19-11-2012, 05:36 PM
Electrical Braking
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Introduction
In any motion control system (especially hoisting applications) braking is necessary
to accurately control the position and speed of the load. Braking can be achieved by
electrical or mechanical means. In traditional vehicle applications, braking is
achieved by using a hydraulically operated mechanical brake (e.g., as in a car). In
most modern motion control systems electrical braking is used on the motor, and the
mechanical brake is only used as a parking brake.
Excessive Energy during Braking
In any system which can overhaul, excess energy will be generated by the motor.
When a load is overhauling (braked lowering) excess energy will be generated. The
motor will act as a generator (e.g., if it is an induction motor it will be running above
the synchronous speed).
In the case of variable frequency inverter drives, this excess energy will be
returned as charge on the terminals of the capacitor. If not removed, this charge
causes the voltage on the DC link to rise, and could cause damage to the system.
Removing this excess charge protects the system, and also achieves the function of
braking the motor. Two methods exist for removing the charge: dynamic resistor
braking and regenerative braking. These are discussed in this Chapter as well, as
two of the methods of electrical braking.
Plugging
Another method of braking is called plugging, which involves applying the reverse
phase sequence to the winding, as if trying to reverse the motor rotation. A typical
set-up is shown in Figure 1. This method does not require a pole changing motor (as
required in the dc injection braking method). However, care has to be taken not to
switch on any of the reverse sequence SCRs until the forward sequence SCR has
ceased to conduct; otherwise a short circuit will result, which would damage the
SCRs and trip the electrical protection. For these reasons, plugging systems will
invariably has a zero current detector fitted in the path of the motor current to check
that the current has dropped to zero before reversing the phase sequence. This
method also suffers from the disadvantage that it can inject high values of current in
the rotor, and rotor bars have been known to rupture due to the high currents induced
in the rotor bars.
Eddy Current Braking
Another method of braking which is not widely used is the so-called Eddy current
braking method. It is based on the principle of inducing a current in a rotating disk,
by which the circulating current will induce a back torque opposing the rotation. “This
method is used to obtain braking torque from the eddy current brake which is
attached on one end of the motor shaft. It has the characteristics of the DC dynamic
brake. The braking torque, which is substantially good enough at longer speed
ranges, becomes zero when the speed becomes zero. (Fukuda, 1979)”. This
method needs a complicated non-standard motor, cannot produce any torque at
standstill and dissipates all the heat in the machine, rather than returning it to the
supply.
Resistor Dynamic Braking
This method is used in variable frequency drives. A resistor is used to dissipate the
extra charge, whereby the resistor is called a dynamic braking resistor. This is
achieved by placing the resistor in parallel with the DC link capacitor, and being
switched (chopped) by using a transistor. This is shown in Figure 5. The transistor is
switched on and off, where the ratio of the on time to the off time is adjusted in order
to achieve the necessary value of current to remove excess charge from the DC link.
Regenerative Braking
The main disadvantage with the resistor braking method, is that the excess energy is
lost as heat. It will be more efficient to attempt to return the energy back in the
supply. This is called regenerative braking.
This method of braking is achieved by connecting a second inverter in parallel
with the rectifier in variable frequency drive systems. When it is detected that the DC
link voltage has exceeded a specified level, the regenerative inverter will go into inverting mode and move the energy back onto the 3 phase main supply. This is
shown in the block diagram of Figure 8.
DC injection braking
The method used to achieve electrical braking is to inject a DC voltage into the low
speed winding of the motor, as shown in Figure 9.
This method has mainly developed because many of the variable voltage system,
when they first appeared where retrofitted on sites which had two speed AC motors
on them. It was easy them to use the low speed windings for braking, by injecting a
DC current in the low speed winding.
Injecting DC in a motor winding will tend to try to stall the rotor. This is because the
magnetic field set up inside the motor is a stationary constant field which is always
pointing in one direction. This will have a braking effect on the moving rotor, trying to
bring it to the synchronous speed of DC (i.e., which is 0 rpm). The disadvantages
with this method is that the energy is dissipated as heat in the windings, and that braking torque approaches zero at the speed approaches zero, making it difficult to
brake the motor at standstill.