How is a solenoid valve able to be opened and closed at different levels with PWM? The resistance of the coil is always the same so wouldn't different voltages at different levels change the heating effect on the coil and ultimately burn it up? I know that over and under voltages can burn up a solenoid so how are they accomplishing this multilevel control? Any help would be appreciated
Porportional valve/pulse width modulation?
- Thread starter stephena
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Over/under voltage burns up a coil that is not designed to handle the over/under given to it. But even then, under voltage generally results in lower magnetic flux and current, which isn't bad for the coil, but if whatever the coil is doing doesn't get done, results in damage because the coil was designed for short time frames at that current.
In a proportional valve, the coil is designed from the outset to be used this way. PWM is used to control the current, which controls the magnetic flux, which exerts the force against the spring in the valve. Full current gives full force for full open, but that is the rated current of the coil. Lower current means lower force, less open, but the coil is designed to do that indefinitely. The PWM is used to control the current by controlling voltage, and an LVDT feedback from the solenoid tells the controller in the valve if the current allowed to the coil was enough (or too much) to accomplish the task.
gar
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Stephena:
Heat and temperature are what destroys most products. Power input times time (energy), physical size, and how heat is dissipated from a device determines its temperature rise, and actual temperature.
In a DC solenoid or relay coil the DC resistance of the coil determines the current flow. The position of the solenoid plunger or relay armature does not affect the coil resistance. Thus, at any position the power input to the coil for a given excitation voltage is the same. If a coil is excited at a level designed for continous operation or lower, then it will not overheat if the armature or plunger is stuck in a partial position.
A solenoid or relay directly powered by AC is quite a different story. Now we are concerned with the AC impedance of the coil to determine current flow. Still current flow is what determines the heating of the coil.
The inductance and thus impedance of a coil is very much dependent upon the magnetic circuit associated with the coil. An air core coil has a much lower impedance than this same coil with a high permeability magnetic material in its magnetic path. Thus, plunger or armature position has a big effect on inductance.
For example: consider an old AB Bul 700 #2 motor starter with a 120 V AC coil. It is not the coil that makes it AC but there is also a shading coil. The coil DC resistance is 40 ohms. A DC resistance at 120 V would draw 3 A and dissipate 40*3*3 W = 360 W. This is a huge amount of power in the small volume the coil occupies. A DC coil in this same relay would have a different resistance.
But with 60 Hz sine wave excitation and because of the AC impedance of the motor starter (contactor or relay) I get the following results:
Plunger in open position,
9.5 V and I = 0.14 A or a Z of 68 ohms. At 120 V the current would be 1.77 A and coil dissipation about 40*1.77*1.77 = 125 W. Still a large amount of power in a small space.
Plunger in closed position,
120 V and I = 0.25 A or a Z of 480 ohms. Power dissipation in the coil about 40*0.25*0.25 = 2.5 W. Now it looks reasonable for the coil space.
This is a very useful characteristic of AC solenoids and relays. Higher initial force and lower holding power. But if something sticks (typical problem on AC solenoid valves), then there is great likelyhood of coil burnout.
For various reasons AC excitation of a proportional valve would not work well.
.
Stephena:
Heat and temperature are what destroys most products. Power input times time (energy), physical size, and how heat is dissipated from a device determines its temperature rise, and actual temperature.
In a DC solenoid or relay coil the DC resistance of the coil determines the current flow. The position of the solenoid plunger or relay armature does not affect the coil resistance. Thus, at any position the power input to the coil for a given excitation voltage is the same. If a coil is excited at a level designed for continous operation or lower, then it will not overheat if the armature or plunger is stuck in a partial position.
A solenoid or relay directly powered by AC is quite a different story. Now we are concerned with the AC impedance of the coil to determine current flow. Still current flow is what determines the heating of the coil.
The inductance and thus impedance of a coil is very much dependent upon the magnetic circuit associated with the coil. An air core coil has a much lower impedance than this same coil with a high permeability magnetic material in its magnetic path. Thus, plunger or armature position has a big effect on inductance.
For example: consider an old AB Bul 700 #2 motor starter with a 120 V AC coil. It is not the coil that makes it AC but there is also a shading coil. The coil DC resistance is 40 ohms. A DC resistance at 120 V would draw 3 A and dissipate 40*3*3 W = 360 W. This is a huge amount of power in the small volume the coil occupies. A DC coil in this same relay would have a different resistance.
But with 60 Hz sine wave excitation and because of the AC impedance of the motor starter (contactor or relay) I get the following results:
Plunger in open position,
9.5 V and I = 0.14 A or a Z of 68 ohms. At 120 V the current would be 1.77 A and coil dissipation about 40*1.77*1.77 = 125 W. Still a large amount of power in a small space.
Plunger in closed position,
120 V and I = 0.25 A or a Z of 480 ohms. Power dissipation in the coil about 40*0.25*0.25 = 2.5 W. Now it looks reasonable for the coil space.
This is a very useful characteristic of AC solenoids and relays. Higher initial force and lower holding power. But if something sticks (typical problem on AC solenoid valves), then there is great likelyhood of coil burnout.
For various reasons AC excitation of a proportional valve would not work well.
.
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