Use the right power for the inverter – Reynolds

Low pressure issues

The VFD reports a low voltage glitch when the drive's DC link voltage drops below 62% of the nominal level at the high setting (480 Vac) and 50% of the nominal value at the low setting (400 Vac).

The +10% and -15% voltage tolerances in most manuals are claimed operating ranges to promise the drive at hand to maintain optimal efficiency and proper motor current. The drive can be lower than these services, but the reduced voltage will have an unpredictable effect on the motor current, motor temperature, and overall function.

More specifically, if the line voltage parameter of the drive is set high and 480 VAC is being applied, the drive will experience a low voltage failure when the DC link voltage drops to 62% of the nominal - such as 480 VAC ·0.62·√2= 421 VDC. The nominal DC link voltage is 480 Vac·√2= 679 Vdc.

　　 Use the right power for the inverter – Reynolds

Use the right power for the inverter – Reynolds

Precautions when switching

While it is accepted practice to switch VFDs from line power to standby generators in many uses, there are some limitations. Importantly, many portable standby generators have a phase-to-phase voltage input voltage swing that is greater than the driver's recommended phase-to-phase voltage tolerance, typically less than 2%. To prevent voltage sloshing, the VFD is equipped with a surge maintainer in the input rectifier circuit – however significant input disturbances may require external maintenance (e.g. surge arresters). Large variances result in large ripple across the DC bus capacitors, which can lead to damage to capacitors and other power components over time.

Another potential problem presents when switching from line power to a standby generator. Most drives take at least two minutes to power up from scratch; Failure to follow this strategy may damage the charging relay circuit, or at least damage the input fuse or trip the circuit breaker.

In the case where the input power exhibits a moderate voltage to induce current surges, a 3% line reactor may improve the situation.

Another solution is to add a voltage monitor with the appropriate time delay. They have a maintenance trip level that shuts down the driver in case of undervoltage, overvoltage, out-of-phase, and phase-to-phase voltage imbalance. These devices ensure that the VFD is not exposed to unbalanced input power or power-up until the voltage is within tolerance and the appropriate delay time is satisfied.

Barrier transformer

After the proposition - *** expensive solution - is the use of barrier transformers. This ensures complete blocking of grounding and noise-related input power issues that may affect the driver. Do barrier transformers provide better maintenance than line reactors? Be. When to Use a Demand Barrier Transformer? When the installation is located close to the substation, the use of such a transformer is advocated.

Penetrating a driver barrier transformer between a VFD and its power supply offers a variety of advantages. Blocking guarantees that there is no direct electrical connection between the source and the load – but this is true for any transformer other than an autotransformer. What drive barrier transformers have in common is the placement of a ground electrostatic (Faraday) shield between and around the primary and secondary windings. This shielding reduces the capacitive coupling involved in transmitting common-mode voltage interference by a factor of one million. Without this shielding, the capacitor promises high-frequency noise and transient voltage spikes through the transformer.

Common-mode transients are transients that are presented between the ground and the neutral point of the communication system. Although these two parts of a circuit are generally combined at one point, they cannot be considered to have the same potential in the entire power system. Common mode transient disturbances arise from the normal operation of equipment such as switching power supplies, drive operation, arc welders, lightning, and even stepper motors. Some barrier transformers are also capable of blocking the "normal form" transients that occur between the line and the neutral.

Consider a use of a substation capacitor bank in an industrial park during the day-to-day utility power supply that causes transient voltage spikes – amplified by reflections from field capacitors in a nearby plant. Let's say a facility in the park has several small drives rated at 7.5 horsepower. Normal form transients can cause these drives to shut down on their own, which can then lead to costly process downtime. Barrier transformers prevent such abortions.

Use the right power for the inverter – Reynolds

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