Mitigating flicker

The European Metal Recycling (EMR) auto shredding facility in Birmingham, England had been struggling for years with an excessive flicker problem. The EMR facility’s 6,000-hp wound rotor motor, with an 11,000 volt primary, used an industry-standard electro-mechanical liquid rheostat control. They had explored many options but none could provide a satisfactory solution. 

Today EMR’s facility is operating more effectively and efficiently than ever. Flicker is no longer a game-changing issue, productivity is up and the company is saving money every month on its power bill. The key to EMR’s dramatic turnaround: Illinois-based Quad Plus’ Clean Power Drive, a fully integrated electrical control specifically designed for auto shredders.

Flicker is often a major concern for utilities, when it comes to installing a large wound rotor motor on their distribution system. This may influence the utility to insist on the installation of a dedicated substation installed at the customer’s expense. In EMR’s case, the local power utility was adamant that they lower their flicker levels, so they needed to identify and implement a new strategy and system. 

“Flicker is a voltage fluctuation on power lines that causes the lights in the neighbourhood to flicker,” explains Joseph Crosetto, Quad Plus. “This is the same as when you turn on an electric saw or grinder at your house and the lights flicker. Only with a shredder application this can be a 6,000 hp motor. Flicker is caused when a motor suddenly changes the amount of current it uses (a current spike). This could be when starting inrush or a load change (for example, when a car is dropped into the mill.)”

Flicker data was recorded at EMR on April 6, 2011, running the facility’s typical production, using a liquid rheostat control. The data clearly documented the problem. For compatibility levels in low voltage and medium voltage systems, AS NZS 61000.3.7-2001 dictates short-term perceptibility (Pst) should be less than 1.0 percent, and long-term perceptibility (Plt) should be less than 0.8 percent. But, EMRs numbers far exceeded those limits. Short-term perceptibility was measured at 2.106 percent with a 10-minute capture and long-term perceptibility was measured at 1.579 percent with a 2-hour capture.

The degree of flicker was a product of frequent load fluctuations, electrode reaction delays inherent in industry typical electro-mechanical liquid rheostat controls, and lack of an effective current limit on the motor.   

Implementing Clean Power Drive
During the week of November 13, 2011, Quad Plus commissioned the Clean Power Drive at EMR’s shredding facility. During production runs similar to those in the initial data collection process, the highest measured Pst value during a 10-minute capture was 0.4464, a significant reduction from the 2.106 Pst reading taken during the initial data gathering in April.

The EMR operations staff immediately noticed an increase in perceived power. Product that normally would have tripped the main stator circuit breaker now ran through their Lindemann shredder without exceeding 135 percent stator current. Productivity was significantly increased because they were spending less time waiting for the rotor to recover speed. In addition, the motor was more effective at producing useful torque – not wasted heat. 

Quad Plus’ Clean Power Drive is a fully integrated electrical control specifically designed for Auto Shredder applications. The technology consists of an input AC line reactor, a fully regenerative source converter, an inverter drive utilizing Pulse Width Modulation (PWM), and an inverter output dv/dt (delta voltage / delta time) filter. Additional components include: a crowbar circuit for overvoltage protection; a combining reactor; a contactor for switching from a starting circuit to Clean Power Drive control; starting circuit; feedback transformer; a Wonderware operator station; and plc.

The Clean Power Drive allows users to enter in a desired rpm for the rotor. This rpm can be sub-synchronous (motor speed below synchronous speed) or super-synchronous (motor speed above synchronous speed), as needed.

This is important because some shredding applications can be optimized using a different rpm than what is normally available based on the number of poles in the motor. 

With proper component selection, the EMR Clean Power Drive was designed for a ± 17 percent speed range. With a 495 rpm base speed, the drive system could control the motor from 411 rpm to 579 rpm. 

EMR determined that 525 rpm would be the optimum operating speed for their shredding application, and the desired operating rpm was entered on the Wonderware operator station. 

After the Clean Power Drive installation, it was verified that EMR’s facility was processing more tons per hour due to the higher rpm, increased kinetic energy (12.6 percent), and the rise in horsepower to 6,359 hp (from 6,000 hp). Depending on the secondary voltage of the wound rotor motor and Clean Power Drive ampacity, a speed range of up to ±30 percent can be provided for a specific application.  

Solution integration
The Clean Power Drive can easily be retrofitted into existing applications or designed specifically for a new installation. At the EMR site, it was decided that the existing motor start/stop circuit would remain nearly untouched. This enabled the operations staff to begin using the system with minimal training. 

The main motor is started the same way it was started prior to the Clean Power Drive installation – with a start button on the operator’s chair. As the motor reaches synchronous speed, a status light on the Wonderware operator station indicates the Clean Power Drive is ready to start. Once the Clean Power Drive Start button is pressed, the sequence is fully automatic. The addition of a contactor panel allows the rotor circuit to be controlled from the liquid rheostat or the Clean Power Drive, and the contactor sequences between the liquid rheostat and the Clean Power Drive. Once the Clean Power Drive contactor is closed, and the liquid rheostat contactor is opened, the Clean Power Drive takes over; accelerating or decelerating the rotor to the desired speed set point.  

In addition, the true current limit in the Clean Power Drive eliminates the highly inefficient 200 percent to 300 percent spikes which are frequent with electro-mechanical liquid rheostat control. 

The bottom line is a system that becomes more efficient at producing scrap, resulting in a lower cost per ton ratio. For most large mills, this savings can be significant.

For EMR, the use of the Clean Power Drive allows for real-time VAR compensation, closed loop speed control, and effective current limit in the stator circuit. That combination reduces flicker to manageable levels and increases the overall efficiency of the system.

In addition to production gains realized through better speed control and increased rpm when needed, the Clean Power Drive can provide EMR owners and staff with a power management tool to limit demand charges, similar to the technology used in Quad Plus’ DC drive systems. The use of Kilo-Watt Demand Limiting Technology will enable more accurate forecasting of monthly expenses, and will yield significant savings on utility bills over the course of a year.