Industrial Motor Control: Overload Relays

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-- Discuss differences between fuses and overloads.

-- List different types of overload relays.

-- Describe how thermal overload relays operate.

-- Describe how magnetic overload relays operate.

-- Describe how dashpot overload relays operate.


Overloads should not be confused with fuses or circuit breakers. Fuses and circuit breakers are de signed to protect the circuit from a direct ground or short-circuit condition. Overloads are designed to protect the motor from an overload condition.

Assume, for example, that a motor has a full-load current rating of 10 amperes. Also assume that the motor is connected to a circuit that is protected by a 20-ampere circuit breaker, FIG. 1. Now assume that the motor becomes overloaded and has a current draw of 15 amperes. The motor is drawing 150% of full-load current. This much of an overload will overheat the motor and damages the windings.

But, because the current is only 15 amperes, the 20-ampere circuit breaker does not open the circuit to protect the motor. Overload relays are designed to open the circuit when the current becomes 115% to 125% of the motor full-load current. The setting of the overload is dependent on the properties of the motor that is to be protected.

Overload Properties

There are certain properties that all overload relays must possess in order to protect a motor:

1. They must have some means of sensing motor current. Some overload relays do this by converting motor current into a proportion ate amount of heat, and others sense motor current by the strength of a magnetic field.

2. They must have some type of time delay.

Motors typically have a current draw of 300% to 800% of motor full-load current when they start. Motor starting current is referred to as locked rotor current. Because overload relays are generally set to trip at 115% to 125% of full-load motor current, the motor could never start if the overload relay tripped instantaneously.

3. They are divided into two separate sections: the current sensing section and the contact section. The current sensing section is connected in series with the motor and senses the amount of motor current. This section is typically connected to voltages that range from 120 volts to 600 volts. The contact section is part of the control circuit and operates at the control circuit voltage. Control circuit voltages generally range from 24 volts to 120 volts, although some controls operate on line voltages of 240 or 480 volts.

Dual-Element Fuses

There are some fuses that are intended to provide both short-circuit protection and overload protection. These fuses are called dual-element time-de lay fuses. They contain two sections (FIG. 2). The first contains a fuse link that is designed to open quickly under a large amount of excessive current. This protects the circuit against direct grounds and short circuits. The second section acts more slowly; it contains a solder link that is connected to a spring. The solder is a highly con trolled alloy designed to melt at a particular temperature. If motor current becomes excessive, the solder melts and the spring pulls the link apart.

The desired time delay is achieved because of the time it takes for the solder to melt even under a large amount of current. If motor current returns to normal after starting, the solder does not get hot enough to melt.

FIG. 1 The circuit breaker does not protect the motor from an overload.

FIG. 2 Dual-element time-delay fuse.

FIG. 3 Construction of a typical solder pot overload.

FIG. 4 Melting alloy thermal overload relay. A spring pushes the contacts open if heat melts the solder and permits the serrated wheel to turn freely. Note the electrical symbols for the normally closed overload contact and the heater element.

Thermal Overload Relays

There are two major types of overload relays: thermal and magnetic. Thermal overloads operate by connecting a heater in series with the motor. The amount of heat produced is dependent on motor current. Thermal overloads can be divided into two types: solder melting type, or solder pot, and bimetal strip type.

Because thermal overload relays operate on the principle of heat, they are sensitive to ambient (surrounding air) temperature. They trip faster when located in a warm area than they do in a cool area.

FIG. 5A Solder melting-type overload heater.

FIG. 5B Solder melting overload heater for an Allen-Bradley overload relay.

FIG. 6 Typical alloy melting type of single-phase overload relay.

Solder Melting Type

Solder melting-type overloads are often called solder pot overloads. To create this type of overload, a brass shaft is placed inside a brass tube. A serrated wheel is connected to one end of the brass shaft. A special alloy solder that melts at a very specific temperature keeps the brass shaft mechanically connected to the brass tube (FIG. 3). The serrated wheel keeps a set of spring loaded contacts closed (FIG. 4). An electric heater is placed around or close to the brass tube. The heater is connected in series with the motor. Motor current causes the heater to pro duce heat. If the current is great enough for a long enough period of time, the solder melts and permits the brass shaft to turn inside the tube, causing the contact to open. The fact that some amount of time must elapse before the solder can become hot enough to melt provides the time delay for this overload relay. A large overload causes the solder to melt faster and the contacts to open more quickly than a smaller amount of overload current.

Solder melting-type overload heaters are constructed differently by different manufacturers, but all work on the same principle. Two different types of melting alloy heater assemblies are shown in FIG. 5, parts A and B. A typical melting alloy-type over load relay is shown in FIG. 6. After the overload relay has tripped, it is necessary to allow the relay to cool for two or three minutes before it can be reset.

This cool-down time is necessary to permit the solder to become hard again after it has melted.

The trip current setting can be changed by changing the heater. Manufacturers provide charts that indicate what size heater should be installed for different amounts of motor current. It is necessary to use the chart that corresponds to the particular type of overload relay. Not all charts present the information in the same manner. Be sure to read the instructions contained with the chart when selecting heater sizes. A typical over load heater chart is shown in FIG. 7.

FIG. 7 Typical overload heater chart.

FIG. 8 A bimetal strip is constructed by bonding two different types of metal together.

Bimetal Strip Overload Relay

The second type of thermal overload relay is the bi metal strip overload. Like the melting alloy type, it operates on the principle of converting motor cur rent into a proportionate amount of heat. The difference is that the heat is used to cause a bimetal strip to bend or warp. A bimetal strip is made by bonding together two different types of metal that expand at different rates (FIG. 8). Because the metals expand at different rates, the strip bends or warps with a change of temperature (FIG. 9). The amount of warp is determined by

1. the type of metals used to construct the bimetal strip.

2. the difference in temperature between the two ends of the strip.

3. the length of the strip.

The overload heater heats the bimetal strip when motor current flows through it. The heat causes the bimetal strip to warp. If the bimetal strip becomes hot enough, it causes a set of contacts to open (FIG. 10). Once the overload contact has opened, about 2 minutes of cool-down time is needed to permit the bimetal strip to return to a position that permits the contacts to be re-closed. The time-delay factor for this overload relay is the time required for the bimetal strip to warp a sufficient amount to open the normally closed contact. A large amount of overload current causes the bimetal strip to warp at a faster rate and opens the contact sooner.

Most bimetal strip-type overload relays have a couple of features that are not available with solder melting-type overload relays. As a general rule, the trip range can be adjusted by turning a knob, as shown in FIG. 10. This knob adjusts the distance the bi metal strip must warp before opening contacts. This adjustment permits the sensitivity to be changed due to changes in ambient air temperature. If the knob is set in the 100% position (FIG. 11), the overload operates at the full-load current rating as determined by the size of overload heater installed. In cold winter months, this setting may be too high to protect the motor. The knob can be adjusted in cold conditions to operate at any point from 100% to 85% of the motor full-load current. In hot summer months, the motor may "nuisance trip" due to high ambient temperatures. For hot conditions, the adjustment knob permits the overload relay to be adjusted between 100% and 115% of motor full-load current.

FIG. 9 A bimetal strip warps with a change of temperature.

FIG. 10 Bimetal strip type of overload relay.

Another difference from the solder melting-type is that many bimetal strip-type overload relays can be set for either manual or automatic reset. A spring located on the side of the overload relay permits this setting (FIG. 12). When set in the manual position, the contacts must be reset manually by pushing the reset lever. This is probably the most common setting for an overload relay. If the overload relay has been adjusted for automatic reset, the contacts re-close by themselves after the bimetal strip has cooled sufficiently. This may be a safety hazard if it could cause the sudden restarting of a machine.

Overload relays should be set in the automatic reset position only when there is no danger of someone being hurt or equipment being damaged when the overload contacts suddenly re-close.

FIG. 11 An adjustment knob permits the current setting to be adjusted between 85% and 115% of the heater rating.

FIG. 12 Many bimetal strip-type overload relays can be adjusted for manual or automatic reset.

FIG. 13 A single-overload relay is used to protect a single-phase motor.

Three-Phase Overloads

The overload relays discussed so far are intended to detect the current of a single conductor supplying power to a motor (FIG. 13). An application for this type of overload relay is to protect a single-phase or direct-current motor. NEC requires only one overload sensor device to protect a direct current motor or a single-phase motor, whether it operates on 120 or 240 volts. Three-phase motors, however, must have an overload sensor (heaters or magnetic coils) in each of the three-phase lines.

Some motor starters accomplish this by employing three single-overload relays to independently sense the current in each of the three-phase lines (FIG. 14). When this is done, the normally closed contact of each overload relay is connected in series as shown in FIG. 15. If any one of the relays should open its normally closed contact, power to the starter coil is interrupted and the motor is disconnected from the power line.

Overload relays are also made that contain three overload heaters and one set of normally closed contacts, FIG. 16. These relays are generally used to protect three-phase motors. Although there is only one set of normally closed contacts, if an overload occurs on any one of the three heaters, it causes the contacts to open and disconnect the coil of the motor starter (FIG. 17).

FIG. 14 Three single-phase overload relays are used to sense the current in each line of a three-phase motor.

FIG. 15 When three single-phase overload relays are employed to protect a three-phase motor, the normally closed contacts of each overload relay are connected in series.

FIG. 16 Three-phase thermal overload relay.

Magnetic Overload Relays

Magnetic-type overload relays operate by sensing the strength of the magnetic field produced by the current flowing to the motor. The greatest difference between magnetic type and thermal type overload relays is that magnetic types are not sensitive to ambient temperature. Magnetic-type over load relays are generally used in areas that exhibit extreme changes in ambient temperature. Magnetic overload relays can be divided into two major types: electronic and dashpot.

Electronic Overload Relays

Electronic overload relays employ a current trans former to sense the motor current. The conductor that supplies power to the motor passes through the core of a toroid transformer (FIG. 18). As current flows through the conductor, the alternating magnetic field around the conductor induces a voltage into the toroid transformer. The amount of induced voltage is proportional to the amount of current flowing through the conductor. This is the same basic principle of operation employed by most clamp-on-type ammeters. The voltage induced into the toroid transformer is transmitted through a connected electronic interface that pro vides the time delay necessary to permit the mo tor to start. Many electronic-type overload relays are programmable and can be set for the amount of full-load motor current, maximum and minimum voltage levels, percentage of overload, and other factors. A three-phase electronic overload relay is shown in FIG. 19.

FIG. 17 A three-phase overload relay contains three overload heaters but one set of normally closed contacts.

FIG. 18 electronic overloads sense motor current by measuring the strength of a magnetic field.

FIG. 19 Three-phase electronic overload relay.

FIG. 20 A dashpot timer consists mainly of a piston, shaft, and container.

FIG. 21 Basic construction of a dashpot timer.

FIG. 22 Setting the opening of the orifices affects the time delay of the dashpot timer.

FIG. 23 Dashpot overload relays contain coils that are connected in series with the motor.

Dashpot Overload Relays

Dashpot overload relays receive their name from the device used to accomplish the time delay that permits the motor to start. A dashpot timer is basically a container, a piston, and a shaft (FIG. 20). The piston is placed inside the container, and the container is filled with a special type of oil called dashpot oil (FIG. 21). Dashpot oil maintains a constant viscosity over a wide range of temperatures. The type and viscosity of oil used is one of the factors that determines the amount of time delay for the timer. The other factor is the setting of the opening of the orifice holes in the piston (FIG. 22). Orifice holes permit the oil to flow through the piston as it rises through the oil. The opening of the orifice holes can be set by adjusting a sliding valve on the piston.

The dashpot overload relay contains a coil that is connected in series with the motor (FIG. 23).

As current flows through the coil, a magnetic field is developed around the coil. The strength of the magnetic field is proportional to the motor cur rent. This magnetic field draws the shaft of the dashpot timer into the coil. The shaft's movement is retarded by the fact that the piston must displace the oil in the container. If the motor is operating normally, the motor current will drop to a safe level before the shaft is drawn far enough into the coil to open the normally closed contact (FIG. 24). If the motor is overloaded, however, the magnetic field will be strong enough to continue drawing the shaft into the coil until it opens the overload contact. When power is disconnected from the motor, the magnetic field collapses and the piston returns to the bottom of the container.

Check valves permit the piston to return to the bottom of the container almost immediately when motor current ceases.

Dashpot overloads generally provide some method that permits the relay to be adjusted for different full-load current values. To make this adjustment, the shaft is connected to a threaded rod (FIG. 25). This permits the shaft to be lengthened or shortened inside the coil. The greater the length of the shaft, the less current is required to draw the shaft into the coil far enough to open the contacts. A nameplate on the coil lists the different current settings for a particular overload relay (FIG. 26). The adjustment is made by moving the shaft until the line on the shaft representing the desired current is flush with the top of the dashpot container (FIG. 27). A dashpot over load relay is shown in FIG. 28.

FIG. 24 Normally closed contacts of a dashpot overload relay.

FIG. 25 The length of the shaft can be adjusted for different values of current.

FIG. 26 The nameplate lists different current values.

FIG. 27 The line on the shaft that represents the desired amount of current is set flush with the top of the dashpot container.

FIG. 28 Dashpot overload relay.

FIG. 29 Overload relay containing both a normally closed and normally open contact. The normally closed contact is labeled OL and the normally open contact is labeled ALAR. (The common contact is labeled COM.)

FIG. 30 The overload relay contains a single-pole, double-throw set of contacts. The normally closed section (NC) protects the motor in the event of an overload condition and the normally open section (NO) turns on an indicator lamp to alert an operator that the motor has tripped on overload.

Overload Contacts

Although all overload relays contain a set of normally closed contacts, some manufacturers also add a set of normally open contacts as well. These two sets of contacts are either in the form of a single pole, double-throw switch or two separate contacts.

The single-pole, double-throw switch arrangement contains a common terminal (C), a normally closed terminal (NC), and a normally open terminal (NO) (FIG. 29). There are several reasons for adding the normally open set of contacts. The starter shown in FIG. 30 uses the normally closed section to disconnect the motor starter in the event of an overload and uses the normally open section to turn on an indicator light to inform an operator that the overload has tripped.

The overload relay shown in FIG. 31 contains two separate sets of contacts, one normally open and the other normally closed. Another common use for the normally open set of contacts on an overload relay is to provide an input signal to a programmable logic controller (PLC). If the over load trips, the normally closed set of contacts opens and disconnects the starter coil from the line. The normally open set of contacts closes and provides a signal to the input of the PLC (FIG. 32). Notice that two interposing relays, CR1 and CR2, are used to separate the PLC and the motor starter.

This is often done for safety reasons. The control relays prevent more than one source of power from entering the starter or PLC. Note that the starter and PLC each have a separate power source. If the power were disconnected from the starter during service or repair, it could cause an injury if the power from the PLC were connected to any part of the starter.

FIG. 31 An overload relay that contains a normally closed and a normally open contact.

FIG. 32 The normally open contacts provide a signal to the input of a programmable logic controller.

Protecting Large Horsepower Motors Large horsepower motors often have current draws of several hundred amperes, making the sizing of overload heaters difficult. When this is the case, it is common practice to use current transformers to reduce the amount of current to the overload heaters (FIG. 33). The current transformers shown in FIG. 33 have ratios of 150:5. This means that when 150 amperes of current flows through the primary, which is the line connected to the motor, the transformer secondary produces a cur rent of 5 amperes if the secondary terminals are shorted together. The secondaries of the current transformers are connected to the overload heaters to provide protection for the motor (FIG. 34).

FIG. 33 Current transformers are used to reduce overload current.

FIG. 34 Current transformers reduce the current to the overload heaters.

Assume that the motor connected to the current transformers in FIG. 34 has a full-load cur rent of 136 amperes. A simple calculation reveals that current transformers with a ratio of 150:5 would produce a secondary current of 4.533 amperes when 136 amperes flow through the primary.

150/5 = 136/X

150X = 680

X = 680/150

X = 4.533

The overload heaters would actually be sized for a motor with a full-load current of 4.533 amperes.


1. What are the two basic types of overload relays?

2. What is the major difference in characteristics between thermal-type and magnetic-type over load relays?

3. What are the two major types of thermal over load relays?

4. What type of thermal overload relay can generally be set for manual or automatic operation?

5. Why is it necessary to permit a solder melting- type of overload relay to cool for 2 to 3 minutes after it has tripped?

6. All overload relays are divided into two sections. What are these two sections?

7. What device is used to sense the amount of motor current in an electronic overload relay?

8. What two factors determine the time setting for a dashpot timer?

9. How many overload sensors are required by the NEC to protect a direct-current motor?

10. A large motor has a full-load current rating of 425 amperes. Current transformers with a ratio of 600:5 are used to reduce the current to the overload heaters. What should be the full load current rating of the overload heaters?

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