The performance characteristics of DC motor are
- Torque versus armature current (T v/s Ia)
- Speed versus armature current (N v/s Ia)
- Torque versus speed (T v/s N)
DC Series Motor Characteristics and Applications
Torque – Current characteristics (T v/s Ia)
For a series motor,
Torque α Armature current x Field flux
T α Iaφ
Before saturation, φ α Ia
Therefore, T α Ia2
After magnetic saturation of core, flux (φ) is independent of Ia i.e. flux does not increase with increase in armature current.
Therefore after saturation, T α Ia
Therefore, on light loads, the torque produced by the series motor is proportional to the square of armature current and hence curve drawn between torque and armature current up to magnetic saturation is a parabola. But after magnetic saturation flux φ is independent of excitation current and so torque is proportional to Ia and hence characteristics become a straight line.
Speed – Current Characteristics (N v/s Ia)
We know that
Eb = (PφNZ)/60A
or N = (60AEb)/PφZ
In above equation, all quantities are constant except Eb and φ.
N α Eb / φ
also Eb = V – Ia Ra
Therefore, N α (V – IaRa) / φ
In a DC series motor, initially the field flux φ rises in proportion to the current but after saturation, it is independent of armature current. Consequently, speed N is roughly proportional to the current. The speed may become dangerously high if load reduces to a small value.
Hence, a DC series motor should not run on no-load. DC series motors are always connected to loads by gears so that minimum load is always maintained to keep the speed with in safe limits.
Speed – Torque Characteristics (N v/s T)
Since a series motor develops high starting torques at low speeds and low torque at high speeds, therefore, speed – torque characteristics of a DC series motor is a hyperbola. High starting torque enables, even a small series motor to start a heavy load.
Application of DC Series Motor
DC series motors are used where high starting torque is required like hoists, cranes, electrical locomotives, elevators etc.
DC Shunt Motor Characteristics and Applications
Torque – Current Characteristics (T v/s Ia)
We know that,
T α Iaφ
But flux of a shunt motor is practically constant.
Therefore, T α Ia
Therefore, torque current characteristics of a shunt motor is a straight line passing through the origin. Although the field current remains practically constant, yet the field flux becomes slightly weaker at heavy loads, due to armature reaction, hence the curve bends slightly bends at heavy loads.
Speed Current Characteristic (N v/s Ia)
Back EMF of shunt motor is given by
Eb = V – IaRa = (PφNZ)/60A
Because P, N, Z and A are constant
Therefore, Eb α Nφ
or V – IaRa α Nφ
or N α (V – IaRa)/φ …….(1)
The field flux of shunt motor is almost constant. Therefore, the numerator of RHS of equation (1) decreases with increase in load (or Ia).
So there is a little fall in speed with the increase in load, hence the curve bends slightly as the load is increased due to increased IaRa voltage drop.
Speed – Torque Characteristics (N v/s T)
The speed torque characteristics are similar to speed current characteristics.
Applications of DC Shunt Motor
These motors are used where constant speed and low starting torque is required like centrifugal pumps, fans, blowers, conveyers etc.
DC Compound Motor Characteristics and Applications
Torque – Current Characteristics
In the case of a cumulative compound motor, as the armature current increases, the series flux increases, so flux per pole increases.
But T α Iaφ
Consequently torque also increases; however, this increase in torque is greater than that of the shunt motor.
Whereas in the case of a differential compound motor, series field opposes the shunt field so the total flux of such motor decreases with increase in current (i.e. load). Hence in a differential compound motor, torque increases with increase in current.
Speed – Current Characteristics
In cumulative compound motors, series field aids the shunt field, so flux per pole increases as the armature current increase and hence speed decreases.
Whereas in a differential compound motor, series field opposes the shunt field, so flux per pole decreases as the armature current increase and hence speed increases.
Speed – Torque Characteristics
In a cumulative compound motor, the series excitation helps the shunt excitation hence speed decreases with increase in torque whereas torque increases very slightly with the speed.
Applications of DC Compound Motor
A cumulative compound motor has a fairly constant speed and good starting torque. Such motors are used where series characteristics are required and the load is likely to be removed totally. These motors are used in driving machines which are subject to the sudden application of heavy loads; they are used in rolling mills, punching and shearing machines, mine-hoists etc.
In a differential compound motor, the motor speed will increase with an increase in the load, which leads to an unstable operation. Therefore, a differential compound motor is rarely used for any practical application.