Key Questions to Ask When Ordering electromagnetic induction heater

Question of the Month: We are planning to use induction to heat composite metallic materials prior to diffusion bonding. Is there any concern with respect to excessive noise? Have you found that certain frequencies are noisier than others? If so, do you have a solution for containing the noise level?

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Answer: In the vast majority of induction heating applications, industrial noise does not reach an appreciable level. Therefore, there is no reason to be concerned about excessive noise, although there are a few exceptions. Lower frequencies typically result in higher coil current thus increasing electromagnetic forces and coil turn vibration. So it is reasonable to expect that a system applying line frequency (60 Hz) would be associated with a greater magnitude of industrial noise compared to 3 kHz or 30 kHz. In addition, systems with a greater amount of kW most likely would also be associated with more noise. Therefore, assuming all other factors are identical, 500 kW can produce greater noise than 100 kW.

When discussing audible noise, we must bear in mind the two main factors that impact how humans are affected by industrial noise&#;magnitude and unpleasantness/discomfort level. For example, low-frequency audible noise (e.g., 50 60 Hz) could have a higher magnitude, but it might not be as unpleasant to the human ear compared to a lower magnitude noise at an elevated frequency (e.g., 1 kHz).

Following are the four main sources of noise generation during induction heating:

• Noise generated by the power supply. Numerous power supplies are available on the market. For induction heating, power and frequency combinations that use single-module inverters vary from line frequency to several hundred kHz and power levels exceeding kW even for a frequency in the 800 kHz range. Design features of modern power sources based on semiconductor technology result in nearly silent operation. Therefore, noise stemming from the power supply is not usually a concern.
• Noise generated from vibration of copper coil turns. The two main approaches to building induction coils can be categorized as either open-wound or refractory-encased [Ref. 1]. The open-wound method provides more convenient repair in the event of failure, but coil turns must be secured using studs and proper fixtures to eliminate or minimize vibration and noise. An encased coil using a castable refractory (for example, special grades of cement) offers durability and longer life, eliminating or dramatically reducing the vibration of coil copper turns. Properly designed induction coils do not exhibit problematic noise levels.
• Noise generated by workpiece vibration or resonant sound waves (amplifying effect). If a workpiece consists of some loose parts, then they may vibrate and produce noise. To assess this possibility, the specific workpiece geometry would need to be reviewed. Pressure can be applied in certain diffusion bonding applications to minimize the possibility of individual component vibrations. When induction heating hollow workpieces (e.g., induction heating of relatively thin-walled pipes or tubes), certain frequencies in combination with sufficiently high power densities could emit resonant sound waves of an appreciable magnitude, exceeding the audible limit. In cases like this, audible noise can also be a dominant factor that greatly affects the selection of frequency. Each tube has its own structural resonant frequency (SRF), which depends on the diameter, wall thickness, material, length, and other factors. When the heating frequency of the induction coil is sufficiently close to the tube&#;s own resonant frequency, then high amplitude vibration and excessive audible noise may occur. In other words, as a radio receiver transforms electromagnetic waves into an audible sound, somewhat similar mechanisms can occur that cause a tube or pipe to act as an amplifier when dealing with certain frequencies. A decision as to whether noise could be reduced by choosing a different (higher or lower) frequency depends on a combination of the structural self-resonant frequency (SRF) of a particular tubular workpiece and the applied frequency of the induction heater. Therefore, in cases like this, it is beneficial to measure the workpiece SRF by simply hitting it with a hammer and measuring the resonant audible noise with some kind of audible receiver with the ability to detect the frequency and amplitude of the measured signal. As a result, more intelligent decisions can be made in determining whether a certain frequency would improve the noise level or not compared to a frequency that produces unacceptable noise. For example, if the SRF of a certain workpiece is 300 Hz, then the further away the selected frequency is, the lower the noise that will be produced. In this case, 6 kHz would produce a noticeably lower noise compared to 1 kHz. In contrast, if the SRF is 5 kHz, then the use of 6 kHz will make it worse compared to 500 Hz. Many times, a protective or reducing atmosphere is used while heating metallic materials. Therefore, if your induction system will be located inside a gas-tight chamber/enclosure, this will substantially reduce the noise level.

• Noise generated by vibration of power cables, buses, and fixtures. There is a very small probability of using the wrong design of fixtures, power cables, and bus bars. However, any concerns may be addressed by an induction heating expert.

More discussion on these points can be found in Ref.1.

Are you interested in learning more about electromagnetic induction heater? Contact us today to secure an expert consultation!

Dr. Valery Rudnev, FASM
Director, Science & Technology
Inductoheat Inc
www.inductoheat.com

Reference

1. V. Rudnev, D. Loveless, and R. Cook, Handbook of Induction Heating, 2nd Edition, CRC Press, .

Beginning in July , the Professor Induction column started a new article series called &#;Induction Heating: Everything You Wanted to Know, But Were Afraid to Ask.&#; The most commonly asked questions related to different aspects of induction heating and heat treating will be reviewed and explained. All are welcome to send questions to Dr. Rudnev at . Selected questions will be answered in this column without identifying the writer unless specific permission is granted.

Induction heater is a fun project. I built a small induction heater then I wanted a more powerful one. A small unit is so simple you can build it in about 2 hours work. A larger more powerful unit will take longer. If you take your induction camping it works great to run it on your 12v car battery.

I built my largest induction heater from a microwave oven transformer. Remove the v secondary coil, replace it with enough #12 solid copper wire to get 12v to 15vdc. I have about 15 MOTs they all have 100 turns on the primary coil. Math is easy for the secondary coil running on 120vac on the primary = about .833 vac per turn on the secondary.

I have not experimented with a flat induction coil the value must be 2. uh minimum value. Buy a, resistor, capacitor, inductor, meter about $35. on ebay to test your coils. The choke coil needs to be large enough not to saturate or it will stop working at a certain power range then current suddenly goes to maximum then mosfets explode. Very low internal mosfet resistance works best it prevents mosfets from over heating easy.

Here is my induction circuit, leave off the transformers so you can run it on a car battery if that is what you want. My small induction heater will heat a 1/4" steel rod red hot in about 3 minutes. My w induction heater heats a 1/4" steel rod red hot in 7 seconds. You don't need w to heat food it will burn before food has time to cook.

My induction heaters all have the same circuit drawing for the exception of larger power supplies and larger L2 choke coil for higher power units. If you change #10 circuit copper wire to a smaller wire skin effect goes up and the circuit efficiency goes down. My induction heat is several times smaller than the w power supply as you can see in the photos.

Be sure to connect the caps in a box shape like a Rail Gun cap bank charge and discharge is much faster. Build the small unit with the 6 yellow color caps first. I use insulated Romex solid copper house wire. Be sure to use insulated wire if you accidently touch the osc coil with metal osc stops then 1 of the mosfets will explode.

You can build the small circuit for $15. The larger circuit only cost be extra for a 100a bridge rectifier and the meter. I salvage parts from old TV and old computer power supplies. 10 mosfets are $6 free shipping on ebay if you order from a seller in China.

If you have not built an induction heater yet it is a fun project. Build the small circuit with 6 yellow capacitors first.Induction heater is a fun project. I built a small induction heater then I wanted a more powerful one. A small unit is so simple you can build it in about 2 hours work. A larger more powerful unit will take longer. If you take your induction camping it works great to run it on your 12v car battery.I built my largest induction heater from a microwave oven transformer. Remove the v secondary coil, replace it with enough #12 solid copper wire to get 12v to 15vdc. I have about 15 MOTs they all have 100 turns on the primary coil. Math is easy for the secondary coil running on 120vac on the primary = about .833 vac per turn on the secondary.I have not experimented with a flat induction coil the value must be 2. uh minimum value. Buy a, resistor, capacitor, inductor, meter about $35. on ebay to test your coils. The choke coil needs to be large enough not to saturate or it will stop working at a certain power range then current suddenly goes to maximum then mosfets explode. Very low internal mosfet resistance works best it prevents mosfets from over heating easy.Here is my induction circuit, leave off the transformers so you can run it on a car battery if that is what you want. My small induction heater will heat a 1/4" steel rod red hot in about 3 minutes. My w induction heater heats a 1/4" steel rod red hot in 7 seconds. You don't need w to heat food it will burn before food has time to cook.My induction heaters all have the same circuit drawing for the exception of larger power supplies and larger L2 choke coil for higher power units. If you change #10 circuit copper wire to a smaller wire skin effect goes up and the circuit efficiency goes down. My induction heat is several times smaller than the w power supply as you can see in the photos.Be sure to connect the caps in a box shape like a Rail Gun cap bank charge and discharge is much faster. Build the small unit with the 6 yellow color caps first. I use insulated Romex solid copper house wire. Be sure to use insulated wire if you accidently touch the osc coil with metal osc stops then 1 of the mosfets will explode.You can build the small circuit for $15. The larger circuit only cost be extra for a 100a bridge rectifier and the meter. I salvage parts from old TV and old computer power supplies. 10 mosfets are $6 free shipping on ebay if you order from a seller in China.

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