Old fixed-grip, AC-powered lifts using banks of resistors to control their speed are noisy, waste energy, have a tendency to overheat and often have to be stopped to load errant skiers. It is often this last trait that makes such lifts candidates for variable frequency drive (VFD) retrofits.
Ski areas that have fit older AC-powered lifts with VFDs report smooth speed changes, the ability to run the lift at constant speeds and better uphill capacities. What makes all this possible is the VFD’s ability to take the existing power supply and turn it into an operator-controlled source of variable voltage.
A VFD—using a properly rated AC motor—works in two stages. In the first stage, the drive’s electronics converts the incoming AC supply (with a fixed frequency of 60 cycles per second) into DC voltage. A second stage then “inverts” the new constant DC voltage into variable voltage with a variable frequency. This is why VFDs are often called inverters. The variable output frequency determines the motor speed. Output voltage is also controlled by the VFD and increased voltage yields increased motor torque.
The Advantages of VFDs
Cost—Most VFD drives don’t contain SCRs (silicon-controlled rectifiers) as do conventional DC drives, which are currently the most popular replacement option. This means AC motors have a simpler design than today’s DC counterparts, which makes them less expensive.
Maintenance—Unlike digital DC motors, AC motors have no brushes and thus, require less maintenance.
Size—Totally-enclosed, fan-cooled (TEFC) AC motors don’t need external blowers for cooling, making them more compact.
Power—AC motors exhibit a more favorable power factor than their DC equivalents. In some cases, the better power factor can result in a more favorable pricing structure from the utility company.
The Disadvantages of VFDs
Voltage Supply—Both AC and DC drives are sensitive to supply voltage changes. If supply voltage dips, neither drive will produce rated power. This shows up as a slower top speed with DC drives and lower torque in AC units.
Noise—VFDs create line noise in power systems. An input line reactor should be installed to dampen or reduce noise that is sent back into the power distribution network. The owner is responsible for conducting any study required to determine if filtering is necessary. If so, it must comply with standards set by the Institute of Electrical and Electronics Engineers (IEEE 519).
Repair Costs—VFDs generally cost more to repair than DC drives and many times are not field repairable. In addition, parts must be purchased directly from the drive supplier, which could cause additional downtime.
Answers to Remaining Questions
These commonly-asked questions and their respective answers should help alleviate remaining confusion:
Q: Can VFDs be used to drive lifts?
A: Yes. However, VFDs purchased from normal supply sources do not meet ANSI B77.1 requirements and should be ordered through a supplier actively involved in the ski industry and then load-tested following installation.
Q: Can a VFD be used with an existing AC motor on a ski lift?
A: This is not recommended. Wound rotor motors were not designed for the high voltage spikes that VFDs can generate causing the motor’s winding insulation to break down. Also, in normal applications, an “across-the-line” wound rotor motor can develop much higher torque at low speed than when using a VFD. As a result, a repowered motor may not be able to start the lift.
Q: What size motor is required to replace the existing one?
A: It is best to contact a lift manufacturer to calculate the required torque.
Q: Can we use the existing wiring if the motor size doesn’t change?
A: The insulation rating of existing wiring may not be adequate to handle voltage spikes created by the VFD and would most likely fail Christmas Day.
Q: What type AC motor should I use?
A: Select a TEFC, inverter-duty motor with at least a 4:1 speed range. Speed range determines how slow the motor can operate at full load and within temperature limits. A typical range would be 1800 to 450 rpm. If rope speed is still too high, ask for a better speed range.
Q: Are bearing types important?
A: Yes. If the motor output shaft is fitted for a belt drive (overhung load), then roller bearings are recommended. On direct-drive motors, standard ball bearings should be sufficient.
Q: Are heat sensors of any value?
A: Yes. For summer use, heat detecting thermistors should be mandatory. Also, in many winter environments, closed motor rooms can experience very high temperatures and motor protection is both advisable and inexpensive.
Q: Can VFD motors control overhauling (downhill) loads?
A: No. Most VFDs don’t contain any type of dynamic braking. Although special “regenerative” VFDs can be built, the cost would be prohibitive. DC drives are better suited and can supply braking with a 100 percent duty cycle.
Q: Can a Soft-Start™ be replaced with a VFD when using smaller horsepower motors?
A: Yes. But, as mentioned above, there could be a loss in starting torque.
Q: Are there any horsepower limitations on ski lift VFDs?
A: Yes. There is a practical limit of about 150 horsepower. Above this, standard DC drives are more cost effective.
Prior to investing in any new drive, VFD or DC/SCR, contact a drive supplier early on in the process to determine the correct application for your lift.
