Low pressure issues
The VFD reports a low voltage issue 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 advocated for the scale of operation to allow the drive at hand to adhere to quality efficiency and proper motor current. Drives can fall below these tolerances, but the voltage drop can have unpredictable effects on motor current, motor temperature, and overall performance.
More specifically, if the drive's line voltage parameter is set high and 480 VAC is being applied, the driver will experience a low voltage problem 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.
Precautions when switching
While it is accepted practice in many applications to switch VFDs from line power to standby generators, there are some constraints. On top of that, many portable standby generators have a phase-to-phase voltage input voltage swing that is greater than the phase-to-phase voltage tolerance of the driver referral, typically less than 2%. To avoid voltage flapping, VFDs are equipped with surge maintainers in the input rectifier circuitry – but significant input disturbances may require external maintenance (e.g. surge arresters). Large variances cause large ripple across the DC bus capacitors, which can cause capacitors and other power components to deteriorate 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 guide may damage the charging relay circuit, or at least damage the input fuse or trip the circuit breaker.
In the case of current surges caused by moderate voltage in the input power, a 3% line reactor may improve the situation.
Another solution is to add a voltage monitor with the right moment delay. They have a maintenance trip level that encloses the driver in the event 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 moment is met.
Barrier transformer
Finally, the most 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. When to use a demand barrier transformer? When the installation location is close to the substation, the use of this type of transformer is advocated.
Inserting a driver barrier transformer between the VFD and its power supply offers several 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 touched by transmission common-mode voltage disturbances by a factor of one million. Without this shielding, the capacitor allows high-frequency noise and transient voltage spikes to pass 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 assumed 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 operations, arc welders, lightning, and even stepper motors. Some barrier transformers can also prevent the "normal form" transients that occur between the line and the neutral.
Consider an application in which a substation capacitor bank in an industrial park causes transient voltage spikes caused by the daily utility power supply – amplified by reflections from field capacitors in a neighboring plant. Let's say a facility in the park has several small drives rated at 7.5 horsepower. Normal form transients may cause these drives to self-enclose, which can then lead to high-value process downtime. Barrier transformers can avoid this abort.
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