Construction and Working:
Though the same purpose can be served by the use of a motor generator set in which the motor is supplied with power from the available source and the driven generator develops the required voltage, but such an arrangement is expensive and inefficient since two machines are required.
Dynamotor is a single machine that functions both as a motor and a generator. It has a single field structure and an armature with two armature windings and two commutators.
Two independent armature windings are usually placed in the same slots, one above the other (in some cases alternate sets of slots are used for each winding) and are connected to separate commutators one at each end of the shaft.
Special dynamotors have been constructed with as many as four armature windings and four commutators, two at each end of the shaft, in such an arrangement one winding is connected to the available source and the remaining three windings develop three different voltages required.
Dynamotors are less expensive and more efficient than motor-generator sets; the higher efficiency results because the rotational (stray) losses consisting of the friction, windage and iron losses, and the field copper losses are due to a single machine.
The dynamotors have the drawback, however, that the generated voltage cannot be controlled and is fixed by the voltage supplied to the motor end.
Commutation in dynamotors is usually excellent because the armature reaction effect results from two sets of ampere-turns in opposite directions.
This is true because the current in the motor winding is always opposite to that in the generator winding; the net armature ampere-turns are exactly those required to provide sufficient torque for the rotational losses. In addition, the shaft has to transmit but little torque.
When interpoles are fitted to such a machine, the windings then would be relatively feeble under normal conditions, as armature reaction is not to be neutralized.
For preventing sparking at sudden momentary overloads on the lt side, it is well to have powerful windings on the interpoles, and the reluctance of the interpole magnetic circuit may with advantage be increased to permit of this, by inserting non-magnetic distance pieces between the interpole and the yoke.
On analysing the dynamotor it is found that the induced emf per conductor is the same for either windings because the same flux is cut at the same speed.
If the two windings are similar (both lap or wave), the counter-emf in the motor and the generated emf in the generator winding will be directly proportional to their respective number of turns.
This implies that any change in speed or any change in flux affects the two windings equally and there will always be a definite ratio between the output voltage to the input voltage if armature drops are neglected.
It can be explained in another way also, if the field is weakened the motor speeds up but since the generator armature winding cuts less flux, the product of speed and flux remains constant so that the generated voltage remains constant.
On the other hand, if the field is strengthened, the motor slows down but since the armature winding cuts more flux, the product of flux and speed remains again constant and, therefore, generated voltage remains constant.
The emf induced in the generator winding is given as the voltage applied to the motor multiplied by the ratio of number of conductors i.e. V = ZG/ZM where V is the voltage applied to the motor end ZG is the number of conductors in generating winding and ZM is the number of conductors in motoring winding.
Applications:
Dynamotors are employed mostly for radio and other communication services when rating is of the order of a few hundred watts.
The power is supplied to the machine from a storage battery at low voltage and the output voltage may be as high as 1500 V.