DC injection braking in three-phase motors

This braking procedure is achieved by disconnecting the motor from the AC power supply and connecting two terminals of its stator to a DC power source. All the considerations discussed on this same topic regarding wound-rotor motors remain valid. Furthermore, wound-rotor motors allow, within certain limits, the selection of the most appropriate speed for a given braking torque.

The power dissipated as heat in the rotor resistors is moderate. Considering that in countercurrent braking, the rotor power that must be dissipated during braking is practically equal to the rotor's nominal power, we can see that in DC injection braking, the dimensions of the rotor resistors can be considerably reduced, which represents a significant advantage in this braking method.

Generally, the value of the DC current injected into the stator is determined by the most unfavorable conditions, that is, a hot motor and the mains voltage at its lowest value. Therefore, an overcurrent occurs when the motor is cold and the mains voltage is overvolted.

When dealing with manual control, it is advisable to include a timer device that cuts off the braking after a predetermined time. Indeed, if the control device were left indefinitely in the braking position, the motor and its power supply would overheat, potentially leading to their destruction; this danger would go unnoticed since the machine would remain at rest.

DC SELF-EXCITATION BRAKING

In DC injection braking, the stator acts as the exciter and the rotor as the alternator. It seems convenient to use this alternating rotor voltage and rectify it to power the stator, thus achieving self-excitation braking.

Following this braking procedure, the scheme shown in Fig. 5.17 was designed. This corresponds to a lifting device with braking during descent, allowing for a reduced and stable speed. The rectifier must continuously withstand a rotor current close to the motor's nominal current, with significant current and voltage spikes during coupling changes.

In ascending or descending motion, contactor c2 closes and the motor starts at the moment c1 (ascending) or c11 (descending) is energized; the rotor resistances are progressively short-circuited by means of contactors c21 to c24.

During descent braking, contactors C1 and C11 are at rest; the motor is disconnected from the power supply, contactor C2 is open, and the starting resistors are disconnected from the circuit. Contactors C3 and C31 connect the rotor to the stator via the rectifier, thus enabling self-priming of the braking system. To facilitate and accelerate priming, a small transformer TR supplies a feed-in current. A resistor R1 limits the current and protects the transformer in case of rotor voltage fluctuations.

The resistors shunted by C32, C33, and C34 allow the desired speed to be regulated during braking. If the motor operates at constant torque, the stator and rotor currents are also constant. An increase in the braking resistance corresponds to an increase in rotor voltage and operating speed.

The advantages of this procedure are significant. First, a slow braking speed can be achieved, less than one-tenth of the nominal speed and independent of the driving torque. Intermediate speeds also offer very acceptable stability. For low driving torques, the motor starts without prior launching; moreover, the torque developed at the beginning of braking from a running motor is very significant because the rotor tension is very high. This precaution is automatically ensured by the current equipment, independently of the operator's input.

Explanatory diagram of a device for braking by self-excitation of current
continuous, of a three-phase asynchronous motor, with wound rotor

Finally, the stator and rotor currents are not high and are adjusted to the value of the drag torque that is intended to be balanced. To use self-excitation braking in a wound-rotor motor, the stator and rotor current ratings must be comparable. Sometimes it is advantageous to reverse the operation, that is, to rectify the stator current if it is high and inject it into the rotor. The results obtained are entirely comparable.