DC motor control of starting in current function

One can see from the starting diagram, that the current oscillates in some limits I₁-I₂. Initial current surge is defined by the values of necessary starting moment and allowable current for the motor. As the speed increases motor current decreases to the I₂ value, and the part of starting resistor is shunted, that leads to the increasing current to the value of I₁.

Switching current I₂ is chosen to provide minimal acceleration at the motor starting with the given load, i.e. this current should be greater than the current specified by resisting moment.

Scheme of automatic starting that constructed by control principle in current function should envisage one or some current relays that are switching directly into rotor circuit of a motor. Relay number depends on number of starting steps of rheostat.

Schematic solution for method to control starting in current function will be clear from considering of such simplified scheme.

Figure 7.10 – Simplified scheme of DCM IE starting in armature current function.

Applied here is the current relay of angular acceleration KI and as well for simplification the starting resistor for one step. When energizing linear contactor KM1 with the help of command apparatus SA simultaneously with armature circuit the coil of blocking relay KN is energized (the coil of current relay KI is energized in the armature coil).

Contact KI is deenergized before contact KH of blocking relay energizing, that`s why the coil of starting contactor KM2 is deenergized (its contact KM2 is opened and the motor is energized by resistor R). Contact KH of blocking relay doesn’t allow energizing the starting contactor KM2 just after energizing of linear contactor KM1. Releasing current of current acceleration relay KI is equal to current I2 that results to energizing of starting contactor KM2, which is to shunting of starting resistor R. At the next current ripple from I2 to I1 the starting contactor KM2 doesn’t open due to the fact that current relay KI is shunted with contact KM2 of starting contactor.

The advantage of this method is that the switching of starting resistor occurs at a given current value I2 and doesn’t depend on oscillations of voltage supply at starting.

Figure 7.11 – Graph of current changing at load increasing at starting.

Hence, this way also has disadvantages (look at the figure). So, when starting the motor the load moment on its shaft for any reasons turns to be higher of specified; then current I2 may for long time exceed for the value of Di, when opening of current relay K1 occurs. For the reason it will not energize and starting resistor will be unprotected (be on). Further drive acceleration will not take place (that is the drive jammed on one of the starting steps), besides such a situation causes the burning of resistor, because it is computed by short-term operation.

Дата добавления: 2014-01-05