Shunt regulators can play a key role in dynamic braking systems, which are commonly used to decelerate electrically-powered vehicles.
The shunt regulator is a type of linear regulator, a system used to maintain a steady voltage in an electrical circuit. In a linear regulator, the level of resistance changes to accommodate the input, producing an output voltage that is constant. The regulator dissipates the difference between the input and output voltages in the form of waste heat.
Linear regulators sometimes are placed between the source and regulated loads. These are called series regulators. In contrast, shunt regulators are placed in parallel with the load.
Shunt regulators can be very simple in design, with some consisting of a single reference diode. They are commonly used in voltage reference circuits and DC power supplies.
Using shunt regulators, designers can control the voltages on their systems to prevent shutdown and component burnout.
Shunt Understanding
Suppliers of shunt regulators include Microchip Technology Inc., Texas Instruments, Fairchild Semiconductor, ON Semiconductor, NXP Semiconductors, and more.
Beyond dynamic braking, there are multiple applications for shunt regulators including:
- Error amplifiers
- Independent source circuits
- Precision current limiters
- Precision voltage references
- Switching power supplies
- Voltage supervision and monitoring
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Step on the Brakes
Dynamic braking is defined as using the electric traction motor of a vehicle as a generator in order to decelerate. In such a system, the electric motor runs backwards to slow the vehicle.
This brings into play a fundamental truth about electrical generation—when motors run forward, they produce motion; when they run backwards, they become generators and make electricity. Thus, dynamic braking transforms the energy used to slow down a vehicle into electricity.
This system brings several benefits, including lowering the wear on friction-based braking components.
Electricity Excess
However, dynamic braking raises an important question. What is to be done with the electricity produced by the motor or generator?
One approach to dynamic braking is known as rheostatic. For rheostatic braking systems used in locomotives, the energy generated by running the motor backwards is dissipated as heat by a resistor. Large train diesel engines use arrays of resistors and large cooling fans to prevent overheating.
This is a relatively inexpensive approach to dynamic braking, although it wastes the energy produced by the system.
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Talkin’ ‘bout Regeneration
Another approach is to use regenerative braking, which is a system used in some hybrid automobiles such as the Toyota Prius. In regenerative systems, the electricity is sent back into the system in order to charge up the batteries.
This is the most efficient approach to dynamic braking, ensuring that more of the electricity produced is used to propel the vehicle. On the downside, regenerative braking is a more expensive way to implement braking compared to the rheostatic approach. However, despite this additional cost, hybrid vehicles use regenerative braking as their primary means of deceleration.
Some cars using conventional internal combustion engines are employing regenerative systems to power electrical components in the car. For example, Mazda’s i-ELOOP system uses a regenerative braking system to charge supercapacitors that, in turn, supply electricity for the electronic equipment.
Feel the Burn
For either approach, the electricity generated by the motor or generator needs to go somewhere. For regenerative systems, this electricity feeds back into the batteries.
In the rheostatic approach, as the motor runs backwards, the voltage on the DC bus of the motor control will start to increase. This could cause a shutdown or component burn out among components connected to the motor’s bus. To protect their circuits, designers must devise a way to deal with this voltage.
One solution is to employ a shunt regulator that consists of a resistor and a voltage regulated switching transistor. Such a shunt regulator dissipates the voltage as heat.
Accelerated Braking
With the use of electrical motors in a variety of systems, dynamic braking systems employing shunt regulators are likely to be prevalent.