Refrigeration Chapter 23

Devices which provide satisfactory overload protection

1.Be available in a wide variety of sizes, capacities, or settings, to closely match the motor 2.Have some time delay in their operation so that they will carry the starting in rush current long enough to permit a normal motor start 3.But must be very little time delay in their operation in breaking the power circuit if motor fails to start.

Overload protection should guard against two conditions which may cause excessive heat.

1.Conditions while the motor is running 2.Conditions associated with Lock-rotor Current Flow

Current-Temp Overload Protectors

1.Current temp overload protectors 2.Have bimetal discs or strips 3.Are mounted inside a motor or in the end bell of a motor 4.They serve and react to motor temperature

Pilot Duty

1.Do not carry line current 2.Carry only the small current necessary to operate a mag. Coil 3.Can be built of light construction, with little mass so that upon a rise in motor temperature they will readily heat enough to operate quickly

Reason for motors to draw locked-rotor current

1.Excessive mechanical load 2.Low voltage 3.Single phasing of three-phase motors Note: All cause failure to start or stall if running

Disc-Type Bimetal Thermal Overload Relay

1.Has a bimetal in the shape of a disc 2.This is the basic element of a number of overload devices having the name Klixon 3.Has the current flowing through the bimetal disc and heats it

Motor temperature affected by:

1.High ambient temperature 2.Heavy mechanical overload 3.Excessive current draw due to low voltage 4.Single phasing of three phase motors

Types of Reset

1.Manual Reset 2.Automatic Reset

In a Hermetic Motor

1.Rapidly rising motor temp can continue to increase for a time after the current is cut off 2.Rising motor temp could be high enough to damage the motor even after an overload trip 3.Therefore rapid response to overload is important in hermetic motors

Overload protection based on the measurement of current sense the magnitude of the current by three ways

1.Sensing the amount of heat when passing through a small resistance element 2.The heating and warping of a bimetal thermostatic disc 3.Sensing the strength of the magnetism it creates in passing through an electromagnet

Generally, the greater the motor horsepower...

1.The faster the temp. will rise on a locked-rotor or short cycling condition. 2.The more slowly the motor winding will cool due to their increased mass 3.The current draw increases as motor HP inc. 4.The heat generated inc in proportion to the square of the current

Advantages of SSMP

1.The speed and sensitivity with which it reacts to changes in motor temperature. 2.And its automatic reset motor protection.

Classification of Motor Overload Protection Devices They are classified into 3 basic types:

1.Types of duty 2.Types of reset 3.Types of devices as related to their operating principle

Klixon

A predetermined amount of current will heat the disc enough to cause it to warp so that it opens a set of contacts When the disc cool down it will require an automatic reset Usually referred to as a four wire protector, two leads carry line current and the second pair is a pilot circuit. Commonly located in the compressor terminal box Ambient temp has negligible effect on the current at which this overload relay will operate Seldom used alone and is often referred to as a supplementary overload

Melting Alloy Thermal Overload Relay

A small spindle is mounted inside of a loosely fitting tube The space between the spindle and the inside of the tube is filled with solder which when solid keeps the spindle from turning. A resistance heater carrying motor current surrounds the tube. If current through the heater exceeds a predetermined value, the tube becomes hot enough to melt the solder The melting solder allows a spring to turn the spindle The turning of the spindle trips a switch to open the control circuit

They are _______ reset devices when internally located they are ______

Automatic, inaccessible

The external thermostats _____ sense changes in motor temp as quickly as internal type and ______ provide protection that is as good as that provided by the internal type

Can not, does not

Internal thermostats in hermetic motors _____ be replaced in the field and it is good practice to protect them against excessive current by ______ the control circuit directly before it enters the motor ___ directly after it leaves the motor

Can not, fusing, and

Line duty internal thermostats ___ be used in small motors. The bimetal _______ carry current.

Can, does not

Fuses and circuit breakers are usually sized as compared to the rated current of the motor and so _____ be expected to ____ the circuit at a ___ enough value of current to protect the motor against overheating

Cannot, open, low

The contacts of internal motor thermostats are normally held _____ by a spring disc, but excess temp will warp the bimetal and cause the spring disc to snap to a position which will _____ the contacts.

Closed, open

The purpose of fuses and circuit breakers is to protect the _______ and switches as well as the motor from ________ and heavy _______.

Conductors, short-circuit, overloads

The principal limitation of a three-phase current-temp OL protector is the amount of _____ it can carry, with corresponding limit on motor size, which means a maximum of about ___ HP.

Current, 7 1/2

Internal type motor thermostats can _____ sense the motor winding temp.

Directly

Locked-rotor current may be ___ or ____ times the value of a motors' normal ____ running current.

Four, Five, Full-load.

The purpose of motor overload protection is to protect the motor from being damaged by excess ___, most importantly the winding

Heat

If upon starting, a motor is able to come up to speed in a normal manner and to run for a considerable length of time, the ___ generated by the heavy starting ____ is dissipated so that it does not hurt the motor.

Heat, Current.

The _____ the current draw, the more heat is generated in the motor. The more heat generated with current flow the _____ the motor temperature will be.

Higher, higher

A magnetically operated relay __ inherently free of the effects of ambient temperature.

Is

The heater of a three-terminal current-temp OL protector __ energized only when the start winding is energized

Is

•Devices which carry the actual line current of the motor or other current-consuming item that is being controlled is referred to as ___ duty.

Line

Generally all control devices as well as motor-overload protectors can be classified as either ___ or ____ duty.

Line, pilot

A motor will draw its maximum current when at its standstill. This is called ________ current and for a specific motor design and size it is determined by actually locking the motor so it wont turn and applying rated ______ and than measuring the _____.

Locked-rotor, voltage, current.

Automatic Reset

Means that if opened in response to an abnormal operating condition, the overload or safety device will automatically re-close and re-establish the circuit to restart the system w/o anyone's personal attention

If current draw for a given motor is limited, _________________will be limited.

Motor temperature

Line duty devices :

Must carry enough current to supply the power to the motor 2.Are limited to the smaller size motors 3.Must be of rather heavy construction as compared to pilot duty devices

Motor current ______ be carried through the bimetal, current would affect the temp setting

Must not

What is meant by NC/NO?

NO: The controlled circuit remains open when the control circuit is de-energized, closes when energized. NC: The controlled circuit remains closed when the control circuit is de-energized, opens when energized.

If external, its mounted on the motor enclosure or shell, and the ______ motor leads are brought out to the protector.

Neutral

Time -Delay Fuse

Operate by sensing the magnitude of the current, also referred to as dual-element fuse. Have normal fuse link which blow immediately on a short-circuit or other very heavy current Are design to carry motor locked-rotor current Have thermal element which contains a small solder pot

Manual Reset

Requires that someone has to physically do something such as push a button or operate a switch to re-establish the circuit and set the system in operation again

After the single phase motor using a current-temp OL w/a heater is up to speed , both the heater and the bimetal snap disc continues to carry _____ current

Running

If incorrect or faulty controls cause a motor to start and stop every min or so, a condition referred to as "_______", the motor will quickly become ______.

Short-cycling, Overheated

____ motors might stay in a _________ condition for some time w/o burning because ____ generated by the small current could be adequately ________ to the surrounding ___

Small, locked rotor, heat, dissipated, air

Motor current passing through a _____ coil of a magnetic type thermal overload relay ___ operate a switch if the current and the strength of the resulting magnetism exceeds a predetermined value

Solenoid, can

When motor thermostats are installed inside hermetic motor-compressors, they must themselves be hermetically sealed to avoid the possibility of the refrigerant being exposed to _____.

Sparks

The cooling effect of ref. ________ enables a hermetic motor to produce as much ______ as a much larger _____ motor.

Suction gas, horsepower, open

Sensitivity as related to overload protection means the ability to sense changes in ________ or _____.

Temperature, current

Some overload devices operate on a combination of both _____ and to the ______ of the current

Temperature, magnitude

Operating Principles of Overload Protectors

The most direct approach to protection is to sense motor is based on temperature A thermostatic device which responds to changes in temp is one operating principle of motor protection Measurement of the current as an indication of the temp that will reached in the motor offers another operating principle of overload protection

Ambient Compensated Thermal Overload Relay

Thermal overloads depend on the temp they attain, and are affected by the ambient temp. Are usually mounted in starters The overloads are remote from the motor Relative location of motor and starter is important

Line duty control devices for motor control.....

They must have contacts of sufficient size to make and break the locked-rotor current of the motor They must have contacts of sufficient size to break the running current. Except in the case of a mal-funcion, the running current would be less than locked-rotor current

The overload protector has ____ sets on contacts. All ____ -phase circuits are broken when disc snaps.

Three, three

Except for the ______ type of fuse, fuses and circuit breakers do not provide adequate motor overload protection.

Time delay

Current-temp OL protectors are avail. In several variations, the simplest being a ___-terminal OL and would be applied to a ____ phase motor.

Two,single

Bimetal Thermal Overload Relay

Uses a small resistance or heater, and a bimetal strip. Motor current passing through the heater in a bimetal relay in excess of a predetermined value will act on the bimetal strip causing it to warp sufficiently to operate the switch and break the control circuit

Possible if the motor windings are ___ connected.

Wye

Some systems using automatic reset safety devices include a lockout relay or recycle timer to limit the number of motor restarts per hour.

automatic

Three-phase current-temp type protectors is also used in internally in hermetic motor compressors, in which case it must have a __________________.

hermetically-sealed housing

Normal application of time-delay fuse is in the power supply to ____ phase motors. They are made in _______ type as well as ____ type.

single, cartridge, plug

•Advantages of these methods are

•1. That the restart button can be in a location remote from the equipment so it can be placed in a convenient and accessible location. •2. The system is protected from repeated automatic restarts, but is restarted easily in case of a nuisance trip

•Reasons why Short-cycling happens

•1. The low pressure switch •2. The high pressure switch •3. Too sensitive a thermostat •4. loose electrical conditions •5. Power interruption and intermittent low voltage.

•Methods of short-cycling prevention are:

•1. Time-off Cycle •2. Lock-out Relay/ Restart Relay

Phase Monitors

•Is a solid state device designed to protect electrical and electronic 3-phase equipment from damage. •And protect against : •1. Phase loss •2. Under voltage •3. Phase sequence

Lock-out or restart Relay

•Most overloads and safety devices are of these automatic reset type and are used to protect the equipment from repeated restarts.