A drawing specified “suction force 50N” but omitted the duty cycle. When an electromagnet continuously operates for ten minutes inside a 70°C enclosure, its output force is often halved.
When designing actuators, R&D personnel often tend to treat these magnetic circuit components as simple mechanical outputs, overlooking the physical limitations during the conversion of electrical energy, thermal energy, and magnetic fields.
What is an Electromagnet? Starting from Structure and Operating Principles
The “Duty Cycle” Specification Most Often Overlooked in Electromagnet Selection
Why are some electromagnets small yet immensely powerful, while others are large but prone to overheating?
Compared to the multi-directional movement of motors, this component focuses solely on pushing or pulling in a single direction. Its structure consists of a coil, an iron core, and a housing. When current passes through the coil, the internal iron core is magnetized, generating a linear pulling force. This structure dictates its extremely fast response time, usually completing its action within milliseconds. However, its disadvantage is that its attractive force decays exponentially as the stroke increases.
The Golden Rule of Magnetic Circuit Design: For every doubling of the stroke, the attractive force typically decays to one-quarter of its original value. Demanding high attractive force at the initial stroke often leads to uncontrolled size and power consumption.
A 100% duty cycle means the component can be energized indefinitely without exceeding its temperature limit, while a 10% duty cycle restricts each energization time to no more than one-tenth of the total cycle. If procurement only considers maximum attractive force but uses intermittent specifications in a continuous energization scenario, the coil will burn out within two minutes due to Joule heating.
Electromagnet Selection Calculation: Estimating Attractive Force Loss from Heat Generation
We must admit that pursuing high attractive force within extreme space constraints inevitably requires compromising on temperature rise.
In a confined space with ambient temperatures exceeding 80°C, using H-class insulated coils paired with brass guides is the only viable solution to ensure the electromagnet operates continuously for over ten thousand hours without burning out.
| Duty Cycle | Maximum Energization Time | Attractive Force Enhancement Factor | Cooling Requirements |
|---|---|---|---|
| 100% (Continuous Energization) | Infinite | 1.0x (Baseline) | Natural convection is sufficient |
| 50% | 120 seconds | 1.4x | Local temperature rise needs attention |
| 25% | 30 seconds | 2.0x | Metal surface cooling recommended |
| 10% | 5 seconds | 3.0x | Forced cooling or intermittent cooling required |
We recorded a set of data during testing: when the coil temperature rose from 20°C to 120°C, the copper wire resistance increased by approximately 40%, which directly led to a 28% drop in current, and a corresponding decline in attractive force.
Custom Electromagnet Mechanical Tolerances and Residual Magnetism Elimination Design
Eliminating residual attractive force after power-off is a critical step to ensure smooth mechanism reset.
Why does the iron core remain sticky after power is cut?
Although highly permeable materials can provide strong attractive force, their physical properties also come with higher residual magnetism, preventing immediate release after power-off. To address this physical limitation, designs typically incorporate non-magnetic media such as copper shims or Teflon coatings on the impact surface, or design return springs internally to overcome the residual magnetic force with physical force.
How to Optimize Electromagnet Procurement Process through Specification Lists
A standard confirmation process includes three steps: first, confirming the stroke and attractive force curve; second, defining the operating voltage and duty cycle; and finally, evaluating the ambient temperature and protection rating.
When collaborating with clients, we found that many procurement cases could save significant time and cost from subsequent sample modifications if a complete stroke-force curve diagram could be provided early on.
Compared to traditional estimation based solely on experience, Shih-Shin Technology (世僖科技) utilizes precise electromagnetic simulation software to calculate dynamic attractive force at different temperatures during the design phase when handling such complex selections, assisting engineers in finding the optimal balance between space constraints and performance.
Choosing an electromagnet is not just about looking at the static maximum attractive force in the catalog, but also about its thermal equilibrium performance at extreme operating temperatures. Only by incorporating duty cycle, temperature rise, and residual magnetism control into the initial design can unexpected failures during subsequent mass production be avoided.
It is recommended that when starting a new project, you first confirm the three key indicators—stroke, operating voltage, and duty cycle—with the engineering team to receive precise preliminary selection recommendations within 24 hours.
