b17-10 lakay the practical effects of variable speed drive use
TRANSCRIPT
Lorinda Lakay DoorsamyWesley Doorsamy
THE PRACTICAL EFFECTS OF VARIABLE SPEED DRIVE USE ON THE CONVEYOR REDUCER
Introduction
• The use of Variable Speed Drives or VSDs are becoming commonplace on conveyor drives.•VSDs provide increased operating
freedom, through speed control, and have inbuilt soft-start capabilities• This paper explores the effects of the VSD
on the conveyor geared reducer.
Background- The Conveyor Drive
Induction motor controlled by a VSD
Flexible Input coupling
Geared Reducer
Background- The VSD
• The VSD commonly used on conveyors are Variable Voltage Variable Frequency convertors.• It controls the speed
through frequency variation.•While, the torque is
controlled through voltage variation.
Background- The VSD
•A motor controlled by a VSD is de-rated for maximum torque operation below 40Hz operation.•A torque limitation exists, which limits
VSD operation to a minimum of 5Hz for continuous operation.
Background- The VSD
• The VSD operation is categorised into four quadrants of operations.
Reverse driving- braking
Reverse driving- accelerating
Forward driving- braking
Forward driving- accelerating
Dynamic Braking
Background- The VSD
• The VSD can gradually increase voltage and frequency, ramping up the motor.• To decrease stopping times, dynamic
braking can be used to maintain a smooth deceleration.
Advantageous of VSD use on Conveyor Drives
• The VSD can limit the peak torque during start-up, limiting the mean peak stress experienced in the reducer• The AGMA relationship for stress:
shows that for a 50% decrease in peak torque, there is a 50% decrease in bending stress and a 70% decrease in the contact stress.
Reduced Mechanical Wear
Advantageous of VSD use on Conveyor Drives
• The reduction in stress translates directly into improvements in life.
Reduced Mechanical Wear
Advantageous of VSD use on Conveyor Drives
• The fault ride-through capability is the ability of the VSD to free-wheel the motor in the event of an electrical disturbance.•Once the event passes, the VSD will ramp
the motor up to full operating conditions.• This avoids the harsh effects of DOL start-
ups after electrical trips.
Fault ride-through capabilities
Disadvantageous of VSD use on Conveyor Drives
•While VSDs have many advantages, this paper is aimed to encourage thinking about the disadvantages so that they can be mitigated.
Disadvantageous of VSD use on Conveyor Drives
•Rotating equipment are often subject to both mechanical and electrical disturbances, which in this case is often seen as torque oscillations.• Shaft failures are induced
by the frequency and not the magnitude of these oscillations.
Induced shaft failure
Disadvantageous of VSD use on Conveyor Drives
• Rotating equipment are often subject to both mechanical and electrical disturbances, which in this case is often seen as torque oscillations.• Shaft failures are induced by the
frequency and not the magnitude of these oscillations.• EVEN THE SMALLEST
OSCILLATIONS, IF CLOSE TO THE SHAFT NATURAL FREQUENCY, ARE DANGEROUS!
Induced shaft failure
Disadvantageous of VSD use on Conveyor Drives
•Bearing failure due to VSD use occurs as a result of induced voltages transmitted from the motor shaft through the input coupling.• These voltages break down
the oil film within the bearing cause pitting, fluting and ultimately failure.
Bearing failure
Disadvantageous of VSD use on Conveyor Drives
• To prevent the voltages from being transmitted to the bearings, it is recommended that an insulating coupling be used as a high-speed coupling.• Insulating bearings include
spider type or pin and bush type couplings.
Bearing failure
Disadvantageous of VSD use on Conveyor Drives
•Conveyor geared reducers are generally splash lubricated.• Splash lubrication is recommended for
tangential gear speeds of 4-15m/s.• The lubrication of the gear unit must be
considered for all operating speeds.
Lubrication failure
Disadvantageous of VSD use on Conveyor Drives
• If the VSD is used for low speed operation, generally input speeds below 800rpm, the use of a shaft-driven oil pump is recommended.• The use of troughs and oil ports must be
investigated to adequate feed oil supplied to the bearings.
Lubrication failure
Pump size Oil Flow [l/min]0,37kW 9-140,55kW 18-22
Disadvantageous of VSD use on Conveyor Drives
•Most conveyor reducers are cooled through convection and radiation.• The convection in generated through a
input shaft driven fan.•Air flow is based on input shaft speed.•At slow speed VSD operation, this speed
may be inadequate to cool the unit leading to over heating.
Thermal failure
Disadvantageous of VSD use on Conveyor Drives
• Although the VSD does not supply maximum load at slow speed operation, the heat generated is usually higher than that which the unit can dissipate through radiation alone.• The AGMA defined heat
transfer coefficient of a reducer is determined relative to airspeed over the unit, which is proportional to the motor speed.
Thermal failureAir speed over unit [m/s]
Heat transfer coefficient [kW/(m2°C)]
2,5 0,0155,0 0,02410,0 0,04215,0 0,058
Disadvantageous of VSD use on Conveyor Drives
• It is important that gear units operate at moderate temperatures.• The higher the operating temperature, the
shorter the life of the lubricating oil.• The optimal operating temperature range
is 60-80°C.
Thermal failure
Case Study
• Lets consider a case study of a typical conveyor reducer.
Nominal motor power 110kW @ 1480rpmAbsorbed power 85%Required ratio 12:1 ± 5%VSD operation 100% speed 100% load
– 20%75% speed 100% load - 20%50% speed 100% load - 20%25% speed empty- 15%
Mechanical service factor
1,5
Thermal service factor 1,0 @ 40°C
Case Study
•Unit selected on nominal operating parameters:
Unit rating 168kW @ 1450rpmService factor 1,53Unit ratio 11,598:1 (3,35%
variation)Unit thermal rating 84kW @ 40°CInput bevel ratio 2,05:1Second reduction gears 19 tooth pinion
42 tooth wheel8 module11°Helix angle
Case Study• Looking at the thermal rating of the unit for
all operating conditions:
• It is recommended that either the unit be upsized, or a secondary cooling system is used.
Percentage of Motor nominal speed
Thermal rating of the Unit
[kW]
Absorbed Power of
Motor [kW]
Thermal
Rating
100 84,0 76,5 75 60,8 56,2 50 35,2 37,5 25 17,9 18,8
Case Study• If lubrication is considered:
Case Study•Considering the VSD operational conditions:
•Hence a shaft driven pump is required for VSD operation.• This case study shows that thermal rating and
lubrication must be investigated for VSD operation.
Percentage motor
nominal speed
Wheel tangential
speed [m/s]
Adequate Splash
lubrication
100 5,85 75 4,39 50 2,93 25 1,46
Conclusion
•While VSD use can be beneficial and it can also be detrimental is not properly investigated.• The correct use of a VS can improve the
life of the conveyor reducer.•While incorrect use can drastically reducer
reliability of the gear unit.• The negative effects of VSD on the gear
unit can, however, be mitigated as shown.
Conclusion
• It is up to the conveyor manufacturer and end user to specify all operating points when requesting a gear unit.• It is the responsibility of the gear
manufacturers to ensure compliant operation at all specified operating points.