The potential relay is getting popularity because it deserve to be adapted easily to most any kind of compressor. The ago electromotive force created by the starting winding the a engine is the managing factor of a potential relay.

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When a single-phase motor is operating, a voltage is produced throughout the starting windings above and beyond the voltage being used to the motor. The starting windings in reality act as a generator to produce the back electromotive pressure of a motor. The ago electromotive pressure of a motor coincides to the engine speed. The potential relay is designed to open, dropping the beginning circuit, as soon as the motor reaches a particular back electromotive pressure that"s predetermined by the manufacturer of the motor.

Figr. 10.6 Potential relays

Figr. 10.7 Schematic diagram of a potential relay

Figr. 10.8 Schematic diagram of a hard-start kit: To starting winding; To power leg feeding operation of compressor; To typical of compressor; Potential relay

ill 10.6 mirrors two potential relays the look lot like ordinary general-purpose relays. The wiring diagram because that a potential relay is shown in ok 10.7. The potential relay has actually an benefit over various other relays due to the fact that its contacts are usually closed when the unit starts, so over there is no arcing. Arcing is an electric spark that"s produced throughout two to adjust of contacts. As the motor rate increases, therefore does the ago electromotive force. Once the speed viewpoints 75% come 80% of full speed, the ago electromotive pressure is big enough come drop the starting circuit by an energetic the potential relay coil. The potential relay is rated by, and operates on, three voltage ratings: pickup voltage—the minimum voltage compelled to energize the coil and also open the NC contacts; dropout voltage—the minimum voltage the coil requires as soon as it"s energized to save the contact from closing; and continuous coil voltage—the best voltage at which the coil have the right to continuously operate.

On numerous permanent split-capacitor hermetic electric motors that space experiencing challenge starting, a hard-starting kit have the right to be installed to get rid of the problem, making the PSC engine a capacitor-start—capacitor-run motor. A hard-starting kit consists of a potential relay and also a starting capacitor. Ill 10.8 reflects the schematic chart of a hard-start kit, and ill 10.9 shows a hard-start kit as it would come indigenous a manufacturer.

Figr. 10.9 Hard-start kit.

Figr. 10.10 commercial potential relay tester.


The potential relay is simple to troubleshoot since only two components of the relay need to be checked. The coil of the potential relay have the right to be checked v an ohmmeter across 2 and 5 (ill 10.7), which could examine out together good, open, or shorted. The contacts of the potential relay across 1 and also 2 have to be checked, and they need to be closed. Motor present will rise if the potential relay does not drop the starting circuit at the appropriate time. This problem is dangerous to the motor and the beginning apparatus. The potential relay should always be checked fully to protect against this condition. A service technician should always check the operation of a unit after ~ the dropout the a starting relay. The correct size relay should constantly be supplied for instead of of a faulty relay. If the starting relay is working properly, there will be no ampere attract through the starting circuit after ~ the motor has actually reached full -speed. Ok 10.10 shows a commercial potential relay tester.

The usage of potential relays top top single-phase hermetic motors has actually increased end the past 10 years. Potential relays vary considerably in number of terminals, coil group, pickup voltage, and also mounting position. One explanation that the number device of the two significant manufacturers the potential relays need to be helpful in the field.

The two major brands of potential relays are general Electric and RBM. Explanations of their numbering codes are provided in ills 10.11 and 10.12, respectively. Pay mindful attention to the coil group and the pickup voltage once you cross-reference a potential relay.

10.4 SOLID-STATE starting RELAYS and also DEVICES

The development of solid-state controls has created another form of current-sensitive relay, a PTC beginning switch. Particular ceramic products (PTC) boost their resistance together they heat up from present passing through them. This is the communication of the solid-state relays that are replacing numerous of the other varieties top top fractional speech motors. A PTC solid- state beginning relay is displayed in ok 10.13. The solid-state beginning relay offered on tiny split-phase and also capacitor-start engine compressors is an easy in that is operation. The device is used to drop out the beginning windings and starting capacitor that a small hermetic compressor motor. Ill 10.14 mirrors a solid-state relay mounted on a fractional-horsepower hermetic compressor. The adaptability of the solid-state relay has actually greatly expanded its usage due to the fact that one relay will cover a variety of horsepowers. Manufacturers are producing a PTC relay with a built-in beginning capacitor, as shown in ill 10.15, and also a built-in starting capacitor and also overload, as displayed in okay 10.16.

Figr. 10.13 Solid-state beginning relay

Figr. 10.14 Solid-state beginning relay set up on a compressor

Figr. 10.15 Solid-state relay with built-in capacitor; Figr. 10.16Solid-state relay with integrated capacitor and overload

Figr. 10.17 optimistic temperature coefficient starting device

Permanent split-capacitor (PSC) motors frequently need help in starting. A potential relay and starting capacitor can be offered to adjust a regular PSC motor right into a capacitor-start—capacitor-run motor with a high beginning torque. On some occasions a optimistic temperature coefficient start machine is supplied to rise the beginning torque the a PSC motor. The PTC start device is easier to install and less expensive than the conventional potential relay and beginning capacitor. Okay 10.17 shows the PTC start device. This machine is wired in parallel through the operation capacitor. The PTC start machine per creates much prefer a tiny starting capacitor; it momentarily boosts the existing in the motor starting winding. As the product heats up, that resistance increases conveniently to the allude where it i do not care a nonconductor and the motor returns come PSC operation. This maker requires a three-minute cool-down period between starts. To examine this device, the company technician require only recognize if it"s dropping out of the circuit or if it"s being energized ~ above startup. It should be changed if it"s not functioning properly.

An digital motor starting relay has actually been presented that have the right to be supplied as a global replacement for many motor starting relays in the industry. This electronic relay operates top top a time role principle rather than sensing voltage or current. The time function design allows this relay to be provided as a instead of for numerous potential and also current beginning relays. This relay is presented in ill 10.18. The wiring chart is presented in okay 10.19 with the relay instead of a potential relay and also in ok 10.20 instead of a current-type relay.

Figr. 10.18 universal motor beginning relay:

Figr. 10.19 Wiring of universal relay replacing potential relay: WIRING together A POTENTIAL RELAY: REPLACES every POTENTIAL MOTOR beginning RELAYS for COMPRESSORS RATED at 120 come 270 VAC solitary PHASE approximately 5 H.P. Any kind of PICK-UP VOLTAGE. Diagram NOTE: A wire from the running TERMINAL of the compressor motor have to be connected to #4 top top the relay.

Figr. 10.20 Wiring of universal relay replacing a current relay: PRO-90 WIRING as A CURRENT starting RELAY


The solid-state beginning relay is placed in series with the start winding the the fractional speech hermetic compressor, as shown in okay 10.21. The solid-state starting relay typically has a very low resistance. As the compressor motor starts, the existing flows to the start winding. The resistance of the solid-state beginning relay swiftly rises really high. Therefore, the current flow to the begin winding is diminished greatly, i m sorry takes the starting winding the end of the circuit. Many solid-state beginning relays can additionally be supplied with a capacitor inserted in series with the relay and also start winding, as shown in okay 10.22. One benefit of the solid-state beginning relay is the one model covers a selection of speech ranges. E.g., one relay can be offered from ½-horsepower to1/5-horsepower, and also another indigenous 1/4 speech to 1/3 horsepower. Part manufacturers produce this type of relay to cover compressor motors above 1/3 horsepower. The major advantage of this form of relay is the the business technician deserve to stock three simple relays and cover many applications and sizes.

These beginning devices room also easily accessible with a built-in beginning capacitor (refer again to ill 10.15). As soon as the relay capacitor is used, the present overload need to remain on the compressor for protection. A three-in-one relay is also available (refer again to ill 10.16) that has the solid- state relay, a capacitor, and also an overload.

The solid-state PTC-type beginning relay is usually minimal to domestic refrigeration applications, however it have the right to be offered on various other applications if the manufacturer’s environment instructions are followed. The solid-state relay usually calls for 3 to 10 minute to cool down between operating cycles, for this reason it"s not feasible because that short-cycling applications.

Figr. 10.21 Schematic chart of compressor and solid-state starting relay (split-phase motor); Figr. 10.22 Schematic chart of compressor and solid-state beginning relay (capacitor-start motor)


The solid-state starting relay is simple to troubleshoot due to the fact that of the simplicity. On plenty of occasions, the relay will display some visible signs that would show the condition of the relay. In some cases, the relay will certainly be charred, severely burned, and also cracked; in various other instances, the relay will only be contempt burned. The relay can likewise be checked with an ohmmeter: If the resistance is very low, the relay is good; a high resistance across the relay shows a negative relay. The technician need to make particular the relay is cool once performing the resistance check. The service technician could also check to make details the relay is dropping the starting winding the end of the circuit by making use of a clamp-on ammeter if a wire is connecting the relay come the compressor terminal. If the relay pushes straight onto the compressor terminals, this check is almost impossible; hence, one amp analysis of the compressor motor should be taken, and a judgment need to be made even if it is the begin winding stayed in the circuit.

10.5 engine BEARINGS

All rotating electrical equipment has actually some form of bearing to allow for smooth and easy rotation. A bearing is that component of a rotating electric an equipment that allows free movement. The two varieties of bearings offered in the heating, cooling, and also refrigeration industry are ball bearings and sleeve bearings, displayed in ill 10.23. The round bearing is the most efficient because it produces much less friction. The sleeve bearing is cheaper, however, and also is more commonly used.

Ball Bearings

Ball bearings are designed v an inner and an outer ring, i m sorry enclose the balls by usage of a separator. The within ring is the bore in i m sorry the tower is pressed.

Ball bearings space lubricated in three ways. The permanently lubricated bearings space sealed in ~ the factory and also only rarely require additional lubrication. The pack lubrication of round bearings is done by hand. The bearing need to be disassembled and hand packed with grease every 2 to five years. Numerous ball bearings space equipped through a grease fitting and should be lubricated every 2 years. Lubrication is that prime prestige to a motor. Keep in mind that over-lubrication is as damaging together under-lubrication. Ok 10.24 mirrors a ball bearing pressed right into an end bell the a motor.

Figr. 10.23 Sleeve and also ball bearings used in the industry; Figr. 10.24 mounted ball bearing

Ball bearings are supplied on most heavy loads. However, sphere bearings can"t be offered in a hermetic compressor due to the fact that of the risk of sparks. Several of the many benefits of using round bearings space mountings in any type of position, antifriction design, versatility, and the capability to bring a big load. Round bearings are often permanently lubricated from the manufacturing facility or call for infrequent lubrication. Thus, they require less maintenance 보다 a sleeve bearing. The life that a sphere bearing is usually long and also trouble free, yet mis use have the right to rapidly damage a ball bearing.

Sleeve Bearings

Sleeve bearings are brass or bronze cylinders in i m sorry a obelisk rotates. The bearings have an ext friction 보다 the round bearing and thus are offered for lighter duty. Lubrication of sleeve bearings is accomplished by oil wick lubrication, yarn-packed lubrication, or oil ring lubrication.

The oil wick lubrication has actually a wick the extends right into an oil reservoir. The wick choose up oil from the reservoir and transfers it to the shaft, as shown in ill 10.25. The reservoir need to be filled v oil twice a year.

The yarn-packed lubrication is merely yarn packed about the column to lubricate it, as displayed in ill 10.26. This bearing have to be lubricated every few months.

The oil ring lubrication is supplied on huge motors. In this device a ring rotates through an oil reservoir, choose up oil and also transferring it come the shaft, as displayed in ill 10.27. The oil level the the reservoir need to be confirm monthly.

Figr. 10.25 Oil wick lubrication the a sleeve bearing

Figr. 10.26 Yarn-packed lubrication that a sleeve bearing; Figr. 10.27Oil ring lubrication the a sleeve bearing

Hermetic compressors use sleeve bearings due to the fact that with sleeve bearings over there is no peril of sparks. Care must be taken in mounting electric motors equipped v sleeve bearings since of the lubrication troubles when the bearings are mounted in a vertical position. Sleeve bearings are generally used in many applications in the industry.

10.6 engine DRIVES

A motor journey is the connection in between an electric motor and also a component that needs rotation. Electric motors are used to journey most gadgets that require rotation. There room two straightforward driving devices: straight drive and belt and also pulley drive.

Direct journey

Direct-drive techniques require the a machine turn v the same revolutions per minute as the motor. Pan motors and pumps space often straight drive; hermetic compressors are always direct drive. Direct-drive hookups require a nearby fit between the motor and also the device. In a hermetic compressor, the crankshaft is do in one piece, through the motor on one end and the compressor portion on the contrary end. Direct-drive applications usually need a coupling, other than with hermetic compressors and fan motors.

The two species of direct-drive couplings provided in the sector are the versatile hose coupling, as supplied on oil burners, and also flange couplings, as provided on some open types of compressors and hot-water pumps. Okay 10.28 shows both generally used varieties of direct-drive couplings.

Figr. 10.28 two widely used direct-drive couplings: (a) Flexible-hose coupling; (b) Flange coupling

Figr. 10.29 V-belt connection between fan and also motor

CAUTION: When removing a maker from the shaft of an electric motor, make sure that the motor pillar or the an equipment being gotten rid of isn"t damaged.


Although many species of belts are offered to journey devices, just the V-belt will certainly be discussed here. It is used virtually exclusively in the industry. Through using 2 pulleys and connecting them through a V-belt, rotation deserve to be moved from the motor to the device, as displayed in ill 10.29. V-belts are provided in numerous applications in the industry, such together driving open species of compressors, fan motors, and pumps.

CAUTION: The technician need to never wear loose clothing or neckties when working approximately rotating equipment. Lengthy hair should additionally be bound back.

There are basically three sizes that V-belts provided at present. Dimension FHP V-belts space 3/8 inch broad and used on fractional-horsepower motors. Size A-section is ½-inch broad and provided for most jobs requiring one- to five-horsepower motors. Size B-section is 21/32 inch vast and offered when the motor wheel is larger than 5 customs in diameter. More than one V-belt is frequently used come pull large devices. When belt alters are needed, a matched collection should be provided in a dual-belt application. The equipment’s changes per minute have the right to be adjusted by changing the pulley-block size or sizes. The following formula can be used:

diameter of equipment pulley = engine rpm X motor pulley size / rpm the equipment

This formula have the right to be transposed to fix for any kind of of the proclaimed values by simple algebra. E.g., if the motor rpm to be 1750 through a 5-inch pulley, what size tools pulley would certainly be required to create a speed of 1000 rpm?

diameter of equipment pulley = motor rpm x motor pulley-block size / rpm that equipment

diameter of tools pulley = 1750x5 / 1000

diameter of equipment pulley = 8.75 inches

V-belts should constantly be appropriately aligned and also the anxiety adjusted. Ok 10.30(a) mirrors an wrongly aligned V-belt. The ideal amount of stress for most belts is displayed in ok 10.30(b). Commonly a V-belt will slip, bring about erratic tools speed or breakage. In this cases, the belt must be replaced and the tension changed correctly.

See more: Words That End With A Consonant, Ending Consonants Worksheets

Figr. 10.30 V-belt alignment: (a) untrue alignment; (b) suitable tension and adjustment

CAUTION: Always usage caution as soon as working roughly an electrical motor that"s delivering rotating motion with V-belts.

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