205 VDC vs 230 VAC coils

[RobertHasty]

Hi,

I came across a hydraulic valve with 205 VDC coil recently.
The coil has a connector with rectifier mounted on it. That means that the voltage applied to the coil is 230 VAC.

I am wondering, why not use an AC valve with 230 VAC coils instead of valve with 205 VDC coils?

 

[waross]

Possibly to avoid the inrush current of a typical AC solenoid type valve.
If a DC coil does not pull in for any reason,low voltage, dirt in the valve port or other cause, there is no damage to the coil.
If an AC coil does not pull in for any reason,low voltage, dirt in the valve port or other cause, the coil often burns out.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Heaviside1925]

Not sure if this will answer your question.

https://www.insanehydraulics.com/letstalk/dccoils.html

https://www.nord.com/media/documents/bw/u35000_motor_brakes.pdf

 

[che12345]

"...The coil has a connector with rectifier mounted on it. That means that the voltage applied to the coil is 230 VAC...I am wondering, why not use an AC valve with 230 VAC coils instead of valve with 205 VDC coils?".
I have the following opinion for your consideration.
1. DC coil is gaining popularity. One of the setbacks of AC coil is it needs a shading ring/pole to avoid "chattering" on every cycle. Dc coil also encounter some problems that it may failed to reset due to residual magnetism etc.
2. AC coil pull-in (VA)/holding(VA) ratio is about = to DC coil pull-in (W)/holding(W).
3. Both AC coil or DC coil would be over-heated (damage) if the armature failed to seal in, due to any reasons. In this case, the current would remain at high pull-in (VA) on AC, or pull-in (W) on DC.
4. AC coil R<<X. DC coil has only R , no X. AC coil R >> DC coil R. Therefore, AC coil is very much less affected by the winding temperature.
Che Kuan Yau (Singapore)
PS: Correction " AC coil R << DC coil R.

 

[waross]

2. AC coil pull-in (VA)/holding(VA) ratio is about = to DC coil pull-in (W)/holding(W).

Is this a typo? An AC coil sees an increase in impedance due to increased inductive reactance as it moves to shorten the magnetic circuit.
The DC coil does not change impedance.
A failure to pull in does not affect the impedance of a DC coil.

Therefore, AC coil is very much less affected by the winding temperature.

That is a design consideration. The strength of a DC electro-magnet depends on the number of turns, and the gauge of the wire within reason.
Select any wire gauge and any number of turns. Calculate the resistance and the Amp Turns.

Now double the number of turns, with the same gauge wire, and recalculate.
Resistance is doubled, current is halved.
Amp turns remains the same.
Magnetic force remains the same.
Heat generated is 50%

Now halve the number of turns, with the same gauge wire, and recalculate.
Resistance is halved, current is doubled.
Amp turns remains the same.
Magnetic force remains the same.
Heat generated is 200%
And this compares with an AC coil how?

The same relationship does not apply to AC coils.
The effect of increasing the number of turns on the inductive reaction of an AC coil is a square function rather than a linear function.

In the above example, increasing the number of turns on the DC coil and the increased induction does affect the buildup of current and thus the pull-in time.
I am not privy to the design of modern DC solenoid coils, but I suggest that if the solenoid may be subject to Pulse Width Modulation control, a designer may opt for as little resistance as possible with the resulting high temperature to get the fastest action possible under PWM control.
Dropout time may be even more affected than pull in time by increased induction. It depends somewhat on the construction of the magnetic circuit.




--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

#1." AC coil pull-in (VA)/holding(VA) ratio is about = to DC coil pull-in (W)/holding(W). Is this a typo?"
1. This is the case in general, that is why is about. In any case, irrespective of whether AC or DC coil, the pull-in/holding ratio is generally > 30...up to .., depending on the magnetic design.
2. Both AC or DC coil, the very heavy pull-in value is only momentary, which must be lowered to holding value, on continuous operation.
Note: In some DC coil designs, an external resistor is inserted in series into the coil circuit, after sealing in. Lowering the pull-in value to holding.
3. Attention: Both AC or DC coil would over-heat and damage with prolong operation on pull-in current value, when the armature failed to seal in; due to any reasons. This is evident by very frequent DC brake coil damage on brake motors.
Che Kuan Yau (Singapore)

 

[waross]

2. Both AC or DC coil, the very heavy pull-in value is only momentary, which must be lowered to holding value, on continuous operation.
Note: In some DC coil designs, an external resistor is inserted in series into the coil circuit, after sealing in. Lowering the pull-in value to holding.

I guess we have seen different things.
I am familiar with heavy solenoids such as automobile starter solenoids, which have two coils, a pull in coil and a holding coil.
A switch is activated at the end of the stroke to insert the holding coil into the circuit.
I have seen a large number of DC solenoids that are designed to operate indefinitely on pull in current.
I did once encounter a DC coil with a series device to drop the holding current.
I forget the details except that it had failed and parts were not available.
I only remember that an alternate solution was found, not what that solution was.
Pulse Width Modulation control is becoming more common with DC solenoids.
Many holding current reduction techniques may not be compatible with PWM control,hence designs suitable for continuous operation at full voltage.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!