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Speed Control of DC Motors

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8.1 Factors controlling the speed

Since, DC motors are readily available for adjustment of their operating speed hence the DC machines are considered as adaptable to the varying speed.  The speed of DC motor varies over the wide range. There are several methods of varying the speed of the DC motors and that is why DC motor has wide range of applications in the modern industries.

 The speed of DC motors can be given as:

V-IaRa

         N α Φ

where, Φ is the electric field, V is voltage, R is Rheostat measure and N is speed of DC motor.

Thus, it can be said that speed of Dc motor can be controlled by Φ, V or R.

8.2 Field control method

This is the method in which there is an advantage of shunt motors.  It is the most common method of controlling the speed of DC motors as well as the compound motors. When the field is excited separately, the field current, flux and speed are adjusted with the adjustment of shunt field circuit resistance. This is done in the most simple and an inexpensive way without causing changes in the motor loss.

 Speed is inversely proportional to the field current,

N α 1/ If α 1/ Φ

When there is a maximum field current, there is a lowest speed. The highest speed of the DC motor is limited electrically due to the armature reaction effects under the weak field. This causes the motor to become unstable.

 A constant output is delivered when the voltage remains constant throughout the motor.

Advantages of field control method

Some of the advantages of using this method for speed control are:

  • Even the minute speed can be controlled
  • Higher efficiency to work
  • Controlling equipments are compact
  • Inexpensive and simple method
  • Can be done automatically as well as manually
  • Access to the smooth and stepless method of controlling speed
  • Speed control is not affected due to field control within the limit. No matter whether it is shunt motor, compound motor or series motor.

Disadvantages of field control method

Some of the disadvantages of using this method for speed control are:

  • speed cannot be control beyond the basic speed
  • Armature reaction causes instability at higher speed
  • Commutation gets difficult and  commutator can be damaged at the higher speeds

Shunt motors

Change in the shunt field current with the help of shunt field rheostat causes changes in the flux of DC shunt motors. The wastage of power while controlling the resistance is quite less because of the small field current.

Speed of the motor can be increased in the ratio of 2:1 with the help of this method in non-interpolar machines.

Series motors

In this type of motor, the afflux can be changes with the help of following methods:

  • Armature Divertor: The divertors across the armature is used to get the speeds lower than the normal speed
  • Field Divertor: This is the invariable resistance and shunts the series windings by adjusting the resistance of this divertor any amount of current can be passed through it. When the flux is decreased, the speed of the motor is increased.
  • Parallel field coils: In this method, by regrouping of the field coils, several speeds can be obtained.
  • Tapped field control: In this method, the numbers of tapping are brought outside from the field. Hence the number of series of filed can be short circuited. The motor works at higher speed when all field turn in the circuit and works at low speed when some series field turns are cut down.

8.3 Rheostatic control

It is the method of speed control of DC motors in which the series resistance is inserted in the armature circuit. This method can be used with the shunt, series and compound motors.  In the shunt and compound motors, resistor is  connected between the armature and shunt. This method of speed control is common in series motors and is used when there is a need of the low load speed.

Advantages of Rheostatic control method

The advantages of this method include:

  • Easy connection and simple to use
  • Speed lower than the basic speed can be achieved
  • Functionality of motor starting can be controlled with the speed control

Disadvantages of Rheostatic control method

The advantages of this method include:

  • Less efficient
  • Speed regulation is not good
  • High cost of operation due to the presence of variable resistors
  • Problem in getting the stepless control of speed in high power

Shunt motors

In this method, the voltage across the armature changes due to the insertion of the variable resistance or rheostat. This type of resistance is also called as controller resistance. When the controller resistance is increased, the potential difference across the armature is decreased which ultimately lowers the speed of the armature. Speed is proportional to the potential difference across armature for getting the constant torque. Thus, it can be said the higher is the resistance, the higher is the reduction in speed.

When there is a certain value of load current and the speed is zero in the motor, this is the condition of maximum current and it is also called as the stalling current.

 This method is quite wasteful and expensive one, also not appropriate when the load is variable. By using the divert or across the armature, the operation of the motor can be made stable. The change in the armature current because of the change in the potential difference is negligible hence there is no effect on the change of speed of the armature.

Voltage applied across the armature can be reduced with the increased in resistance in series. The speed will also be reduced with the decrease in voltage across the armature. A very low power is wasted due to the passage of full motor current through resistance. Variable voltage supply is helpful in controlling the motion of the DC motors.

Series parallel control

In electrical tractation, this method is widely used. In this, coupled motors are used that are mechanically similar. The motors which are at the low speed are joined in series and when the motors attain the specific speed the additional resistance is cut down with the help of the controller.  There is no wastage of power because no additional resistance in the circuits hence the motor is highly effective.

8.4 Voltage method

In this method, the speed is controlled by regulating the motor’s terminal voltage when the constant field current is maintained. The change in speed is proportional to the change in voltage. Though, the output changes directly with the change in speed when the torque is constant. The application of voltage control is limited as the voltage is needed to be regulated without causing changes in the field.

 The variable DC Voltage source can be the DC Vacuum tube, there at on tube or gas tube, three phase operating, semi conductor amplifier and single phase AC supply. By using this method, the motors of upto 75kW can be controlled by using magnetic amplifiers as a source of an adjustable DC voltage. Large DC motors can be controlled with rotary amplifiers. Ward Leonard control system is the example.

Advantages

  • It helps in eliminating the need of series armature starting resistance
  • Uniform acceleration
  • Better regulation of the speed

Disadvantages

  • Arrangement cost of expensive
  • Efficiency of the overall system is low

Application of this method

 It is used in steel mills for reversing the rolling mills, high and medium speed elevators, paper machines, seamless tube mills and shovels.

Ward Leonard control system

This method   of speed control provides an array of speed of operating speeds and reduces the wastage of energy.

In the system of Ward Leonard control, there is a main motor whose speed is controlled, a separately excited generator that feed the armature of the motor, an exciter which offer excitation of the field and the driving motor with a constant speed. The motor is the shunt motor when there is a DC supply and the driving motor is used when AC supply is used. Excitor is not important during the DC supply.

Working of the Ward Leonard control system

Generator and excitor are driven by the motor and the voltage is fed with controlled varying settings. Change in voltage is applied for the change in motor’s speed.  This speed can be adjusted from zero to maximum in any of the direction with the help of rheostat and switch. When the sliding contact of the rheostat is at the extreme hand side the motor runs at the full speed and when the slider is on the left hand side, the speed of the motor becomes slowest or zero. The sliding contact of the rheostat is shifted to the extreme left to reverse the motor’s speed.

 There is a modification in this type of control system by the use of small motor generator which reduces the fluctuations in the power’s demand from the supply circuit.

Advantage of Ward Leonard control system

The main advantages include;

  • Instant reversal without causing high armature currents
  • Variable speed from zero to highest, can be obtained
  • Stepless control is there
  • Easily adaptable to the intermediate electronic, magnetic amplifier and semi conductor for the amplification of the large motors.
  • Ideal for the speed regulation

Disadvantages of Ward Leonard control system

The main disadvantages include;

  • High cost of installation
  • Effectiveness and efficiency of the system is less as compared to the filed control method.

 

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