at its minimum position, the rated current flows through the field winding due to a rated supply voltage, and as a result, the speed is kept normal. When the resistance is increased gradually, the current through the field winding decreases. This in turn decreases the flux produced. Thus, the speed of the motor increases beyond its normal value.

Armature Resistance Control Method for DC Shunt Motor

With this method, the speed of the DC motor can be controlled by controlling the armature resistance to control the voltage drop across the armature. This method also uses a variable resistor in series with the armature.

When the variable resistor reaches its minimum value, the armature resistance is at a normal one, and therefore, the armature voltage drops. When the resistance value is gradually increased, the voltage across the armature decreases. This in turn leads to a decrease in the speed of the motor.

This method achieves the speed of the motor below its normal range.

Armature Voltage Control Method for DC Shunt Motor (Ward Leonard Method)

The Ward Leonard technique of DC motor speed control circuit is shown as follows:

In the above picture, M is the main motor where its speed is to be regulated and G corresponds to an individually excited DC generator where this is driven by using a three-phase motor and it may be of either synchronous or induction motor. This pattern of DC generator and AC driven motor combination is termed as M-G set.

The generator voltage is varied by altering the field current of the generator. This voltage level when provided to the armature section of the DC motor and then M is varied. In order to keep the flux of the motor field constant, the motor field current has to be maintained as constant. When the motor speed is regulated, then the armature current for the motor is to be the same as that of the rated level.

The delivered field current will be different so that the armature level of voltage varies from ‘0’ to the rated level. As the speed regulation corresponds to the rated current and with the persistent field flux of the motor and the field flux till when the rated speed is achieved. And as the power is the product of speed and torque and it has a direct proportion to the speed. With this, when there is an increment in power, the speed increases.

Both the above-mentioned methods cannot provide speed control in the desirable range. Moreover, the flux control method can affect commutation, whereas the armature control method involves huge power loss due to its usage of a resistor in series with the armature. Therefore, a different method is often desirable – the one that controls the supply voltage to control the motor speed.

Consequently, with the Ward Leonard technique, the adjustable power drive and the constant value of torque are acquired from the speed level minimal to the level of the base speed. The field flux regulation technique is mainly employed when the speed level is more than that of the base speed.

Here, in the functionality, the armature current is kept at a constant level at the specified value and the voltage value of the generator is maintained at constant. In such a method, the field winding receives a fixed voltage, and the armature gets a variable voltage.

One such technique of voltage control method involves the use of a switchgear mechanism to provide a variable voltage to the armature, and the other one uses an AC motor-driven Generator to provide variable voltage to the armature (the Ward-Leonard System).

The advantages & disadvantages of the ward Leonard method are:

The benefits of using the Ward Leonard technique for DC motor speed control are as follows:

• In both directions, one can control the speed of the device in a smooth manner for an extended range
• This technique has intrinsic braking ability
• The trailing reactive volt-amperes are counterbalanced through a drive and the extensively excited synchronous motor acts as the drive so there will be an increment in the power factor
• When there is a flashing load, the drive motor is the induction motor having a flywheel which is used to lessen the flashing load to a minimal level

The disadvantages of Ward Leonard technique are:

• As because this technique has a set of motor and generator, the cost is more
• The device is complicated to design and has heavyweight too
• Need more space for installation
• Requires regular maintenance and foundation is not cost-effective
• There will be huge losses and so the efficiency of the system is reduced
• More noise is generated

And the application of the Ward Leonard method is smooth controlling of speed in the DC motor. A few of the examples are mine hoists, paper mills, lifts, rolling mills, and cranes.

Apart from these two techniques, the most widely used technique is the speed control of dc motor using PWM to achieve speed control of a DC motor. PWM involves the application of varying width pulses to the motor driver to control the voltage applied to the motor. This method proves to be very efficient as the power loss is kept at a minimum, and it doesn’t involve the use of any complex equipment.

The above block diagram represents a simple electric motor speed controller. As depicted in the above block diagram, a microcontroller is used to feed PWM signals to the motor driver. The motor driver is an L293D IC which consists of H-bridge circuits to drive the motor.

PWM is achieved by varying the pulses applied to the enable pin of the motor driver IC to control the applied voltage of the motor. The variation of pulses is done by the microcontroller, with the input signal from the pushbuttons. Here, two pushbuttons are provided, each for decreasing and increasing the duty cycle of pulses.

So, this article has given a detailed explanation of various techniques of DC motor speed control and how speed control is most important to be observed. It is furthermore recommended to know about the 12v dc motor speed controller.