Optimizing Electrical Motor Control for a Wood Grinder Application Using

Reduced Voltage Soft Starters

Dacian Zainea, Radu Mihai Campeanu Faculty of Electrical Engineering and Computer Science

Transilvania University of Brasov dacian.zainea@unitbv.ro, radu.campeanu@unitbv.ro

Abstract - Most common used method of starting large medium voltage induction motors is Reduced Voltage Soft Starts (RVSS). The paper will present an application example and will describe some of the challenges encountered when controlling electrical motor speed for maximum energy savings. For better understanding the challenges that practitioners face on site, a real life application was analyzed. A waste management facility integrated in their recycling flow a wood grinder controlled by a RVSS. Two electrical motors drive the wood grinder. Depending on the flow of material the grinder is driven by one 250 HP electrical motor or by two same size motors working together.

I. INTRODUCTION

Today’s technology is evolving at a very fast and steady pace. Electrical motor control is a very good example for that matter. Today we have available a large array of technologies used for electrical motor control.

Variable frequency drives (rated as best solutions on the market but very expensive), inverters, drives and variable speed drives are all commonly used in different industrial areas as a way to vary the speed or torque delivered by a motor.

Even though most applications today use permanent magnet AC (PM), Switched Reluctance (SR) or induction motors this paper will focus on other concepts and will ignore these technology differences agreeing that each has its benefits and strengths.

The need for electrical motors controls surfaced the moment the electrical motors were first invented. It is generally agreed that different technologies used for manufacturing motors have an impact on the behavior of the motor itself.

The manufacturing technology of an electrical motor will be ignored and the paper will focus only on two technologies that can be used to control the electrical motor.

In most cases, electrical motors will need large amounts of energy when accelerating to full speed. Variable Frequency Drives (also called VFDs) and Reduced Voltage Soft Starts (also known as RVSSs or just Soft Starts) are both used to regulate the flow of current and voltage to an electric motor.

Applications can use both torque and current limit, thus protecting valuable equipment and preserving the life of the

motor by reducing heating of the motor caused by frequent starts and stops.

A VFD (also referred to as adjustable frequency drives or AFD) is a device that regulates the voltage and frequency in order to control the speed of motor. By keeping V/F constant, the motor speed can be controlled from stop to full power keeping full torque constant.

Functioning of VFDs require regulating the frequency of the AC current going into the motor to adjust its speed and torque. The VFD has control over this frequency at all times, and it can be used to initiate a soft start of soft stop to reduce motor wear and to ease mechanical stress over time.

The most suitable applications for using a VFD are applications where complete speed control is required, where energy savings is a goal or where custom control is needed.

For the application described in this paper a VFD was considered expensive by the beneficiary and so the solution chosen was controlling the grinder application using a soft starter.

What was considered for this application regarding the differences between VFD’s and RVSS is also generally true. Better performances come at a cost and in the end it is all about the type of application and industry where is used.

II. WHAT IS A RVSS?

Soft starters are AC motor controllers that ramps the motor up to full speed slowly and gently with less starting current over a longer (although limited) time, but it always results in full speed operation of the motor.

A RVSS is a device where the user controls the voltage applied to the motor, and thus reduces the starting current of the motor to optimum level. Not only mechanical stress on motor /load is reduced but also the voltage drop in system from which the subject motor draws its power.

The system voltage drop, otherwise due to heavy inrush of current drawn by motor affects the starting and running performance of other motors working near main motor on the same system.

RVSS can provide a soft ramp up to full speed. They are used only at startup, but if equipped can be used for stop also. A gradual start is produced by ramping up the initial voltage to the motor. RVSS are most commonly referred to as soft starters.

426978-1-4799-5183-3/14/$31.00 ' 2014 IEEE

Most soft starters use a series of thyristors or silicon controlled rectifiers (SCRs) to reduce the voltage. In the normal OFF state, the SCRs restrict current, but in the normal ON state, the SCRs allow current.

The SCRs are engaged during ramp up, and bypass contactors are pulled in after maximum speed is achieved. This helps to significantly reduce motor heating.

III. WHERE IS A RVSS BEST SUITABLE FOR USE?

Soft starters work well with motors that draw large currents and that are connected to extensive mechanical systems (belts, conveyors, gears, engines, etc.) A RVSS is used in large systems with many connected mechanical components.

Soft starters are best appropriate to be used in applications where speed and torque control are required only during startup or stop cycle if equipped with soft stop. If a mechanical system requires a gentle start to relieve torque spikes and tension associated with normal startup such as conveyors, belt driven systems or gears then soft starts are a good option.

Soft starters also work well with pumps used to eliminate pressure surges caused in piping systems by fluids that can change often and rapid the direction of flow. If an application requires a large motor then soft starters can be used to reduce associated large inrush currents.

Soft starters offer some benefits because they are often the more economical choice for applications that require speed and torque control only during motor startup.

Additionally, they are often the ideal solution for applications where space is a concern, as they usually take up less space than variable frequency drives.

IV. WOOD GRINDER CASE STUDY

To better demonstrate and compare the capabilities of RVSSs measurements were conducted in a real life scenario for a usual industrial application consisting wood grinder installed at a waste management / material recycling company two electrical motors were used.

The two motors are rated as 250 HP each. The total horsepower of the wood grinder is 500 HP. The two electrical motors were 3-phase induction motors.

According to the performance data sheet supplied by the manufacturer the motor has the following specifications and some of the typical performances:

• Output: 250 HP / 187 KW • Number of poles: 4 • Rated Voltage: 460V • Time rating: continuous • Service factor: 1.15 • Full load RPM: 1775 • Maximum power factor correction 41 KVAR • No load current: 55 A • Full load current: 300 A • Locked rotor current: 1825A

A real picture of the equipment can be observed in the figure one below.

Fig. 1. Wood grinder application – real picture For this particular application, the most relevant parameter

for the customer is current. All the measurements and analysis were performed having this parameter in consideration.

In order to highlight RVSS properties, theoretical across the line start-up were is shown for comparison.

It is easy to spot the differences in behavior regarding current drawn for each part of the cycle when analyzing the measurements screenshots displayed in the following pages.

Each solution allows different parameters to be adjusted in order to make functioning as efficient as possible.

Start, running and stop cycles were studied and measurements were performed accordingly and after an analysis the results were gathered as case study for the wood grinder.

In order to observe the differences, measurements were performed on the installation with only one motor connected to the RVSS and after with both motors connected to the RVSS.

In general, a full voltage start is used in applications requiring across-the-line starting as displayed in figure two. Full inrush current and locked-rotor torque are realized.

This method can take a short time, even ¼ second and was used for comparison.

Overload protection must be used to protect the motor based on the actual nameplate amperes of the motor.

Fig. 2. Current drawn when starting a motor across the line

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For the application, motor control was pABB PSBT 720-600 RVSS, seen in connected to each electrical motor usindepicted in figure four.

Fig. 3. PSBT 720-600 RVSS According to the theoretical principle sh

when starting one motor across the line,even as much as 500% its nominal current.

After a very short time of around 0.25 sdraws its nominal current of around 50A aon no load until after 0.5 seconds when loto draw more current until reaching full loa

.

Fig. 4. Electrical schematic for one motor controlle After 5 seconds, the load decreases and

not draw that much current. At this point,the grinder working at nominal speed.

The current drawn by the motor directlwas logged as seen in the pictures below twith CNX i3000 iFlex AC Wireless Curren

For the measurements a Fluke CNX 300and data was recorded using the CNX approvided by Fluke.

Measurements were performed directly grinder is installed as displayed in figure fiv

All results gathered while measuring u

performed with an figure three, and ng the schematic

hown in figure two the motor draws

seconds, the motor after which it runs oad presence starts ad of around 280A

ed by the RVSS

d the grinder does , the motor makes

ly from the RVSS that show currents

nt Module. 00 measure system pplication software

on site where the ve. sing the assembly

were logged using the softwarmeasuring kit.

Fig. 5. Current measurements assemb

V. EXPERIMENTAL ME

Experimental measurementsgrinder in two different modgrinder is designed and built in driven by only one of the twpower is needed for large quaelectric motors running making

Experimental measurements wthe grinder,

The current drawn for the sofdepending on the case, were mfunctioning, including start, different load going through the

The first measurements wecycle was initiated for the grindfirst and both electrical motors f

The starting current measurethe grinder being driven by oresults measured are shown in fi

Fig. 6. Current drawn when soft start When starting one 250 H

configured to initiate the proc

re provided by Fluke for this

bly

EASUREMENTS RESULTS

s were performed with the des of operation. The wood

such a manner that it can run wo 250HP motors or if more antities of material with both the grinder a 500HP one. were performed when starting

ft start by the motor or motors, measured in different cycles of

stop and functioning under e grinder. ere performed when starting der using one electrical motor for the second start cycle. ement is performed first with nly one 250 HP motor. The

figure six below.

ting the grinder with one motor

HP motor the RVSS was cess of drawing current and

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energizing the motor after around 35 secclimbs up to 10 times the no load current 20 times the no load current.

After around 200 seconds the current dris a little over the no load current rate from

The next step was to measure startinggrinder being driven by both 250 HP mmeasured are depicted in figure seven.

It can be easily seen when compared starting method as seen in picture two, usinmore convenient for the motors that significant shock in order to break the inerspike of time. Even though the current migdrawn over a much longer period.

Fig. 7. Current drawn when soft starting the grinder When running the grinder driven by the t

the RVSS used was configured to initiadrawing current and energizing the motoseconds. Figure seven shows the current times the no load current and peaking even load current.

After around 100 seconds, the current dris a little over the no load current rate fromconsidering that the two motors are contrRVSS.

Once the starting cycle for the wood grina new measurement was performed.

Fig. 8. No load current drawn by the grinder runnin

conds. The current and peaks even to

rawn by the motor the manufacturer.

g current with the motors. The results

to across the line ng a RVSS is much

do not suffer a rtia in a very short ght be higher, it is

r with two motors

two 250 HP motor ate the process of or after around 30

climbing up to 3 to 10 times the no

rawn by the motor m the manufacturer rolled by the same

nder was complete,

ng with one motor

Some spikes can be seen in tthere is no material going thrcaused by the mechanical time reduced and even eliminated wi

In figure nine current was medriven only by one motor and going through the machine.

Fig. 9. Grinder with one 250 HP mot Figure ten shows how the m

when a stopping cycle is initiate

Fig. 10. Current diagram for grinder For this application the RVS

and torque during stop cycle sstop. A more expensive RVSS that if equipped with soft stop.

Figure eleven shows a measuillustrate how the grinder wouboth 250 HP motors meaning 50

It can be observed that thdepending on the load with spikof wood are grinded.

The differences from the grinrunning on two electric motors ameasurements are performed du

the measurement even though rough the grinder. They are constant. The spikes could be th low pass filter. easured while the grinder was

wood of different sizes was

tor running with load

motor thus the grinder behaves ed regarding current drawn.

starting the stopping cycle

SS does not control the speed so the motors are coasting to

version would be able to do

urement performed in order to uld perform when running on 00 HP. he current drawn fluctuates

kes showing when large pieces

nder running on one motor and are very easy to spot when the uring running on load.

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Fig. 11. Current diagrams for grinder with two 2

with different size loads Figure twelve shows how the grinder

stopping cycle is initiated.

Fig. 12. Grinder with two 250 HP motors stopping Same observation as when stopping one

The RVSS does not control the speed andstop cycle

The differences between the two operatinand stopping, are more that obvious but thresult from these measurements is that one very successfully the same grinder in two conly one 250 HP motor or if the grinder wtwo identical motors.

50 HP motors running

behaves when a

motor is available. d torque during the

ng modes, running he most important RVSS can control

configurations with would be driven by

VI. RVSS - ADVANTA

GRINDING

When choosing a RVSS therequirements, cost for initial scycle of the whole equipmeconsideration.

Soft starters are smaller and controlling large horsepower mmore space and are usually mustarters. It is important to note two to three times more thanconstant acceleration and torquethe application requires current soft starter may be a better solThis was the case in the applalways be the case when tamachines. A RVSS provide redreduced energy consumption electrical motors and other mecwhen looking for value, usersinitial cost and determine if functionality of a RVSS is enouproject.

In the study case presentedperform in order for the customcurrent drawn in different rugrinder. As it was expected, thcontrolling electrical motors currents is perfectly verified. Tfull load currents were of intebacked up by measurements thacknowledge that the whole anticipated when the project wa

ACKNOWLE

The authors would like to Industries, Inc. for the opportuwith wood grinding equipment Company, Inc. for the help acommissioning of the waste masite.

REFERE

[1] Marian P. Kazmierkowski, RaElectronics” Academic Press, pp.

[2] Mengze Yu; Cuihua Tian;, “A Based on Magnetically ControlleApplications, 2006 1ST IEEE Con

[3] Lukitch, W.J.,”Soft start vs AC Textile, Fiber and Film Industry T

[4] Choosing between Soft Starters anhttp://www.yaskawa.com/site/dmd8GAL4E/$file/PR.AC.01.pdf

[6] Choosing between Soft Starters an [7] Variable Frequency Drive, hvacpa

http://www.eaton.com/ecm/idcplg [8] ABB PST Soft Starter Extreme Du

http://www.clrwtr.com/PDF/ABBSoftstarters.pdf

AGES WHEN USED IN WOOD

G APPLICATIONS

e type of application, system startup and cost over the life nt are factors to take into

less expensive than VFDs for motors. Large VFDs take up uch more expensive than soft that a VFD can initially cost

n a soft starter. Therefore, if e control is not necessary, and limiting only during startup, a lution from a cost standpoint. lication presented and it will alking about wood grinding duced voltage motor starting, and reduced stress on the

chanical equipment. However, s may need to look beyond the added performance and

ugh for the requirements of the

d all the measurements were mer to see the real values of

unning modes for the wood e theory of functioning when with soft starters regarding

The no load currents and the erest for this application and hey were verified in order to

equipment is behaving as as started.

EDGMENT

thank Continental Biomass unity to study and experiment

and New England Recycling and support provided during anagement system installed on

ENCES

amu Krishnan, “Control in Power 494-450, August 2002. Novel Induction Motor Soft Starter d Reactor” Industrial Electronics and nference on, pp. 1-6, May 2006. drives – understand the differences”,

Technical Conference, May 1999. nd Drives, automation.com; drive.nsf/link2/SKUS-

nd Drives, automation.com; artners.com; g?IdcService=GET_FILE&dID=91892 uty -Controls/ABB-Extreme-Duty

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