DC Motor Characteristics

DC MOTOR CHARACTERISTICS

The performance of a DC motor can be determined from its characteristics curves known as motor characteristics. Following are the three important characteristics of a DC motor.

(i) Torque versus Armature current characteristics (T Vs I_a) ═>Electrical Characteristics.

(ii) Speed versus Armature current characteristics (N Vs I_a).

(iii) Speed versus Torque characteristics (N Vs T) ═> Mechanical characteristics.

Characteristics of DC Shunt Motor

(i) Torque Versus Armature Current  (T Vs I_a)

This characteristics give the relationship between the torque and armature current of

a DC motor. This is known as electrical characteristics.

Torque, Τ α φ I_a

where, φ  →  Flux of the motor, I_a → Armature current.

From the expression for the torque of a DC motor, torque is directly proportional to the product of flux per pole 'φ' and armature current.

Since in case of DC shunt motor, the flux per pole 'φ' is remains constant.

Torque increases with the increase of load current.

Torque-armature current characteristics is a straight line passing through origin "0".

Due to armature reaction, friction and windage losses this torque time droops slightly.

(ii) Speed Versus Armature Current (N Vs E_a)

where, N is the speed in rpm

I_a is the armature current in amperes

R_a is the armature resistance in ohms

φ is flux/pole in webers.

If applied voltage 'V' is kept constant, the field current will remain constant.

Hence, flux will have maximum value on no-load, but will slightly decrease due to armature reaction as the load increases, mostly the flux is considered to be constant.

From the above speed equation, speed 'N' is directly proportional to back emf (E_b) (or) (V – I_a R_a) and inversely proportional to the flux 'φ'.

If the flux 'φ' is constant, as armature current I_a increases, the drop I_a R_a also increases. Thus (V – I_a R_a) decreases, so the speed 'N' decreases.

Since voltage drop in armature at full-load is very small as compared to applied voltage. So the drop in speed from no-load to full-load is very small. The dotted line, drawn parallel to the X-axis shows the level of no-load speed. The solid line shows the actual speed versus armature current characteristics curve.

The % of speed change will be about 5% at full-load due to armature resistance drop.

(iii) Speed Versus Torque Characteristics

The points for drawing the speed (N) versus torque (T) can be obtained from speed (N) versus armature current (I_a) and torque (T) versus armature current (I_a).

For example, I_a = 0, T = 0 in T Vs I_a curve and N = N₀ from N Vs I_a

This point therefore gives the first point on N Vs T characteristics as N = N₀ speed when T = 0.

If I_a increases, torque increases, but speed N decreases.

We may say,

Ν α 1/T

Therefore, the N versus T characteristics plotted. This shows a slightly fall in speed from no-load to full-load as the torque developed increase.

Conclusions:

There is a slightly change in the speed of shunt motor from no-load to full-load. Hence, it is a constant speed motor. The starting torque is not high, because T_a α I_a.

Characteristics of DC Series Motor

Torque Versus Armature Current Characteristics:

T_a α φ I_a

Upto magnetic saturation, φ α I_a, so that

Torque (T_a) α I_a²

After magnetic saturation, is constant, so that

Torque (T_a) α I_a.

Thus upto magnetic saturation, the armature torque is directly proportional to the square of armature current. Therefore, torque versus armature current characteristics curves is a parabola upto magnetic saturation. After magnetic saturation, torque is directly proportional to the armature current. The curve after magnetic saturation is a straight line.

Conclusion:

The initial portion of the curve T_a α I_a². This means that starting torque of a DC series motor will be very high as compared to a shunt motor.

(iv) Speed Versus Armature Current Characteristics

The speed 'N' of a series motor is given by:

Ν α E_b / φ

where, E_b = V - I_a (R_a + R_se).

When the armature current increases, the back emf decreases due to (R_a + R_se) drop, but the flux φ increases.

However, I_a (R_a + R_se) drop is small under normal operating conditions. It a neglected during light load conditions

N  α 1 / φ

upto magnetic saturation

N  α 1 / I_a (I_a = I_se = I_L)

Thus, upto magnetic saturation, the N versus I_a curves follows the hyperbolic shape.

After saturation, the flux becomes constant and so does the speed.

Why DC series motor is always started with load?

Speed is inversely proportional to armature current.

From the characteristics curves, we can able to understand that the speed decreases as the load on he motor increases.

At a very low load, the speed is dangerously high. Thus, if a series motor is allowed to run at a very light load or at no-load, its speed will become much higher than its normal speed, which may cause damage to the motor.

For this reason, series motors are never started on no-load and are not used in applications where there is a chance of the load being completely removed when the motor remains connected to the supply.

The load on the series motors is to be connected gears and not through a belt pulley arrangement.

(v) Speed Versus Torque Characteristics

The speed versus Torque characteristics of a series motor can be derived from its speed-Armature current and Torque-Armature current characteristics as follows.

The series motor develops high torque at low speed and vice-versa.

It is because an increase in torque requires an increase in armature current, which is also the field current.

The result is that flux strengthened and hence the speed drops.

Ν α 1 / φ

If the flux is weakened, the torque is low:

Ν α V - I_a R_a

If I_a R_a drop is negligible

Ν α V / φ

T α φ I_a, T α I_a², I_a² α φ²

φ α √T

N α V / √T

From the equation, we find that speed is inversely proportional to torque. Hence the characteristics curve is hyperbolic in shape.

Characteristics of DC Compound Motor

A compound motor has both series field and shunt field. The shunt field is always stronger than the series field.

Compound motors are of two types:

(i) Cummulative - Compound motors in which series field aids the shunt field.

(ii) Differential - Compound motors in which series field opposes the shunt field.

So the characteristics curves are intermediate between the shunt and series motors.

Speed Versus Armature Current Characteristics:

In the case of a cummulative compound motor the N versus I_a characteristics is slightly more drooping than that of a shunt motor as there is increase in flux with load. As flux increases speed decreases.

↓ Ν α 1 / φ ↑

In differential compound motor, the curve is above the shunt motor curve, because the series field flux opposes the shunt field flux, so the net available flux decreases.

↑ Ν α 1 / φ ↓

If flux decreases, speed increases.

Torque Versus Armature Current Characteristics:

Torque α φ . I_a

In cummulative compound motor, the series field flux aids the shunt field flux so the net available flux increases, because of that the torque increases.

The torque developed by a cumulative compound motor increases with sudden increase in load.

Cummulative compound motors are therefore suitable in situations where there is sudden application of heavy loads like shears, punches, rolling mills, etc.

In differential compound motor, the fluxes established by shunt and series, field are opposite to each other. So the net available flux decreases, because of that the torque decreases.

The torque developed by a differential compound motor, decreases with increase in load.

Speed-Versus Torque Characteristics Curves:

The speed of a differential compound motor remains more or less constant with increase in load, but its torque decreases with load.

Since the shunt motor also develops a speed torque and the speed does not vary 1x 21 ms and oft grille g appreciably will increases in load.

Differential compound motors are not preferred over shunt motors and hence are rarely used.