I design many small panels that use 24 VDC as the primary control voltage. The incoming line is usually 230VAC, 3 phase. I have typically used a small control transformer to power the PC and the motion controller. Both components have built-in switching power supplies and can run on 115VAC or 230VAC. I would like to eliminate the transformer to save space and reduce the weight of the panel.

I am wondering if I can leave out the transformer and run the PC and the motion controller directly from the 230VAC line? I suspect that switching
supplies do not isolate the ground from the line. Is this a problem?

I am also planning to start using DIN rail mounted switching power supplies for my 24VDC supply. Will this cause any isolation problems? I have always used linear supplies with a built in transformer, but they are large and heavy also.

Thanks,

Bill Sturm

 

I have been using switchers in my panel designs for about 3 years now and have not encountered any problems, but I have stuck to two brands of switcher power supplies, Phoenix Contact's Quint line and Sola's line so I don't have any
experience with others. I prefer to avoid the higher voltages in control and have only used 230V in panels I build for European installations, and there only when current demands of a 24V device are impractical, such as a very large motor contactor. A high quality switcher will generally provide you with cleaner more efficient power than a linear. But a switcher can cause line side harmonics. The Phoenix Quint -F series addresses the harmoncs problem and may be required (or other similar switcher) in some areas.

 

Weidmuller makes a line of 24VDC power supplies that take 230VAC 3 Phase input power. These would allow you to eliminate your control transformer and power all of your panel components off 24VDC.

 

If the transformer is both primary and secondary fused, supplemental circuit protection will be required for each device-typically a minibreaker
appropriately rated for the devices. A small CPT is relatively cheap when compared to minibreakers, and unless the cpt is quite large, nominal weight savings should be expected.

> I am wondering if I can leave out the transformer and run the PC and the
motion controller directly from the 230VAC line? I suspect that switching
> supplies do not isolate the ground from the line. Is this a problem?

If supplemental protection is provided (appropriate curve and ampacity) this should not be a problem. Considerations for GFI should be explored especially if exporting.

> I am also planning to start using DIN rail mounted switching power supplies
for my 24VDC supply. Will this cause any isolation problems? I have always
used linear supplies with a built in transformer, but they are large and heavy also.
>

Switching power supplies run the gamut in both quality and price when compared to a linear supply. Usually the greater the efficiency the greater the cost. The best switching power supplies should not cause much emi or rfi, if that is the isolation you are worried about

Best regards,

Malcolm

 

Bill,

There are requirements in Intrinsically Safe circuits for isolation of the DC supply from AC mains.

Bruce.

 

On March 25, 2002 11:08 am, Bill Sturm wrote:
<clip>
> I am also planning to start using DIN rail mounted switching power
> supplies for my 24VDC supply. Will this cause any isolation problems? I
> have always used linear supplies with a built in transformer, but they
> are large and heavy also.
<clip>

We've had some problems with ground loops in instrumentation when used with switching supplies. It may be due to some sort of capacitive coupling to ground. The problem would come and go over time without any apparent cause.
These have all been cured by changing to a linear supply (the instrumentation systems had their own power supplies).

The instruments we used in these applications were not three way isolated, which may be a consideration in whether this could be a problem for you or not.

************************
Michael Griffin
London, Ont. Canada
************************

 

I'm not exactly sure what you're trying to do but a switcher primary side runs off a dc bus and the secondaries are typically isolated e.g. motor-drive supply: primary coil pwm circuit runs off the dc bus of the ac/dc converter stage. Secondary coils for 5V, 15V etc. power distribution are isolated from primary (secondary voltage sensing for pwm control via isolated technique). Note that primary not isolated from ac/dc power source. While switchers are used almost exclusively now in modern power electronic
products e.g. drives, the EMI they generate at the higher pwm freqs can cause headaches in certain applications/installations.