The good traction characteristics of DC electric machines have made them an integral part of most industrial and household mechanization devices. But at the same time, there is also a significant problem of significant starting currents, in comparison with asynchronous electric motors operating on alternating voltage. That is why many experts study in detail how to start a DC motor before turning on the unit.
Starting with a starting rheostat
In this case, a variable resistance is introduced into the circuit, which at the initial stage provides a decrease in the current load until the rotation of the rotor reaches the set speed. As the amperage stabilizes to a standard value, the resistance in the rheostat decreases from the maximum value to the minimum.
The calculation of the electrical quantity in this case will be made according to the formula:
I = U / (Rbm + Rreostat)
In laboratory conditions, the load can be reduced manually - by moving the rheostat slider. However, this method is not widely used in industry, since the process is not consistent with the current values. Therefore, regulation by current, by EMF or by time is applied, in the first case, the measurement of the value in the field windings is used, in the second, a time delay is applied to each stage.
Both methods are used to start electric motors:
- with sequential;
- with parallel excitation;
- with independent excitement.
Launching DPT with parallel excitation
Such a start of the electric motor is carried out by turning on both the excitation winding and the armature to the supply voltage of the mains, they are located in parallel relative to each other. That is, each of the windings of the DC motor are at the same potential difference. This starting method provides the hard operating mode used in machine tools. The current load in the auxiliary winding at start-up has a comparatively lower current than the stator or rotor windings.
To control the starting characteristics, resistances are introduced into both circuits:
At the initial stage of rotation of the shaft, the positions of the rheostat provide a decrease in the load on the electric motor, and then they are brought back to the position of zero resistance. With prolonged starts, automation and a combination of several stages of starting rheostats or individual resistors are performed, an example of such a switching circuit is shown in the figure below:
- When the supply voltage is applied to the electric motor, the current flowing through the working windings and the winding excitation, due to the resistance box Rstart1, Rstart2, Rstart3 the load is limited to the minimum magnitudes.
- After reaching the threshold value of the minimum current value, relays K1, K2, K3 are sequentially activated.
- As a result of the closure of the contacts of relay K1.1, the first resistor is bypassed, the performance in the power supply circuit of the electric motor rises abruptly.
- But after falling below the set limit, contacts K2.2 are closed and the process is repeated again until the electrical machine reaches the rated speed.
The DC motor can be braked in the reverse order using the same resistors.
Starting DPT with sequential excitation
The figure above is a schematic diagram for connecting an electric motor with series excitation. Its distinctive feature lies in the series connection of the excitation coil L of the excitation and the motor itself, the variable resistance R of the armature is also introduced in series.
The same current value flows through the circuit of both coils, this circuit has good starting parameters, therefore it is often used in electric transport. Such an electric motor must not be switched on without force on the shaft, and the frequency is controlled in accordance with the load.
DCT start with independent excitation
The connection of an electric motor to a circuit with independent excitation is made by powering it from a separate source.
The diagram shows an example of an independent connection, here the L-excitation coil and the resistance in its R-excitation circuit are powered separately from the motor windings by the current of an independent device. For the motor windings, the armature adjustment rheostat is also turned on. With this starting method, the DC machine should not be turned on without load or with minimal force on the shaft, as this will lead to an increase in speed and subsequent breakdown.