Types of Dc motor (self excited, separately excited DC motor)
Types of Dc motor
Types of DC motor … We said that Dc motors are the latest machines used to convert electrical energy into mechanical energy so it is axiomatic that the power hundreds of devices we use everyday robotics, electric shaver, automobiles, small and also medium motoring applications, and of course this great number of devices can’t be powered with only one type of motors to be more identical they can’t powered by only one type of Dc motors so there is the Multiplicity of types of Dc motor as we have:
1- separately excited Dc motor represented in:
2- self-excited Dc motor represented in:
Of course we talk about these types before (working principle of dc motor), but in Brief, today we will try to understand which is suitable for any application and the advantage of each type; let’s start:
Separately excited Dc motor:
In the separate motor, we supply the field winding from a separate Dc power source.
the torque of this motor depends upon the field flux and we discover that from the torque equation of Dc motor
Tg =Ka Q Ia.
and the torque is independent o the armature current.
The most popular of the separately excited motor is:
Permanent magnet Dc motor:
In permanent magnet Dc motor we use a permanent magnet to create the field flux so it’s called permanent, we can get a great starting torque and a good speed regulation from this motor but this torque is limited so it’s better with low horsepower applications.
the torque of this motor and according to the equation T= Ka Q Ia will change with only changing by controlling the armature supply because the flux is constant as we chose the permanent magnet of required flux density with construction and it can’t be changed.
I will ask you to think in this example to cheek you understand me good or need more information
Self-excited Dc motor:
In the self-excited motor, the field winding is energized by the current produced by the motor itself with the help of residual magnetism, and we connect the field and armature windings in a way help achieving the performance characteristics that means field and armature windings can connect in parallel or series, so self-excited motor classified into:
1-Series wound Dc motor:
In series Dc motor, here we can get a large amount of starting torque, but we can’t regulate the speed and unfortunately if running with no load it can be damaged so it’s suitable for small electrical appliances and versatile electric equipment.
The advantages of the series motor are high starting torque, simple construction, easy in design, easy maintenance, and effective cost.
The speed in series motor varies with load and we discover that by applying KCL:
I= Ise =I a
While Ise is the series current
And from KVL:
V= E+ I(Ra + Rse )
To get the power we multiply the equations so;
VI= EI+ I^2 (Ra +Rse ).
And as we know: input power= mechanical power+ armature losses + field losses
VI= P + I^2 Ra +I^2 Ra
Finally, you can fully understand this example:
2- Shunt wound Dc motor:
Shunt motor is the most common types of Dc motor where we connect the field winding parallel with the armature windings so this motor can be self-excited from the armature windings and this gives it the feature of greatest speed regulation, simplified reversing control and low starting torque , so shunt motor is suitable for belt-driven applications in industrial and automotive applications.
It has the advantages of simple control performance, high availability, smooth running, wide control range and low speeds.
The shunt motor is a constant speed motor as the speed doesn’t change with varying the mechanical load and it’ll be clear with equations.
By applying KCL;
I= Ia +Ish.
While Ish is the shunt field current.
And by applying KVL the voltage for field winding will be:
V= Ish Rsh.
And for armature winding:
V= E + Ia Ra
So, the power will be:
Input power= mechanical power+ armature losses+ field losses.
VI= Pm +Ia^2 Ra + Ish^2 Rsh
VI = Pm + Ia^2 Ra +V Ish
Pm = VIa- Ia^2 Ra =( V- IaRa) Ia.
To get the electrical power supplied to the armature (VIa)
V Ia= E Ia+ Ia^2 Ra .
V Ia= Pm+ Ia^2 Ra.
Finally, we will put your hands on a great example containing what you can need:
3-Compound wound Dc motor:
In the compound motor, we have both series and shunt field winding, this motor has a good starting torque.
This motor also subdivided into cumulative compound motor where the produced flux by both windings is in the same direction ( Φ= Φsh + Φse), and differential compound motor where the flux produced by the series winding oppose the flux produced by the shunt winding( Φ= Φsh- Φse).
Also cumulative and differential motors can be long or short type depending on the arrangement nature:
Long shunt Dc motor:
In the long shunt motor, the shunt field winding parallels both the armature and the series field windings.
Short shunt Dc motor:
In short shunt motor the shunt field winding is parallel to the armature winding and both shunt and armature windings series the series field winding.
And to be fair there is the abandoned type of Dc motor which is Brushless Dc motor.
Brushless Dc motor:
It is a special type of motors as it doesn’t contain brushes, it has a high efficiency typically around 85-90% in producing large amounts of torque over a vast speed range.
And as it has high power to weight ratio, high speed and electronic control we use it in many applications as:
- In computer peripherals (disk drives, printers).
- Hand-held power tools.
- Vehicles ranging from aircraft to automobiles.
- In Small cooling fans.
- And for gramophone records in direct-drive turntables.
It also has many advantages like;
- It’s more efficient as the velocity is determined by the frequency which depends on the current, not the voltage.
- We have less mechanical energy loss as the friction is less and that enhance the efficiency.
- It can operate at high-speed under any condition.
- There is no sparking so we have less noise during operation; and so on.
And unfortunately, we will have some disadvantages in this motor as:
- BLDC motor cost is higher than in a brushed motor.
- Insulation of winding may get damaged as heat weakens the magnets because we can supply limited high power to the motor.
Finally, I hope you like this article (Types of DC motor)