Fractional / Integral Horsepower Manual Motor Starters

Home | Articles | Forum | Glossary | Books


• 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.


Ill. 1 Open-type starter with overload heater.

One of the simplest types of motor starters is an on-off, snap action switch operated by hand on a toggle lever mounted on the front of the starter, Ill. 1. The motor is connected directly across the matching line voltage when the handle is turned to the START position.

This situation usually isn't objectionable with motors rated at 1 hp or less. Because a motor may draw up to a 600 percent current surge on starting, larger motors shouldn't be connected directly across the line on startup. 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) manual motor starters are used whenever it's desired 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 FHB motors be provided with overload protection whenever they are started automatically or by remote control. Basically, a manual starter is an on-off switch with motor overload protection.

Because manual starters are hand-operated mechanical 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 isn't possible with manually operated starters. This automatic restart action is an advantage when the starter is used with motors that run continuously, such as those used on unattended 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 disadvantage 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 avail able space is limited. The unenclosed, or open starter, can be mounted in a standard switch or conduit box installed in a wall and can be covered with a standard 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, Ill. 1.

Application FHB manual starters have thermal overload protection, Ill. 1 and Ill. 2.When an overload occurs, the starter handle automatically 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 2 minutes for the heater to cool. Should the circuit trip open again, the fault should be located and corrected.

FHB 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. Enclosures 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, Ill. 3, or to prevent the possibility of an explosion when the starter is used in hazardous locations. These different types of enclosures are discussed in more detail in Unit 3.

Ill. 2 Diagram of single-pole manual starter shown in Ill. 1.

Ill. 3 Watertight and dust-tight manual starter enclosure.


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 FHB manual starters.

The schematic diagram shown in Ill. 4 illustrates a fractional horsepower an FHB motor controlled automatically by a float switch that's remotely 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 Ill. 5 and Ill. 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 contact, and starts the motor.

When the motor pumps the sump or tank empty, the float lowers and breaks electrical contact with the motor thus stopping the motor.

This cycle of events will repeat when the sump fills again, automatically, without a human operator.

Ill. 4 Automatic control with FHP-single-pole, single-phase manual starter for FHP motor without selector switch.

Ill. 5 Manual FHP-double-pole, single-phase starter with automatic control for FHP motor using selector switch.

Ill. 6 FHP manual starter with selector switch mounted in general-purpose enclosure.

Note that a double-pole starter is used in Ill. 5. This type of starter is required when both lines to the motor must be broken such as for 240 volts, single phase. The double-pole starter is also recommended for heavy-duty applications because of its higher interrupting capacity and longer contact life when using a two-pole motor starter in 120 volts. Normally, the single-pole motor starter is used for 120 volts.


These 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 DC 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 Ill. 7 and Ill. 8.

Ill. 7 Three-phase line voltage manual starter.

Ill. 8 Wiring diagram for the three-pole line voltage push-button manual starter seen in Ill. 7.

Ill. 9 (A) Wiring diagram for integral hp manual starter with low-voltage protection. (B) Manual starter with low-voltage protection.

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 thermal unit to cool.

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

Manual Starter with Low-Voltage Protection

Integral horsepower manual starters with low-voltage protection (LVP) to prevent automatic startup of motors after a power loss. This is accomplished with a continuous-duty electrical solenoid, which is energized whenever the line-side voltage is present and the start but ton is pressed, Ill. 9. If the line voltage is lost or disconnected, the solenoid de-energizes, opening the starter contacts. The contacts won't automatically reclose when the line voltage is restored. To close the contacts to restart the motor again, the device must be manually reset. This manual starter won't function unless the line terminals are energized. This starter shouldn't be confused with magnetic starters described in the next unit. This isn't a magnetic starter, but a lower cost starter.


Typical applications include conveyor lines, grinders, metal working machinery, mechanical power presses, mixers, woodworking machinery, and wherever job specifications and standards require low-voltage protection, or wherever machine operator safety could be in jeopardy. Therefore, this manually operated, push button starter with low-voltage protection is a method of protecting an operator from injury using automatic restart of a machine on resumption of voltage after a power failure. This is normally accomplished with a magnetic starter with electrical (three-wire) control.


Thermal overload units are widely used on both the fractional and integral horsepower manual starters for protection of motors from sustained electrical overcurrents that could result from overloading of the driven machine or from excessively low line voltage.

Heater elements that are closely calibrated to the full-load current of the motor are used on the solder-pot and the thermobimetallic 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 over load 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; 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 stopping the motor.

Normal motor starting currents and momentary overloads won't cause thermal relays to trip because of their inverse-time characteristics. 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's impossible to hold the contacts closed against an overload.

Only one overload relay is required in either the single-pole or double-pole motor starter, be cause the starter is intended for use on DC or single-phase AC service. When the line current is excessively high, these relays offer protection against continued operation. Relays with meltable alloy elements are nontemperable and give reliable overload protection. Repeated trip ping 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 valid relay heater units for a particular overload relay are inter changeable and are accessible from the front of the starter. Because the motor current is connected in series with the heater element, the motor won't 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 en closure. However, the disconnect switch and starter should be turned off first for safety reasons. Additional instruction on motor overload protection is given in Unit 3.


1. If the contacts on a manual starter cannot be closed immediately after a motor over load has tripped them open, what is the probable reason?

2. If the handle of an installed motor starter is in the center position, what condition does this indicate?

3. How may an automatic operation be achieved when a manual motor starter is used?

4. What does the term trip-free mean as it relates to manual starters?

5. If the overload heater element isn't installed in a two-pole manual starter, what is the result? Select the best answer for each of the following.

6. A fractional horsepower manual motor starter

a. starts and stops motors under 1 hp

b. has a toggle-switch type of handle and size

c. cannot offer low-voltage protection

d. does all of these

7. An automatic operation can be achieved with

a. an integral horsepower manual starter with LVP coil

b. a push-button-type manual starter

c. an FHP manual starter wired in conjunction with a pressure switch

d. a fractional hp manual starter

8. Low-voltage (or undervoltage) protection (LVP) is available on some

a. integral hp starters

b. FHP manual starters

c. two-pole toggle switches

d. all of the above

9. LVP is used mainly for

a. fans

b. air compressors

c. pumps

d. protection of the operator

10. Thermal overload protection

a. aids motor warm-up

b. prevents motors from freezing

c. protects motors and conductors from mechanical damage

d. provides overcurrent protection for motors

Top of Page

PREV: General Principles of Electric Motor Control NEXT: Magnetic Line Voltage Starters Article Index HOME