Study of Accelerating Torque Requirements of a Reciprocating Compressor

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Purdue University

Purdue e-Pubs

International Compressor Engineering Conference School of Mechanical Engineering

1996

Study of Accelerating Torque Requirements of a

Reciprocating Compressor

V. K. Rao

Siel Refrigeration Industries

P. S. Murty

M. S. Reddy

S. A. Sundaresan

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Rao, V. K.; Murty, P. S.; Reddy, M. S.; and Sundaresan, S. A., "Study of Accelerating Torque Requirements of a Reciprocating Compressor" (1996).International Compressor Engineering Conference.Paper 1083.

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STUDY OF ACCELERATING TORQUE REQUIREMENTS OF A RECIPROCATING COMPRESSOR

AUTHORS

v

_{Koteswara Rao, P Satyanarayana Murty,}

M Srinivasa Reddy, S A Sundaresan Siel Refrigeration Industries

Hyderabad - 500 037, India

ABSTRACT

During startability tests, a reciprocating _{compressor can}

either 1) start and run without any problem or 2) not _{start at}

all or 3) turns over but does not start or 4) starts _{and crosses}

the _{critical point and then stalls. Of the above}

phenomena,

accelerating torque of the compressor depends on _{the pressure}

built up

in

_{the system . In the present paper, the torque}

requirements

are studied and the performance of the theoretical

motor design is compared at different voltages. _And _also

different

starting phenomena are then explained from the

experimental results.

INTRODUCTION

One _{of the important parameters that is considered}

during

the design of a motor for a hermetic compressor _{is the} _starting

torque _requirement.

Using this starting torque, the lowest

voltage down to which the compressor can start can be estimated.

However, the starting torque alone is not sufficient _{for starting}

and running the compressor. The information _needed _{is the}

accelerating torque characteristic of the pump.

For sustained running, the motor pull-up torque has _{to be}

equal _{or higher than the accelerating torque characteristic}

of

the pump at all speeds till the compressor crosses the critical

point. Parameters such as the balanced pressure _{at starting,} _the

rates at which the suction and discharge pressures _{change depend}

on the system and operating conditions. The other _{parameters are}

fixed _{for a given compressor design. Friction, however,}

is a

function of both the design and the operating _{conditions such as}

the amount of refrigerant dissolved in the oil and the _{speed. For}

a _{given motor design, only the winding temperature,}

and the voltage affect the pull-up torque.

An _{experimental method is described for evaluating}

the

accelerating torque characteristic of the compressor _{and also a}

theoretical model for estimating this is presented.

At different supply voltages the motor torques are compared

with torque requirements of compressor and the different _starting

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EXPERIMENTAL SET UP

The experimental set up consists of the following items: - Test compressor without housings

- Torque and Speed sensor coupled to compressor shaft - Drive motor coupled to Torque sensor shaft

- Special housing to enclose above items with air tight lid

The diagram showing the experimental set up is in fig 1.0.

The suction and discharge tubes of the test compressor are

connected to a system. The system is charged after evacuation to

normal charge. The leads of the drive motor are connected to the power supply and leads of the speed and torque sensor are

connected to the indicators and indicators output connected to a

storage oscilloscope. When the drive motor is energised the speed

and torque values are recorded in the oscilloscope for a given

time. Similarly the transient behaviour of the suction ·and

discharge pressures are also recorded using storage oscilloscope.

The observations of the·experiment are listed in table l.O

The behaviour at lower speeds could not be recorded properly

as the sensor used is not precision enough to record quick

responses. However by using the precision instrumentation this

can also be recorded.

The Frictional Torque is measured from first principles.

The experiment is repeated for a Low Back Pressure

application compressor for which.the recorded readings are given

in table2.0.

THEORETICAL MODEL

A.. STARTING TORQUE

The starting torque requirement depends on the crank angle

position (Reference [l]),the cylinder volume at that crank angle,

system equalised pressure at the time of starting, piston area,

the differential pressure required to open the discharge valve,

the polytropic index for the refrigerant and the static friction.

The starting torque can be expressed as

Ts

=

f(Pe,V(¢) ,Pc,Ap,n,x) + Tf

V(~)

=

Vc+Ap

Cr{1+Cos(~)+

Cl/Cr [l-

~ 1-(Cr/Cl~

Tf is calculated based on Reference[2].

-2

Sin(¢) ] }

- (l)

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Ts _{Starting Torque}

Tf _{Frictional Torque}

Piston Area _v_({l)) _{Cylinder Volme}

Ap

n _{Polytropic Index}

X Piston linear displacement

vc _{clearence volume}

Cr _{Connecting rod radius}

Cl _{Connecting rod length} _¢

Crank Angle

Pe _{Equalised pressure at the time of starting}

Pc _{Pressure in the cylinder to open the Discharge Valve}

~ _{ACCELERATING TORQUE}

The _{transient behaviour of the suction}

and discharge

pressures in the system are calculated _{using References [3-5] .The}

Indicated work is calculated from the polytroic _equation

n-1/n Wi = _{(n/n-1) Ps Vs ( (Pd/Ps)} - 1) - (3) Ps _{Suction pressure} Pd Discharge pressure Vs Volume of Vapour sucked

The accelerating torque is calculated _{as shown below} Ta

=

Wi*C + Tdf

- (4)

C _{Conversion factor}

Tdf _{Torque required to overcome Dynamic friction}

Here Tdf is assumed to be 5% of the calculated _Torque.

DESCRIPTION OF RESULTS

As shown in Fig 2 the motor selected for the HBP compressor behaves

in the following manner at different voltages. _At _120V

motor does not start{No Start) at all. At 140V _{stalls(Turns on No Start). At 160V motor starts}motor turns _and and stalls after

reaching the full speed. Above 160V it starts and _{runs without}

any problem.

As shown in Fig 3 the motor selected for the LBP compressor

behaves in the following manner. At 120V motor _{does not start at}

all. At 140V and 160V the motor starts and reaches _{full speed and}

stalls. Above 160V it starts and runs without _{any problem.}

CONCLUSIONS

The values calculated from the proposed _{theory are matching}

with the experimantal values satisfactorily. _{Further studies in}

this direction may extend the applicability _{of this} _theory

to

different applications to predict the Torque _{requirements of the}

compressor and hence motor can be designed _{appropriately to suit}

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REFERENCES

1. Gatecliff, G.

w.

,

Ted 0.

"Analytical Prediction of the

Refrigeration compressors

Experiment",Proceedings of

Technology Conference,1974 ,

Phil and Ronald , R. Wisner,

Startability of Reciprocating

including comparison with

the 1976 Purdue Compressor

pp. 221-225.

2. Venkateswarlu, K. , Rao, N. J. , Venugopal, E. V. , Akella,

S.,"An Integrated Approach to Reciprocating Compressor

Design",Proceedings of the 1990 Purdue Compressor

Engineering Conference, 19-90 , pp. 282 -291.

3. G.T. Kartsounes and R.A. Erth, "Calculation of Thermodynamic

properties of Refrigerants R12,R22

&

R502 using a Digital

Computer", Proceedings of the 1972 Purdue Compressor Technology Conferance, 1972 , pp. 285

4. Rajendran, N. and Pata, M.B., "A Computer model of the

start-up transients in a vapor compression Refrigeration

systemn. Preprints of the 1986 International Institute of

Refrigeration Meeting, Purdue, pp. 129-140, August 5-8, 1986.

5. Niels Jorgen Josiassen , ••simulation of Condition sequence

during start-up of an Evaporation Refrigeration system",

Proceedings of the 1978 Purdue Compressor Technology

Conference, 1978 , pp. 309-316.

6. Veinott, C . G. , "Theory and Design of Small Induction

Motors", McGraw-Hill Book Company, Inc., New York ,NY, 1959

EVAPORATOR

DISCHARGE MUFFLER

COMPRESSOR

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COMPARISION OF THEORETICAL AND EXPERIMENTAL _{DATA OF HBP COMPRESSOR} SUCTION PRESSURE DISCH. PRESSURE _TORQUE

TIME _(Psia)

(Psia) _(Oz-Ft)

SPEED

(Sec)

(RPM) THEORE- _EXPERI-

THEORE-EXPERI- THEORE- EXPERI-TICAL _MENTAL _TICAL _MENTAL

TICAL _MENTAL

I

0 134.7 _134.7 _134.7 134.7 _9.16 _8.6 0 0.50 ₁₃₄ ₁₃₄ ~40 ₁₄₅ _3.455 3.6 ₁₀₀ 1.00 ₁₃₂ _131.5 162 168.4 _15.982 _16.6

I

305 I I 1.50 ₁₃₀ ₁₂₉ 198 203 32.45 _33.9

I

933

I

_l

2.00 ₁₂₆ ₁₂₅ 230 _234.6

!

45.04 46.2

I

_I

1600

_I

3.00 ₁₂₄ _122.8 274 ₂₈₀ _58.31 60.1 ₂₅₉₀ 4.00 ₁₁₈ ₁₁₆ 296 ₃₀₃ _64.19 66.0 _I ₂₉₀₀ Table 1.0

COMPARISION OF THEORETICAL

_AND

EXPERIMENTAL DATA OF LBP COMPRESSOR SUCTION PRESSURE DISCH. PRESSURE _TORQUE

TIME _(Psia)

(Psia) _(Oz-Ft)

SPEED {Sec}

(RPM)

THEORE- _EXPERI-

THEORE-EXPERI- THEORE- EXPERI-TICAL _MENTAL _TICAL

MENTAL _TICAL _MENTAL

0 69.7 _69.7 _69.7 69.7 _9.16 _8.8 0 0.37 _68.1 _69.2 108.7 _110.4 _3.0146 3.1 ₇₅ 0.89 _66.67 _68.1 114.7 _119.2 _3.4275 3.6 ₂₃₉ 1.39 _65.2 _67.1 11.9.7 _124.0 _3.754 3.9 730 1.90 _63.7 _65.73 129.7 _133.1 _4.295 4.5 ₂₀₂₀ 2.30 _63.2 _64.5 134.7 _139.8 _4.5216 4.7 ₂₈₀₀ 90.00 _25.7 _26.8 165.7 _171.5 _4.298 4.5 ₂₉₀₀ Table 2.0

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w ..--, ;:; ::c c "<:f 0 X I--LL N 0 z w ~ a a: 0

1--SPEED VS TORQUE CURVES OF rlBP COMPRESSOR AND MOTOR

200"1~---,----~---~

lliE----* COtv1PRESSOR TORQUE

J

150 100 12

---8 1000 SPEED IN RPM FIG 2 \ 2000 \ ' _\

SPEED VS TORQUE CURVES OF LBP COMPRESSOR AND MOTOR (CSIR)

---

---MOTOR

CUR~~-~:~?Y~---~,

'

---

' --~- ' ' '

'

\ . ' ' .... ~, ... ...

...

_... ' ' ' . ' MOTOR CURVE -160V \ -~---~ \

_________

..-- ... \ 3000

- - , ; . - - - MOTOR CURVE -i40V "'

~-4

.

r--.;:_· _____________ MOTOR CURV~-120;----~~~--... , ... __ -...:... \

---

""'---

"-

' !,;,.., - - - - -- -- -- -- -- - - ' 1;~---"" . ' ... ~. \

~---~~--- C?_~B_ES_?_OR_J069U~-

C\lf3Vf. _______

;._-..=_-::-::~~~\

: . . . '

\

0 L---~---~--~--~--~---M 0 1000 SPEED IN RPM FIG 3 2000 3000