With thousands of power supplies in the field, Magna-Power Electronics frequently works with customers to insure proper power supply operation for their applications. The following discusses five frequently seen problems and measures for correcting them.

Power Supply Grounding

Power systems in industrial facilities generally have three phase power consisting of connections to each phase, neutral, and ground. Our power supplies do not use and need a neutral connection. Our control circuits are referenced to earth ground through a high impedance resistor and a parallel connected capacitor. This allows connections to all user analog, digital, and communications circuitry without concern for ground loops in the power system. The resistor/capacitor combination enables user interface circuitry to be slightly offset from ground.

By electrical code and from a safety viewpoint, there should only be one connection to earth ground; the ground connection should be made at the electrical entrance of the building, the location of the metering equipment. It is at this point where ground and neutral are connected and a ground rod is driven into the earth. If the facility’s equipment is properly wired, there should only be a small current flowing in the ground path. Under such conditions, a lightning strike will raise the facility to the same voltage potential thereby protecting objects or personnel from voltage across any two ground points.

Unfortunately, not all power systems are wired to code and a common problem is that the ground used for the computer and instrumentation equipment is not at the same voltage potential as the power equipment. While the power supply attempts to adjust for such conditions with its high impedance ground connection, sometimes a poor ground connection between user and power equipment ground can cause strange power supply behavior. The most common problem is loss of communication between the power supply and computer equipment. In most cases, bonding grounds between user interface equipment and the power supply corrects this problem.

Line Voltage

Connecting the power supply to suitable power mains is unfortunately one problem we frequently see. Connecting a 208 V rated power supply to a 480 V mains damages the power section of the unit. Repairs for this issue are expensive and not covered under warranty. To avoid this catastrophic event, read the power input label on the back of the unit and measure the voltage to be applied. If the measurements do not match the rating, do not make the connection and correct the mains wiring.

Switching Remote Sense Leads

Most of our supplies provide the means for sensing voltage at the load negating voltage drops in the connecting power cables. Utilizing this feature creates the possibility of configuring the power supply for remote sense operation and not sampling the voltage at the load with the remote sense leads. Unprotected with no feedback signal, the power supply will drive the output voltage upward until it damages its own output stage. To protect the power supply from this possibility, a smart remote sense detector has been implemented to switch the remote sense lines between the output bus bars and the remote sense terminals during startup. The remote sense detector is disabled after a predetermined voltage is measured across the remote sense terminal inputs.

Switching the remote sense leads after startup places the power supply in an uncontrolled and potentially dangerous state. Magna-Power Electronics has frequently seen field installations where contactors/relays have been deployed to switch outputs and remote sense lines between different loads. The response of the power supply usually exceeds that of the mechanical contactors causing the output voltage to increase quickly during the switching period. The result is either damage to the output stage or tripping from over current due to uncontrolled charging of output capacitors. Customers attempting to work around this problem with resistors connected between the output bus bars and load create further potential problems. The recommendation from the factory is do not attempt to switch remote sense leads.

Over Current Trip

All Magna-Power Electronics’ power supplies have an internal safety provision to protect it from abnormal operating conditions. This feature, internal over current trip, is triggered when internal currents exceed a safe condition in the power circuits. When triggered, the fault condition cannot be cleared with the clear key and control power must be recycled. Internal over current trip contrasts external over current trip, an over current loading state that can be cleared by the clear key. Some of the conditions causing internal over current trip are:

• A sudden dip and recovery of input voltage.
• An output load change from open circuit to near short circuit. Internal over current trip is usually triggered when the power supply is set to full scale output voltage and current and a sudden load change from open circuit causes the output voltage to drop to less than 10% full scale voltage.
• Excessive noise and abuse on user interface connections. User interface connections at the rear of the power supply are directly connected to the supply’s control circuits. With unintentional customer abuse, abnormal voltage at user interface connections can cause component failure or internal over current trip.
• Component failure. If a power component or control circuit fails, the power supply will be uncontrolled causing an internal over current trip.

Environment

About three fourths of all field returns are due to poor environmental conditions. Magna-Power Electronics’ products are either air cooled or water cooled. Water cooled units were developed for installations with poor air quality or for higher density rack mount installations that cannot meet airflow requirements.

For air cooled power supplies, cabinets should be equipped with blowers that equal air flow of the power supplies installed. To prevent trapped air or air flow restrictions, cabinet air intake should be placed below the lowest installed power supply and exhaust should be above the highest installed power supply. Fan or blower ratings should be that recommended in the user manual. Openings for area intake and exhaust should be approximately equal.

Care should be taken not to place power supplies in poor environments. For example, an installation in an environment of paint and dust will eventually cause deposits of paint and dust on printed circuit boards designed with spacing to support high voltages. Over the course of time, these printed circuit boards will eventually break down. Common sense is usually the best cure to solve environment problems.