Motor control relays: Workhorses of the control world

Motor control relays are heavy-duty relays used to control motor starters and other industrial components. More specifically, they are typically used to energize the coil of a motor starter or contactor, which in turn starts a motor. A motor protective relay is a type of motor control relay used to prevent the coil of a motor starter or contactor from being energized.

02/01/2009


Motor control relays are heavy-duty relays used to control motor starters and other industrial components. More specifically, they are typically used to energize the coil of a motor starter or contactor, which in turn starts a motor. A motor protective relay is a type of motor control relay used to prevent the coil of a motor starter or contactor from being energized. These relays prevent equipment damage by detecting overload, over- and under-voltage, over-current and phase-loss conditions.

Motor control relay benefits

The main advantage motor control relays offer over general purpose relays is the ability to add accessories and additional poles. They also offer the benefit of selecting motor control relays with 600 Vac coils. The ruggedness of motor control relays make them preferable in manufacturing applications.

Motor control relays allow for a variety of accessories including:

  • Transient surge suppression

  • Pneumatic and solid-state timers

  • Mechanical and permanent magnet latching controls

  • Convertible contacts.

    • To protect sensitive instruments and solid-state devices, transient surge suppression directly mounts to coil terminals to limit high transient voltages that result from de-energizing relay coils. Pneumatic timers mount directly to the motor control relay in place of auxiliary contacts, and are convertible from on- to off-delay or the other way around.

      More reliable than pneumatic timers and with similar functionality, solid-state timers improve upon the overall accuracy of the timing function. Latches are important to keep the motor control relay contacts closed during a loss and return of power. Convertible contacts can be changed from normally-closed to normally-open or vice versa. By adding auxiliary contacts mounted directly to the top or side of the motor control relay, users are able to add additional poles.

      Manufacturing applications

      Motor control relays are part of the control circuit. For example, an application could include two motor starters, where the second motor is started and stopped after a time delay. The second motor could be a cooling fan or pump in this application. Other applications include priming pumps, conveyor systems, machine jogging, manufacturing processes, safety circuits, surge and backspin protection for pumps and float controls. Motor control relays can also be used to sequentially start motors to prevent excessive starting loads due to motors starting simultaneously.

      To select the appropriate control relay, it is important to determine the system voltage, the load currents, number of poles required and the expected life before replacement. The motor control relay coil should be selected based on the system voltage that energizes the coil. The coil ranges offered typically go to 600 Vac, which is useful for legacy systems. The motor control relay contact rating should be high enough to make and break the coil load of the motor contactor or starter it is controlling. Since coils are inductive loads, the designer must be sure the contacts can handle the inrush currents present when energizing the motor starter coil.

      As with all electro-mechanical devices, motor control relays have electrical and mechanical lives. The mechanical life is based on opening and closing the contacts of the relay under no-load conditions; the electrical life is based on duty cycle and making and breaking currents.

      Alternative motor starter control methods

      Additional methods of controlling motor starters and contactors include:

      • Using general-purpose relays

      • Using intelligent relays

      • Direct control using a programmable logic controller.

        • It is possible to use a general-purpose relay with a control power transformer or power supply. The control power transformer steps the voltage down to a level usable by the general purpose relay coil, while the power supply steps the voltage down and rectifies the voltage to dc. The general purpose relay’s contacts are typically rated for a resistive load. Since motor starter coils are an inductive load, the general purpose relay’s contacts must be de-rated for use in this application.

          Intelligent relays allow for the ability to combine the features of a general-purpose relay in a programmable electronic device. Common features include timers, counters, real-time clocks and displays. Intelligent relays are programmable from the front of the unit, which makes it easy to make changes on the plant floor. Importantly, the designer must ensure suppression is installed on the motor starter coil to protect the relay from the collapsing field of the inductive load. Additional advantages of intelligent relays include reduced labor and assembly costs, less troubleshooting time due to fewer components and the ease of modifying relay logic. Intelligent relays can be cost-effective when replacing two time-delay relays.

          With advances in electronic coils in motor starters, the motor control relay can be eliminated from the circuit design and the motor starter can be switched directly from a PLC. Electronic coils in motor starters are sometimes more efficient than motor control relays, especially for the smaller motor starters (less than 40 hp). The electronic coils can be switched from a low current dry circuit. This can be accomplished from a transistor or relay output. For relay outputs, the designer must take into consideration the expected life of the relay and the number of relays on the PLC output module. If one relay fails, will the entire relay module need to be replaced? To determine whether the PLC relay can switch the motor starter coil, the designer must ensure the motor starter inrush coil current is less than the PLC relay rated switching current.

          Motor control relays are the heavy-duty workhorses of the control world. Engineers must weigh voltage/current handling capability, reliability, endurance, assembly, cost, component size and maintenance issues. With proper selection and application, the motor control circuit can be as reliable as your trusty old stable horse.

          The intelligent relay is another option for controlling motor starters.


          <table ID = 'id1278413-0-table' CELLSPACING = '0' CELLPADDING = '2' WIDTH = '100%' BORDER = '0'><tbody ID = 'id1278063-0-tbody'><tr ID = 'id1278065-0-tr'><td ID = 'id1278067-0-td' CLASS = 'table' STYLE = 'background-color: #EEEEEE'> Author Information </td></tr><tr ID = 'id1278242-3-tr'><td ID = 'id1278244-3-td' CLASS = 'table'> David Brandt is a product specialist at Eaton Corp. </td></tr></tbody></table>


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