VARIABLE SPEED (VFD) MECHANICAL & HYDRAULIC VARIABLE SPEED
B. Apron Feeders, Belt Feeders and Conveyors are constant torque applications.
Apron & Belt Feeders are normally used in applications where there is vertical load applied such as under a crusher, surge bin or reclaim stockpile. In order to start the flow of material downstream of the feeder (with a full load), the ore must pass under a shear bar to reduce the height of material to the designed “bed depth”. The bed depth and inside width of the skirtboards determines the x-section area of material moved. The speed of the pans/belt will determine the volume delivered—factoring in the bulk density of the ore will give the tonnage being delivered.
A variable speed drive is normally used to provide a variable delivery based on the process requirements. Under normal operation, the feeder starts slowly and delivers the required starting torque to shear the ore at the shear bar to control the bed depth. IEM designs equipment for a minimum 200% of MFLT (Motor full load running torque) at starting. When the speed has reached desired setting, the power draw will be what the load demand is (up to the set maximum running current) —running torque remains constant unless more load (higher bed depth, different width or increased bulk density) is introduced than what the design called for.
Feeder speed is normally controlled remotely during operation with the option of local control for maintenance. The application may require speed encoders, zero speed switches for apron feeders as well as belt feeders. Belt feeders may also require belt alignment switches and belt rip detectors . These instrumentation items are normally connected to the control system for the feeder to give the operator status details of the feeder in operation.
B.SPEED RANGE/DRIVE DESCRIPTIONS:
Apron feeders are typically designed to run continuously up to 50 FPM (0.25 m/s) whereas belt feeders would be up to 100 FPM (0.50 m/s). Under normal operation, the speed range used is normally a 3 or 5:1 turndown. For 1800RPM synchronous speed, motor speed would be down to 600 RPM for 3:1 and 360 RPM for 5:1. These ranges would apply without any excessive heat generation. For VFD controlled mechanical drives, VFD and motor are recommended to be supplied by the same manufacturer—items are totally compatible and designed to operate together. Some manufacturers offer systems that have a 10:1 turndown—provision is made to dissipate the heat generated continuously at the lower speeds . All VFD’s are infinitely variable and can work effectively at above speeds but do not operate as constant torque devices above synchronous speed. Starting at essentially zero speed for 20 seconds is not an issue providing the VFD is able to produce the required amperage to provide necessary amperage to allow the motor to produce 200% of MFLT.
VFD can be operated above synchronous speed by increasing the frequency, i.e running at 65 Hz instead of 60Hz will increase the speed from 1800 RPM to 1950 RPM. In this case the torque is no longer constant and does decrease. However, this increase may be acceptable based on the application.
VFD controlled mechanical drives have overall efficiencies in the order of 90 to 95%. The VFD drive system generally includes: Electric motor , flexible input coupling, right angle bevel planetary speed reducer with hollow shaft mounting (approx 6-9” shaft engagement) c/w shrink disc mount with torque arm. Reducers are available with oil heaters or synthetic oil for cold weather operation. Fan cooling is avaiiable for high temperature operation.
For Hydraulic Drives (Typically HAGGLUNDS)
Hagglunds uses a hollow shaft mounted hydraulic motor c/w shrink disc. Depending on the size, the hydraulic power unit can be mounted on the torque arm (TADS) as a Torque Arm Drive System or as a separate hydraulic motor mounted on feeder and connected (using hoses and/or hydraulic tubing) to a separate hydraulic power unit usually in a cabinet. A “SPIDER CONTROL” is supplied with the unit to allow settings to be made and to interface with plant control system. The Hagglunds motor has excellent characteristics at low speed operation, i.e. speed ripple is minimal.
Although Hagglunds MB1600 Hydraulic Motor Data sheets show overall motor efficiency, at selected temperature, to be between 82 to 97% depending on speed and operating pressure. Overall Hydraulic Drive Systems typically have an overall efficiency of 80 to 85% and normally require cooling of the hydraulic fluid if the ambient temp is over 15-20 Deg C. The overall efficiency is relatively low due to pressure drops through components including piping, hoses, fittings, filters, coolers, variable delivery closed loop pumps as well as mechanical efficiency in rotating components such as pumps. Because of the lower overall efficiency of the hydraulics system, compared to VFD/Mechanical drive, a larger HP electric motor is generally required for hydraulics. If VFD drive requires 150HP, hydraulics would typically use 200 t0 250 HP motor for the power unit. The difference in HP requirements has an impact on power consumption. For a VFD 372 Kw, power usage would be 8,928 KWH/day, for the same feeder with Hydraulic 522 KWH, power usage would be 12,528 KWH/day.
Typically, the system includes: hydraulic motor, speed encoder and torque arm. The hydraulic power unit includes; reservoir, air breather, inlet filter, return line filter, visual and electrical indicators for filters, electrically controlled pump(s),electric drive motor(s), oil level switch, temperature switch, pressure switch, pressure gauges (Qty 4), air/oil cooler (may need oil/water cooler for high ambient temp). For cold weather operation, oil immersion heater is used in the power unit. Cold weather packages are offered which includes heat tracing of plumbing and insulation of suction lines. In extreme cold weather (-15 Deg C and lower), it is common to incorpprate a heating and flushing system to circulate warm oil before system is started up.
C.VFD / HYDRAULIC DRIVE COMPARISON
|VFD CONTROLLED SPEED REDUCER DRIVE
|2.Electric Motor (Directly connected to reducer input shaft.
||2. Electric motor (s) in power unit to drive hydraulic pump (s)
3.Hydraulic Power Unit c/w
- • Electric Motor (s)
- • Pump (s)
- • Reservoir c/w hydraulic oil
- • Hydraulic Filters
- • Instrumentation (approx 6 items)
- • Cabinet (Remote)
- • Air/Oil Cooler with motor driving fan
|3. Plumbing/Piping----NOT REQUIRED
||3. Piping and hoses required to connect Hydraulic Motor pressure, drain and flushing circuits.
|4. Normal maintenance---check oil level every month (do not have to shut down equipment) and flexible coupling every 3 months.
||4. Replace filters (3) every month or less frequently depending on environment. Must shut equipment down—estimate 1 to 3 hrs to change filters
|5. Maintenance expertise required:
Millwright for mechanical and electrical/instrumentation technician for electric motor and VFD control.
Normally plant has qualified staff.
|5. Maintenance expertise required:
Hydraulic technician experienced with servo controlled pumps operating in a closed loop mode. Control system requires electrical/instrumentation technician.
Oftentimes Hagglunds factory technicians are called in to troubleshoot/repair systems..
2 Year Operating Spares---Coupling Insert
Capital Spares---Speed Reducer, Electric Motor
Commissioning-----Filter Elements, Air Filter elements.
2 Year Operating Spares----filter elements, air breather elements, Spider Control Card, Pressure gauges, oil level gauge, charge pressure switch, temperature sensor, pressure transducer, temperature transmitter, level transmitter, suction level switch, filter visual and electrical indicator, speed encoder, tank bladder, servo pressure gauge, solenoids for servo valve control.
Capital spares:---Hydraulic pump(s), Air/oil cooler, oil heater, filter housings, hydraulic motor(s), charge pump, electric motor(s).