RPE Services is the Midwest Sales Representative for Emerson Control Techniques offering AC and DC variable speed drives for ski lift and snowmaking applications including the CTSKISYSTEMS® drive monitoring and safety testing hardware / software package.

We specialize in retrofitting Sabina, Fincor and other older analog DC drives. Analog drives progressively deteriorate with time and usage increasing your lift downtime and more importantly, exposing your customers to malfunctions which could affect their safety. Our retrofits utilize either a Unidrive SP module for AC systems or a Mentor MP module for DC systems and can be engineered to mount directly into your existing enclosure or provided as a completely new system in a NEMA 12 enclosure.

AC Variable Frequency Drives are the perfect retrofit for your single speed / fluid coupling or drum controller installations. VFD’s offer greater safety, reliability and speed control flexibility than these older systems and are more economical to operate and maintain. 

We offer a complete line of Ski Lift Duty motors in both AC and DC versions as well as vertical pump motors at very competitive prices.  RPE Services is authorized by Emerson Control Techniques to install and service their full line of AC and DC drive and control products. 

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Retrofitting your existing wound rotor lift drive with a Variable Frequency Drive can result in significant energy savings for applications with frequent operation below full speed.  As an example we’ll look at a typical 460V, 125A, 100HP system.  A few assumptions will simplify the calculations:

 

            Existing motor power factor:            0.85

            Existing motor efficiency:                88%                                        

            Y connected stator winding

            Load at 75% capacity ( 75% full load current )

            Three phase input power = SQRT(3) x VLL x IL x Load% x Power Factor

                                                = (1.732) x 460 x 125 x (0.75) x (0.85)

                                                =  63.5 KW @ full speed 

 

For the wound rotor system, input power is essentially constant for any speed since the current demand remains unchanged. The shaft power required for the constant torque load ( indicative of ski lifts, conveyors etc. ) decreases with speed. The difference in P(in) vs. P(out) is energy lost though heat in the rotor resistor grid.  Figure A illustrates the increase in wasted energy with decrease in speed – anytime you start, stop or slow down your lift.

                                          FIGURE A


For the Variable Frequency Drive system, the output power again decreases with speed due to the constant torque nature of the load.  However, due to the reduced applied stator voltage the input power demand decreases with a decrease in speed. The difference in P(in) vs. P(out) is relatively constant throughout the speed range as illustrated in Figure B.

                                          FIGURE B


For lifts which require frequent stop / start cycles or periods of reduced speed operation, VFD’s can provide substantial energy savings as the graph in Figure C illustrates. Coupled with their inherent capacity to provide better speed regulation and built in ANSI code safety functions, they are the best choice for retrofitting existing wound rotor or single speed AC lift drives.  In many cases, the existing wound rotor motor can be modified and used in the new system.  The addition of a high efficiency inverter motor will extend the energy savings to include application at full speed.  Typical efficiency for a complete system with new motor is 93 to 95% vs. 85 to 88% for existing wound rotor systems ( at full speed ).

                                          FIGURE C

Give us a call and let us help you calculate the payback time on your investment in a VFD system.