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  1. #1
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    How to control a electric magnet

    Hi all,
    I want to use a functional generator to control a electric magnet, the functional generator could generate a sine/cosine signal, and I have a L298N motor driver to drive my 12v-600N electric magnet. What should I do to connect the functional generator with the motor driver in order to let the magnet generate field according to the signal I have?
    Because the signal could be positive and negative voltage, I hope the driver could switch the current and make the polarity reversed.
    If you have better idea for the control circuit, please let me know. Thanks a lot!

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    Re: How to control a electric magnet

    Hi,

    It's done many million times: and audio speaker and an audio amplifer.
    So why don't you use this system?

    Klaus
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    Re: How to control a electric magnet

    Quote Originally Posted by KlausST View Post
    Hi,

    It's done many million times: and audio speaker and an audio amplifer.
    So why don't you use this system?

    Klaus
    Hi Klaus,
    That's because the speaker will be used underwater so I need to increase its power, so I purchased a large electric magnet. Actually I am not familiar audio amplifier, could you give me more information about that? Thanks !



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    Re: How to control a electric magnet

    Quote Originally Posted by KlausST View Post
    Hi,

    It's done many million times: and audio speaker and an audio amplifer.
    So why don't you use this system?

    Klaus
    Another problem is that what we want is a low frequency circuit, maybe 2Hz-40Hz regime, so the normal amplifier might not have good response at this region.



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    Re: How to control a electric magnet

    If you have better idea for the control circuit, please let me know. Thanks a lot!
    You should also implement a feedback system, perhaps with a simple coil around the magnet, perhaps using Hall sensors, so that you know the actual period and actual amplitude.



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    Re: How to control a electric magnet

    Hi,

    You know: for electrical designs you need electrical specifications.

    Now I try to summarize your given information:
    * functional generator, sinewave. (No voltage, no current)
    * electric magnet "12v-600N". I'm not sure if this is the type, or does it mean 12 Volts? 600Newton is useless. You need impedance or current.
    * L298N.

    I still vote for audio amplifiers. Nowadays many classD types are available. They combine a lot of features you need:
    * analog input
    * PWM output (you may use filters if you like)
    * your output voltage range
    * DC input supply voltage
    * AC output voltage (negative output. Often by using bridge topology)
    * relatively high output current
    * frequency range may be adjusted by external circuitry
    * you may buy cheap, ready to use modules (and modify them for your needs)
    * low power dissipation
    * capable of driving inductive loads

    I recommend to specify the current range first, then do an internet search for device names, read through datasheets and application notes provided by the manufacturers.

    Klaus
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    Re: How to control a electric magnet

    The issue with class D or any AC output amplifier is their wasted power which would be reflected and not be consumed by the electromagnet perdue its enormous inductance.

    This seems to be a task for a ZVS NMOS with the magnet inbetween the power supply to NMOS drain and an fast recovery/schottky snubber across the magnet.



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    Re: How to control a electric magnet

    The issue with class D or any AC output amplifier is their wasted power which would be reflected and not be consumed by the electromagnet perdue its enormous inductance.
    Your fears are partly unfounded.

    If you are worried about "enormous inductance" of the electromagnet, recall that any loudspeaker is essentially a motor and cannot move fast enough without large input of power.

    If the magnet is without any load, your point is somewhat valid. But in the voicecoil of any speaker, the movement is the load (rather fast accelerations or decelerations).

    If the magnetic field does no work (except perhaps producing some EM radiation), then what is the objective of the whole exercise? You need to clarify the setup. The magnetic field must see some load.

    Your design should have a high coupling between the magnetic field and the target load.

    For the time being, you can consider the electromagnet as a stator winding of a motor.

    I do not recall seeing any snubber network even for high power speaker coils; as long as the load is well coupled, you need not worry much.



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    Re: How to control a electric magnet

    Quote Originally Posted by c_mitra View Post
    Your fears are partly unfounded.

    If you are worried about "enormous inductance" of the electromagnet, recall that any loudspeaker is essentially a motor and cannot move fast enough without large input of power.

    If the magnet is without any load, your point is somewhat valid. But in the voicecoil of any speaker, the movement is the load (rather fast accelerations or decelerations).

    If the magnetic field does no work (except perhaps producing some EM radiation), then what is the objective of the whole exercise? You need to clarify the setup. The magnetic field must see some load.

    Your design should have a high coupling between the magnetic field and the target load.

    For the time being, you can consider the electromagnet as a stator winding of a motor.

    I do not recall seeing any snubber network even for high power speaker coils; as long as the load is well coupled, you need not worry much.
    An amplifier with AC output will be very nonlinear in its frequency band since its energy storage and usage is effected by electromagnets inductance. The speakers you mentioned is nothing akin to the mentioned electromagnet capable of lifting 134 pounds. Speakers simply do not have that much windings.

    A ZVS method will permit far more linearity for energizing the magnet and rapidly discharging it which cannot be done with an AC output amplifier without enormous voltage across the magnet to overcome its very large inductance. The rise and fall times of the ZVS should be adjusted to not cause damage to the electromagnet by charging or discharging it too rapidly. If a sine waveform is what is intended for the magnet to make the MOSFET can be linearly driven in its linear region swinging from base line to power supply peak.



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    Re: How to control a electric magnet

    The speakers you mentioned is nothing akin to the mentioned electromagnet capable of lifting 134 pounds.
    Please provide more details; voltage, current, power and, most important, your drive scheme.

    What is the inductance of the electromagnetic? Without numbers, it is going no where...



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    Re: How to control a electric magnet

    Quote Originally Posted by c_mitra View Post
    Please provide more details; voltage, current, power and, most important, your drive scheme.
    Electromagnets will have far more inductive reactance to the whole audio frequency bands especially that which can pull 134 pounds.

    The function generator should be isolated from the circuit such which can be done with an isolated gate driver.
    Last edited by Zak28; 24th August 2019 at 01:37.



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    Re: How to control a electric magnet

    Electromagnets will have far more inductive reactance to the whole audio frequency bands especially a that which can pull 134 pounds.
    Your specifications do not make any sense to me.



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    Re: How to control a electric magnet

    Quote Originally Posted by c_mitra View Post
    Your specifications do not make any sense to me.
    There aren't any specifications provided other than a function generator and an electromagnet.



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    Re: How to control a electric magnet

    There aren't any specifications provided other than a function generator and an electromagnet.
    Well, you can replace the electromagnet with a motor or the primary of a transformer. Of course they should have similar current and voltage ratings.

    There are many SMPS circuits that can be adapted for similar job. There are also circuits that can be used to control DC power motors.

    Perhaps an electromagnet will not respond mechanically to audio frequency. But audio amplifiers can be modified to respond to lower frequencies.



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    Re: How to control a electric magnet

    Hi,

    The issue with class D or any AC output amplifier is their wasted power which would be reflected and not be consumed by the electromagnet perdue its enormous inductance.
    The benefit of class D is that they don't waste power. Almost all energy stored in the magnet will be pushed back into the power supply (which may cause problems.. of increasing the supply voltage, if not decoupled or limited)
    For sure the designer has to take care about that.

    Neither the "back" energy, nor the "back" current (of any inductor) is higher than the energy / current you push "in".

    We can't change the requirements of the OP, like the used electro magnet and the sine shaped waveform with it's frequency range.
    There is no requirement for high voltage nor for fast turn OFF...

    Klaus
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    Re: How to control a electric magnet

    I think that you should use a simple coil around the magnet . That would be useful



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    Re: How to control a electric magnet

    I did miss your post #3. So your electromagnet is basically a low frequency loudspeaker (2-40Hz). And you want to use it underwater. I presume that the speaker diaphragm will be in physical contact with water. But why you describe this as an electromagnet? By the way, 2-40Hz is subsonic for many of us but you can easily modify regular audio amplifiers to use only this range of frequencies.



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