How to Select and Apply Electromechanical Contactors for Heavy-Duty Three-Phase AC Motors

Engineers tasked with isolated switching of relatively high voltages and currents using a small voltage signal typically turn to relays. A conventional low-voltage switch activates the relay which in turn switches on the high-power supply. Electromechanical relays (EMRs) are low cost and can handle relatively high voltages, while solid-state relays (SSRs) eliminate contact wear and arcing.

However, when dealing with frequent switching of several hundred volts and tens of amps (and above), both types are challenged. Arcing at these high loads quickly wears out the EMR’s contacts, while leakage currents in SSRs cause overheating. Designers need an alternative option for these high-demand applications.

The less-familiar electromechanical contactor (EMC) offers a hardy replacement for relays. The devices are proven technology and readily available from many reputable suppliers. As there are dozens of options, the selection process soon becomes confusing without a detailed insight of the EMC’s operation.

This article briefly explains the difference between EMRs and contactors, how contactors operate, and then focuses on how a given application influences the choice of product as the first step toward a successful design. Design choices will be illustrated by reference to the Siemens SIRIUS 3RT series power contactors used in an IE3 electric motor implementation.

The difference between electromechanical relays and contactors

Because it is exposed to the full circuit current when closed, using a switch to turn a high-power device such as a large three-phase motor on and off is impractical. The switch arcs dangerously when flipped and overheats in operation. The solution is to use a low-power circuit, turned on and off by a conventional switch, to trigger the high-power circuit. This is the purpose of the EMR.

EMRs use a coil that is energized by the low-power circuit to create a magnetic field which then provides an impulse to a moveable core that in turn opens or closes the (normally closed (NC) or normally open (NO)) contacts. The EMRs can switch an AC or DC load up to their maximum rating. The key benefits of EMRs are low cost and guaranteed isolation at any applied voltage below the device’s dielectric rating. (See, “How to Apply Specialized Low-Noise Solid State Relays to Limit EMI and Meet Critical Standards”.)

However, there is a limit to the power that an EMR can handle. When the load is, for example, a three-phase motor developing more than a few kilowatts (kW), switching using an EMR generates excessive arcing and rapidly wears out the relay. The alternative is the EMC, a heavy-duty, rugged industrial equivalent of a relay, designed to reliably switch high loads over tens of millions of cycles (Figure 1).

How to Select and Apply Electromechanical Contactors for Heavy-Duty Three-Phase AC MotorsFigure 1: Electromechanical contactors replace relays in heavy-duty switching applications. (Image source: Siemens)

EMCs can be safely connected to high-current demand devices and are typically designed with features to control and suppress the arc produced when switching under a heavy load. The devices use the same energized coil/moving core activation as relays and are almost exclusively fitted with NO contacts, although NC contacts are available. NO contacts ensure that when the power to the EMC is removed, the contacts switch to open, cutting off the supply to the high current-draw device. The devices feature either one or multiple pairs of contacts, also called poles.

EMC selection

It’s relatively straightforward to decide on the choice of an EMC over an EMR. While EMCs are more expensive, they are the only option for high-load applications. Once it’s determined that an EMC is needed, selecting the best EMC for the job is more difficult. The best place to start is to determine the peak load current (also referred to as full-load amperage (FLA)) requirement at the application’s operational voltage. This will then determine the current load capacity of the contactor required.

In the case of a three-phase motor, for example, the manufacturer typically specifies the operational voltage and FLA in the datasheet. But if that information is not available, then an engineer can refer to resources such as the U.S.-based National Electrical Code’s (NEC) chart which details FLA for a range of three-phase motors of nominal power and input voltage. The motors are categorized according to International Electrotechnical Commission (IEC) motor classifications. For example, a 375 watt, three-phase motor with a 110-volt operational voltage has an FLA of 4.4 amperes (A), and a 1.1 kW motor with a 220-volt operational voltage has an FLA of 6 A.

Next, the engineer must determine the control voltage required for the EMC. This can be the same voltage as that used to power the associated motor, but often a lower voltage is used for safety reasons. EMC control voltages are typically always below 250 volts AC.

Consideration should then be given to how the motor will perform in the application. For example, two different applications might use a three-phase motor with the same specifications. But an application that requires the motor to be on or off for long periods needs a different EMC than one that is frequently switched on and off. The latter will be subject to repetitive current loads and will therefore need to be a more robust product.

IEC utilization categories or ‘codes’ are a good guide to choosing the correct EMC for a given application. For example, if the EMC is coded “AC-3”, it is suited to ‘squirrel cage’ electric motors (a common type of electric induction motor) in applications where the motor is regularly switched on and off, while “AC-20” is suited to connecting and disconnecting loads under zero-current conditions. While an incorrectly specified IEC-coded EMC may work in a given application, it is likely to have a much shorter lifespan than a correctly coded one.

The IEC codes are also useful to take into account the type of load—resistive or inductive—as this also has a significant influence on the choice of EMC. For example, electric motors are inductive loads, while a heater presents a resistive one.

It is also important to consider how many poles might be required in a single EMC and whether they should be NO or NC. For example, an application might demand three poles using NO contactors for each phase of an electric motor, and a further NC pair to light an LED to indicate a motor is receiving power but not rotating.

Moreover, as EMCs often carry relatively high voltages and currents, it is also important to ensure the isolation rating of the device meets all the safety criteria of the application.

Because motors consume a significant proportion of generated electrical power, the U.S. and E.U. have passed legislation to ensure they operate as efficiently as possible. The E.U.’s energy efficiency levels are expressed in International Energy (IE) efficiency classes (Figure 2). Under the current regulation, motors must reach the IE2 (high efficiency), IE3 (premium efficiency), or IE4 (super premium efficiency) level, depending on their rated power and other characteristics. The EMC has an impact on electric motor efficiency, so if the control system is destined for use in the E.U., it is important for it to be designed to the appropriate IE efficiency class. In the U.S., motors must comply with the National Electrical Manufacturers Association (NEMA) premium efficiency program, which demands compliance with standards like those specified for IE3. Requirements in Australia are similar to those in the U.S.

How to Select and Apply Electromechanical Contactors for Heavy-Duty Three-Phase AC MotorsFigure 2: IE efficiency requirements for electric motors show how efficiency enhancements are greater for lower power motors: IE1 and IE2 motors are no longer allowed under U.S. and E.U. regulations. (Image source: Siemens)

Commercial products

There is a wide range of high-quality EMCs available for almost any high-load application. For example, the Siemens Sirius 3RT2 range of EMCs demonstrates the capability of contemporary products for electric motor switching and other applications. The devices have been designed for high operational reliability, high contact reliability, elevated temperature operation and long service life. These power contactors can be used at up to 60°C without derating—even when mounted side-by-side. The range includes EMCs categorized for AC-1 (non-inductive or slightly inductive loads such as heaters), AC-3 (squirrel-cage electric motors that frequently switch), and AC-4 (squirrel-cage electric motors: starting, plugging, inching) operation. All SIRIUS 3RT2 products are designed for IE3 and IE4 motor operation.

The 3RT20152AP611AA0 from the SIRIUS 3RT2 range is an NO three-pole EMC with S00-sized contactors and is coded for AC-3 applications. The control supply voltage is 220 to 240 volts AC. It features a 400- or 690-volt output voltage, and a maximum current of 7 A at 400 volts or 4.9 A at 690 volts for a nominal maximum power of 3 kW at 400 volts or 4 kW at 690 volts. The contacts close in under 35 milliseconds (ms) and open in under 14 ms.  It has a maximum switching frequency under load of 750 cycles per hour. Service life is 30 million cycles with a failure rate of once per 100 million. When using this EMC, the FLA for an attached three-phase motor is 4.8 A for a 480-volt rated motor and 6.1 A for a 600-volt rated motor; that’s sufficient to power a 2.2 kW (480 volts) motor or a 3.7 kW (600 volts) motor (Figure 3).

How to Select and Apply Electromechanical Contactors for Heavy-Duty Three-Phase AC MotorsFigure 3: The 3RT20152AP611AA0 EMR features three poles that are NO, making it a suitable configuration for switching a three-phase motor. (Image source: Siemens)

At the other end of the SIRIUS range is the 3RT20261AP60. This too is an NO three-pole EMC and coded for AC-3 applications, but with S0-sized contactors. The control supply voltage is 220 to 240 volts AC. The device features a 400- or 690-volts output voltage and a maximum current of 25 A at 400 volts, or 13 A at 690 volts for a nominal maximum power of 11 kW at both output voltages. The FLA for an attached three-phase motor is 21 A for a 480-volt rated motor and 22 A for a 600 volt rated motor; that’s sufficient to power an 11.2 kW (480 volts) motor or a 14.9 kW (600 volts) motor.

The Siemens SIRIUS 3RT2 EMCs are suitable for a range of applications but are optimized for switching IE3 or NEMA premium efficiency-compliant motors. Part of this compliance requires the EMC to be an efficient part of the motor’s control system. To meet this requirement, the EMCs are designed with features such as permanent magnets to reduce coil power consumption and Electronic coil control. This enables the holding power (used to keep the contactor closed) to be reduced to a minimum. Intrinsic power loss of the EMCs has been reduced by 92% compared with previous devices.

For example, the 3RT20171BB41 power contactor—which can switch 2.2 kW to 7.5 kW three-phase motors depending on the EMC’s output voltage—features a loss of 1.2 watts per pole for an overall loss of 3.6 watts when supplying full power to an electric motor.

Using an EMC to start an IE3 motor

A motor drive train comprises several components to ensure safe and reliable operation. For example, a comprehensive set-up could comprise the following components:

  • Protective device (for example, a motor protector starter and/or overload relay)
  • Starting unit (for example, an EMC)
  • Controller (for example, a motor management system)
  • Control unit (for example, a frequency converter)
  • Electric motor
  • Gearbox
  • Cabling
  • Driven machine

The SIRIUS 3RT2 EMCs are designed as modular devices that mount onto a DIN rail (or are screwed in place) together with the other components. The EMCs are designed to fit together with sister modules to build up the desired control section of the motor drive train (Figure 4). The modular design helps to limit the amount of wiring required in the cabinet, and connections are made via spring-loaded contacts, so no special tools are needed.

How to Select and Apply Electromechanical Contactors for Heavy-Duty Three-Phase AC MotorsFigure 4: The SIRIUS 3RT2 series are modular devices making it straightforward to implement a motor control system. Here, a 3RT20171BB41 EMR—which is switched with a 24 volt DC signal—is used with a protection device and an overload relay to control a conveyor motor. (Image source: Siemens)

Providing the EMC has been carefully selected, it becomes a plug-and-play element of the control system. 3RT2 power contactors have been optimized for switching IE3 electric motors in the 1 to 15 kW range and can be used without further constraints for direct-on-line and reversing starting applications. There are, however, some important design considerations for engineers more familiar with IE2 electric motors than IE3 types when using 3RT2 EMCs. Characteristics that affect control system design for IE3 motors include lower-rated currents, increased starting current ratio, and increased inrush current (Figure 5).

How to Select and Apply Electromechanical Contactors for Heavy-Duty Three-Phase AC MotorsFigure 5: The inrush, starting, and rated motor current are key parameters to consider when selecting an EMC for a three-phase AC motor. (Image source: Siemens)

Key to the increased efficiency of IE3 electric motors is lower-rated motor currents. However, IE3 does not specify a linear increase in efficiency across the electric motor power range. Instead, it requires the efficiency of lower power electric motors to increase much more when compared with IE2 types than higher power units (see Figure 2 above). That means for lower power electric motors, the rated motor current has been considerably lowered compared to the IE2 type. Note that like-for-like power is maintained by increasing the operating voltage.

The flip side of reduced rated current is an increase in the starting current ratio (starting current/rated current) for the more efficient motors. It occurs because although the starting current for an IE3 motor is lower, the difference between IE2 and IE3 equal-power motors is not as pronounced for starting current as it is for rated current. For lower power-efficient motors, the starting current ratio is higher than that for higher power alternatives.

The impact of increased starting current ratio is an increase in inrush current. Inrush current is essentially a dynamic compensation event that results from factors such as the connection of an inductive load (such as a motor), and dynamic current transients and saturation effects in the laminated cores of the motor. Inrush current, which can be up to five times higher than FLA, can damage the motor and other systems (Figure 6).

How to Select and Apply Electromechanical Contactors for Heavy-Duty Three-Phase AC MotorsFigure 6: Inrush current is higher for more efficient motors, and it is greater for lower power units. Appropriate control system design can mitigate the effects. (Image source: Siemens)

Together with other modular control components, 3RT2 EMCs can be used in a wye-delta (“YΔ”) starting system to limit inrush current. By starting the motor using the full-line voltage across the unit’s Y windings, around 58% of the line voltage reaches each motor phase, lowering the current and keeping the inrush peak down. Once the motor reaches its rated speed, operation switches to the Δ mode, at which time the full voltage is applied (without danger of any inrush current) to each phase and the motor can produce full power.

This arrangement calls for an overload relay located directly in the motor feeder cable U1, V1, W1 (Figure 7). This ensures overload protection is effective for all three EMCs. The full implementation requires the relay and three 3RT2 EMCs.

circuit comprising an overload relay” alt=”How to Select and Apply Electromechanical Contactors for Heavy-Duty Three-Phase AC Motors”>Figure 7: YΔ circuit comprising an overload relay in the motor feed cable and three EMCs to switch power during motor starting. (Image source: Siemens.)

In operation, the Y part of the sequence is triggered by closing the K1 and K3 EMCs together. After a preset time (at about 80% of the motor’s full speed), a timer triggers K3 to open and K2 to close to initiate the delta part to apply full power to the motor.

Conclusion

When switching high-power loads such as three-phase AC motors, EMCs are the recommended alternative to EMRs. EMCs are designed for high reliability switching over tens of millions of operations. The devices are available for a wide range of motor outputs ranging from a few to hundreds of kilowatts.

As shown, the Siemens SIRIUS 3RT2 EMCs are suited to switching three-phase AC motors from 2 to 25 kW, and their modular design ensures ease of installation into control systems. While the SIRIUS EMCs are relatively straightforward to install, care must be taken with control system implementation to avoid motor damage from excessive inrush current.