Speed Control of the DC motor: Essential Tips

For an electrical and electronics engineering student here are some tips for Speed control of the DC motor. if you are looking for this, then you are at the right place.

By the end of this article, you will be able to know everything about the Speed control of the DC machine. So be there and have patience.

Before writing this article I have been through many textbooks and the internet and then extracted the best of the knowledge below. As I am an electrical and electronics engineering graduate I think I have explained it in the best way as below.

Why do we need the speed control of the DC motor?

Speed control is needed for a user if he/she is using a DC motor or machine, as every application does not need the full speed of the motor. So users should know all the basic things about it.

Since DC machines or motors are of various types based on their connection between the field winding and the armature winding. So each motor has different kinds of speed control methods. Below are some speed control methods for the DC Shunt/ Compound motor and DC series motors.

Analogy behind the speed control of DC machine:

From the previous knowledge base, I think you already know the below-mentioned formula.

As~ we~know,~~N~\propto ~ \frac{V- I_a~R_a}{\phi}

Where,

  • N = Speed of the motor.
  • V = voltage applied
  • Ia = armature current
  • Ra = Armature resistance
  • Φ = flux in weber

By this formula, you can see that the Speed of the motor is directly proportional to the voltage applied, armature current, and armature resistance. Whereas it is inversely proportional to the Field flux. Conclusively, a motor’s speed can be changed by changing the values of these parameters as mentioned below.

The Armature current value depends on the load current being drawn. That means if we increase the load, motor speed reduces as it is directly proportional to the motor speed.

Speed control of DC Shunt/ Compound motor:

Generally a DC shunt or Compound motor has 3 types of speed control methods as mentioned below. We will see them one by one.

  • Field control OR Field Weakening method.
  • Rheostatic OR Armature Resistance Control method.
  • Voltage control method.

1. Flux control method or field weakening method

The external resistance is connected in series with the Field winding, it will reduce the field current and hence produces a reduced flux also. Which in the end reduces torque and increases the speed. This is known as the flux control method or the field weakening method. All the connections are mentioned below in the image for this method.

This method is applied only to vary the speed above the Base value or Rated speed.

it is an efficient speed control method (Because I2sh (Rext+Rsh) losses are less).

Requires the 4-point starter, especially to vary speed above the base speed over a wide range.

There is an additional armature reaction, which affects more as compared to the normal case. Because we reduced the main flux with the help of ‘Rext‘.

For optimal starting, field Rheostat should be at the minimum position. “Because (τ ∝ Ia), at the starting Ia will be very large, hence τ will be very large. But we won’t prefer increased torque with increased Armature current. So we should adjust the flux to the maximum level to get the maximum torque at the start of the machine”.

The flux control method is also called as “constant power variable torque” speed control method.

Flux should not be reduced in large steps but gradually reduce in small steps to prevent a “High Inrush current” drawn by the motor.

Rheostatic OR Armature Resistance Control Method:

In this method, we connect an external resistance or Rheostat in series with the armature. Hence voltage across the armature is reduced. Conclusively it will reduce the speed.

This method is used to vary the speed from rated to below speed only.

It is a less efficient speed control method as the loss associated with this rheostat is very high due to armature current.

As Eb varies, it is considered as variable power but constant torque as flux is unchanged (for rated current).

During starting of the machine, the Armature rheostat should be at a max position. (At the start Eb=0, since I=V/R. In order to control or limit the maximum starting current ‘Rext‘ should be maximum (similar to starter).

If we increase the ‘Rext‘ to some maximum value then the voltage across the armature will reduce to zero. That is Eb=0 (known as stalling condition).

For stalling (zero speed) condition Eb=0. Since V=Ia(Ra+Rext).

Note: problems related to the DC shunt or field flux

In shunt motor, if we vary the terminal voltage ‘ϕsh‘ will get varied.

So we voltage control method is introduced as mentioned below.

Voltage Control Method:

It requires multiple voltage sources to vary the supply voltage and hence speed. However, there is a practical difficulty in directly varying supply voltage in speed in the shunt motor. So it needs to be separately excited for this purpose.

separately excited motor

In large machines to vary speed below base without rheostats, efficiently in both directions, voltage control method called ‘Ward Leonard system‘ is used. It requires a separately excited DC Generator, the control is through a crossover switch with variable resistance. By varying resistance and crossover switch the generator produces variable voltages in both polarities to achieve speed control.

There is a flywheel across the shaft, to reduce the speed fluctuation with varying loads then it is called a “Ward Leonard system”.

Speed control of DC series motor:

Field Diverter: (Flux control)

In this method, the motor has a variable diverter resistance (Rd) in parallel with the field winding of the motor. That is why it is known as a field diverter.

Depending on this Rd or diverter resistance, the current through the field winding (hence flux) can be controlled.

  • If diverter resistance is maximum, then Ise (series current) is maximum hence ϕse (series flux) is maximum. Therefore speed will be minimum.
  • If diverter resistance is minimum, then Ise (series current) is minimum hence ϕse (series flux) is minimum. Therefore speed will be maximum.

Note: There are limitations to the value of diverter resistance. It cannot be varied so much.

This method is preferred to control the above base speed.

Armature Diverter: (flux control)

In this method, we connect the diverter resistance in parallel with the armature winding. That is why it is called an armature diverter.

When a diverter is connected across the armature winding, Ia diverts into it which reduces the torque.

If the motor runs at constant torque condition, then Ia increases which increase the flux, and hence the speed of the motor reduces.

This is preferred below the base speed.

Tapped field coils: (flux control)

In this method filed winding contains tappings and tap changers. So it is known as tapped field coils method of speed control.

By adjusting the tappings, speed can be varied easily.

This is practically used in some traction applications.

Parallel Field Coils: (flux control)

series field coils

In this method, the field winding contains different sets, which can be switched externally to series and parallel depending on the speed requirement.

  • For series connection: ϕse ∝ IaNse/4.
  • For parallel connection: ϕse ∝ IaNse/2.
parallel field coils

Armature Resistance:

In this method, we add a variable resistor or rheostat in series with the field winding.

By adding external resistance in series with the armature, voltage across the armature will be reduced to reduce the speed of the DC series motor. However, efficiency is greatly affected. So this method is rarely used.

Voltage Control:

It requires a variable DC voltage source, which makes this method expensive.

By directly varying supply voltage, the speed is varied.

Motors in Series and Parallel:

series connection of motors

In this method, motors are connected in either series or parallel according to the need of the Ia or armature current.

If you wanted to reduce the Ia or armature current then use the parallel connection of the motors. Whereas if you want to increase the armature current then use the series connection of the motors.

parallel connection of motors

Relation between the speeds of series-connected and parallel-connected motors-

  • As we know, N ∝ Eb/ϕ.
  • For series connection: Nseries ∝ V/2/Ia ∝ V/2Ia.
  • For parallel connection: Nparallel ∝ V/Ia/2 ∝ 2V/Ia.

Hence we conclude the relationship between the speeds of the series method and the parallel method as Nparallel = 4×Nseries. This means the speed of the parallel connected motors are 4 times the speed of the series connected motors.

Relation between the Torque of series-connected and parallel-connected motors-

  • As we know, 𝜏 ∝ ϕ×Ia.
  • For series connection: 𝜏series ∝ Ia×Ia ∝ Ia2.
  • Similarly for parallel connection: 𝜏parallel ∝ Ia/2×Ia/2 ∝ Ia2/4.

Hence we conclude the relation between the Torque of the series method and parallel methods as 𝜏parallel = 𝜏series/4. This means the Torque of the series connected motors are 4 times the Torque of the parallel connected motors. Here Ia is calculated from the current division formula for the parallel circuit.

Application: This method is used in traction.

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