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Electrically testing Compressor motors and keeping

records of the motors winding resistances can some-

times help in preventing the early death of a compressor.

Prior to installing a new compressor the motor windings

resistance and their insulation value should be checked

to make sure the motor has suffered no damage duringits transportation to site.

Compressor manufacturers often list the motor resist-

ance data for their various compressor types and this

information is invaluable in testing if a machine is good

or damaged.

Single Phase Motors

Single phase motors have two windings, called the

START and RUN windings which are commoned through

a third pin. This connection sometimes has a Thermal

Klixon device at the common point to the Start winding,

so if the klixon /overload device trips, the Start windings

will be open circuited to try to prevent a burnout.

The compressor motor pins are

Termed as C= Common, S = Start , and

R = Run. The resistance of the Startwinding is normally of a higher ohmic

value than the Run Winding. This is

due to the Start winding being

wound in a lighter gauge wire than

the Run winding.

Measuring the Winding resistance is simply done using

an Ohmeter . The meter probes are connected to two

of the three motor pins in turn. And the measurements

noted.

Measuring the Start & Run winding resistance of a MTZ18-5

Why Compressors FailPart 9 Electrical Testing of Compressor Motors

MAKING MODERN LIVING POSSIBLE

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Using a Maneurop Compressor type MTZ28JE-5 (230v

1ph 50 Hz) as a reference, the design resistances for the

Run and Start windings are 1.16 Ohm (RUN) & 3.24 Ohm

(Start). When measuring the resistance between the Run

& Common Pins the resistance should read 1.16 Ohms

and between the Start & Common pins the resistance

should be 3.24 Ohms. When measuring between the

Run & Start pins the sum of the Run and Start winding

resistances should be measured ie 1.16 + 3.24 =4.4

Ohms.

These readings confirm that the motor windings are

good to run.

A further test of the compressor motor is the insulation

test . This test is to make sure that there is no earth leak-

age through the motor insulation.

This test is carried out using a Megger insulation tester,

a 500 volt dc megger is ideal. This involves simply put-

ting one of the meter probes onto the compressors earth

terminal or other part of its metallic shell, and the other

meter probe onto the motor pins, one at a time. The test

on all three pins should all show an insulation value of

at least 1 Meg ohm or higher, values showing infinity

are the desired value. Values lower than 1 Meg are acause for concern and usually mean the motor windings

are damaged and need further investigation.

Insulation test of a MTZ18-5 indicates there is no problemwith the motor windings.

NOTE :-

Care should be taken when carrying out Insulation test-

ing as the meter will generate 500 volts dc into the wind-

ings and compressor earth/shell . By touching the com-

pressor shell whilst under this test can cause the person

to receive a nasty electric shock.

Single phase compressor electrical components com-

prise of Potential Relays and Start and sometimes Run

capacitors.

here we have both PSC and CSR starting kits for single phasecompressors.

If there is any doubt as to the electrical components

integrity then they should be changed immediately.

Capacitors in general give good life time operation, but

capacitors are designed to store electricity and the Start

Capacitor is there to enhance the compressors starting

ability when pressure differences across the compressor,

during start up or quick re starts to the motor are

required. Pressure differences between the suction and

discharge of the compressor above 7 Bar should be

avoided, as even single phase machines designated asan HST ( HIGH START TORQUE) motor will only cleanly

start a machine where the pressure difference is less

than 7 Bar. Failing to ensure that the starting PD is less

than 7 Bar can cause the compressor to struggle to start,

and as such the potential relay will hold the START wind-

ing in circuit longer than desired and will eventually

cause the START winding to overheat and quickly burn.

( For every second that the START winding is in circuitits winding temperature will rise approx. 10C).

Start winding burnout

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A weak Capacitor or a leaky capacitor will often com-

promise the compressors motor.

Capacitors can be checked with various CAPACITOR test-

ers on the market.

A quick test will tell if the capacitor is OK to be used or

thrown away and replaced. Capacitors have a lifetime

and deteriorate if not used for some time.

Capacitor tester, this model indicates the capacitors size

in f

Ohmeters and Meggers can be used for checking

capacitors, but it must be remembered that

Capacitors store electricity and after testing with a

Designated Capacitor checking device, the device will

automatically drain the charge making it safe to touch,

whereas if using a Megger, the charge that the Megger

puts into the capacitor will be stored in the capacitor if

its a good one. To remove the charge, simply touch the

two capacitor wires together being careful not to touch

the wires with your fingers as you will receive a nasty

electrical shock that is not at all pleasant. Capacitor

testers can be just simple indicator testers with a LED

indicator or a more sophisticated type that actually

tests the capacitor and can also measure the capacitors

size in f or microfarads. Capacitors can weaken over

time and should be checked during service visits.

Three Phase Motors

Three phase motors are wound

differently where all the three

windings have an equal resistance

value unlike their single phase

alternative. At the Star point of

the three phase windings is a KLIXON device which

is both current and temperature sensitive. If this trips

there will be no resistance measured between the

compressor motor terminal pins as the klixon breaks

the power circuit between the three phases creating

an open circuit.

Note:- On Maneurop Compressors if the Klixon open

circuits during an overload or temperature problem ,

it may take up to 8 hours to reset, depending on the

motors temperature.

Here we have a Klixon fitted to a 3-phase motor.The Klixon is the compressors last line of defence andshould not be used to cycle the compressor.

These machines are connected to the three phase

supply via a three phase contactor and sometimes a

close coupled overload is also fitted.

Testing a three phase machine is similar to the single

phase machine, only this time when resistance

measurements are taken across the 3 motor terminals

, two at a time, the results should be the same for

the three measurements. The three terminals are

designated T1, T2, T3.

If the measurements vary from winding to winding

then the motor has been damaged or burnt due to

over current/temperature, or has been running with a

loss of a phase voltage possibly a blown line fuse. This

causes only two phases to run the compressor motor,

but these two phases will draw extra current and run

in an overloaded condition. This situation often causes

the machine to cycle on the machines internal klixon

leading to eventual klixon failure or the motor burns

out.

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Using the same size compressor but a three phase

version, the resistance across the three windings is 7.11

ohms.

Resistance from T1-T2 =7.11ohm, T2-T3 = 7.11ohm, T3-

T1= 7.11 ohm. This motor is OK.

Insulation testing is the same as for the single phase

version.

If electrical Test records are kept as to the resistance of

the windings and also its Insulation value, then every

6 or 12 months the condition of the motor can be

checked against its previous condition. If for instance

the resistance values and especially the insulation

value, starts to drop, in a compressor motor this could

be a sign of moisture or oil degradation in the system.

A visual Indication of this can be seen in liquid

line moisture indicating sight glasses, and also

discolouration of the oil in the compressors oil sight

glass are easy visual checks to make during a service or

maintenance visit.

Sight glasses are quipped with sensitive indicators that re-flects a color, which depends on the moisture content ofthe refrigerant.

At this stage fitting a new drier and /or changing the

compressors oil charge, could potentially save the

compressor motor. As moisture, and weak acids plus

heat increase in the system due to age and poor

maintenance such as leaky systems, these acids will

start to attack the motors insulation. If left, further

deterioration to the insulation will occur and eventually

the insulation will break down at its weakest point,

causing a spot burn in the windings and then complete

failure of the motor will occur soon after.

Moisture along with week acids and heat can cause aspot burn in the windings.

Danfoss can accept no responsibility for possible errors in catalogues, brochures and other printed material. Danfoss reserves the right to alter its products without notice. This also applies to products

already on order provided that such alterations can be made without subsequent changes being necessary in specifications already agreed.

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Field Service Notes are produced by Glenn K Moore

Field Support Manager, Danfoss Ltd. Danfoss Ltd 9-2013Field Service Note 018