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GOALS:
-- Discuss the operation of manual motor starters.
-- Discuss low-voltage release.
-- Connect a manual motor starter.
-- Check a circuit to determine whether a motor is drawing excessive current.
Manual starters are characterized by the fact that the operator must go to the location of the starter to initiate any change of action. There are several different types of manual starters. Some look like a simple toggle switch with the addition of an over load heater. Others are operated by push buttons and may or may not be capable of providing low voltage protection.
FIG. 1 Single-phase manual motor starter.
Fractional Horsepower Single-Phase Starters
One of the simplest manual motor starters resembles a simple toggle switch with the addition of an overload heater (FIG. 1). The toggle switch lever is mounted on the front of the starter and is used to control the on and off operation of the motor. In addition to being an on and off switch, the toggle switch also provides overload protection for the mo tor. An overload heater is connected in series with the motor (FIG. 2). When current flows, the heater produces heat in proportion to the amount of motor current. If the heater is sized correctly, it never gets hot enough to open the circuit under normal operating conditions. If the motor should become overloaded, however, current will increase, causing a corresponding increase in heat production by the heater. If the heat becomes great enough, it causes a mechanical mechanism to trip and open the switch contacts, disconnecting the motor from the power line. If the starter trips on overload, the switch lever moves to a center position. The starter must be reset before the motor can be restarted by moving the lever to the full OFF position. This action is basically the same as resetting a tripped circuit breaker. The starter shown in this example has only one line contact and is generally used to protect single-phase motors intended to operate on 120 volts.
Starters that are intended to protect motors that operate on 240 volts should contain two load contacts (FIG. 3). Although a starter that contains only one contact would be able to control the operation of a 240-volt motor, it could create a hazardous situation. If the motor were switched off and an electrician tried to disconnect the mo tor, one power line would still be connected directly to the motor. The National Electrical Code (NEC) requires that a disconnecting means open all un grounded supply conductors to a motor.
Manual starters of this type are intended to control fractional horsepower motors only. Motors of 1 horsepower or less are considered fractional horsepower. Starters of this type are across-the line starters. This means that they connect the motor directly to the power line. Some motors can draw up to 600% of rated full-load current during starting. These starters generally do not contain large enough contacts to handle the current surge of multi-horsepower motors.
Another factor that should be taken into consideration when using a starter of this type is that it does not provide low-voltage release. Most manual starters are strictly mechanical devices and do not contain an electrical coil. The contacts are mechanically opened and closed. This simply means that if the motor is in operation and the power fails, the motor will restart when the power is restored. This can be an advantage in some situations where the starter controls unattended devices such as pumps, fans, blowers, air conditioning, and refrigeration equipment. This feature saves the maintenance electrician from having to go around the plant and restart all the motors when power returns after a power failure.
However, this automatic restart feature can also be a disadvantage on equipment such as lathes, milling machines, saws, drill presses, and any other type of machine that may have an operator present. The unexpected and sudden re start of a piece of equipment could be the source of injury.
FIG. 2 Schematic diagram of a single-pole manual starter.
FIG. 3 Schematic diagram of a two-pole manual starter.
Mounting
Mounting a fractional horsepower, single-phase starter is generally very simple because it requires very little space. The compact design of this starter permits it to be mounted in a single gang switch or conduit box or directly onto a piece of machinery.
The open type starter can be mounted in the wall and covered with a single gang switch cover plate.
The ON and OFF markings on the switch lever make it appear to be a simple toggle switch.
Like larger starters, fractional horsepower starters are available in different enclosures. Some are simple sheet metal and are intended to be mounted on the surface of a piece of machinery.
If the starter is to be mounted in an area containing hazardous vapors or gasses, it may require an explosion-proof enclosure (FIG. 4). Areas that are subject to high moisture may require a water proof enclosure (FIG. 5). In areas that have a high concentration of flammable dust, the starter may be housed in a dustproof enclosure similar to the one shown in FIG. 6.
FIG. 4 explosion-proof enclosure.
FIG. 5 Waterproof enclosure.
FIG. 6 Dustproof enclosure.
Automatic Operation
It is sometimes necessary to combine the manual starter with other sensing devices to obtain the de sired control. When using a sensing pilot device to directly control the operation of a motor, you must make sure that the type of pilot device is equipped with contacts that can handle the rated current of the motor. These devices are generally referred to as "line-voltage" devices. Line-voltage devices have larger contacts than those sensing pilot devices intended for use in a motor control circuit that employs a magnetic motor starter. The smaller pi lot devices intended for use with magnetic motor starters have contacts that are typically rated from 1 to 3 amperes. Line-voltage devices may have contacts rated for 15 to 20 amperes. A good example of how a line-voltage sensing device can be used in conjunction with the manual starter is shown in FIG. 7. In this circuit, a line-voltage thermostat is used to control the operation of a blower motor. When the temperature rises to a sufficient level, the thermostat contacts close, connecting the motor directly to the power line if the manual starter contacts are closed. When the temperature drops, the thermostat contact opens and turns off the motor. A line-voltage thermostat is shown in FIG. 8.
Another circuit that permits the motor to be controlled either manually or automatically is shown in FIG. 9. In this circuit, a manual-automatic switch is used to select either manual or automatic operation of a pump. The pump is used to refill a tank when the water falls to a certain level.
The schematic is drawn to assume that the tank is full of water during normal operation.
In the Manual position, the pump is controlled by turning the starter on or off. An amber pilot light is used to indicate when the manual starter contacts are closed or turned on. If the manual-automatic switch is moved to the Automatic position (FIG. 10), a line-voltage float switch controls the operation of the pump motor. If the water in the tank drops to a low enough level, the float switch contact closes and starts the pump motor. If water rises to a high enough level, the float switch contact opens and disconnects the pump motor from the line.
FIG. 7 A line voltage thermostat controls the operation of a blower
motor.
FIG. 8 Line voltage thermostat.
FIG. 9 Pump can be controlled either manually or automatically.
FIG. 10 Moving the switch to the Auto. position permits the float switch
to control the pump.
Manual Push Button Starters
Manual push button line-voltage starters are manufactured with two or three load contacts. The two contact models are intended to control single-phase motors that operate on 240 volts, or direct-current motors. The starters that contain three contacts are intended to control three-phase motors. Push button type manual starters are integral, not fractional, horsepower starters. Generally, they can control single-phase motors rated up to 5 horse power, direct-current motors up to 2 horsepower, and three-phase motors up to 10 horsepower.
A typical three-contact, manual push button starter is shown in FIG. 11. A schematic diagram for this type of starter is shown in FIG. 12.
If any one of the overloads should trip, a mechanical mechanism opens the load contacts and disconnects the motor from the line. Once the starter has tripped on overload, it must be reset before the motor can be restarted. After allowing enough time for the overload heaters to cool, the operator resets the starter by pressing the STOP push button with more than normal pressure. This causes the mechanical mechanism to reset so that the motor can restart when the START push button is pressed. These starters are economical and are generally used with loads that are not started or stopped at frequent intervals. Although this type of starter provides overload protection, it does not provide low-voltage release. If the power should fail and then be restored, the motor controlled by this starter restarts without warning.
FIG. 11 Three-phase line-voltage manual starter.
FIG. 12 Schematic diagram for a three-pole line-voltage manual starter.
FIG. 13 A solenoid coil senses the line voltage.
Low-Voltage Release and Low-Voltage Protection
Manual starters can be obtained that provide low voltage release or low-voltage protection. Both employ a solenoid coil connected across the incoming power that senses the line voltage, FIG. 13. If the incoming voltage should drop to an abnormally low level, the motor disconnects from power. The difference between low-voltage release and low voltage protection is that starters equipped with low-voltage release automatically restart when power is restored to its normal level, and starters equipped with low-voltage protection must be manually reset when power is restored. Low-volt age release should be used only when the sudden restarting of a motor will not endanger personnel or equipment. A manual starter with low-voltage protection is shown in FIG. 14.
Troubleshooting
Anytime a motor has tripped on overload, the electrician should check the motor and circuit to determine why the overload tripped. The first step is generally to determine whether the motor is actually overloaded. Some common causes of mo tor overloads are bad bearings in either the motor or the load the motor operates. Shorted windings in the motor can cause the motor to draw excessive current without being severe enough to blow a fuse or trip a circuit breaker. The simplest way to deter mine whether the motor is overloaded is to find the motor full-load current on the name plate and then check the running current with an ammeter (FIG. 15). When checking a single phase motor, it is necessary to check only one of the incoming lines. When checking a three-phase motor, check each line individually. The current flow in each line of a three-phase motor should all be close to the same. A small amount of variation is not uncommon, but if the current is significantly different in any of the lines, that is an indication of internally shorted windings. Over loads are generally set to trip at 115% to 125% of motor full-load current, depending on the motor.
If the ammeter reveals that the motor is drawing excessive current, the electrician must determine the reason before the motor can be put back into operation.
FIG. 14 Manual starter with low-voltage protection.
FIG. 15 Checking motor current.
Excessive current is not the only cause for an overload trip. Thermal overloads react to heat, so any heat source can cause an overload to trip. If the motor is not drawing an excessive amount of cur rent, the electrician should determine any other sources of heat. Loose connections are one of the greatest sources of heat. Check the wires for insulation that has been overheated close to terminal screws. Any loose connection on the starter can cause an overload trip; make sure that all connections are tight. Another source of heat is ambient, or surrounding, air temperature. In hot climates, the surrounding air temperature combined with the heat caused by motor current can be enough to cause the overload to trip. It may be necessary to set a fan that blows on the starter to help remove excess heat. Manual starters that are installed in a switchbox inside a wall are especially susceptible to ambient temperature problems. In this case, it may be necessary to install some type of vented cover plate.
QUIZ
1. A single-phase, 120 volt motor is controlled by a manual motor starter. The motor is not running, and the switch handle on the starter is found to be in the center position. What does this indicate?
2. Referring to the above question, what action is necessary to restart the motor, and how is it accomplished?
3. A single-phase motor operates on 240 volts. Why should a starter that contains two load contacts be used to control this motor?
4. A push button manual starter has tripped on overload. Explain how to reset the starter so the motor can be restarted.
5. What is meant by the term line voltage on some pilot sensing devices?
6. Explain the difference between manual motor starters that provide low-voltage release and those that do not.
7. What is the simplest way to determine whether a motor is overloaded?
8. Refer to the circuit shown in FIG. 7. What type of switch is connected in series with the motor, and is the switch normally open; normally closed; normally open, held closed; or normally closed, held open?
9. Refer to the circuit shown in FIG. 10. When would the amber pilot light be turned on?
a. When the manual-automatic switch is set to the man. position.
b. When the float switch contacts are closed.
c. Anytime the manual starter is turned on.
d. Only when the manual-automatic switch is set to the MAN. position.
10. Refer to the circuit shown in FIG. 10. Is the float switch normally open; normally closed, normally open, held closed; or normally closed, held open?