Non-regenerative & Regenerative DC Drives

Non-regenerative DC drives, also known as single-quadrant drives, rotate in one direction only & they have no inherent braking capabilities. Stopping the motor is done by removing voltage & allowing the motor to coast to a stop. Typically nonregenerative drives operate high friction loads such as mixers, where the load exerts a strong natural brake. In applications where supplemental quick braking and/or motor reversing is required, dynamic braking & forward & reverse circuitry, may be provided by external means.

Dynamic braking (DB) requires the addition of a DB contactor & DB resistors that dissipate the braking energy as heat. The addition of an electromechanical (magnetic) reversing contactor or manual switch permits the reversing of the controller polarity & therefore the direction of rotation of the motor armature. Field contactor reverse kits can also be installed to provide bidirectional rotation by reversing the polarity of the shunt field.

All DC motors are DC generators as well. The term regenerative describes the ability of the drive under braking conditions to convert the generated energy of the motor into electrical energy, which is returned (or regenerated) to the AC power source. Regenerative DC drives operate in all four quadrants purely electronically, without the use of electromechanical switching contactors:

• Quadrant I -Drive delivers forward torque, motor rotating forward (motoring mode of operation). This is the normal condition, providing power to a load similar to that of a motor starter.
• Quadrant II -Drive delivers reverse torque, motor rotating forward (generating mode of operation). This is a regenerative condition, where the drive itself is absorbing power from a load, such as an overhauling load or deceleration.
• Quadrant III -Drive delivers reverse torque, motor rotating reverse (motoring mode of opera tion). Basically the same as in quadrant I & similar to a reversing starter.
• Quadrant IV -Drive delivers forward torque with motor rotating in reverse (generating mode of operation). This is the other regenerative condition, where again, the drive is absorbing power from the load in order to bring the motor towards zero speed.

A single-quadrant nonregenerative DC drive has one power bridge with six SCRs used to control the applied voltage level to the motor armature. The nonregenerative drive can run in only motoring mode, & would require physically switching armature or field leads to reverse the torque direction. A four-quadrant regenerative DC drive will have two complete sets of power bridges, with 12 con trolled SCRs connected in inverse parallel. One bridge controls forward torque, & the other controls reverse torque. During operation, only one set of bridges is active at a time. For straight motoring in the forward direction, the forward bridge would be in control of the power to the motor. For straight motoring in the reverse direction, the reverse bridge is in control.

Cranes & hoists use DC regenerative drives to hold back "overhauling loads" such as a raised weight, or a machine's flywheel. Whenever the inertia of the motor load is greater than the motor rotor inertia, the load will be driving the motor & is called an over hauling load. Overhauling load results in generator action within the motor, which will cause the motor to send cur rent into the drive. Regenerative braking is summarized as follows:

• During normal forward operation, the forward bridge acts as a rectifier, supplying power to the motor. During this period gate pulses are withheld from reverse bridge so that it’s inactive.
• When motor speed is reduced, the control circuit withholds the pulses to the forward bridge & simultaneously applies pulses to reverse bridge.

During this period, the motor acts as a generator, & the reverse bridge conducts current through the armature in the reverse direction back to the AC line. This current reverses the torque, & the motor speed decreases rapidly.

Both regeneration & dynamic braking slow down a rotating DC motor & its load. However, there are significant differences in stopping time & controllability during stopping, & safety issues depending on how one defines what should happen under emergency conditions. Regenerative braking will stop the load smoothly & faster than a dynamic brake for fast stop or emergency stop requirements. In addition regenerative braking will regenerate power to the supply if the load is overhauling.