A manual starter is a contactor with an added overload protective device. Manual starters are used only in electrical motor circuits. The primary difference between a manual contactor and a manual starter is the addition of an overload protective device. See Figure 1.
Figure 1. A manual starter is a contactor with an added overload protective device.
The overload protective device must be added because the National Electrical Code (NEC) requires that a control device shall not only turn a motor on and off, but it shall also protect the motor from destroying itself under an overloaded situation, such as a locked rotor.
A locked rotor is a condition when a motor is loaded so heavily that the motor shaft cannot turn.
A motor with a locked rotor draws excessive current and its windings and other components will burn up if the motor is not disconnected from the line voltage. To protect the motor, the overload device senses the excessive current and opens the circuit.
Manual Motor Starter Working
A motor goes through three stages during normal operation: resting, starting, and operating under load. See Figure 2. A motor at rest requires no current because the circuit is open. A motor that is starting draws a tremendous inrush current (normally six to eight times the running current) when the circuit is closed.
Fuses or circuit breakers must have a sufficiently high ampere rating to prevent the immediate opening of the circuit caused by the large inrush current required for a motor when starting.
Figure 2. The three stages a motor goes through during normal operation include resting, starting, and operating under load.
A motor may encounter an overload while running. While it may not draw enough current to blow the fuses or trip the circuit breakers, it is large enough to produce sufficient heat to burn up the windings and other components in the motor.
The intense heat concentration generated by the excessive current in the windings causes the insulation to fail and burns the motor. It is estimated that every 1°C (1.8°F) rise over normal ambient temperature ratings for insulation can reduce the life expectancy of a motor by almost a year.
Ambient temperature is the temperature of the air surrounding a motor. The normal rating for many motors is about 40°C (104°F).
Fuses or circuit breakers must protect the circuit against the very high current of a short circuit or a ground fault.
An overload relay that does not open the circuit while the motor is starting but does open the circuit if the motor gets overloaded and the fuses do not blow is required. See Figure 3.
Figure 3. An overload relay is required that does not open a circuit while a motor is starting but opens the circuit if the motor gets overloaded and the fuses do not blow.
To meet motor protection needs, overload relays are designed to have a time delay to allow harmless, temporary overloads without disrupting the circuit.
Overload relays must also have a trip capability to open the circuit if mildly dangerous currents that could result in motor damage continue over a period of time. All overload relays have some means of resetting the circuit once the overload is removed.
Melting Alloy Overloads
Heat is the end product that destroys a motor. To be effective, an overload relay must measure the temperature of the motor by monitoring the amount of current being drawn.
The overload relay must indirectly monitor the temperature conditions of the motor because the overload relay is normally located at some distance from the motor.
One of the most popular methods of providing overload protection is to use a melting alloy overload relay.
A heater coil is a sensing device used to monitor the heat generated by excessive current and the heat created through ambient temperature rise. Many different types of heater coils are available. The operating principle of each is the same.
A heater coil converts the excess current drawn by a motor into heat, which is used to determine whether the motor is in danger. See Figure 4.
Figure 4. A heater coil is a sensing device used to monitor the heat generated by excessive current and the heat created through ambient temperature rise.
Most manufacturers rely on a eutectic alloy in conjunction with a mechanical mechanism to activate a tripping device when an overload occurs.
A eutectic alloy is a metal that has a fixed temperature at which it changes directly from a solid to a liquid state. This temperature never changes and is not affected by repeated melting and resetting.
Most manufacturers use a ratchet wheel and eutectic alloy tube combination to activate a trip mechanism when an overload occurs.
The eutectic alloy tube consists of an outer tube and an inner shaft connected to a ratchet wheel.
The ratchet wheel is held firmly in the tube by the solid eutectic alloy. The inner shaft and ratchet wheel are locked into position by a pawl (locking mechanism) so that the wheel cannot turn when the alloy is cool. See Figure 5. An excessive current applied to the heater coil melts the eutectic alloy. This allows the ratchet wheel to turn freely.
Figure 5. Most manufacturers use a ratchet wheel and eutectic alloy combination to activate a trip mechanism when an overload occurs.
The main device in an overload relay is the eutectic alloy tube. The compressed spring tries to push the NC overload contacts open when motor current conditions are normal. The pawl is caught in the ratchet wheel and does not let the spring push up to open the contacts. See Figure 6.
Figure 6. In a manual starter overload relay, the compressed spring tries to push the normally closed (NC) contacts open under normal operating condition.
The heater coil heats the eutectic alloy tube when an overload occurs. The heat melts the alloy, which allows the ratchet wheel to turn. The spring pushes the reset button up, which opens the contacts to the voltage coil of the contactor. The contactor opens the circuit to the motor, which stops the current flow through the heater coil. The heater coil cools, which solidifies the eutectic alloy tube.
Only the NC overload contacts open during an overload condition. The NC overload contacts can be manually reset to the closed position.
The actual heating elements (heaters) installed in the motor starter do not open during an overload. The heaters are only used to produce heat. The higher the current draw of the motor, the more heat produced.
Resetting Overload Devices
The cause of an overload must be found before resetting an overload relay. A relay trips on resetting if the overload is not removed.
Once the overload is removed, the device can be reset. The reset button is pushed, which forces the pawl across the ratchet wheel until the contacts are closed and the spring and ratchet wheel are returned to their original condition. The start pushbutton can then be pressed to start the motor. See Figure 7.
Figure 7. The overload relay is reset by pressing the reset button, which forces the pawl across the ratchet wheel until the contacts are closed and the spring and ratchet wheel are returned to their original condition.
Nothing requires replacement or repair when an overload device trips because the heaters do not open like a fuse would open. Once the cause of the overload is removed, the reset button may be pressed. Normally, a few minutes should be allowed for the eutectic alloy to cool.
The same basic overload relay is used with all sizes of motors. The only difference is that the heater coil size is changed. For small horsepower motors, a small heater coil is used. For large horsepower motors, a large heater coil is used. The NEC should be consulted for selecting appropriate overload heater sizes.
Selecting AC Manual Starters
Electricians are often required to select AC manual starters for new installations or replace ones that have been severely damaged due to an electrical fire or explosion. In either case, the electrician must specify certain characteristics of the starter to obtain the proper replacement. See Figure 8.
Figure 8. AC manual starters are selected based on phasing, number of poles, voltage, starter size, and enclosure type.
AC manual starters are selected based on phasing, number of poles, voltage, starter size, and enclosure type.
Starter sizes are given in general motor protection tables. General motor protection tables indicate motor protective device sizes based on motor horsepower, current, fuse classification, and wire size.
AC manual starters/contactors can be divided into single-phase and three-phase contactors. See Figure 9.
A 120 V, 1φ power source has one hot wire (ungrounded conductor) and one neutral wire (grounded conductor).
A 230 V, the 1φ power source has two hot wires, L1 and L2 (ungrounded conductors), and no neutral.
A 3φ power source has three hot wires, L1, L2, and L3, and no neutral.
Figure 9. AC manual contactors can be divided into 1φ and 3φ contactors.
Single-phase manual starters are available as single-pole and double-pole devices because the NEC requires that each ungrounded conductor (hot wire) be open when disconnecting a device. A single-pole device is used on 120 V circuits and a double-pole device is used on 230 V circuits.
Single-Phase Manual Starters
Single-phase manual starters have limited horsepower ratings because of their physical size and are normally used as starters for motors of 1 HP or less.
Single-phase manual starters are often available in only one size for all motors rated at 1 HP or less. The size established for 1φ starters is classified as NEMA size 00. IEC manual starters/contactors are horsepower rated.
Single-phase manual contactors and starters are normally used for 1φ, 1 HP or fewer motors where low-voltage protection is not needed. They are also used for 1φ motors that do not require a high frequency of operation.
Three-Phase Manual Starters
Three-phase manual starters are physically larger than 1φ manual starters and may be used for motors of 10 HP or less. Three-phase manual contactors are normally pushbutton-operated instead of toggle-operated like 1φ starters.
Motor circuits require a manual starter that has overloads. Contactors, however, can be used in certain applications, such as in lighting circuits, without overload devices. In those cases, the fuse or circuit breaker in the main disconnect provides the overload protection.
Three-phase devices are designed with three-pole switching because 3φ devices have three hot wires that must be disconnected. Similar to 1φ devices, 3φ devices use contacts and have quick-make and quick-break mechanisms.
Three-phase contactors and starters are normally designed to be used on circuits from 115 V up to and including 575 V.
Three-phase starters are normally used for 3φ, 7.5 HP and fewer motors operating at 208/230 V or 3φ, 10 HP and fewer motors operating at 380/575 V.
Three-phase starters are also used for 3φ motors where low-voltage protection is not needed, for motors that do not require a high frequency of operation, and for motors that do not need remote operation by pushbuttons or limit switches.
Enclosures provide mechanical and electrical protection for the operator and the starter.
Although the enclosures are designed to provide protection against a variety of contaminants such as water, dust, and oil, as well as contaminants from hazardous locations, the internal electrical wiring and physical construction of the starter, remain the same.
The NEC and local codes should be consulted to determine the proper selection of an enclosure for a particular application.
For example, NEMA Type 1 enclosures are intended for indoor use primarily to provide a degree of protection against human contact with the enclosed equipment in locations where unusual service conditions do not exist.
Manual Starter Applications
Manual motor starters are used in applications such as conveyor systems and drill presses. See Figure 10.
In most applications, the manual starter provides the means of turning on and off the device while providing motor overload protection.
Figure 10. Manual motor starters are used in applications such as conveyor systems and drill presses.