Preventing VFD faults and failures
Following these best practices can help prevent voltage drops, overload trips, and other common VFD faults and serious failures before they occur.
Variable frequency drive (VFD) technologies support an expansive range of machine tasks and robotics in automated warehouses, logistics, manufacturing, and process industries. Properly sized and configured VFD systems can help optimize performance, save energy, and permanently lower machine and robotic lifecycle costs. Conversely, faults and failures can escalate into costly downtime. Operators must quickly identify and resolve problems. Identifying a fix may be simple or reveal a complex problem, which is why fault and failure prevention is always the best strategy.
Properly derate VFDs
Preventing faults and failures starts by right-sizing the VFD for the machine task. Single-phase input voltage on a VFD with 3-phase input is common in many automation applications. Depending on the horsepower and voltage rating, drives can accept single-phase input voltage without derating the output current. If ratings are exceeded, a larger 3-phase drive is required when using single-phase input voltage. Most 3-phase drives up to 30 hp at 240 V, 60 hp at 480 V, and 60 hp at 590 V input can have single-phase power applied. However, it is important to be aware that single-phase power will cause higher dc bus voltage ripple.
For proper drive sizing, the output current should be derated by 50%. When derating, double the current but not the horsepower rating. For example, a 10 hp, 240 V drive is typically rated at 29 A continuous current, but the derated output current will be half, or 14.5 A, because of the single-phase input to the drive. The input current rating for the drive remains the same. Drive software doesn't recognize input power as single-phase. Therefore, the motor overload parameter must be scaled to limit output current. The motor overload parameter for the drive should be set according to the drive's true output rating, not the derated value. In the aforementioned scenario, assuming the motor's full load amp (FLA) rating is 12 A, the VFD's overload parameter should be set to 12 A divided by 29 A, or 42%, plus additional derating that may be required for the application.
VFDs have an adjustable overload parameter to protect the motor. Drives come standard with electronic thermal overload protection allowing the VFD to deliver 150% of the rated output current for 1 minute and higher current levels for shorter periods. The overload can be adjusted to protect smaller motors. When using a larger VFD on a smaller motor, the output current must never exceed the motor's FLA rating under normal operation. This requires scaling down drive output current capacity by identifying the motor's FLA rating and dividing that value by the drive's output current rating to get the correct percentage for the motor overload parameter. The minimum setting for most VFDs is 25% to 30%. If the motor rating is lower, the drive won't fully protect the motor.
Wiring and filter protection
Additional precautions apply to wiring, isolation, grounding, and shielding. A VFD generates a pulse-width modulated output waveform containing high frequency components including radio frequency interference (RFI) and electromagnetic interference (EMI). Cable lengths exceeding 33 ft can pick up noise, stray capacitive effects, and have resistive voltage drops. It is important to not exceed the maximum recommended cable length between a VFD and a remote analog input speed reference signal. As wiring distance increases, the exposure to higher noise levels, resistive voltage drops, or parasitic capacitance increases. Consult the drive manual to ensure proper wire gauge ratings. A separate 10 V dc power supply may be needed if the voltage drop across the long cable length is significant. This also holds true for a 4-20 mA signal, in which case a shielded cable with a minimum of 300-V jacket insulation should be used.
VFD power output connections carry high levels of high frequency voltage contributing to EMI. Screening (or mesh) of the output power cable is needed at both the VFD and motor ends. A copper braid screen with full 360-deg clamp coverage works to minimize emissions. The motor cable screen must be terminated to the drive heat sink or mounting plate and to the motor frame.
Typically, a line filter to the drive input is unnecessary, except in certain cases, such as to meet CE compliance or when noise from the drive reflects back onto the input power and causes interference. If the motor cable length exceeds 100 ft, an RFI filter should be connected on the VFD input to attenuate high frequency noise (see Figure 1).
Common fault parameters
Knowledge about common fault parameters is key to prevention. Whether a stock VFD in conveyors, fans, and cooling towers, or a specialized unit designed for presses, extruders, roll-forming machines, lathes, and routers, a VFD will generate a low-voltage fault when the voltage drops below set parameters. A VFD may report a low-volts fault when the drive dc link voltage drops below 62% of the nominal level for the high setting (480 V ac) and 50% of nominal for the low setting (400 V ac). This can be stated as:
480 V ac x 0.62 x √2 = 421 V dc
The nominal dc link voltage is:
480 V ac x √2 = 679 V dc
The +10% and -15% voltage tolerance in most manuals is the recommended operating range to allow the drive to maintain premium efficiency and proper motor current. Drives can run below these tolerances, but reduced voltages can have unpredictable effects on motor current, temperature, energy use, and overall performance.
Some automated processes rely on generators to maintain seamless operation. Switching from line power to a backup generator is a common practice. Most portable backup generators have a larger than ideal voltage swing on their phase-to-phase voltage outputs, compared with drives, which generally should have less than 2% imbalance on their input voltage terminals. Larger variances can induce greater ripple on the dc bus capacitors, which can damage the capacitors and other power components.
Protective devices are designed to prevent VFD exposure to unbalanced input power and to allow for power-up only when voltage is within tolerances and proper delay times are met. VFDs come equipped with surge guards in the input rectifier circuit. Additional safeguards against voltage swings, such as surge arrestors and other external protection, can prevent severe input disturbances. During a switch from line power to a standby generator, most drives need a minimum of 2 minutes before reapplying power. Ignoring this guideline can blow the input fuses, trip the breaker, or even damage the charge relay circuit.
A 3% line reactor can improve the situation when input power exhibits moderate voltage spikes. Another option is to add a voltage monitor with a time delay, which can provide protection trip levels to shut down the drive in the event of under-voltage, over-voltage, loss of phase, and voltage imbalance between phases.