Double Drive System at Main Conveyor Chain Implementation Project

Role and Responsibility

In this project, I am responsible to design and convert the conventional single drive

conveyor chain to double drive conveyor chain system. I do survey and study on the

double drive application that available in market then try run using prototype to find

out the feasibility and reliability of the application.

1. Introduction

The crucial part of rubber glove manufacturing process is the main drive conveyor

chain. This main drive conveyor chain is the “heart” to the whole production machine.

It moves all the formers into all process cycles to produce gloves.

The production lines have now been modified and increased the production output

from 18,000 pcs/hr to 26,000 pcs/hr by running the conveyor chains in faster speed.

However, the conveyor chains and main drive motors are still the same. Hence, the

load factor for main drive motors has increased due to the increasing in motor speed

and former weight.

Currently, one main drive motor is running a 942 meter length conveyor chain; the

total weight of the conveyor chain complete with formers is around 56318kg. In table

1 shows the overall performance for main drive motors in the factory. The load factor

for production line 18 until 25 is almost 80% of the full load.

Line Freq AmpInput

Power

Motor

torque

Output

voltage

Motor

Power

Load

factorW/h

18 48 22.54 12.16 72 396 10 80 706.3

19 48 22.28 14.64 88 399 12.59 91 615.5

20 - -

21 48 14.75 6.84 46 399 6.53 47 700.1

22 - -

23 48 21.35 12.55 80 398 11.55 82 799.9

24 50 19.87 12.38 82 404 13.16 80 644.1

25 53 20.94 14.33 78 409 13.2 92 684.1

1 40 8.08 3.11 35 335 3.08 35 148.4

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2 36 9.13 3.46 42 303 2.95 44 233.3

3 41 7.16 2.72 30 340 2.02 39 401.6

4 36 7.02 2.08 55 249 2.27 43 389.6

5 31 7.29 2.25 53 202

6 35 3.84 1.32 27 238

8 40 20.95 9.54 152 337 8.22 142 1250

14 28.5 15.08 5.43 110 244 4.58 111 618.4

15 28.5 16.39 5.38 81 243 5.52 86 154.6

16 28.5 19.21 6.93 102 243 5.77 92 145.1

17 28.5 15.81 5.77 106 245 5.03 120 488.9

10 10 12.94 1.88 80 98 1.26 88 721.8

11 24 16.3 5.45 119 209 4.34 121 77.25

13 30 11.29 3.57 68 257 3.24 78 684.6

Table 1: Main drive inverter reading for every production line

2. Objective:

Currently main drive motors are having high load factor and high operating ampere

due to main drive motors are running in a very heavy load causing motors are in high

temperature condition. Hence, this project purpose is to:

1. Reduce load factor for main drive motor, by using double main drive motors

to distribute driven load evenly.

2. Synchronize system for both main drive motors, to avoid chain out issue.

3. Concept and Design for Double Drive System

To reduce down the load factor for the main drive motors, the concept is to add in

additional main drive motor to share the load. By running with two units of main

drive motor, theoretically the load will be shared among motors. Hence, the operating

ampere for main drive motor will be lower down and the motor life span can be

extended.

Currently main drive motors are controlled by inverters (variable frequency drive

VFD) where inverters provide frequency to motors to run in different speed based on

the frequency setting. Therefore, the design for double drive system come out with

using one unit of inverter as the supply source to two units of main drive motor. The

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inverter is giving the same frequency signal to both main drive motors. Thus, motors

will run in the same speed. The main purpose of using this design is to reduce the

control system complexity. It can save up the troubleshooting step to align both

motors speed.

3.1 Inverter Sizing Design

With this double drive system design, one additional main drive motor which is same

HP to existing main drive motor will be added into the production line main conveyor

chain. The existing main drive motor is 20HP. Hence, an additional 20HP will be

added.

To run both main drive motors with 20HP respectively, the existing inverter which

provides 15kW output is not sufficient to run two units of 20HP motor

simultaneously. Therefore, inverter must be sized properly based on its connected

motors.

I. The total HP of the connected motors to the inverter is 40HP.

II. Referring to NEC Table 430.250 the full load current for one 20HP main drive

motor is 27Amp.

III. Adding up both main drive motors the total of full load current will draw from

inverter is 54Amp.

IV. The estimate output power from inverter during full load is as below:

Power = (√3 x 415V x 54Amp x 0.8) /1000 = 31kW

Typical induction motor exhibit a full load running power factor of 80

to 90 percent

V. Therefore the most suitable inverter size available in market is 37kW.

Hence, for the main conveyor chain system, the application design is by using one

unit of 37kW inverter to control two units of 20HP motor. The double drive system

design circuit diagram is in Appendix.

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4. Double Drive System Prototype Test Run

A double drive system prototype that mimics the circuit design is constructed and to

test run on the performance. This prototype consists of one unit of 3.7kW inverter

connected with two units of 1HP motor as shown in picture 1. The inverter is tested

with different frequencies to observe the motors’ performance as shown in picture 2.

Pic 1: double drive system prototype with one unit inverter to run two units of motor

Pic 2: The speed of both motors is tested using one conveyor chain in different

frequency values

Initially, both motors are running in low frequency, the conveyor chain is driven

normally. The gap between upper and bottom layer conveyor chain is wide as shown

in picture 3. Both motor amperes are measured using clamp meter and the value is

0.3Amp respectively as shown in picture 4. Adding up both motor amperes should be

0.6 Amp. The inverter ampere reading is 0.8Amp as shown in picture 5. This is due to

the decimal error in clamp meter where causing the ampere between motors and

inverter has slight different.

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Pic 3: both motors are running in low frequency value

Pic 4: both motor amperes are measured using clamp meter

Pic 5: inverter displays the outgoing ampere is 0.8A

When the inverter frequency value has been increased gradually until 50Hz, the

motors start to run in different speed where the gap between upper and bottom layer

conveyor chain become narrow. This means that one unit of motor is running slightly

faster than another causing bottom conveyor chain to be tighten and upper layer

conveyor chain become loosen as shown in picture 6. Hence, this prototype can

conclude that both motors have different motor slip.

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Pic 6: motor on the left hand side is running slower than motor on the right one

5. Design Constraint

Double main drive system will initially run in the same speed for both motor if the

mechanical structure for the machine is well tuned. After running for some time, the

conveyor chain will elongate due to continuous heating and cooling process. Then, the

chain will start jerking; one motor will run slightly faster than other. The mechanical

part should do alignment to balance up the load between motors by changing the

sprocket ratio. However, this condition is continuous, meaning that every time when

chain start jerking a new sprocket has to replace to slow down the faster motor.

To reduce further on the chain elongation issue, the double drive conveyor chain

comes with one tensional load and one spring to absorb the elongation as shown in

picture 7. However, this system will potentially be more worsen. The tensional load

will absorb the chain elongation effect by lowering down the dead load and the spring

will pull back further to tighten back the chain if the chain elongated. But, tensional

load will react faster than spring system to absorb the elongation part. If the tensional

load is after the faster motor (A), then the conveyor chain will become loosen even

faster at the bottom part as shown in picture 7. Spring has little reaction of elongation

on this situation. Then the situation will happen where the conveyor chain on spring

system part become tighten and on the tensional load will become loosen. Hence, the

load in the whole conveyor chain will no longer be even.

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Pic 7: Side effect if one motor run slightly faster than other in spring and tensional

load system

If one main drive motor was burned and repaired by rewinding, the performance of

both motors will not be the same, the rewinding motor will have lower efficiency and

the speed may not be the same anymore. Hence, it may cause load uneven issue too.

6. Future Study

To eliminate the design constraint, the double drive system concept has to be

changed. Instead of providing same speed to both motors, the new design is by using

two units of inverter to run two units of main drive motors.

Both inverters will communicate with each other on the load factor, torque and speed.

To link up both inverters, master-slave design should implement. One inverter will act

as master that providing frequency to main drive motor and provide torque and load

factor data to slave inverter. Slave inverter will base on the signal provided and adjust

the frequency provide to the downstream main drive motor.

Hence, master inverter will make sure its downstream main drive motor running in

constant torque and speed while slave inverter will vary the torque and speed to

support master main drive motor.

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