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Magnetic Motor Starter: Working Principle & Wiring Diagram

A magnetic motor starter is an electrically operated switch (contactor) that includes motor overload protection. Magnetic motor starters include overload relays that detect excessive current passing through a motor and are used to switch all types and sizes of motors.

Magnetic motor starters are available in sizes that can switch loads of a few amperes to several hundred amperes. See Figure 1.

magnetic motor starter

Figure 1. A magnetic motor starter is a contactor that includes overload protection added.

Overload Protection

The main difference between the sensing device for a manual motor starter and a magnetic motor starter is that on a manual motor starter a manual overload opens the power contacts on the starter. The overload device on a magnetic motor starter opens a set of contacts to the magnetic coil, de-energizing the coil and disconnecting the power.

Overload devices include melting alloy, magnetic, and bimetallic overload relays. The overload unit (heater) does not open as a fuse or CB does, but it produces the heat required to open the overload contacts.

Melting Alloy Overload Relays

 The melting alloy overload relays used in magnetic motor starters are similar to the melting alloy overload relays used in manual motor starters. They consist of a heater coil, eutectic alloy, and mechanical mechanism to activate a tripping device when an overload occurs.

Magnetic Overload Relays

 Magnetic overload relays provide another means of monitoring the amount of current drawn by a motor. A magnetic overload relay operates through the use of a current coil.

At a specified overcurrent value, the current coil acts as a solenoid, causing a set of normally closed contacts to open. This causes the circuit to open and protect the motor by disconnecting it from power. See Figure 2.

Magnetic overload relay

Figure 2. Magnetic overload relays use a current coil that, at a specific overcurrent value, acts like a solenoid and causes a set of normally closed contacts to open.

Magnetic overload relays are used in special applications such as steel mill processing lines or other heavy-duty industrial applications where holding a specified level of motor current is required.

 A magnetic overload relay is also ideal for special applications such as slow-acceleration motors, high-inrush-current motors, or any use where normal time/current curves of thermal overload relays do not provide satisfactory operation. This flexibility is made possible because the magnetic unit may be set for either instantaneous or inverse time-tripping characteristics. The device may also offer independently adjustable trip time and trip current.

Magnetic overload relays are extremely quick to reset because they do not require a cooling-off period before being reset. Magnetic overload relays are much more expensive than thermal overload relays.

Bimetallic Overload Relays

In certain applications such as walk-in meat coolers, remote pumping stations, and some chemical process equipment, overload relays that reset automatically to keep the unit operating up to the last possible moment may be required.

A bimetallic overload relay is an overload relay that resets automatically. Bimetallic overload relays operate on the principle of the bimetallic strip. A bimetallic strip is made of two pieces of dissimilar metal that are permanently joined by lamination.

Heating the bimetallic strip causes it to warp because the dissimilar metals expand and contract at different rates. The warping effect of the bimetallic strip is used as a means of separating contacts. See Figure 3. Once the tripping action has taken place, the bimetallic strip cools and reshapes itself.

In certain devices, such as circuit breakers, a trip lever needs to be reset to make the circuit operate again. In other devices, such as bimetallic overload relays, the device automatically resets the circuit when the bimetallic strip cools and reshapes itself.

The motor restarts even when the overload has not been cleared and trips and resets itself again at given intervals. Care must be exercised in the selection of a bimetallic overload relay because repeated cycling eventually burns out the motor.

The bimetallic strip may be shaped in the form of a U. The U-shape provides a uniform temperature response.

bimetallic overload relay

Figure 3. The warping effect of a bimetallic strip is used as a means for separating contacts.

Trip Indicator

 Many overload devices have a trip indicator built into the unit to indicate to the operator that an overload has taken place within the device. See Figure 4.

A red metal indicator appears in a window located above the reset button when the overload relay has tripped.

The red indicator informs the operator or electrician why the unit is not operating and that it is potentially capable of restarting with an automatic reset.

Trip indicator

Figure 4. Trip indicators indicate that an overload has taken place within the device.

Overload Current Transformers

Large-horsepower motors have currents that exceed the values of standard overload relays. To make the overload relays larger would greatly increase their physical size, which would create a space problem in relation to the magnetic motor starter. To prevent such a conflict, current transformers are used to reduce the current in a fixed ratio. See Figure 5.

 A current transformer is used to change the amount of current flowing to a motor but reduces the current to a lower value for the overload relay. For example, if 50 A were flowing to a motor, only 5 A would flow to the overload relay through the use of the current transformer.

Standard current transformers are normally rated in primary and secondary rated current such as 50/5 or 100/5.

Because the ratio is always the same, an increase in the current to a motor also increases the current to the overload relay.

 If the correct current transformer and overload relay combination is selected, the same overload protection can be provided to a motor as if the overload relay were actually in the load circuit.

The overload relay contacts open and the coil to the magnetic motor starter is de-energized when excessive current is sensed. This shuts the motor off.

Several different current transformer ratios are available to make this type of overload protection easy to provide.

Standard overload relays

Figure 5. Standard overload relays may be used on very large motor starters by using current transformers with specific reduction ratio.

Overload Heater Sizes

Each motor must be sized according to its own unique operating characteristics and applications. Thermal overload heaters are selected based on the full-load current (FLC) rating, service factor (SF), and ambient temperature (surrounding air temperature) of the motor when it is operating.

Full-Load Current Rating

 Selection of thermal overload heaters is based on the FLC rating shown on the motor nameplate or in the motor manufacturer specification sheet.

The current value reflects the current to be expected when the motor is running at specified voltages, specified speeds, and normal torque operating characteristics. Heater manufacturers develop current charts indicating the heater that should be used with each full-load current.

Service Factor

 In most motor applications, there are times when the motor must produce more than its rated horsepower for a short period of time without damage.

A service factor (SF) is a number designation that represents the percentage of extra demand that can be placed on a motor for short intervals without damaging the motor.

Common SFs range from 1.00 to 1.25, indicating that the motor can produce 0% to 25% extra demand over that for which it is normally rated.

A 1.00 SF indicates that the motor cannot produce more power than it is rated for and to do so would result in damage.

A 1.25 SF indicates that the motor can produce up to 25% more power than it is rated for, but only for short periods of time.

The excessive current that can be safely handled by a given motor for short periods of time is approximated by multiplying the SF by the FLC rating. For example, if a motor has an FLC rating of 10 A with an SF of 1.15, the excess short-term current equals 11.5 A (10 × 1.15 = 11.5 A). The motor could handle an additional 1.5 A for a short period of time.

Ambient Temperature

 A thermal overload relay operates on the principle of heat. When an overload takes place, sufficient heat is generated by the excessive current to melt a metal alloy, produce movement in a current coil, or warp a bimetallic strip and allow the device to trip.

The temperature surrounding a thermal overload relay must be considered because the relay is sensitive to heat from any source.

The ambient temperature is a factor when considering moving a thermal overload relay from a refrigerated meat packing plant to a location near a blast furnace.

Overload Heater Selection

Overload heater coils for continuous duty motors are selected from manufacturer tables based on the motor nameplate full-load current for maximum motor protection and compliance with Section 430.32 of the NEC.

The class, type, and size information of a magnetic motor starter are found on the nameplate on the face of the motor starter. See Figure 6.

The phase, service factor, and full-load current of the motor are determined from the motor nameplate. Common applications use 40°C as the ambient temperature.

Questionable ambient temperatures should be measured at the job site or determined by some other method.

nameplate of a magnetic motor starter

Figure 6. The nameplate of a magnetic motor starter includes the class, type, and size of the starter.

It is important to always refer to the manufacturer instructions on thermal overload relay selection to see if any restrictions are placed on the class of motor starter required. See Figure 7. For example, unless a class 8198 starter is used, motors with service factors of 1.15 to 1.25 may use 100% of the motor full-load current for thermal overload selection.

Manufacturer instructions on thermal overload relay

Figure 7. Manufacturer instructions on thermal overload relay selection detail restrictions that are placed on classes of starters.

Manufacturer Heater Selection Charts

 Manufacturers provide charts for use in selecting proper thermal overload heaters. The correct heater selection chart must be used for the appropriate size starter. See Figure 8. This information is also found within the enclosure of many motor starters. Each motor starter manufacturer has a chart that applies to their specific brand.

selecting proper overload heaters

Figure 8. Manufacturers provide charts to use for selecting proper overload heaters.

For example, a thermal unit number B2.40 is the correct overload heater for controlling a 3φ motor with an FLC rating of 1.50 A.

 Column three in the heater selection chart is used because all three phases of the 3φ motor must have thermal overload protection. The heater must provide protection of approximately 1.5 A (1.44 to 1.62) based on the motor full-load current.

Manufacturers have different numbers that relate to their specific heaters, but the selection procedure is similar.

Checking Selections

 Section 430.32 of the NEC indicates that a motor must be protected up to 125% of its FLC rating. Because the minimum full-load current of a B2.40 overload device is 1.44 A, the device trips at 125% of this value or 1.8 A (1.44 × 1.25 = 1.8 A).

Dividing the minimum trip current (1.8 A) by the full-load current of the motor (1.5 A) and multiplying by 100% determines if this range is acceptable (1.8 / 1.5 × 100% = 120%). The heater selection is correct because the trip current is less than the NEC limit of 125%.

Ambient Temperature Compensation

 As ambient temperature increases, less current is needed to trip overload devices. As ambient temperature decreases, more current is needed to trip overload devices.

Most heater manufacturers provide special overload heater selection tables that provide multipliers to compensate for temperature changes above or below the standard temperature of 40°C.

The multipliers ensure that the increase or decrease in temperature does not affect the proper protection provided by the overload relay. See Figure 9.

Special overload heater selection tables

Figure 9. Special overload heater selection tables provide multipliers to compensate for ambient temperatures above or below the standard temperature of 40°C.

For example, a multiplier of 0.9 is required for an ambient temperature increase of 10°C to 50°C. Multiplying the motor full-load current (1.5 A) by the correction factor (0.9) determines the compensated overload heater current rating of 1.35 A (1.5 A × 0.9 = 1.35 A). Using the heater selection chart, the acceptable current range is 1.28 A to 1.43 A. A B2.10 heater is required based on the increase in ambient temperature. This is one size smaller than the heater required (B2.40) at a 40°C ambient temperature.

The temperature surrounding an overload heater is 30°C if the ambient temperature is decreased by 10°C. The correction multiplier is 1.05 for a 10°C decrease in ambient temperature. The corrected current is 1.575 A using a full-load current of 1.5 A (1.5 A × 1.05 = 1.575 A). Using the heater selection chart, the acceptable current range is 1.44 to 1.62 A. In this case, the same size heater could be used.

Manufacturer specifications and tables should always be consulted for proper heater sizing.

In rare instances, such as older installations or severely damaged equipment, it may be impossible to determine a motor full-load current from its nameplate. Manufacturers provide charts listing approximate full-load currents based on average motor full-load currents. See Figure 10.

ampere ratings of induction motor

Figure 10. Most manufacturers provide charts for approximating full-load current when motor nameplate information is not available.


These charts should be used only as a last resort. This technique is not suggested as a standard procedure because the average rating could be higher or lower for a specific motor and, therefore, selection on this basis always involves risk.

For fully reliable motor protection, heat coils should be selected based on the motor full-load current rating shown on the motor nameplate.

The full-load current of a motor stated on charts should be used in the selection of a heater using the same procedure as if it were the motor nameplate information.

These charts provide approximately the same information that may be found on the motor nameplate, but they should be used only if motor nameplate information is not available.

About Ahmed Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.

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