Applications such as heating, ventilation and air conditioning (HVAC ) , compressors, pumps, material handling and packaging require the safe deployment and control of large motors operating at high voltages and currents . Controlling these large motors is a challenge for designers because adequate isolation must be provided between the motor and the control circuitry. Additionally, high voltages and currents can generate large electromagnetic transients that can damage electronic controls.
Electromagnetic relays can provide remote control with isolation protection, but have their own limitations. High power motors can create arcs when energized and de-energized, which will wear down the contact surfaces of the relay and shorten contact life.
A magnetic contactor can solve this problem, which is a special type of relay designed for motor control. In addition to being more robust than relays, with larger and more stable contacts, magnetic contactors feature arc suppression technology, including special materials and faster contact opening and closing.
This article describes the basics of electromagnetic motor contactors and their advantages over other motor control methods. Then it discusses how to select and apply the actual configuration of Easy TeSys series of Schneider Electric as an example.
How the contactor works
A magnetic contactor consists of an electromagnet with an "E" shaped iron core. Specifically, an electrically isolated coil is wound concentrically around the central leg of the core. The coil is energized by a control power source, which may be AC or DC. When the coil is energized, an electromagnetic force attracts the armature at the open end of the core (Figure 1).
Figure 1: Simplified functional schematic of a contactor showing the de-energized and energized states of the contactor. (Image source: Art Pini)
The electrical contacts are mechanically coupled to the armature. The contact arrangement varies depending on the contactor model: it may be normally open (NO) or a combination of normally open and normally closed (NC). There may be multiple isolated contacts. For example, a three-phase contactor will have 3 sets of power contacts, 1 set for each phase. When the armature is drawn in, the NC contact opens and the NO contact closes. In addition, many contactors include a set of low power auxiliary contacts to monitor the energized or de-energized state of the contactor.
The contacts are made of materials with high strength and excellent electrical conductivity, which can withstand arc and oxidation. The contact geometry is designed to handle expected power levels and suppress arcing.
All elements of the contactor are contained in a housing which electrically insulates the contacts while providing easy access to power, load and coil wiring. The enclosure also provides mounting support for panel or DIN rail mounting (Figure 2).
Figure 2: Examples of typical contactor housings; panel mount (left) and DIN rail mount (right). (Image source: Schneider Electric)
Schneider Electric Easy TeSys ( DP E series) contactors have a compact housing with a width of only 45 mm and can be mounted on a panel or on a DIN rail. The enclosure has an ingress protection rating of IP20, which means it can prevent the insertion of fingers. All contactors in this series contain a normally open auxiliary contact. The three-phase contactors in the series are UL/CSA listed and rated up to 32 A, 20 HP/480 VAC and 25 HP/600 VAC, and are available in a variety of control coil excitation voltages (Table 1).
77 x 45 x 86 (mm)
DPE32G7 3 690 32 15 HP/480 V 15 HP/440 V 120 AC AC1, AC3, AC4 3.35 x 1.77 x 3.62 (in) 85 x 45 x 92 (mm) DPE38G7 3 690 38 20 HP/4 80 V 18.5 HP/440 V 120 AC AC1, AC3, AC4 3.35 x 1.77 x 3.62 (in) 85 x 45 x 92 (mm) Table 1: Example of selected Schneider Electric's Easy TeSys DPE contactor series showing the range of operating current and control coil voltage options
for
the
series
. (Table source: Art Pini)
The service life of these devices is approximately 1 million electrical operations. Easy TeSys contactors are suitable for the applications described in the usage categories specified in the standard IEC 60947. The rated current of each contactor depends on its usage category. For example, the AC-1 category describes applications where the load is non-inductive or only slightly inductive, such as a resistance furnace. These applications are primarily resistive loads where transient voltage and current are less of a problem.
The AC-3 category covers applications for squirrel cage induction motors where, after the motor has started, it may sometimes be necessary to remove power to stop the motor. The motor is an inductive device, and the operation of starting and stopping will cause a transient inductance, which will cause a large pressure on the contactor.
Applications in category AC-4 place greater stress on the contactor. This category includes squirrel cage induction motors and slip ring motors that require reverse current braking and inching or jogging. Jogging or inching is "quick and repeated application of power to start the motor from a standstill to complete small movements of the motor". Jogging generally refers to starting a motor with a short power pulse at full voltage. Similarly, inching refers to starting a motor with a short pulse at low voltage. Multiple applications of power place the greatest stress on the contactors.
A specific Easy TeSys DPE contactor is matched to a motor or similar high power application primarily based on the current to be handled. Schneider Electric's Easy TeSys catalog includes sizing assistance based on motor power, category of use and required service life (Figure 3).
Figure 3: Easy TeSys DPE selection guide for AC-3 usage category motors based on motor power, usage category and required lifetime. (Image source: Schneider Electric)
Figure 3 shows one of 3 selection guidelines related to the category of use of the equipment being controlled. It's for the AC-3 usage category, which is basically a motor that may not stop very often. When the motor stops, the current is equal to the operating current. For example, consider looking for an Easy TeSys DPE contactor for a 5.5 kW three-phase motor operating at 400 V at 11 A with an expected life of 2 million cycles. Starting from the 400 V line, the designer needs to find 5.5 kW and project a line up from there until it intersects the line for 2 million operations. The closest DPE model locus (blue) to the intersection point is DPE 18.
Another example is the AC-4 usage category, where the motor is stopped and restarted frequently, and the worst case current is high. Consider a three-phase 5.5 kW motor in an AC-4 application, operating at 400 V and 11 A, which is de-energized when the motor stalls. Expected operating life is 300,000 operations.
The stall current of this motor is six times the operating current, requiring a contactor with a higher current rating (Figure 4).
Figure 4: Easy TeSys DPE selection guide for AC-4 usage category. Note that the worst case current could be much higher, because when the motor stalls, the power may be cut off. (Image source: Schneider Electric)
To find a recommended contactor, start with a stall current of 66 A, which is 6 times the operating current of 11 A. Project upwards from the current axis until you intersect the line representing 300,000 operations. The closest site to the intersection is DPE32.
Easy TeSys DPE series contactors cover the most common motor configurations and applications such as conveyors, packaging machines, pumps, compressors, HVAC, refrigeration equipment and more.
The Easy TeSys range also includes a complementary range of thermal overload relays designed to protect AC circuits and motors against conditions such as overload, phase failure, extended start-up time and rotor stall. These relays monitor the motor current and when the current exceeds the programmed current limit, the contacts open and stop the motor. There are 15 different models, each with a range of programmable current trip levels. Overload protection models are compatible with selected Easy TeSys contactors DPE09 to DPE38. They connect directly to the bottom terminals of a three-phase contactor using the contactor's screw clamp terminals. The combination is available in a common 45 mm width and can be mounted on a DIN rail or screwed to a panel with a DPE contactor bracket (Fig. 5).
Figure 5: The overload protection relay is mounted directly under the DPE contactor and is secured with the contactor's screw-clamp connector . (Image source: Schneider Electric)
The Easy TeSys DPER32 Thermal Overload Relay is rated at 32 A/690 V, has a programmed thermal trip range of 23-32 A, and a trip class of 10 (the overload protector will trip within 10 seconds when the overload is 6 times the preset level), and is designed to protect a three-phase motor rated at 15 kW at 400 V. This is a differential device capable of detecting phase failures and load imbalances. The device has a thermal adjustment dial, manual/automatic reset selector, test selector for simulated trip, reset and stop buttons, flag indicator light, and two auxiliary contacts (1 NO, 1 NC) for fault signaling. User settings are protected with a lockable transparent cover. The entire range of thermal overload protectors is certified to multiple standards including IEC, UL and CUL.
Summary
For motor applications operating at higher voltages and currents, designers need a reliable method to isolate the associated control circuits and protect the circuits from electromagnetic radiation. Easy TeSys 3-pole DPE contactors paired with DPER Easy TeSys thermal overload relays are designed to switch and protect the most common motor use cases. The wide range of models in this series covers a wide range of current and voltage levels and can be easily configured to meet the requirements of a specific application.