First of all we will understand three important characteristics of a DC motor then we will understand in detail Shunt Motor, Series Motor and Compound Motor characteristics. But before that let us know what is motor characteristics.
The performance of a DC motor can be judged its characteristic curves known as motor characteristics.
The characteristic curves of a motor are those curves which show the relationship between the following quantities.
1. Torque and armature current characteristics (Ta / Ia ) – It is a curve between armature torque Ta and armature current Ia of a DC motor. This is also known as the electrical characteristic of the motor.
2. Speed and armature current characteristics (N / Ia ) – It is a curve between speed N and armature current Ia of a DC motor. This is a very important characteristic as it is often the deciding factor in the selection of a motor for a particular application.
3. Speed and torque characteristics – (N / Ta ) – It is a curve between speed N and armature torque Ta of a DC motor. This is also known as mechanical characteristic.
Note – When discussing motor characteristics, two relationships must always be kept in mind.
and
See also this : What is DC motor? – Principle, Working, Types, Back emf, Significance
What are the characteristics of a DC shunt motor?
Ta / Ia Characteristic
We know that in a DC motor, Ta ∝ Φ Ia
Since the motor is operating from a constant supply voltage, the flux Φ is constant (neglecting armature reaction).
Therefore, Ta ∝ Ia
So the Ta/ Ia characteristic is a straight line that passes through the origin as you can see in the figure-1 below. The shaft torque (Tsh) is less than Ta and is shown by the dotted line.
Now it is clear from the curve that a lot of current is require to start a heavy load. Hence shunt motor should not be started at heavy load.
N / Ia Characteristic
The speed N of the motor is given by, N ∝ Eb / Φ
The flux Φ and back emf Eb in a shunt motor are almost constant under normal conditions. Hence, speed of the shunt motor will remain constant as the armature current varies (see dotted line AB in figure-2).
When the load is increase, ( Eb = V−Ia Ra ) and Φ decrease due to armature resistance drop and armature reaction respectively. However, Eb decreases slightly more than Φ so that the speed of the motor with the load (line AC) decreases slightly.
N/Ta Characteristic
This curve is obtained by plotting the values of N and Ta for different armature currents (see figure-3). It can be seen that the speed decreases somewhat as the load torque increases.
Conclusion :
Two important conclusions can be drawn from the above characteristics.
- There is a slight change in the speed of a shunt motor from no-load to full-load. Therefore, it is essentially a constant-speed motor.
- The starting torque is not high because Ta ∝ Ia.
See also this : What is torque in DC motor? – Define armature torque and shaft torque of DC motor
What are the characteristics of DC series motor?
Ta / Ia Characteristic
We know that : Ta ∝ Φ Ia
Up to magnetic saturation, Φ ∝ Ia so that Ta ∝ I2a.
After magnetic saturation, Φ is constant so that Ta ∝ Ia.
Thus up to magnetic saturation, the armature torque is directly proportional to the square of the armature current. If Ia is doubled, Ta is almost quadruples. Hence, the Ta/Ia curve is a parabola up to magnetic saturation (see the portion OA of the curve in figure-1).
However, after magnetic saturation the torque is directly proportional to the armature current. Hence, after magnetic saturation the Ta/Ia curve is a straight line (part AB of the curve).
It can be seen that in the initial part of the curve (i.e. up to magnetic saturation), Ta ∝ I2a. This means that starting torque of a DC series motor will be very high as compared to a shunt motor (where Ta ∝ Ia).
N / Ia Characteristic
The speed N of a series motor is given by, N ∝ Eb / Φ (where Eb = V − Ia (Rse + Rse )
When the armature current increases, the back emf Eb decreases due to the drop of Ia (Rse + Rse ) while the flux Φ increases. However, the Ia (Rse + Rse ) drop is quite small under normal conditions and can be neglected.
Therefore, N ∝ 1/Φ
∝ 1/Ia up to magnetic saturation
Thus, up to magnetic saturation, the N/Ia curve follows a hyperbolic path as you can see in figure-2. After saturation, the flux becomes constant and so does the speed.
N/Ta Characteristic
You can see the N/Ta characteristic of a series motor below in Fig-3.
It is clear that the series motor develops high torque at low speed and vice-versa. This is because an increase in torque requires an increase in armature current, which is also field current. The result is that the flux is stronger and hence the speed decreases (because N ∝ 1/Φ). The opposite happens torque should be less.
Conclusion :
Three important conclusions can be drawn from the above characteristics of the series motor.
- Its initial torque is high because initially Ta ∝ I2a.
- It is a variable speed motor (see N/Ia curve in figure-18.17) that is it automatically adjusts the speed as the load changes. Thus if the load is reduced, its speed automatically increases and vice versa.
- At no-load, the armature current is very small and so is the flux. Therefore, the speed increases to an excessive high value (because N ∝ 1/Φ). It is dangerous for the machine which can be destroyed due to the centrifugal forces set up in the rotating parts. Hence, series motor should never be started at no-load. However, to start a series motor, first a mechanical load is applied and then the motor is started.
Note : The minimum load on a DC series motor must be sufficient to keep the speed within limits. If the speed becomes dangerously high, the motor should be disconnected from the supply.
See also this : What is the DC generator? : Construction, Working principle, Types, EMF equation, Losses
Compound motors
A compound motor has both a series field and a shunt field. Shunt field is always stronger than series field. There are two types of compound motors :
- Cumulative-compound motors
- Differential-compound motors
In a cumulative-compound motor, the series field assists the shunt field. and in a differential-compound motor, the series field opposes the shunt field.
Differential-compound motors are rarely used because of their poor torque characteristics at heavy loads.
Cumulative compound motor characteristics
In the figure below you can see the connections of a cumulative-compound motor. Each pole has a series as well as shunt field winding, the series field assisting the shunt field.
Ta / Ia Characteristic
As the load increases, the series field increases but the strength of the shunt field remains constant. As a result, the total flux increases and therefore the armature torque (because Ta ∝ Φ Ia ). The torque of a cumulative-compound motor is higher than that of a shunt motor for a given armature current due to the series field (see figure-1).
N / Ia Characteristic
As we understood above, as the load increases, the flux per pole also increases. As a result, the speed of the motor (N ∝ 1/Φ) falls as the load increases see figure-2.
It may be note that as the load is added, the increased amount of flux causes the speed to decrease more than does the speed of the shunt motor. Thus the speed regulation of a cumulative-compound motor is poorer than that of a shunt motor.
Note : Due to the shunt field, the motor has a fixed no-load speed and can be operated safely at no-load.
N/Ta Characteristic
In figure-3 you can see the N/Ta characteristic of a cumulative compound motor. For a given armature current, the torque of a cumulative compound motor is higher than that of a shunt motor but lower than that of a series motor.
Conclusion :
A cumulative compound motor has characteristics between series and shunt motors.
- Due to the presence of shunt field, the motor is prevent from running at no-load.
- Due to the presence of series field, the starting torque increases.
See also this : What are the characteristics of DC generator?
What are the comparison of three types of motors?
- The speed regulation of shunt motor is better than that of series motor. However, the speed regulation of a cumulative-compound motor lies between that of shunt and series motors (see figure-1).
- For a given armature current, the starting torque of a series motor is more than that of a shunt motor. However, the starting torque of a cumulative compound motor is between that of series and shunt motors (see figure-2).
- Both shunt and cumulative compound motors have a fixed no-load speed. However, a series motor has a dangerously high speed at no-load.
See also this : What are the applications of DC generator?
I hope this information (DC motor characteristics) will help you. Thank You