Electronics > Beginners
Did I get screwed on these electromagnets?
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Jwillis:
Should say right on it what voltage it is .Is it possible that your current is to low? See attached picture.


oops sorry wrong one.my bad
SL4P:

--- Quote ---however I'll be using the magnets to send on/off pulses in the 100-500 Hz range.
--- End quote ---
Interesting to see how they work in that role.
You’ll want a substantial snubber to quench the back EMF operating like that.
T3sl4co1l:
More to the point, you'll need a lot of drive voltage, both turning on and turning off.  Probably quite a lot more than rated voltage (regardless of whether that rating is correct or not :) ).

Example, stepper motors typically being rated for, say, 3 or 5V, but driven at 30V or more, to get the required current slew rate (dI/dt) to run at reasonable frequencies (~kHz).  The driver is a switched constant current (usually hysteretic control), so excessive real power is not delivered to the motor (it never delivers, e.g., (30V)^2 / (DCR) watts, continuously, to a winding), but enough reactive power is delivered to deal with stray inductance of the windings.

Tim
TakeItUpANotch:

--- Quote from: TERRA Operative on July 23, 2018, 10:38:22 am ---Can you measure the pull of the magnets at various voltages and check against the specs from the seller?

That would tell you what you got for sure.

--- End quote ---

I don't really have the equipment to do proper measurements of force, but it would be cool to do.


--- Quote from: Jwillis on July 23, 2018, 03:04:32 pm ---Should say right on it what voltage it is .Is it possible that your current is to low? See attached picture.


oops sorry wrong one.my bad

--- End quote ---

The sticker on the physical magnets I received specifically say 12V.


--- Quote from: SL4P on July 23, 2018, 05:40:33 pm ---
--- Quote ---however I'll be using the magnets to send on/off pulses in the 100-500 Hz range.
--- End quote ---
Interesting to see how they work in that role.
You’ll want a substantial snubber to quench the back EMF operating like that.

--- End quote ---

My plan is to drive them with a standard L293D H bridge which has built-in protection if I'm not mistaken. On the few tests I've done they seem to work quite well.



--- Quote from: T3sl4co1l on July 23, 2018, 07:22:08 pm ---More to the point, you'll need a lot of drive voltage, both turning on and turning off.  Probably quite a lot more than rated voltage (regardless of whether that rating is correct or not :) ).

Example, stepper motors typically being rated for, say, 3 or 5V, but driven at 30V or more, to get the required current slew rate (dI/dt) to run at reasonable frequencies (~kHz).  The driver is a switched constant current (usually hysteretic control), so excessive real power is not delivered to the motor (it never delivers, e.g., (30V)^2 / (DCR) watts, continuously, to a winding), but enough reactive power is delivered to deal with stray inductance of the windings.

Tim

--- End quote ---

That's interesting and is actually relevant to my project. I'm using the magnets to vibrate strings (hence the 100-500Hz range) and ideally I want the strings to start vibrating as quickly as possible when pulsed, so I might have to look into spiking the voltage for the first few hundred milliseconds or so. I'm controlling the magnets with an ardunio and L293D H bridges that are fed a constant 12V from a fairly hefty power supply (so no noticeable voltage drop under load). Do you have any suggestions for how I would go about spiking the driving voltage for the initial pulses?

IconicPCB:
PWM is a possibility however dual spike and pedestal driver is probably the way to go.

With PWM use a higher voltage ( say double the rated voltage ) and modify the PWM to alter average DC to energise the coil in a two step fashion.

EDIT:

The alternative is to operate the coil switching at a highvoltage but include series resistance to ensure coil current is safe. This will also alter the electrical time constant of coil favourably.
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