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.