Motor Starters And Pilot Devices–Fractional and Integeral Horsepower Manual Motor Starters

Objectives

After studying this unit, the student will be able to:

• Match simple schematic diagrams with the appropriate manual motor starters

• Connect manual fractional horsepower motor starters for automatic

and manual operation

• Connect integral horsepower manual starters

• Explain the principles of operation of manual motor starters

• List common applications of manual starters

• Read and draw simple schematic diagrams

• Briefly explain how motors are protected electrically

FRACTIONAL HORSEPOWER MANUAL MOTOR STARTERS

One of the simplest types of motor starters is an on-off, hand-operated, snap action switch. A toggle lever is mounted on the front of the starter, figure 12-1. The motor is connected directly across the line voltage when the handle is turned to the START position. This situation usually is not ob jectionable with motors rated at one horsepower (hp) or less. Since a motor may draw up to a 600- percent current surge on starting, larger motors should not be connected directly across the line on start-up. Such a connection would result in large line surges that may disrupt power services or cause voltage fluctuations which impede the normal operation of other equipment. Proper motor starters for larger motors are discussed later.

Fractional horsepower (FHP = 1hp or less) manual motor starters are used whenever it is de sired to provide overload protection for a motor as well as “off’ and “on” control of small alternating current single-phase or direct-current motors. Electrical codes require that fractional horsepower motors be provided with overload protection whenever they are started automatically or by re

mote control. Basically, a manual starter is an on-off switch with motor overload protection.

Since manual starters are hand-operated me chanical devices (requiring no electrical coil), the contacts remain closed and the lever stays in the ON position in the event of a power failure. As a result, the motor automatically restarts when the power returns. Therefore, low-voltage protection (see Glossary) is not possible with manually oper ated starters. This automatic restart action is an advantage when the starter is used with motors that run continuously, such as those used on un attended pumps, blowers, fans, and refrigeration processes. This saves the maintenance electrician from running around the plant to restart all these motors after the power returns. On the other hand, the automatic restart feature is a disadvan tage on lathes and machines that may be a danger to products, machinery, or people. It is definitely a safety factor to be observed.

The compact construction of the manual starter means that it requires little mounting space and can be installed on the driven machinery and in various other places where the available space is limited. The unenclosed, or open starter, can be mounted in a standard switch or conduit box in stalled in a wall and can be covered with a stan dard flush, single-gang switchplate. The ON and OFF positions are clearly marked on the operating lever, which is very similar to a standard lighting toggle switch lever, figure 12-1.

Application

Fractional horsepower manual starters have thermal overload protection, figures 12-1 and 12-

When an overload occurs, the starter handle au tomatically moves to the center position to signify that the contacts have opened and the motor is no longer operating. The starter contacts cannot be reclosed until the overload relay is reset manually. The relay is reset by moving the handle to the full OFF position after allowing about two minutes for the heater to cool. Should the circuit trip open again, the fault should be located and corrected.

Fractional horsepower manual starters are provided in several different types of enclosures as well as the open type to be installed in a switch box, flush in the wall, or on the surface. Enclo sures are obtained to shield the live starter circuit components from accidental contact, for mounting in machine cavities, to protect the starter from dust and moisture, figure 12-3, or to prevent the possibility of an explosion when the starter is used in hazardous locations. These different types of enclosures will be discussed in more detail in Unit 13.

AUTOMATIC AND REMOTE OPERATION

Common applications of manual starters pro vide control of small machine tools, fans, pumps, oil burners, blowers, and unit heaters. Almost any small motor should be controlled with a starter of this type. However, the contact capacity of the starter must be sufficient to make and break the full motor current. Automatic control devices such as pressure switches, float switches, or thermostats

rated to carry motor current may be used with fractional horsepower manual starters.

The schematic diagram shown in figure 12-4 illustrates a fractional horsepower motor con trolled automatically by a float switch that is re motely connected in the small motor circuit as long as the manual starter contact is closed. When the float is up, the pump motor starts.

In figures 12-5 and 12-6, the selector switch must be turned to the automatic position if the float switch is to take over an automatic operation, such as sump pumping. A liquid-filled sump raises the float, closes the normally open electrical con tact, and starts the motor. When the motor pumps the sump or tank empty, the float lowers and breaks electrical contact with the motor thus stop ping the motor. This cycle of events will repeat when the sump fills again, automatically, without a human operator.

Note that a double-pole starter is used in figure 12-5. This type of starter is required when both lines to the motor must be broken such as for 230 volts, single phase. The double-pole starter is also recommended for heavy-duty appli cations because of its higher interrupting capacity and longer contact life, when using a two-pole mo tor starter on 115 volts. Normally, the single-pole motor starter is used for 115 volts.

MANUAL PUSH-BUTTON LINE VOLTAGE STARTERS

Manual push-button line voltage starters are integral horsepower motor starters (not fractional). Generally, manual push-button starters may be used to control single-phase motors rated up to 5 hp, polyphase motors rated up to 10 hp and di rect-current motors rated up to 2 hp. They are available in two-pole for single-phase and three pole for polyphase motors. A typical manual three-phase push-button starter and diagram are shown in figures 12-7 and 12-8.

When an overload relay trips, the starter mechanism unlatches, opening the contacts to stop the motor. The contacts cannot be reclosed until the starter mechanism has been reset by pressing the STOP button, after allowing time for the ther mal unit to cool.

These starters are designed for infrequent starting of small ac motors. This manual starter provides overload protection also, but cannot be used where low or undervoltage protection is re quired or for remote or automatic operation.

Manual Starter with low Voltage Protection

Integral horsepower manual starters with Low Voltage Protection (LVP) prevent automatic start-up of motors after a power loss. This is ac complished with a continuous-duty electrical sole noid which is energized whenever the line-side voltage is present, figure 12-9. If the line voltage is lost or disconnected, the solenoid de-energizes, opening the starter contacts. The contacts will not automatically re-close when the line voltage is re stored. To close the contacts to restart the motor again, the device must be manually reset. This manual starter will not function unless the line ter minals are energized. This starter should not be confused with magnetic starters described in the next unit. This is not a magnetic starter, but a lower cost starter.

Applications

Typical applications include conveyor lines, grinders, metal working machinery, mechanical power presses, mixers, woodworking machinery and wherever job specifications and standards re quire low-voltage protection, or wherever machine operator safety could be in jeopardy.

Therefore, this manually operated, push-but

ton starter with low-voltage protection is a method of protecting an operator from injury using auto matic restart of a machine upon resumption of voltage, after a power failure. This is normally ac complished with a magnetic starter with electrical (three-wire) control.

THERMAL OVERLOAD PROTECTION

Thermal overload units are widely used on both the fractional and integral horsepower man ual starters for protection of motors from sus tained electrical overcurrents that could result

from overloading of the driven machine or from excessively low line voltage.

Heater elements which are closely calibrated to the full load current of the motor are used on the solder-pot and the thermo-bimetallic types of overload relays. On the solder-pot overload relays, the heating of the element causes alloy elements to melt when there is a motor overload due to excess current in the circuit. When the alloy melts, a spring-loaded ratchet is rotated and trips open a contact which then opens the supply circuit to the motor starter coil. This stops the motor. On the bimetallic overload relays, the heating from the thermal element causes the bimetallic switch to open, opening the supply circuit and thereby stop ping the motor.

Normal motor starting currents and momen tary overloads will not cause thermal relays to trip because of their inverse-time characteristics (see Glossary). However, continuous overcurrent through the heater unit raises the temperature of the alloy elements. When the melting point is reached, the ratchet is released and the switch mechanism is tripped to open the line or lines to the motor. The switch mechanism is trip-free, which means that it is impossible to hold the con tacts closed against an overload.

Only one overload relay is required in either the single-pole or double-pole motor starter, since the starter is intended for use on de or single phase ac service. When the line current is exces sively high, these relays offer protection against continued operation. Relays with meltable alloy elements are nontemperable and give reliable overload protection. Repeated tripping does not cause deterioration nor does it affect the accuracy of the trip point.

Many types of overload relay heater units are available so that the proper one can be selected on the basis of the actual full-load current rating of the motor. The applicable relay heater units for a particular overload relay are interchangeable and are accessible from the front of the starter. Since the motor current is connected in series with the heater element, the motor will not operate unless the relay unit has the heater installed. Overload units may be changed without disconnecting the wires from the starter or removing the starter from the enclosure. However, the disconnect switch and starter should be turned off first for safety reasons. Additional instruction on motor overload protection is given in Unit 13.