Preventing downtime, protecting PLCs, VFDs

Throughout the past 15 years, microprocessor-based control systems such as robotic system, variable frequency drives (VFDs), and programmable logic controllers (PLCs) have altered manufacturing. The advancement of very large scale integration (VLSI) in integrated circuits makes process control systems in manufacturing even more dependent on microprocessors. The stoppage of a VFD or PLC can interrupt manufacturing, causing a financial impact on producers. Here are two examples:

• An aerospace and auto glass manufacturer uses conveyers to move rubber molding under heat at a speed that evenly dries the mold onto the glass. PLCs control the on/off and speed of the motors on the conveyor belt, following a programmed sequence of on/off and speed events as the item moves down the conveyor. Power anomalies, such as under voltage or power outage, could interrupt the PLCs stopping the motors and causing the molding to remain under the heating element as the element ramps down in temperature. All windshields with rubber molding under the heating element are wasted, the molten rubber requires cleanup of the conveyer, and the PLCs potentially require reprogramming.

A quality power system – such as AC or DC uninterruptible power system (UPS) with batteries, or a UPS with supercapacitors – would have avoided these interruptions.

• A multi-story production facility with a large service elevator moves sub-assemblies and people from one floor to another. VFDs control large 3-phase motors at programmed speeds and torques for lifting and lowering the elevators. A VFD rectifies input 60Hz or 50Hz (depending on country) AC voltage to DC voltage, with an output circuit that takes the DC energy and converts it back to AC voltage at various frequencies. Variations in AC input voltage will impact the DC voltage that creates the output frequency sent to the elevator motors. With no protection, low or interrupted power could cause the lifting or lowering process to stop until the VFD is reset.

Power stability

Motor controllers and VFD systems require a stable DC voltage for the manufacturing process to continue. A loss of quality DC power would disable the speed and torque control of manufacturing motors. The quality of DC power is a function of the quality of the AC power at the input, so stability at the DC and AC levels is crucial.

To define power anomalies and identify their sources, first examine an ideal waveform by the utility company on an oscilloscope, shown in Figure 1.

In the U.S., the magnitude of that voltage would be either 480V 60Hz, 240V 60Hz, or 208V 60Hz measured from phase-to-phase on a 3-phase system. As that voltage is distributed, it may be left the same or transformed down from 480V to 208V, 208V to 120V, or 240V to 120V depending on the size and sensitivity of the load.

Power disturbances

Seven primary power disturbances can occur in a power network. The graphic to the left represents each by a waveform as seen on an oscilloscope.

1. Transient – anomaly during switching of high current loads
2. Interruption – momentary or long-term power outage
3. Sag/under-voltage – Ramping up high current loads overload the power source, damping voltage
4. Swell/over-voltage – Rapid load reduction provides surplus power in the utility
5. Waveform distortion – Fed to the utility by loads such as motors
6. Noise/electromagnetic interference (EMI) – Caused by high current radio frequency waves
7. Frequency variation – Caused by acceleration/deceleration of generators supplying the utility

Connected-load impacts

As AC voltage moves along a production facility’s power network, connected loads can impact power quality. The on-off switching for a motor, for example, can cause transients. A low-input power supply from a PLC can send distortion to the power network. The high-current draw of a VFD can cause the power network to have a low-voltage condition. Acceleration or deceleration of a backup generator can cause frequency deviation, and the utility supply can be interrupted.

In information technology (IT), switch-mode power supplies have become rugged to where only the most severe anomalies 3 through 7 will impact server operation – providing output DC voltage if the input voltage deviates from 100V to 240V and the frequency deviates from 49Hz to 60Hz. Quality power systems, such as UPS, primarily protect against outages. This is not the case in the process control world.

VFDs are typically installed in rugged environments where switching constantly occurs, causing transients, sags, and distortion. These anomalies impact the VFD’s DC output. DC output at various magnitudes recreates a waveform that varies in frequency, which then varies the speed and torque of the motor.

Any deviation in the DC levels at the DC bus impacts the regenerated frequency and motor speed and torque. The challenge is to maintain stability at the DC source or at the AC input. DC bus voltage is directly impacted by voltage sags that occur with ramping up of high current loads, typically seen with manufacturing machinery.

Power quality options

With stable utility grids, no protection against outages is needed, and supercapacitors (also referred to as ultracapacitors) can be used for energy storage to compensate for sags. (See Figure 2) In environments where outages do occur, batteries can be used. Batteries require maintenance and replacement, whereas capacitors do not, but batteries can provide 6 minutes to 30 minutes of ride-through, whereas supercapacitors offer 7 seconds to 24 seconds.

Stability at the DC bus can be achieved in two ways:

1. 1. With uninterruptible power for drives (UPD) that rectify the AC voltage, store energy in capacitors, and supply that energy to the DC bus when a sag is detected. Bonitron, a Riello UPS partner, supplies such systems.
2. 2. In the case where the DC bus is not accessible or multiple VFDs on the same electrical network need to be protected, a UPS can provide quality power at the input of the VFD, again resulting in a stable DC voltage. While UPS systems are typically provided with batteries, Riello UPS offers a supercapacitor solution. In addition to stability, these capacitors can provide up to 15 seconds of outage ride-through.

Each solution has advantages and disadvantages. UPD consumes little to no energy unless the capacitors or batteries need to recharge after discharging. The system is on standby. UPS is online, operating at 95% efficiency, consuming 5% of the input capacity. UPD requires one system per drive, whereas UPS can supply a network of drives. Finally, UPS may be the only solution in an application where the DC bus is not accessible by the electrical contractor doing the installation.

Adding an AC and/or DC UPS systems between utility and the various loads that support the manufacturing process will assure that downtime and resulting financial impact will be avoided.

Riello UPS

www.riello-ups.com

Bonitron

www.bonitron.com

About the author: Yves Bouhadana has more than 26 years in the quality power industry, having worked for Emerson Network Power and currently Riello UPS. Yves is a graduate of University of South Florida with a BS in Electrical Engineering. He can be reached at y.bouhadana@riello-ups.com or 513.282.3777.