1. Field
The disclosed concept pertains generally to motor control drives, and, more particularly, to a motor control drive that includes a regenerative front end.
2. Background Information
There are numerous settings wherein motors are employed to drive heavy machinery. For example, multiple high horsepower electric motors are used in a pumping system, such as, without limitation, a water pumping system. As is known in the art, in such settings, there are a number of devices that can be used to control the motors. In particular, contactors, soft starters, and variable frequency drives (VFDs) (also referred to as adjustable frequency drives or AFDs) are different types of devices that can be used to control a motor in such a setting.
A contactor simply connects the motor directly across the AC line. A motor connected to the AC line will accelerate very quickly to full speed and draw a large amount of current during acceleration. Thus, use of a contactor only to control a motor has many drawbacks, and in many industrial settings will not be permitted by the electric utility.
A soft starter is a device used to slowly ramp up a motor to full speed, and/or slowly ramp down the motor to a stop. Reducing both current draw and the mechanical strain on the system are big advantages of using a soft starter in place of a contactor. Soft starters are more common on larger horsepower systems.
A VFD is a solid state electronic power converting device used for controlling the rotational speed of an alternating current (AC) electrical motor by controlling the frequency of the electrical power supplied to the motor. Typically, a VFD first converts an AC input power to a DC intermediate power using a rectifier circuit. The DC intermediate power is then converted to a quasi-sinusoidal AC power using an inverter switching circuit. A VFD not only has the ramping ability of a soft starter, but also allows the speed to be varied while at the same time offering more flexibility and features.
When the rotor of an AC motor turns more slowly than the speed set by the applied frequency from, for example, a VFD, the motor is in a condition referred to as “motoring” wherein the motor transforms electrical energy into mechanical energy at the motor shaft. In contrast, when the rotor turns faster than the speed set by the applied frequency, the motor is in a condition referred to as “regeneration” wherein the motor acts like a generator and transforms mechanical energy from the motor shaft into electrical energy. Regeneration conditions may be caused by, for example and without limitation, an overhauling load, a reduction in commanded speed, or a ramp to stop. For a VFD and motor in a regenerative condition, the AC power from the motor (i.e., the regenerative current) from the motor flows back into the VFD. In most instances, the rectifier front end of a VFD only permits current to flow in the motoring direction. As a result, regeneration will cause the charge on the DC bus capacitors of the VFD to increase and will therefore cause the DC bus voltage to rise. Unless this back flow of regenerative current is addressed, the VFD will protect itself by initiating a high DC bus voltage trip.
There are three main ways to handle such regenerative energy in order to avoid a high DC bus voltage trip. In one method, the regenerative energy is released in the form of heat through a voltage regulated switching transistor and resistor, often called a “chopper,” a “snubber” or a “dynamic brake.” In another method, the DC buses of several drives are tied together such that the regenerative energy from one VFD/motor can be absorbed and used by another VFD/motor on the same DC bus. In the third method, a regenerative rectifier or regenerative bridge converter (often referred to as a regenerative front end) is provided in the drive and converts the regenerative energy on the DC bus into AC energy that may be provided back to the utility power grid.
One known and currently available VFD is the SC9000 motor drive sold by Eaton Corporation, the assignee of the present invention. The SC9000 includes a multi-pulse diode rectifier having four AC to DC rectifier bridges on the secondary of a phase shifting isolation transformer. The bridges are arranged in series connection creating two twelve pulse rectifiers which result in twenty four pulse harmonic mitigation on the primary of the transformer. The rectifiers are constructed with line frequency general purpose puck/capsule diodes fixed in a baseplate module. Since the rectifier is single way, the power from the drive is limited primarily in the motoring direction in either forward or reverse speed (i.e. two quadrant). When the sum product of torque speed is negative, power flows back into the drive (i.e., a regeneration condition exists) and increases the voltage on the split DC capacitor bank. In this scenario, the drive's SPX speed/current loop controls go into over-voltage limit and the deceleration plus regeneration is fairly limited. This regenerative torque/power is constrained to the losses of the rotating machine and inverter conduction/switching and DC bus discharge resistor losses. This amounts to less than 2-3% of rated power at full speed.
It would be advantageous to be able to provide a regenerative bi-directional fundamental front end (FFE) input power stage for the SC 9000 and similar drives that works in parallel with the existing line frequency diode modules and that allows power to flow to and from the machine so that it can both motor and generate on deceleration or overhauling loads.