We know that the armature current is decided by the load. On light load or no load, the armature current is very small. In DC series motor, Ǿ∝Ia, Ǿ is Flux produced in Armature circuit. Ia is Armature current.
Hence on no load, Ia is small and hence flux ( Ǿ) produced also very small. According to speed equation, N∝(1/ Ǿ).
So on no load, as flux is very small, the motor tries to run at dangerously high speed which may damage the motor mechanically. This is the reason why series motor should never be started on light loads or no load conditions.
Application of DC Motor
Separately excited motors
Very accurate speeds can be obtained by these motors. Moreover, these motors are best suited where speed variation is required from very low value to high value. These motors are used in steel rolling mills, paper mills, diesel electric propulsion of ships, etc.
Shunt motors
From the characteristics of a shunt motor we have seen that it is almost constant speed motor. It is, therefore, used;
Where the speed between no-load to full load has to be maintained almost constant.
Where it is required to drive the load at various speeds (various speeds are obtained by speed control methods) and any one of the speed is required to be maintained almost constant for a relatively long period.
As such the shunt motors are most suitable for industrial drives such as lathes, drills, grinders, shapers, spinning and weaving machines, line shafts in the group drive, etc.
The characteristics of a series motor reveal that it is variable speed motor i.e., the speed is low at higher torques and vice-versa. Moreover, at light loads or at no-load, the motor attains dangerously high speed. It is, therefore, employed:
Where high torque is required at the time of starting to accelerate heavy loads quickly.
Where the load is subjected to heavy fluctuations and speed is required to be adjusted automatically.
As such the series motors are most suitable for electric traction, cranes, elevators, vacuum cleaners, hair driers, sewing machines, fans and air compressors, etc.
Compound motors
The important characteristic of this motor is that the speed falls appreciably on heavy loads as in a series motor, but at light loads, the maximum speed is limited to safe value. It is, therefore, used;
Where high torque is required at the time of starting and where the load may be thrown off suddenly.
Where the load is subjected to heavy fluctuations.
As such the cumulative compound, motors are best suited for punching and shearing machines, rolling mills, lifts and mine – hoists, etc
Selection of DC Motors
Selection of power rating
If the size (HP or kW capacity) of a motor is more than the load to be picked-up by it then it will operate at lighter load than its rating. Hence, there will be more losses (i.e., motor will operate at poor efficiency) and unnecessarily there will be wastage of power. Thus extra expenditure will incur during operation. At the same time its initial cost will be more.
On the other hand, if the size of the motor is less than the load to be picked-up by it, then it will be over loaded and it will be heated-up beyond its limits. Hence the motor may be damaged if operated continuously. Otherwise, also the protective devices will disconnect the motor under over-load conditions and this affect the production.
Thus, a motor should be chosen of a size for its maximum power utilisation. During operation, it should be heated-up within the permissible temperature limits and it should never be over[1]heated. But at the same time it should be capable to take-up the over-loads temporarily.
Characteristics of the motor
For satisfactory operation, selection of power rating of a motor is not only the criteria, rather one should know the characteristics of a motor (i.e., behaviour of a motor under different load conditions) so that a work can be done quickly and efficiently without any breakdown.
Therefore, before selection of motor, one should know the following particulars of the work.
- Torque requirement during starting and running at different loads.
- Requirement of accelerating and braking torque
- Frequency of switching
- Temperature at the work place
- Environmental conditions, etc.
After knowing the above particulars at the work place, a suitable motor (with proper enclosures) to meet with the requirements is selected.