2258 |
CHAPTER 28. VARIABLE-SPEED MOTOR CONTROLS |
28.3.3Regenerative braking
Regenerative braking takes the concept of dynamic braking one step further, in converting the DC bus over-voltage into usable AC power to be placed back on the AC line for other AC devices to use. Rather than regulate DC bus voltage via a shunt resistor switched on and o by a special transistor, a regenerative drive manages the same task by augmenting the bridge rectifier diode array with a set of six more power transistors, then switching those transistors on and o synchronously with the line voltage (the AC power source). This line-synchronized switching takes the DC bus voltage and “inverts” it to AC so that the drive may send real power back into the AC power system from whence it originated:
Sending power to AC line through inverter transistors
From |
Positive (+) DC bus |
|
three-phase |
||
|
||
AC power |
|
|
source |
|
|
|
AC |
|
|
motor |
Negative (-) DC bus
Rectifier circuits equipped with a set of line-synchronized power transistors are often referred to as an active front end to the motor drive. The term “active” refers to the transistors (diodes are “passive” devices), and the term “front end” simply refers to the bridge being at the incoming (front) side of the VFD power circuit. In such a drive, the front end’s transistors are sequenced as needed to clamp the DC bus voltage to reasonable maximum levels, just like the braking transistor is pulsed in a drive with dynamic braking to shunt-regulate DC bus voltage. If DC bus voltage in a regenerating drive rises too high, the active front end transistors will pulse for longer periods of time (i.e. with greater duty cycles) to apply more of that braking energy to the AC power grid.
Regenerative braking enjoys the unique advantage of putting the kinetic energy lost through braking back into productive use. No other method of motor braking does this. The cost of doing this, of course, is increased component count and complexity in the motor drive itself, leading to a more expensive and (potentially) fault-prone VFD. However, in applications where the recovered energy is significant, the cost savings of regenerative braking will rapidly o set the additional capital expense of the regenerative drive.
28.3. AC MOTOR BRAKING |
2259 |
A simpler and cheaper way to enjoy the benefits of regenerative braking without adding a lot of complexity to the VFD circuitry is to take multiple VFDs and simply connect their DC bus circuits in parallel. If one of the drives slows down its motor, the raised DC bus voltage will be available at the other motor drives to help them drive their motors.
The following schematic diagram shows two interconnected VFD circuits, with the upper drive braking and the lower drive motoring (driving):
From |
(Braking) |
three-phase |
|
AC power |
|
source |
|
|
AC |
|
motor |
From |
(Driving) |
three-phase |
|
AC power |
|
source |
|
|
AC |
|
motor |
The major disadvantage to regeneratively braking in this fashion is that the braking energy is only recoverable by the other motor(s) with their DC busses paralleled, and only at the exact same time one or more of those motors are braking. This is not as convenient or practical as AC line regenerative braking, where a virtually unlimited number of loads exist on the grid to absorb the braking energy at any time. However, for certain applications11 it may be practical, and in those applications the installed cost of the VFDs will be less than a comparable installation with AC line regeneration.
As with dynamic braking, motor heating is reduced (compared to DC injection braking) because the kinetic energy is dissipated elsewhere.
11One such application is machine motion control, where one part of the machine always needs to slow down while another part is accelerating. Another application is coupling the drive motors of two conveyor belts together, where one conveyor always lifts the load uphill and the other conveyor always lowers the load downhill.
2260 |
CHAPTER 28. VARIABLE-SPEED MOTOR CONTROLS |
28.3.4Plugging
Plugging is the most powerful method of braking an electric motor, consisting of actively applying power to the motor in the opposite direction of its rotation. This is analogous to reversing the engine thrust of a power boat or an airplane in order to quickly bring it to a halt. For a VFD, this means a reversal of phase rotation while carefully applying power to the AC induction motor.
Like DC injection braking, plugging requires power be applied to the motor in order to make it stop, and it also results in all the kinetic energy being dissipated in the rotor. The advantage held by plugging over DC injection braking is that the braking torque may be maintained and precisely controlled all the way to zero speed.