What are the Characteristics of DC motors
Monday, June 1, 2020
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Characteristics of DC motors-
Characteristics of DC Motor means the relation(or graph) between different parameter like Armature Torque, Armature Current & Speed of the motor.
There are three characteristic of DC motors which are:
- Torque speed characteristics
- Torque current characteristics and
- Current speed characteristics.
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| DC Motor |
These are explained step by step for each type of the motor (DC Motor). These graph of DC motor are determined by keeping two things in mind first back emf equation and second torque equations. For a DC motor, magnitude of the back emf of DC motor is same as emf equation of a dc generator.
The back emf equation of DC motor are given as: Eb=(PɸNZ)/(60A)-----(i)
and armature torque equation of DC Motor is given as: Ta= 0.159(PZ/A)ɸIa-----(ii)
where P= number of poles,
Z= Total number of conductor and
A= Number of parallel path,
Hence once a motor get manufactured then P, Z and A remains constant and Therefore above equations (i) and (ii) can be written as Ta ∝ ɸ.Ia and N ∝ Eb/ɸ........(iii)
Characteristics of DC series motors- In a DC series motor the series field winding is always connected with the armature winding and DC supply is applied across it. The applied voltage or terminal voltage (supply voltage) in DC series motor is given by : Vt=Eb+IaRa+IseRse+Brush drop
Vt=Eb+Ia(Ra+Rse)+Brush drop, Eb= Vt-Ia(Ra+Rse)-Brush drop...................(iv)
The load current is given by IL=Ise=Ia and the back emf is given by equation (i).
(a)Armature Current Vs Speed Characteristic (Ia Vs N): In DC series motor the flux, which is produced by magnetic field is proportional to load current . As per the above equation(i) the speed of the motor is proportional to back emf , but back emf of DC Series motor can be given in equation (iv), Eb= Vt-Ia(Ra+Rse)-Brush drop.
In a DC series motor the voltage drop across brush terminals is very small. The resistance of the motor is also negligible, hence a change in the back emf will remains constant if we maintain terminal voltage constant. It means rotational motion i,e; speed of DC series motor is inversely proportional to load current as stated in above equation(iii) (N ∝ Eb/ɸ).
As ɸ ∝ Ise=Ia= IL, Hence N ∝ 1/ɸ ∝ 1/Ia
There will be dangerously very high speed at no load (no armature current) condition of the motor and current in the field winding is low hence flux is low. Hence DC series motor should never operates without mechanical load. The Armature Current and Speed Characteristic (Ia Vs N) is shown in figure(a).
(b)Armature Current Vs Torque Characteristic(Ia Vs Ta):From above equation(ii), we know that torque is directly proportional to the product of armature current and magnetic flux, Ta ∝ ɸ.Ia. In DC series motor the flux is directly proportional to current carried by field winding (hence load current), ɸ ∝ Ise=If (Field current).
If we increase the field current, flux will also increase up to a certain point, known as a magnetic saturation point of field core. The magnetic saturation is a capacity of a field core to hold maximum magnetic flux. Once core get saturated, after that if you increase field current then flux will remains constant. It means there will be no change in flux after saturation point.
Therefore, before magnetic saturation of the field core, ɸ is directly proportional to armature current, Hence torque is proportional to square of armature current,Ta ∝ Ia*Ia= (Ta ∝ Ia2). Hence before saturation point characteristic is a parabola.
After magnetic saturation of the field core, magnetic flux is independent of armature current, Ia. Hence, the torque directly proportional to Ia , Ta ∝ Ia. Therefore, armature current- torque curve becomes a straight line after magnetic saturation of the field core.
Before magnetic saturation torque increases as the square of armature current, therefore DC Series motors are used where high starting torque is required. The Armature Current and Torque (Ia Vs Ta) characteristics is shown in figure(b).
(c)Torque Vs Speed Characteristic (Ta Vs N):Torque is proportional to square of the armature current, Ta α Ia2. The speed of DC series motor is N ∝ Eb/ɸ. Hence if the torque on the DC shunt motor increased the armature current also increases, thus flux increases and speed will be dropped as N ∝1/square root Ta .
As the load goes on increasing speed of DC series motor will drop rapidly. This characteristic is also known as mechanical characteristic of DC motor. The Torque and Speed Characteristic (Ta Vs N) is shown in figure(c).
Characteristics of DC Series motor.
Characteristics of DC shunt motor
In DC Shunt motor the shunt field winding and the armature is connected in parallel and DC supply voltage is applied across it. The applied voltage or terminal voltage,Vt in DC shunt motor is given by : Vt=Eb+IaRa+Brush drop
Eb= Vt-IaRa-Brush drop
Vt=IshRsh Ish= Vt/ Rsh ................. ...............(v)
The line current is given by IL=Ish+Ia and the back emf, Eb is given by equation (i).
(a)Armature Current Vs Speed Characteristic (Ia Vs N): As we know that flux ɸ, is directly proportional to field current. In DC shunt motor field current ,If=Ish= Vt/ Rsh ,hence in DC shunt motor if supply voltage and shunt field resistance is constant then the flux is constant as current Ish, carried by field winding is constant.
Therefore DC shunt motor is also known as constant flux motor. Hence equation (iii) speed is proportional to back emf, N ∝ Eb/ɸ ∝ ( Vt-IaRa-Brush drop)/ɸ. The shunt field resistance and voltage drop across brush is very less, hence change in back emf Eb will remains constant if we maintain terminal voltage Vt constant. This indicate that the speed of DC shunt motor is almost constant with respect to armature or load current.
Hence DC shunt motor can be assumed as a constant speed motor. The characteristic is slightly dropping due to presence of voltage drop in back emf by IaRa drop which is very small. The Armature Current and Speed Characteristic (Ia Vs N) is shown in figure(d).
(b)Armature Current Vs Torque Characteristic(Ia Vs Ta): In DC shunt motor the flux is constant as current carried by field winding is constant. From equation (iii), we know that the torque is proportional to product of field flux and armature current, Ta ∝ ɸ.Ia .But as we have stated above that the flux of DC shunt motor is assumed to be constant.
Hence the torque is proportional to armature current, Ta ∝ Ia , and therefore current and speed characteristic is linear passing through origin. The Armature Current and Torque (Ia Vs Ta) characteristics is shown in figure(e).
(c)Torque Vs Speed Characteristic (Ta Vs N): In DC shunt motor the flux is assumed to be constant hence to armature current, Ta ∝ Ia .
The speed of DC shunt motor is N ∝ Eb/ɸ∝( Vt-IaRa-Brush drop)/ɸ . Hence as the torque on the DC shunt motor increases the armature current increases and speed will be dropped by some value. But the characteristic is slightly dropping.
This characteristic is also known as mechanical characteristic of DC motor. The Torque and Speed Characteristic (Ta Vs N) is shown in figure(f).
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| Characteristics of DC Shunt Motor |
Characteristics of DC compound motor-DC compound motors have two field winding, series as well as shunt winding. Series field winding is always connected in series with armature winding. Based on the total flux in motor there are two types of compound motor.
1.Cumulative compound motor
2.Differential compound motor
In a compound motor if the nature of both flux is additive, if series and shunt windings are connected in such way that the direction of series flux is same as the shunt flux then the motor is said to be cumulative compound motor.
And a compound motor if the nature of both flux is subtractive, if series and shunt windings are connected in such way that the direction of series flux is opposite to the shunt flux then the motor is said to differential compound motor.
Characteristics of these compound motors are shown below:
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| characteristics of compound motor |
