Electrical motors are integral to nearly all industrial processes. Conveyors, furnaces, HVAC systems, bottling systems, refrigeration units, packaging machines, and many other applications rely on dependable motor control to function. One such motor controller is a motor contactor, an electrical device which is used to energize or de-energize a motor. A manual motor controller (also called a manual starter) provides both power switching and protective functions to prevent high currents, fault conditions, and thermal conditions which might damage a motor and other electrical components. This article discusses when a motor contactor or a manual motor starter should be used, depending on the conditions for starting and stopping the motor or load.
An electrically operated motor contactor is a special relay used to switch a motor load on or off in a high voltage or high current electrical circuit. The contactor itself is made of three key parts: an enclosure, a coil or electromagnet, and power contacts. Figure 1 illustrates a magnetic electrically operated contactor.
The operating principle of a motor contactor is fairly basic. The contactor’s start circuit applies power to the electromagnet (A2-A1). The energized electromagnet builds a magnetic field around it. This magnetic field causes an armature to move towards the coil. As a result, the main power contacts change states, from open to closed. Input power is then applied to the load as long as the contactor coil is energized. When the coil is de-energized, the contacts change states again, and the load is de-energized. Contactors are a useful tool to power high current loads, but by themselves they do not offer protective features, nor do they have internal start and stop circuitry.
Motor contactors can be fitted with accessories, typically overload relays and auxiliary contact blocks. Easy TeSys thermal overload relays are designed to protect AC circuits and motors from overloads, phase failures, Iong starting times, and prolonged stalled rotor conditions. The thermal relay permanently controls the current pulled by the motor. The overload relay doesn’t open the motor circuit directly. Typically, the NC auxiliary contact of the overload relay is wired in the contactor coil circuit, so that in the event of an overload trip, the contactor opens the motor circuit. The Easy TeSys DPER overload relay also includes a normally open (NO) auxiliary contact. Additional contacts can be obtained for different start and stop conditions using an auxiliary contact block.
Figure 2: Accessories: (a) and (b)
When to Use a Motor Contactor
Selecting a motor contactor depends upon the conditions for starting and stopping the motor or load. Here are typical conditions for using a motor contactor:
- Pushbutton or selector switch control – Some applications need to be controlled by the operator when they are away from the equipment electrical panel. In this scenario, a separate pushbutton control station is located near the operator. A contactor is ideal for this situation, as it enables control of the load using a pushbutton or selector switch operation wired to the contactor coil circuit.
- Sensor control – Some motors or loads need to operate based on the input from a sensor. In this scenario, the sensor contact can be used to close the contactor coil circuit, thus controlling the operation of the motor/load.
- PLC output signal – Some motors or loads need to operate dependent upon more complex conditions. For this scenario, Programmable Logic Controllers (PLCs) are often used to define the logic of when a motor or load should operate. The coil circuit of the contactor can be used to control the operation of the motor/load.
- Drop out protection – In the event of power loss, some equipment should not automatically restart once power is restored. Contactors can be wired to force the operator to intentionally restart the load after a power loss.
- Manual reset after a motor overload trip – In the event of a motor overload trip, care should be taken to determine the cause of the overload trip. For this scenario, a manual reset can be accomplished using a DPER overload relay or a GP2E manual motor controller.
- Difficult to reach locations – Some equipment is located in remote areas or in areas that are difficult for maintenance personnel to reach. Should a motor overload trip occur in these scenarios, an automatic reset of the overload protection device may be desired or necessary. For this need, the Easy TeSys DPE contactor with the DPER overload relay provides an ideal solution. The DPER overload relay can be set to automatically reset, instead of the more traditional manual reset setting.
- Group motors – In addition to motor control and overload protection devices, motor circuits must also have overcurrent protection, such as would be provided by a circuit breaker or fuse. An Overcurrent Protective Device (OCPD) must be provided for each circuit individually, except in cases where a group motor installation is permitted (see NEC 430.53). Group motor protection is when a single OCPD is used to provide overcurrent protection for a group of loads. For small motors (1HP or smaller), the NEC limits circuit protection size to 15 amps (20 amps if 120V). Easy TeSys DPE contactors and DPER overload relays can be a good fit for this application.
Manual Motor Controllers (or Manual Starters)
As opposed to an electrically operated motor contactor that can be operated locally or remotely, a manual starter is operated manually. Typically, it will have a local pushbutton control right on the manual starter. A manual starter is ideal for simple motor control applications, due to its compact footprint, low cost, and internal motor overload protection.
The Easy TeSys GP2E Series Manual Motor Controller includes a 3-pole thermal-magnetic circuit breaker conforming to IEC 60947-2 and IEC 60947-4-1. Motors must have a disconnect, as per the National Electrical Code (NEC) 430.102. If the disconnect is not in line of sight from the motor, a local disconnect is required. The Easy TeSys GP2E manual motor controller is UL rated as “Suitable as Motor Disconnect.”
Easy TeSys manual motor controllers are designed to control and protect motors. Unlike a contactor alone, the Easy TeSys Manual Motor Controller consists of a circuit breaker, push button control, and thermal-magnetic protection elements (overloads). The circuit breaker is designed for connection by screw clamp terminals to ensure secure, permanent, and durable clamping that is resistant to harsh environments, vibrations, and impacts. Motor energisation is controlled manually by using the Start button. De-energisation is controlled manually by using the Stop button, or automatically by the thermal-magnetic protection elements, or by a voltage trip attachment.
Motor protection is provided by the thermal-magnetic protection elements incorporated in the motor circuit breaker. As per IEC 60947-4-1, the magnetic elements (short circuit protection) have a non-adjustable tripping threshold, which is equal to about 13 times the maximum setting current of the thermal trips. The thermal elements (overload protection) include automatic compensation for ambient temperature variations. The rated operational current of the motor is displayed by means of a graduated knob.
Easy TeSys contactors may be used on the load side of the Easy TeSys Manual Motor Controllers in Group Installations based on available short-circuit current ratings. All live parts are protected against direct finger contact. Easy TeSys manual motor controllers are easily mounted on din rail or directly to the panel.
For control panels controlling one or two motors, it is common for the devices to be wired using electrical cable wire. However, this approach can be time consuming for control panels with several motor loads. The Easy TeSys includes busbar links that quickly distribute power from a single source to multiple motor circuits, using the GP2E manual motor controllers in each circuit. A wiring adapter to connect the GP2E manual motor controller to the DPE contactor is also available to expedite installation time.
When to Use a Manual Motor Controller (or Manual Starter)
A manual starter is ideal for simpler motor starting/stopping conditions, such as:
- Manual operator control – The GP2E manual motor controller is a simple manual starting option, available with an optional enclosure accessory if needed.
- Manual reset after a motor overload trip – In the event of a motor overload trip, care should be taken to properly address the reason for the overload trip. For this scenario, a manual reset can be accomplished using the DPER overload relay or the GP2E manual motor controller. Either scenario can be used with a contactor, depending upon the application need.
- Group motors: There are also requirements regarding the conductors when a group motor is used. In general, group motor devices have a lower Short-Circuit Current Rating (SCCR). The Easy TeSys GP2E manual motor controllers with DPE contactor have up to 35kA 480V SCCR.
Selecting a contactor or a manual starter starts first with understanding the conditions for starting and stopping the motor or load. A motor contactor is a good choice for more complex motor starting and stopping conditions, as well as for remote operation, especially in difficult to reach installations. A manual motor controller or manual starter is ideal for simple motor starting and stopping conditions, where the need for manual operator control is adequate, and when the convenience of having internal motor overload protection in a compact footprint at a low cost.