VFD Drive Considerations
Voltage reflection, which can destroy a motor, may occur when the motor cable is longer than or equal to the critical length; lead lengths as short as 25 ft are sufficient for such an event (Figure). Thus, voltage reflection is a function of the lead length of the power cable between the drive and motor.
We can predict voltage reflection using the following equation:
Eq. 1: Lcritical=Vcable × tr
where, Lcritical is the critical cable length in feet, Vcable is the pulse speed from the drive to the motor in feet per microsecond (ft/µsec), and tr is the rise time of the output pulses from the VFD under consideration in microseconds (µsec). Vcable is often estimated as 500 ft/µsec. The Vcable, or “propagation factor,” is the speed at which the pulse travels from the drive to the motor in ft/µsec. An independent study of 17 different drive manufacturers suggests rise time of the reflected pulse may be as short as 0.117 µsec. Plugging these numbers (0.117 µsec and 500 ft/sec) into Equation 1 gives 29.25 ft as the critical distance for voltage reflection. In other words, motor cable lengths must not exceed 29 ft in most installations if VFDs are located in the MCCs. You can alleviate this situation with several solutions like using output dv/dt filters or a definite-purpose inverter-fed motor. If 230V power is available, you can also use a 230V VFD.
The first two solutions add cost and complexity. For example, dv/dt filters are large and hard to install in MCCs. How would wiring a 230/460V motor for 230V solve anything? If voltage reflection does occur, the voltage peak will be less than 1,000V, and the motor insulation guarantee level will be 1,000V. On 460V drives, 650V (DC bus voltage)×2=1,300V. On 230V drives, 325 (DC bus voltage)×2=650V — well below the insulation rating of 1,000V. You can avoid all of these drawbacks by putting VFDs close to their motors, not in MCCs.
Thermal considerations. Putting a VFD in an MCC requires a special enclosure. Any savings you might expect from reduced wiring costs will disappear in the face of special engineering charges and cooling requirements. Solid-state switching devices typically generate losses in the range of 4% to 6%, requiring heat dissipation within the MCC.
Calculate the VFD heat loss using the following two equations:
Eq. 2: Heat loss=hp×746W/hp×[(1-VFD efficiency)÷(motor efficiency)]
Eq. 3: BTU load=heat loss×3.41
For example, a 15-hp drive with an efficiency of 96% and connected to a 90% efficient motor will yield these results:
Heat loss=15 hp×746W/hp×[(1-0.96)÷0.9]×3.41
Heat loss=11,190W×[(.04)÷0.9]
Heat loss=497.3W lost
BTU load=497.3W×3.41
BTU load=1,696 BTUs/hr
Since VFD losses are nearly proportional to horsepower, you can estimate losses at 33.3W or 113 BTUs/hr per horsepower. For example, the losses on a 30-hp drive would be about 1,000W. This is like having a 1,000W heater in the enclosure. So the challenge is to get the heat out of the MCC bucket without compromising the VFD heat sink design or restricting VFD cooling air.
What about front ventilation to prevent a heat-stacking effect from multiple VFDs in a common vertical enclosure? This is easy to do in a NEMA 1 enclosure, but not so easy in a NEMA 12 enclosure. If the heat sinks are mounted on the front door of the enclosure, load conductors must route to the door. Prudent control panel design limits the number of conductors traveling across a hinge; an unwritten rule instructs you to never run power conductors across a hinge. You can flip the drive over to shove the heat sink fins through the door's gasketed opening, but this solution puts the drive in an awkward position for service technicians.
Many specifications call for input line reactors and/or output filters, which are heat-producing devices that require large mounting areas. Most VFD manufacturers have pre-engineered designs for placing filters inside the drive's enclosure. Twelve-pulse drives, often a means for eliminating harmonics, don't fit well into MCC enclosures because their large phase-shifting transformers produce too much heat. It's much easier to retrofit harmonic mitigation devices to freestanding or wall-mounted VFDs than it is to mount and connect these devices to drives inside MCCs.