Everything you need to know about motor starters

Magnetic Motor Starters

The magnetic contactor is primarily used to switch power in resistance heating elements, lighting, magnetic brakes, and heavy industrial solenoids. It can also be employed to switch motors if separate overload protection is provided. A magnetic motor starter, in its simplest form, is a contactor with an overload protective device called an overload relay (OL) physically and electrically connected. The overload relay safeguards the motor against overheating and damage due to excessive heat. Typically, magnetic starters are equipped with manufacturer-installed control wiring that may include:

• A wire connected from the overload relay contacts to the starter coil
• A wire connected from the other side of the starter coil to the holding contacts
• A wire connected from L2 to the other side of the overload relay contacts (note that this wire must be removed when a control transformer is used)

Magnetic motor starters are the most common type of motor starter. They consist of a two-, three-, or four-pole magnetic contactor and an overload relay mounted in an enclosure. Enclosures are boxes that protect the motor control devices inside them from the environment. They can be made of general-purpose sheet metal, dust-tight, water-tight, or explosion-resistant materials, depending on the needs of the installation.

Start and stop push buttons may be mounted on the cover of the enclosure, or they may be mounted separately. In the latter case, only the reset button for the overload relay would be mounted on the cover. Some motor starters are built in skeleton form, without an enclosure. These are typically mounted in a motor control center or control panel on a machine.

The control circuit of a magnetic motor starter is very simple. When the start button is pressed, the starter coil is energized. This causes the contactor contacts to close, which allows current to flow to the motor. When the stop button is pressed or when the overload relay trips, the starter coil is de-energized and the contactor contacts open, stopping the motor.

Motor Overcurrent Protection

Motor branch circuits are electrical circuits that power motors. They have special requirements to protect the motor from damage and to prevent fires.

One important requirement is overload protection. Overload protection devices prevent the motor from drawing too much current and overheating. This can happen if the motor is overloaded, if the line voltage is too low, or if a phase is lost on a three-phase motor.

Motor overload protection devices are typically integrated into the motor starter. Motor starters are devices that switch the power to the motor on and off.

Another important requirement is short-circuit and ground-fault protection. This protects the motor circuit from the high currents that can flow during a short circuit or ground fault. Short-circuit and ground-fault protection is typically provided by fuses or circuit breakers.

Fuses and circuit breakers connected to motor circuits must be able to withstand the high inrush current that flows when the motor is first started. This inrush current is often 20 times the normal full-load current of the motor.

In addition to overload and short-circuit/ground-fault protection, motor circuits also require a disconnecting means. This is a device that allows the motor circuit to be safely disconnected from power. Disconnect switches are typically located near the motor.

Contactors are not suitable for use with motors because they cannot withstand the high starting currents and other load characteristics of motors.

The purpose of overload protection is to protect the motor windings from overheating due to overloading. If the motor is overloaded for a short period of time, it will not be damaged. However, if the overload persists, the overload relay will trip and shut off the motor to prevent damage.

Contactor use is restricted to fixed lighting loads, electric furnaces, and other resistive loads that have set current values. Motors are subject to high starting currents and periods of load, no-load, short duration overload, and so on. They must have protective devices with the flexibility required of the motor and driven equipment. The purpose of overload protection is to protect the motor windings from excessive heat resulting from motor overloading. The motor windings will not be damaged when overloaded for a short period of time. If the overload should persist, however, the sustained increase in current should cause the overload relay to operate, shutting off the motor.

Motor Overload Relays

Overload relays are designed to meet the special protective needs of motor control circuits. Overload relays:

• Allow harmless temporary overloads (such as motor starting) without disrupting the circuit.
• Will trip and open a circuit if current is high enough to cause motor damage over a period of time.
• Can be reset once the overload is removed.

Overload relays are classified into trip classes, which determine the time it takes for the relay to trip during an overload condition. The most common trip classes are Class 10, Class 20, and Class 30. For instance, a Class 10 overload relay must trip the motor offline in 10 seconds or less at 600 percent of the full-load amperes, which is typically sufficient time for the motor to reach full speed. The class designation is a crucial factor when applying overload relays in motor-control circuits. In certain cases, a high-inertia industrial load may necessitate a Class 30 overload relay that trips in 30 seconds instead of a Class 10 or 20. The overload relay itself will be marked to indicate its class .

Overload protection devices typically incorporate a trip indicator to notify the operator when an overload has occurred. Overload relays can be equipped with either manual or automatic reset functionality. A manual reset necessitates operator intervention, such as pressing a button, to restart the motor. Conversely, an automatic reset enables the motor to restart automatically, usually after a cooling-off period, allowing sufficient time for the motor to cool down. After an overload relay has tripped, it is important to investigate the cause of the overload. Repeated attempts to reset the relay without addressing the underlying cause of the overload can lead to motor damage. The nominal current setting allows the relay to be adjusted to match the full-load current specified on the motor rating plate and can be further fine-tuned to achieve the desired trip point.

External overload protection devices, which are mounted in the starter, aim to monitor the motor’s heating and cooling by sensing the current flowing through it. The current drawn by the motor is a reasonably accurate measure of the motor’s load and, consequently, its heating. Overload relays can be categorized as thermal, magnetic, or electronic.

THERMAL OVER LOAD RELAYS

Thermal overload relays are a type of overload protection device that uses a heater to detect and protect against motor overload. The heater is connected in series with the motor supply, so current flowing to the motor must also flow through the heater. If the motor becomes overloaded, the heater will generate more heat. This heat causes a set of contacts in the overload relay to open, interrupting the circuit to the motor. Thermal overload relays are very effective because they closely approximate the actual temperature of the motor windings. They also have a thermal memory, which means that they will not reset immediately after an overload. This prevents the motor from being restarted too soon and being damaged.

Thermal overload relays can be further subdivided into two types: melting alloy and bimetallic. The melting alloy type, utilizes the principle of heating solder to its melting point. It consists of a heater coil, eutectic alloy, and mechanism to activate a tripping device when an overload occurs. The term eutectic means easily melted. The eutectic alloy in the heater element is a material that goes from a solid to liquid state without going through an intermediate putty stage. The operation of the device can be summarized as follows:

• When the motor current exceeds the rated value, the temperature will rise to a point where the alloy melts; the ratchet wheel is then free to rotate, and the contact pawl moves upward under spring pressure, allowing the control circuit contacts to open.
• After the heater element cools, the ratchet wheel will again be held stationary and the overload contacts can be reset.
• The thermal overload relay uses an integral heater element, the trip setting of which is adjustable to match the motor’s full-load amperes.

The bimetallic type of thermal overload relay uses a bimetallic strip made up of two pieces of dissimilar metal that are permanently joined by lamination. The operation of the device can be sum-marized as follows:

• Heating the bimetallic strip causes it to bend because the dissimilar metals expand and contract at different rates.
• Overload heating elements connected in series with the motor circuit heat the bimetal tripping elements in accordance with the motor load current.
• The movement/deflection of the bimetallic strip is used as a means of operating the trip mechanism and opening the normally closed overload contacts.

Thermal overload relays protect motors from overload by using a heater to detect too much current. The heater is connected in series with the motor, so the same current that flows to the motor coils also flows through the heater. If the motor becomes overloaded, the heater will generate heat. This heat causes a set of contacts in the overload relay to open, interrupting power to the motor.

The heater size for thermal overload relays is critical for ensuring maximum motor protection. The heater size is selected based on the motor’s full-load amperage (FLA) and the motor starter size. The FLA rating is found on the motor’s nameplate.

Thermal overload relays react to heat, regardless of the origin of the heat. This means that the ambient temperature can affect the tripping time of a thermal overload relay. Cooler temperatures increase tripping times, while warmer temperatures decrease tripping times.

Ambient compensated bimetal overload relays are designed to overcome this problem. They use a compensating bimetal strip along with the primary bimetal strip. As the ambient temperature changes, both bimetals will bend equally and the overload relay will not trip.

ELECTRONIC OVERLOAD RELAYS

Unlike electromechanical overload relays, which simulate motor heating indirectly by passing motor current through heating elements, an electronic overload relay directly measures motor current using a current transformer. It employs a signal from the current transformer and precision solid-state measurement components to provide a more precise indication of the motor’s thermal state. By monitoring the starting and running currents, the electronic circuitry calculates the average temperature within the motor. When a motor overload is detected, the control circuit opens the normally closed (NC) overload relay contacts.

Incorporated with a separate phase loss detection circuit, the overload relay promptly responds to phase loss conditions. The self-enclosed latching trip relay is equipped with a set of isolated normally closed (NC) and normally open (NO) contacts, which facilitate trip and reset functions for control circuits. When an overload motor condition is detected, these contacts change state and activate a control circuit that interrupts the current flow to the motor. The electronic design’s low energy consumption mitigates temperature rise concerns within control cabinets. Dual-in-line package (DIP) switch settings enable the selection of trip class (10, 15, 20, or 30) and reset mode (manual or automatic).

Advantages of solid-state electronic overload relays over thermal-overload types include the following:

• No buying, stocking, installing, or replacing of heater coils
• Reduction in the heat generated by the starter
• Energy savings (up to 24 W per starter) through the elimination of heater coils
• Insensitivity to temperature changes in the surrounding environment
• High repeat trip accuracy (±2 percent)
• Easily adjustable to a wide range of full-load motor currents

Solid-state overload relays are more popular than bimetal and eutectic trip mechanisms for newer motor control installations. Despite the differences between NEMA and IEC motor controls, the two types share a major similarity – the solid-state overload relay. There is little difference between solid-state overload relays used for either type, and in some applications, the same solid-state overload relay can be used in NEMA and IEC units, with the contactor and enclosure being the main differences between the two.

Another form of electronic overload relay is the microprocessor-based modular overload relay. They provide better motor control management by eliminating unnecessary trips and isolating faults. In addition to motor overload protection, other protective features may include overtemperature, instantaneous overcurrent, ground fault, phase loss/phase reversal/phase unbalance (both voltage and current), overvoltage, and undervoltage protection. Some units can tabulate the number of starts per programmed unit of time and lock out the starting sequence, preventing inadvertent excessive cycling.

DUAL-ELEMENT FUSES

Dual-element (time-delay) fuses protect motors from both overload and fault conditions. They have two fuse elements: an overload element and a short-circuit element.

The overload element has a calibrated fusing alloy that melts at a specific temperature. If the motor is overloaded, the overload element will melt and open the circuit, protecting the motor from damage.

The short-circuit element has a restricted portion that vaporizes when a short-circuit fault occurs. This creates an arc, which is quenched by the special granular arc-quenching filler material. This creates an insulating barrier that forces the current flow to zero, protecting the motor and other components from damage.

NEMA and IEC Symbols

Electrical control symbols vary from country to country, so motor control panels may contain products from different parts of the world with different symbols. It is important to recognize and understand these symbols in order to read motor control diagrams.

IEC devices have power terminals marked 1, 3, 5 and 2, 4, 6 corresponding in function to L1, L2, L3 and T1, T2, T3 on NEMA controllers. The terminals of the IEC auxiliary contacts are marked with a two-digit number. Terminals that belong together are marked with the same location digit (first digit). The second digits (called the function digits) indentify the function of each contact. For example, on the motor control IEC schematic:

• The numbers 13 and 14 represent an auxiliary contact.
• The number 1 identifies that this is the first contact in the sequence.
• The numbers 3 and 4 identify this is as normally open contact.
• The numbers 21 and 22 represent another auxiliary contact.
• The number 2 identifies that this is the second con-tact in the sequence.
• The numbers 13 and 14 represent an auxiliary contact.
• The numbers 1 and 2 identify this as a normally closed contact.