Q: Protecting against Reneration

When a motor is spinning faster than it would normally at the applied voltage, such as when moving downhill or decelerating, the motor acts like a generator. In such cases, the current will flow in the opposite direction, back to the power source.

It is therefore essential that the controller be connected to rechargeable batteries. If a power supply is used instead, the current will attempt to flow back in the power supply during regeneration, potentially damaging it and/or the controller.

Regeneration can also cause potential problems if the battery is disconnected while the motors are still spinning. In such a case, the energy generated by the motor will keep the controller On, and depending on the command level applied at that time, the regenerated current will attempt to flow back to the battery. Since none is present, the voltage will rise to potentially unsafe levels.

The oscilloscope capture to the right shows the voltage at the input of a MBL1660 controller connected to a power supply.  Here we are driving a high current Hub Motor for an electric bicycle.  When suddenly stopping this motor, which was running at full speed, the voltage quickly rises from 34V to over 60V. The voltage could rise even higher under certain conditions. When the motor is finally stopped, the voltage that accumulated in the controller's capacitors starts decaying after about 350ms.

The controller includes an overvoltage protection circuit to prevent damage to the output transistors. The oscilloscope capture on the right shows the voltage at the input of an MBL1660 controller connected to a power supply with the overvoltage protection set to 36V.

The overvoltage protection circuit will cut off (float) the motor, quickly stopping the regeneration and voltage surge. Once the motor is floating, the voltage accumulated in the controller's capacitor will decay. When the voltage goes level below the overvoltage threshold, the transistors reactivate. If the motor is still spinning because of inertia, regeneration will resume and the voltage quickly rise again above the overvoltage limit. This cycle will continue until the motor has stopped. Because the power transistors disconnect, there is no braking taking place. The motors are freewheeling down.  The motor controller may shut down in this process with an overvoltage error and will have to be reset to continue operation if the internal electronics have not been damaged.

If the motor inertia is very high, the voltage jump at the time the transistors turn back on can be dangerously high. The overvoltage protection can therefore not entirely be depended upon to protect the controller and the power supply. This protection will work best when the overvoltage limit is set as close as possible above the supply voltage. For example, 25V on a 24V supply system.

A safer technique and one that will cause the motor to brake instead of freewheeling is to place a resistive load in parallel with the power supply, with a circuit to enable that load during regeneration. This solution is more complex but will provide a safe path for the braking energy into a load designed to dissipate it. The block diagram to the right shows an example of such a circuit.  Roboteq has developed and optimized this concept into a product, the SR5K70V25R Smart Shunt Regulator.

The latest release of the Roboteq Shunt Regulator is a fast, highly accurate, high wattage resistive load with multiple user selectable input thresholds, status indicator lamps, and advanced i²t and thermal protection implemented into its fast microcontroller to prevent large overloads from damaging the loading resistors.  The onboard controller monitors the input voltages every millisecond during the time that the shunting resistors are active.  If the accumulated shunting energy exceeds the manufacturers limits for the shunting resistors, the loading is deactivated to prevent damage to the resistors – which depends on the time and applied voltage.  This type of protection technique is known as “i²t Protection” and is commonly used industry wide with resistive loading and motors.

The oscilloscope capture to the right shows the effect of this circuit. Here the SR5K70V25R Shunt Regulator was connected with the threshold voltage set for 35v.  With a loading resistor value that switches as needed from 2.5 to 5 Ohms, the power supply and regeneration voltage suddenly see a load of 14 amps, causing a rapid drop in voltage. The resistor is then quickly disconnected and the voltage rises as the controller’s capacitors recharge. When the voltage rises again above the overvoltage threshold, the resistive load reconnects again, repeating the cycle until all the motor’s kinetic energy is dissipated.  The motor controller, and any other devices which share the power supply with it are now protected from the damaging effects of sudden and potentially catastrophic over voltage conditions encountered if regeneration conditions are present.

In summary, here are some key points to consider about the SR5K70V25R Smart Shunt Regulator:

• i2t technology for minimizing load resistor damage from unplanned extreme overloads.
• Over heating protection can be cycled repeatedly without replacing an inline fuse or circuit breaker.
• Fast easy connection to unit using Fast-On type connectors and standard crimp style lugs.
• Voltage loop through connections makes the use of “Y” cables unnecessary.
• Solid 6061 aluminum base plate for easy mounting and superb heat transfer.
• 5000 Watt instantaneous, 200 Watts continuous load capability.
• Flashing Status Lamp to indicate the operational mode including Scan, Loading ON, Resistor Cooling, and Overvoltage modes.
• Active Solid State Relay switching indication lamps during shunting operation.
• Compact, Low Cost with Simple Operation.

Operation

The Shunt Regulator is connected across the DC power source to the operating electronics.  It then monitors the line continuously, and when the voltage exceeds the clipping preset the on-board controller activates the switching which connects the shunt resistors across the load.  For the time that the measured voltage exceeds the preset value, the shunt remains connected.  When the voltage levels drop to a level just below the threshold value minus a preset hysteresis value, the shunt disconnects.  Constant monitoring of the shunt resistor temperature ensures that long term shunting does not exceed the thermal limits of the two 100W load resistors.  To ensure that the proper braking load is applied at the correct time, the Shunt Regulator uses a Dual Resistor configuration.   As the voltage rises, the first load resistor is made active, then the second resistor about 2 volts higher.  This allows more accurate damping of the overvoltage condition.  If the amount of energy dissipated by the loading resistors exceeds their maximum limit, the loading is disconnected and a thermal recovery period is active.  Once the residual energy is dissipated to a safe level, the loading is allowed to continue if the need arises.

Connection to the shunt regulator is accomplished by looping through the Fast-On connections as shown. Two sets of Fast-On connections are provided to simplify system integration.  

Selecting the Clipping Voltage

Voltage selection is accomplished by setting a rotary PCB mounted switch, including an AUTO mode.  Typically, the user must select a voltage that is greater than the power supply nominal voltage, and greater than any small amplitude noise that may be present on the power line.  For most applications, this ranges from 1 – 5 volts over Vin.  Below is a table of available presets on the SR5K70V25R including “AUTO” mode which samples the power line 3 seconds after turn on and generates a threshold voltage that is 5 volts higher.

The preset voltages for shunting can be set at any time during operation.  The selection of a proper shunt clip voltage above the power supply voltage is crucial for proper operation.  Standard shunting voltages from 25 volts to 70 volts maximum are available on the switch.  A small chart on the board also indicates the switch settings clip voltage values.

Auto Mode is active when switch position “0” is selected.  The onboard processor reads the power line voltage upon power up and sets a fixed preset threshold (about 5v) above that value.  This will allow for intermediate values to be obtained for shunting.

Overload voltage versus i2t Fuse Protection cutoff time.

Two curves are shown below – Shut off time in seconds (y-axis) vs Input Voltage (x-axis) - One for the 5 Ohm load and the second for the 2.5 Ohm loading.  As an example, if our preset is for 50v and the input voltage exceeds this voltage by less than 2 volts continuously, the left graph shows that the controller will shut off the load after about 13 seconds.  For the full loading condition, where we exceed the threshold voltage by more than 2v, the time allowed is about 2.2 seconds.  The allowable on time is defined by the square of the current ratios and thus the response defined by the non-linear curve.  Once the input voltage drops below the pre-set threshold, the decay of the energy accumulator proceeds.  After a period of up to 10 seconds or more, the energy accumulation bleeds off and will allow the loading resistors to be used again without damage.   This bleed off process is designed to emulate the thermal cooling of the actual resistors.