NEMA Ratings
Contactors are devices that switch electrical currents on and off. They have to follow certain guidelines set by the NEMA and the IEC, which are two organizations that regulate electrical standards. These guidelines are not the same for both organizations, so it is important to know the differences.
One of the principles of the NEMA standards is to make sure that contactors from different manufacturers can work together if they have the same NEMA size. This is because customers may not know the exact details of how they will use the contactor, such as the type of load or how often they will switch it. Therefore, the NEMA contactor is designed to have enough capacity to handle a wide range of situations.
The NEMA size of a contactor depends on its current rating and horsepower rating at different voltages. Some contactors have copper contacts, which can get oxidized if they are not used for a long time. To prevent this, these contactors have to be switched at least once every 8 hours. This is called the 8-hour open rating. Other contactors have silver or silver-alloy contacts, which do not have this problem. They have a continuous rating, which means they can be used without interruption. The NEMA current rating applies to each main contact separately, not to the whole contactor. For example, a Size 00 three-pole AC contactor rated at 9 A can switch three 9 A loads at the same time. There are also ratings for total horsepower for each contactor. When choosing a contactor, always make sure that its ratings are higher than the load that it will control. NEMA contactors come in different coil voltages to suit different needs.
Magnetic contactors are also rated for the type of load they are used with. The following are the four load utilization categories:
- Nonlinear loads such as tungsten lamps for lighting. These loads have a large hot-to-cold resistance ratio, typically 10:1 or higher. The current and voltage in these loads are in phase.
- Resistive loads such as heating elements for furnaces and ovens. These loads have a constant resistance, so the current and voltage are in phase.
- Inductive loads such as industrial motors and transformers. These loads have a low initial resistance until the transformer becomes magnetized or the motor reaches full speed. The current in these loads lags behind the voltage.
- Capacitive loads such as industrial capacitors for power factor correction. These loads have a low initial resistance as the capacitor charges. The current in these loads leads the voltage.
The load utilization category of a magnetic contactor must be selected to match the type of load it is used with. This will ensure that the contactor can safely and reliably switch the load.
IEC Ratings
IEC contactors are designed to be physically smaller than NEMA contactors, while still providing the same or higher ratings. This is achieved by using different manufacturing methods and materials. IEC contactors are not defined by standard sizes, but are instead rated according to their utilization category. The utilization category indicates the type of load that the contactor is designed to switch, and the maximum current and voltage that it can handle.
AC CATEGORIES
- AC-1: Inductive loads with low inrush current, such as heating elements.
- AC-2: Inductive loads with high inrush current, such as motors.
- AC-3: Motor loads, such as squirrel cage motors.
- AC-4: Resistive loads, such as lamps.
- AC-5: Resistive loads with high inrush current, such as welding machines.
- AC-6: Motor loads with high inrush current, such as wound rotor motors.
- AC-7: High-frequency inductive loads, such as fluorescent lighting ballasts.
- AC-8: High-frequency resistive loads, such as electronic components.
DC CATEGORIES
The IEC ratings for DC contactors are similar to the IEC ratings for AC contactors, but there are some key differences. The most important difference is that the DC ratings are based on the average current, rather than the peak current. This is because DC currents do not have a sinusoidal waveform, so the peak current is much higher than the average current.
The following are the most commonly used utilization categories for IEC DC contactors:
- DC-1: Constant resistive loads, such as heating elements.
- DC-2: Constant inductive loads, such as motors.
- DC-3: Resistive loads with inrush current, such as welding machines.
- DC-4: Inductive loads with inrush current, such as wound rotor motors.
Contactor Enclosures
Enclosed magnetic contactors must be housed in an approved enclosure to provide mechanical and electrical protection. The type of enclosure required depends on the environment in which the contactor will be operating.
The following are some of the environmental factors that must be considered when selecting an enclosure:
- Exposure to damaging fumes: Fumes can corrode the contactor and its components, so the enclosure must be airtight.
- Operation in damp places: Moisture can cause the contactor to rust and short-circuit, so the enclosure must be watertight.
- Exposure to excessive dust: Dust can clog the contactor’s moving parts, so the enclosure must be dust-tight.
- Subject to vibration, shocks, and tilting: The enclosure must be strong enough to withstand these forces.
- Subject to high ambient air temperature: The enclosure must be able to dissipate heat without overheating the contactor.
There are two general types of NEMA enclosures: non-hazardous-location enclosures and hazardous-location enclosures.
- Non-hazardous-location enclosures are used in environments where there is no risk of explosion or fire. They are further subdivided into the following categories:
- General-purpose: This is the least costly type of enclosure. It is used in locations where there are no unusual service conditions.
- Watertight: This type of enclosure is designed to keep water out. It is used in locations where the contactor may be exposed to water, such as outdoors or near a water source.
- Oiltight: This type of enclosure is designed to keep oil out. It is used in locations where the contactor may be exposed to oil, such as in a machine shop.
- Dust-tight: This type of enclosure is designed to keep dust out. It is used in locations where the contactor may be exposed to dust, such as in a factory.
- Hazardous-location enclosures are used in environments where there is a risk of explosion or fire. They are much more expensive than non-hazardous-location enclosures, but they are necessary to protect the contactor and the people working around it. Hazardous-location enclosures are classified into two categories:
- Gaseous vapors: This type of enclosure is used in locations where there are flammable gases, such as acetylene or hydrogen.
- Combustible dusts: This type of enclosure is used in locations where there are flammable dusts, such as metal dust or coal dust.
All industrial electrical and electronic enclosures must conform to standards published by the National Electrical Manufacturers Association (NEMA) to meet the needs of the location conditions. Figure 6-24 shows typical NEMA enclosure types.
Enclosures are cases that protect electrical equipment from different situations. However, the way the device is wired and built inside does not change. To choose the right enclosure for a specific use, check the National Electrical Code (NEC) and the local codes.
The IEC has a system to describe the enclosures of electrical equipment based on how much protection they offer. The IEC system is different from NEMA, because it does not cover protection from things like rust, ice, oil, and coolants. That is why IEC and NEMA enclosure numbers are not exactly the same. The table at the bottom of the page shows how to convert from NEMA enclosure numbers to IEC enclosure numbers. The NEMA enclosures pass or exceed the test requirements for the IEC enclosures they match; but this does not mean that the table can be used to convert from IEC enclosures to NEMA enclosures and the conversion from NEMA to IEC should be tested
Solid-State Contactor
Solid-state switching means using electronic devices to turn power on and off without any moving parts. Unlike a magnetic contactor, an electronic contactor does not make any noise, and its “contacts” do not get worn out. Solid-state or static contactors are good for situations that need frequent switching, such as heating circuits, dryers, single- and three-pole motors, and other industrial uses.
Solid-state contactors usually use a type of semiconductor called a silicon controlled rectifier (SCR) to switch power on and off. An SCR has three terminals (anode, cathode, and gate) and works like the power contact of a magnetic contactor. A gate signal, instead of a coil, is used to turn the device on, letting current flow from cathode to anode. Figure below shows three kinds of SCR shapes for higher-current uses: the disk (or puck type), stud mount, and module. Stud-mounted SCRs with flexible leads have a gate wire, a bendable cathode lead, and a smaller cathode lead that is only for control. The SCR makes heat when it works, so all contactors have a way to cool the SCR. Usually an aluminum heat sink, with fins to make the surface bigger, is used to get rid of this heat to air.
The SCR is a two-state device, like a contact. It can be either on or off. When it is on, it conducts current like a closed contact. When it is off, it blocks current like an open contact. SCRs are normally off devices. They can be turned on by a small current pulse applied to the gate terminal. Once turned on, the SCR will stay in the on state even if the gate signal is removed. It will only turn off if the current through the SCR falls below a certain minimum value, or if the direction of the current is reversed.
In this respect, the SCR is similar to a latched contactor circuit. Once the SCR is turned on, it will stay on until the current through it decreases to zero.
Here is a diagram of the SCR’s on and off states:
The SCR is a versatile device that can be used in a wide variety of applications. It is a reliable and efficient way to switch high currents and voltages.
To test the SCR, first connect the DC voltage source to the circuit. Then, momentarily close the “on” pushbutton switch. This will connect the gate of the SCR to the anode, and the SCR will turn on. The light bulb will light up. Now, momentarily open the “off” pushbutton switch. This will disconnect the gate from the anode, but the SCR will remain turned on. The light bulb will stay lit.
If the light bulb stays lit even after the “off” pushbutton switch is opened, this is an indication that the SCR is shorted. If the light bulb fails to light up when the SCR is triggered into operation, this is an indication that the SCR is faulty open.
An inductive load is a device that stores energy in a magnetic field when current flows through it. When the current is turned off, the magnetic field collapses and creates a voltage spike. This voltage spike can be thousands of volts and can damage the SCR.
A snubber circuit is used to suppress the voltage spike and protect the SCR. The snubber circuit consists of a resistor and capacitor connected in series. The resistor limits the current flow, and the capacitor absorbs the energy from the magnetic field.
The snubber circuit is placed in parallel with the SCR. When the current is turned off, the voltage spike across the SCR is applied to the snubber circuit. The resistor limits the current flow through the capacitor, and the capacitor absorbs the energy from the magnetic field. This prevents the voltage spike from damaging the SCR.
The abrupt switching of an SCR, particularly at higher current levels, can cause unwanted transients on the power line and create electromagnetic interference (EMI). Zero-fired control is a scheme that can help to mitigate these problems.
In zero-fired control, the SCR is turned on at or near the zero crossing point of the AC sine wave. This means that the SCR is not switched when there is current flowing through the load. This eliminates the switching transients and EMI that can be caused by abrupt switching.
Zero-fired control is not without its drawbacks. It can be more complex to implement than other control schemes, and it can also be more expensive. However, it can be a valuable tool for applications where EMI and transients are a concern.
Solid-state contactors are also typically more expensive than magnetic contactors, especially in high current ratings. This is because solid-state contactors require more sophisticated components and manufacturing processes. However, solid-state contactors offer a number of advantages over magnetic contactors, including longer lifespan, lower maintenance requirements, and reduced EMI emissions.
Here are some of the key differences between zero-fired control and other control schemes:
- Zero-fired control eliminates switching transients: When an SCR is switched abruptly, it can cause a voltage spike on the power line. This voltage spike can damage other equipment on the same circuit. Zero-fired control eliminates this problem by turning on the SCR at the zero crossing point of the AC sine wave.
- Zero-fired control reduces EMI emissions: EMI is a type of electromagnetic interference that can be caused by electrical equipment. It can interfere with the operation of other equipment and can even be a health hazard. Zero-fired control reduces EMI emissions by turning on the SCR at the zero crossing point of the AC sine wave.
- Zero-fired control can be more complex to implement: Zero-fired control requires a more sophisticated control circuit than other control schemes. This can make it more difficult to implement and troubleshoot.
- Zero-fired control can be more expensive: Zero-fired control requires more sophisticated components and manufacturing processes than other control schemes. This can make it more expensive to implement.
