Speed control of Dc motor
Speed control of dc motor is the most important feature we interest in, when we control the speed we vary it according to the requirements and the operation we need.
You can sense the effect of speed control of dc motor in the movement of robotic vehicles, movement in elevators, movement in paper mils and so on when different types of Dc motors are used.
The principle of speed control of dc motor:
You should remember the working principle of Dc motor that:
Eb=(PΦ N Z)/60A
P: number of poles.
A: just a constant.
Z: number of conductors.
N: speed of the motor.
And by substituting in the voltage equation:
V=(PΦ N Z)/60A + Ia.Ra
And the speed will be;
N= (P.Z /60A)(V-Ia.Ra) /Φ.
And we know that P, Z, and A are constants which we can’t change in working so the speed equation can be:
N= K.(V-Ia.Ra) /Φ.
We deduce that:
- The motor speed is directly proportional to the supply voltage.
- The speed is inversely proportional to the armature voltage drop.
- Also, the motor speed is inversely proportional to the flux.
This means that we can control the speed by:
- Varying the supply voltage.
- We can Vary the flux.
- Also, Varying the armature voltage.
And as there are many types of Dc motors we should have many types of speed control of Dc motor like:
Speed control of shunt motor:
We can control the speed of a shunt motor by:
- Flux control method.
- Armature and Rheostatic control method.
- Voltage control method (Multiple voltage control & Ward Leonard system).
Speed control of series motor:
We can also control the speed of a series motor in many ways such as:
1- Variable resistance in series with the motor (Armature control of Dc series motor).
2- Flux (Field) control method: (Field diverter – Armature diverter – Tapped field control – Paralleling field coils)
3- Series-parallel control method.
Let’s start with speed control of Dc series motor:
1- Armature control of Dc series motor:
We can use more than one technique in armature control like:
- Armature resistance control method:
It’s the most common methods where we connect a controlling resistance in series with the supply.
- Shunted armature control:
In this method, we use a combination of a rheostat shunting the armature and a rheostat in series with the armature, and here the voltage applied to the armature varies with varying the series rheostat, and also the exciting current varies with varying the armature shunting resistance.
Unfortunately, this method isn’t economical due to power losses in speed controlling resistance and we also the speed control is obtained blow normal speed.
- Armature terminal voltage control:
We rarely use this method because it involves high cost as we supply the power to the motor from a separate variable voltage supply.
2- Field control of Dc series motor:
We can also control the speed by:
- Field diverter method:
In this method, we use a diverter and we vary the field flux by varying the current and in turn vary the speed.
And this method is most common in electrical drives because we have a speed above the normal.
- Armature Diverter method:
If we have constant load torque … If we reduce Armature current the flux will increase.
As we know that the torque is directly proportional to flux and current.
So, if current increase the flux will also increase and the speed will decrease.
- Tapped field control:
Here we also increase the speed by reducing the flux, but with lowering the number of turns of field winding where the current flows.
- Paralleling field coils:
In this method, we can control the speed of the motor by adding groups of coils as shown in fig.
3- Series-parallel control method.
In this method, we use two or more mechanisms coupled series motors.
If we require lower speed we must join motors in series because in this case, the motors have the same current passing through them.
If we require higher speed we must join motors in parallel because in this case, the motors have the same voltage across them.
And this method usually used in electric traction.
Speed control of Dc shunt motor:
Field control of Dc shunt motor:
We can control the speed by:
- Field rheostat control method:
We control the speed by adding a variable resistance in series with the shunt field, when we increase the resistance the field current reduces and by the way, the flux reduces and the speed increases.
This method is independent of the load and the power wasted in controlling the resistance is very less, we also use this method of control in the Dc compound motor
- Field voltage control:
In this method, we need a variable voltage supply for the field circuit and we separate it from the main power supply and that can be obtained by an electronic rectifier.
Armature control of Dc shunt motor:
We have two ways:
- Armature resistance control:
We add a variable resistance to the armature circuit and the field connected directly to the supply so it doesn’t change with the variation of series resistance.
We usually use this method in the printing press, cranes, hoists and so on where we use a speed lower than the normal.
- Armature voltage control:
In this method, we use a variable source of voltage separated from the source supplying the field current so this method has a poor speed regulation and low efficiency of armature resistance control methods.
And to make the armature voltage control method more efficient we use an adjustable voltage generator called Ward Leonard system as we use a motor generator set, which makes it suitable for steel rolling mills, paper machines, elevators,…
to be fair that’s too much so I brief all (speed control of dc motor) in three points which are:
Flux control method:
In flux control method rather it’s of shunt or series type we vary the flux produced by the field winding in order to vary the speed of the motor.
And as we know the magnetic flux depends on the current flowing through the field winding so we can vary the flux by varying this current and that can happen by using variable resistance in series with the field winding resistor.
When we keep the resistance in its minimum position we have a rated current due to a rated voltage so the speed will be in its normal value.
And when we gradually increase the resistance the current decreases and in turn the flux produced decreases, so the speed of the motor increases its normal value.
Armature control method:
In armature control method we can control the speed by controlling the armature resistance by controlling the voltage drop across the armature, and we also use a variable resistor in series with the armature.
When we keep the resistance to its minimum value we have a normal armature voltage drop.
And when we gradually increase the resistance the voltage across the armature decreases.
and in turn the speed of the motor decreases and here we can reach a speed below the normal range.
Voltage control method:
If we focused in flux control method and armature control method we found that we can’t provide speed control in the desirable range, which makes us think in a different method which controls the speed by controlling the supply voltage.
In voltage control method we fix the field winding voltage and vary the armature voltage and that can happen by:
- Using a switchgear mechanism to provide the variable voltage to the armature.
- Or using an Ac motor driven generator to provide a variable voltage to the armature(Ward_Leonard system)
And the most widespread technique used in those two ways is the use of pulse width modulation which involves the application of varying width pulses to the motor driver to control the supply voltage, and it’s the most efficiently used method because the power losses are kept at a minimum value.
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