Dynamic Brakes

     Last week we looked at how a mechanical brake stopped a rotating wheel by converting its mechanical energy, namely kinetic energy, into heat energy.  This week, we’ll see how a dynamic brake works.

     Chances are you have directly benefited by a dynamic braking system the last time you rode in an elevator.  But, to understand the basic principle behind an elevator’s dynamic brake system, let’s first take a look at the electric braking system in Figure 1 below. 

Figure 1 – A Simple Electric Braking System

     Here the brake consists of an electric generator wired via an open switch to an electrical component called a resistor.  The weight is attached to a cable that is wound around a pulley on the generator’s shaft.   As the weight freefalls, the cable unwinds on the pulley, causing the pulley to turn the generator’s shaft.

     Unlike last week’s mechanical brake which required a good deal of effort to employ, a dynamic braking system requires very little.  All that needs to be done is to close a switch as shown in Figure 2 below.  When the switch is closed, an electrical circuit is created where the resistor gets connected to the generator.  The resistor does as its name implies: it resists (but doesn’t stop) the electrical current flowing through it from the generator.  As the electrical current fights its way through the resistor to get back to the generator, the resistor gets hot like an electric heater.  This heat is dissipated to the cooler surrounding air.  At the same time, the weight begins to slow down in its descent.  But how is this happening?

     The electric braking system can be thought of as an energy conversion process.  We start out with the kinetic, or motion energy, of the freefalling weight.  This kinetic energy is transmitted to the electrical generator by the cable, which spins the generator’s shaft as the cable unwinds.  Electrical generators are machines that convert kinetic energy into electrical energy.  This energy travels from the electric generator through wires and a closed switch to the resistor.  In the process the resistor converts the electrical energy into heat energy.  So, kinetic energy is drawn from the falling weight through the conversion process and leaves the process in the form of heat.  As the falling weight is drained of kinetic energy, it slows down. 

 Figure 2 – Applying the Electric Brake   

     Okay, now let’s get back to dynamic brakes on elevators.  An elevator is attached by a cable to a hoist that is powered by an electric motor.  When it’s time to stop at the desired floor, the automatic control system disconnects the elevator’s electric motor from its power source and turns the motor into a generator.  The generator is then automatically connected to a resistor like the one shown in the electric brake above.  The kinetic energy of the moving elevator is converted by the generator into electrical energy.  The resistor converts the electrical energy into heat energy which is then dissipated into the surrounding environment.  The elevator slows down in the process because it’s being robbed of kinetic energy.  When the dynamic brake slows the elevator down enough, a mechanical brake is introduced, taking over to bring the elevator to a complete stop.  This two-fold process serves to reduce wear and tear on the mechanical brake’s parts, lengthening the operational lifespan of the system as a whole.

     Next time, we’ll tie everything together and show how mechanical and dynamic brakes work together in a diesel locomotive.

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