Regenerative braking employs motor controller’s H bridges and motor’s internal inductance to form a boost or step up converter. The main principle is to step-up the voltage at the motor bus over the supply so that the current flows from motor to the power source. The nature of this technique is shown in figure 3. When regenerative braking is enabled, instead of switching the high side of one H bridge, driver starts to switch on/off low side switch. One of the other bridges also has a low side MOSFET turned on so that when second MOSFET is turned on, the terminals of the motor shorted, building up a current over the motor inductance. When low side MOSFET is turned off, a voltage is created over the motor inductance adding to the motor BEMF. Then the motor terminal voltage becomes higher than bus voltage, causing the motor current to flow to the bus.
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a- Switching MOSFET ON Cycle |
b- Switching MOSFET OFF Cycle |
Figure 3 – Regenerative Braking Operation Principle
Regenerative braking differs from other techniques in the sense that; instead of dissipating the energy created due to braking; the method stores the energy on the motor bus and connected storage elements (battery, supercapacitor etc.) if available. For motor loads with high inertia, this energy is very valuable such that at least half of the energy required to run the motor at a certain speed can be restored using regenerative braking.
However, since the energy is “gained” back from the motor, this energy should either be dissipated or stored. Therefore, the power source for motor should be able draw back energy. If the source is a rechargeable battery, the energy will naturally flow and charge the battery during braking. If the source is a bidirectional or 2-quadrant power supply, that will also allow reverse energy flow. Either for a battery or for such power supplies; the main consideration is the fact that the braking power/current should not be more than the power source can handle. For example; lead acid batteries can discharge instantaneous currents which are up to 5 times (namely 5C) the battery capacity. However, for charging, the current is generally limited to half of the capacity (C/2) at most. Therefore, the braking power is limited with 1/10 of the maximum instantaneous power. There exists a similar situation for lithium batteries. Some polymer chemistries allow up to discharging current which is 40 times (40C) the capacity. However, charging current is limited to only 2 times (2C) the capacity. For power supplies; especially which can act as a load and uses a resistive load for reverse current operation, the maximum allowed current for sourcing mode and sinking mode differ. Furthermore, all types of batteries and capacitors have a peak terminal voltage, beyond which the battery or capacitor is damaged. The control circuitry in a sense should follow both the current flowing to the power source and the voltage at the bus and should both limit the maximum current and bus voltage.
If the power source for motor is not capable of sinking power, the regenerative braking will cause the voltage on the bus to rise and if it is not monitored by the controller, it can extend beyond the ratings of components connected to the bus and damage components. For cases where a power sinking source is not available, capacitor banks can be added to the bus so that the regenerative energy is stored and voltage rise is limited to safe levels for the system. Since the power source will also be connected to the banks, maximum capacitive loading limit of the supply should not be exceeded in that case.
The regenerative braking is advantageous in the sense that the braking current on the motor is limited to . The on resistance of the MOSFET’s are much lower than the motor’s internal resistance so that motor currents can reach stall currents which means the braking force is maximized. Regenerative power in that sense is much effective than dynamic braking in terms of applicable speed range. However regenerative braking has system level requirements which may not be available in all applications.
In the next part we will discuss plug braking or plugging.