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TERMINOLOGY

There are two types of AC electromagnets. One is DC electromagnets with built-in AC to DC converters (rectifiers). Another is true AC electromagnets made of laminated electrical steel core (lamination). While applying AC voltage, true AC electromagnets generate alternate magnetic field. Laminated core eliminates eddy current due to alternating power source. So true AC electromagnets can work with both AC and DC power source.

Air-gap or air-gaps are the non-magnetic distance in the magnetic field path between an electromagnet and a plunger (workpiece). Air-gap(s) can be physical air distance or any non-magnetic materials. Air-gap(s) will exponentially reduce holding value.

Magnetech round, rectangular, and square electromagnets are constructed by a ferromagnetic center pole with electrical coil surrounded by an outer shell. When activated by a DC power supply, magnetic field in the center pole radially returns to the outer shell with minimum leaking field. The end result is a strong concentrated holding power on the center pole and an evenly distributed holding power on the outer shell. This unique and optimized design delivers higher holding value and less power consumption then conventional design. These high holding value electromagnets are flat-faced and available in a variety of shapes and sizes. They ideally suit for applications in which the electromagnet comes directly in contact with a smooth and flat surface plunger (workpiece). They are the best choice for ferrous material lifting, holding and positioning. They can be used in manually operated or automated application with 12 V.DC, 24 V.DC, or 110 V.DC operation voltage.

Magnetech parallel pole (bi-polar) electromagnets are constructed by a ferromagnetic parallel side poles with an electrical coil between them. When activated by a DC power supply, magnetic field in one side pole returns to other side pole. The end result is a strong even holding power on the parallel poles. And since the distance between poles usually are greater, magnetic field is able to reach out further for applications with air-gaps. This unique and optimized design delivers higher holding value and less power consumption then conventional design. These electromagnets are flat-faced and available in a variety of shapes and sizes. Although they ideally suit for applications in which the electromagnet comes directly in contact with a smooth and flat surface plunger (workpiece), they can work on uneven surface plunger (workpiece) with air-gaps. They can be adapted with custom pole shoes to any curved surface. They are the best choice for ferrous material lifting, holding and positioning. They can be used in manually operated or automated application with 12 V.DC or 24 V.DC operation voltage.  

Operation duty cycle is the percentage of total on-time over one complete on and off cycle. The maximum on-time in a cycle is determined by the physical size of an electromagnet. The smaller the electromagnet, the short the maximum on-time. For example, 25% duty cycle with 2 minutes max. on-time means that every 2 minutes on-time needs 6 minutes off-time.

An electromagnet rated continuous duty cycle (100% duty cycle) can run continuously at normal room temperature with convection heat dissipation.

An electromagnet rated with intermittent duty cycle (not 100% duty cycle) must run within specified duty cycle in order to avoid overheating the electromagnet. Overheating will lead to premature failure.

Electromagnet
Magnetism is generated by electrical current.  So magnetism presents while electrical current flowing. An electromagnet generates heat, but the magnetism does not change by heat. The more the electrical current and winding turns, the more the magnetism.

Permanent Magnet
Magnetism is retained after magnetized by electrical current. So is a residual magnetism, but very strong residue. A permanent magnet does not generate heat, but the magnetism reduces by surrounding heat. A permanent magnet eventually will be de-magnetized by operation over time.

Which one has stronger magnetism?
It depends on an application and physical and environmental restraints. In general with given physical size, an electromagnet with continuous Duty_Cycle is little weaker than a strong permanent magnet. But an electromagnet can be stronger with intermittent duty cycle. In other words, an electromagnet can be made very strong, if you can take the heat away from the electromagnet by lowering duty cycle or forced cooling. 

How far magnetic field can reach out? 
The answer is not far. Since magnetic field or path is a loop with no beginning nor ending. The magnetic path typically consists of a magnetic field inside an electromagnet core and a magnetic field in air. The magnetic field in air is sometime a desired field for application. Since magnetic field is a loop that can be considered from North pole of an electromagnet to air, from air to South pole of the electromagnet, from the South pole through inside of the electromagnet returns to the North pole. The path of magnetic field in the air follows the rules of the least reluctance path (shortest smoothed curve for easy interpretation). This is why that magnetic field can not project far away. Never think that the magnetic field can be like an invisible rope shooting out.

What is the strength of magnetic field?
The projected Magnetic field in the air reduces its strength (flux density) exponentially over the distance.

What is the practical ratio of field in air vs. physical size?
In continuous duty cycle, one unit magnetic field distance requires about 4 unit diameter electromagnet, e.g. a 2" diameter electromagnet usually generates magnetic field that projects about 1/2" above the electromagnet. Magnetic field strength, duty cycle, cooling method, and shape of an electromagnet could dramatically alter the ratio.

Listed holding value is the actual readings of breaking away a 1/2" steel plate plunger (workpiece) with no air-gap(s) between them. Holding value is an axial break away force per diagram. Pry-open force, e.g. by opening one edge first to break away, is much smaller than holding value. Shearing force (sliding force) is not holding force, but a frictional force and much smaller than holding value. Holding value will be exponentially reduced with presence of any air-gap(s). For safety in holding applications, do not use electromagnets at more than 1/2 of rated value. In lifting applications, do not use at more than 1/4 of rated value. Do not use electromagnets over people.

Testing Diagram:
Holding Value Test

Operation temperature is also called working temperature or ambient temperature in technical term. Magnetech standard electromagnets are designed to work between -10°C (14°F) and 40°C (104°F) ambient temperature. Too low ambient temperature will cause epoxy crack that leads to a break inside the magnet coil or to cause insulation of lead wires to be brittle. Too high ambient temperature will cause the inside magnet coil to overheat. If higher ambient temperature is desired, an electromagnet can be custom-made of high temperature insulation material. 

An electromagnet, when energized, has a polarity of a north pole and a south pole. Magnetic flux lines go from the north pole to the south pole, then through the steel core of the electromagnet, and return to the north pole. Actually, magnetic flux is a closed loop without starting nor finishing. Adjusting the polarity of DC source to a DC electromagnet will adjust the polarity of the north pole and the south pole.

Core material of electromagnets is low carbon steel with very low residue. Magnetech DC electromagnets have very low residual magnetism left on electromagnets, when DC power is turned off. But the parts to be held by an electromagnet could retain some residual magnetism depending on the material of the parts. Usually, a soft steel with low carbon has less residue and a tool steel with high carbon ends up with high residue.

What if parts stick on an electromagnet?
Sometimes, the gravity of a part is not big enough to overcome the residual magnetism, so that the part sticks on the electromagnet and can't fall. This usually happens only on small, light weight parts in an automatic pick-and-place application. Electrically solving this problem, use a special DC Off-release power supply or DC power supply with reversing current to cancel the residue. Or mechanically, use electromagnets with auto release. The parts might still end up with some residual magnetism left.

How to remove residual magnetism on parts?
Use a de-magnetizer to remove residue. A common demagnetizer is a AC powered magnetic coil. Parts must go through the coil and physically move away from the coil while the coil is kept on AC power, as if the magnitude of alternating magnetic field gradually reduces to zero to completely remove the residual magnetism on the parts. If there is no physical movement while demagnetizing, the magnitude of AC power source must gradually reduce to zero either by using a Varica or a automatic damping circuitry.