Can I operate my three phase power supply from WYE and Delta AC inputs?

Before addressing how we connect, or even if we can connect, a three phase power supply to an AC voltage source, I think we should review some background information.

AC-DC power supplies that are rated higher than 2.5kW frequently have a three phase AC input.  In the US the voltage can be 208/220Vac or 480Vac.  In Europe it is a “harmonized 400Vac” which in actuality is 415Vac in the UK and 380Vac for Europe.  A higher input voltage allows more power to be drawn from the incoming AC at a lower current.  These three phase AC voltages can be one of two configurations – Delta or Wye (pronounced “why”).

The following should also clarify what three phase input voltage would be best suited for a large power system.  Just as important, how to read a manufacturer’s datasheet to make sure that power supply can be used in the US, Europe and globally.

Typically very high voltage power is transmitted from the power generation plants to local substation transformers (where it is reduced in voltage) and then to facilities in a Delta configuration (Figure 1).  Note that a Delta configuration only uses three wires and has no neutral or ground wire.  This saves the cost of a fourth wire, which is not needed during transmission.

Figure 1: Delta wiring configuration (US voltage shown)

I will start with the US first.  Figure 2 shows a basic overview of what a manufacturing facility receives from the Grid, at what point it is lowered in voltage and how it is distributed to the loads.

Figure 2: Typical US facility power distribution

Starting at the left, a 480Vac Delta three-wire feed enters the facility from the substation.  From the incoming distribution panel, 480Vac Delta is supplied to electrical equipment needing a large amount of power.  Large ovens, test equipment for semiconductors, burn in chambers and machines fabricating metal (including laser cutting and additive manufacturing) would typically use 480Vac Delta.  It is important to note that equipment connected to this voltage feed can reduce the size of the Delta-Wye step down transformer, saving cost, energy losses and floor space.

To supply the rest of the facility, the three phase is reduced from a 480Vac Delta configuration to a 4 wire 208Vac phase to phase Wye configuration (Figure 3) via a step-down Delta-Wye transformer.  

Figure 3: 208Vac phase to phase Wye configuration

From the distribution panel, in addition to being able to supply 208Vac phase to phase, 120Vac is available by connecting to either one of the Lines (L1, L2 or L3) and neutral N.

As very rough order of magnitude, 208Vac three phase would be used for mid-sized loads, greater than 5kW and less than 25kW and single phase 208Vac for smaller loads greater than 1.5kW.  We are all aware of the 120Vac wall outlet which can support around 1kW.  The amount of power depends on the wiring size and fusing, consult your local qualified electrician!

There may also be a second Delta-Wye transformer in some facilities.  As discussed in another blog, this provides 277Vac feeds to lighting and HVAC (Heating, Ventilation and Air Conditioning) equipment.

In Europe the arrangement and voltages are different than the US, see Figure 4.

Figure 4: Typical European facility

Again starting on the left, high voltage (11kVac in a Delta configuration in the UK) is provided by from the Grid and a step down transformer delivers three phase in a four wire Wye configuration to the facility’s distribution panel.  See Figure 5.  As explained earlier 380V/220Vac is mainly used in mainland Europe, 415/240Vac in the UK.

Figure 5: 380/415Vac phase to phase Wye configuration

From the distribution panel, in addition to being able to supply 380/415Vac phase to phase, 220/240Vac is available by connecting to either one of the Lines (L1, L2 or L3) and neutral N. 

Returning to the subject of three phase AC-DC power supplies, we shall review some examples from TDK-Lambda’s product offering.

The HWS1800T-24 is a 1.8kW rated power supply accepting a 170-265Vac 3 phase input.  This would be suitable for operation on a standard US type of 208Vac three phase Wye input.  It could also be operated in Europe, but would require a 400Vac to 220Vac three phase Wye-Wye step down transformer.

The TPS4000-24 is a 4kW rated power supply accepting a 350-528Vac 3 phase input, either Delta or Wye.  This would be suitable for operation in the US and in Europe without the need to change connections to the power supply, or additional transformers.

The Genesys+™ series of programmable power supplies has a large number of models ranging from 1.5kW to 15kW.  Depending on the power level the units have different input voltages, covering most of the global input voltages.

GH1.5kW / G1.7kW:                     1ø 85 to 265Vac

G2.7kW / G3.4kW:                       1ø 170 to 265Vac or 3ø 208Vac or 3ø 400Vac

G5kW / GSP10kW & 15kW:        3ø 208Vac, or 3ø 400Vac or 3ø 480Vac

Ensure that the manufacturer has internal fuses fitted, as some low cost power supplies do not.  A high voltage fuse is required for each phase.  They are bulky and are not inexpensive.

After reading this blog, you might even take a second glance at those big grey mystery boxes surrounded by chain link fencing and high voltage warnings in the company parking lot!

Power Guy

Fan cooling power supplies, which airflow direction is better?

Even though the efficiency levels of new power supply designs are now routinely in the 94 to 95% range, the push for higher power densities continues. Fan cooling is one option to reduce the product’s overall size for mid to high power requirements. Should the fan blow air into the power supply or extract air out?  This depends on several factors.

In rack mounted products like the Genesys+™ programmable power supplies the airflow is drawn through the front and out the rear. This is to offer the best possible cooling for these products, to optimize the overall system’s performance and avoid the operator being subjected to hot exhaust air.

For enclosed power supplies, air flow direction can depend upon which direction the system air is being directed.  Having the power supply’s air flowing in the opposite direction to larger system’s fans can cause the airflow to reduce dramatically due to the system’s pressure, and cause overheating.

Where the fan is positioned in a power supply is another consideration.  The lifetime of an electrolytic capacitor is extremely sensitive to heat.  Each 10^oC rise in the capacitor’s temperature will halve its operating life, thus cool air has to be directed accordingly.

Figure 1 shows the top view of a typical product (the cover removed) and the location of the capacitors and fan.  In the case of this power supply, as the input and output connectors are both located on the left side (front) for system wiring access, the fan is situated on the right hand side (rear).

Figure 1: Electrolytic capacitors and fan location

Should the fan blow air out, or in? As with any design, there are advantages and disadvantages.

Figure 2 shows the fan blowing air out (exhaust). 

Figure 2: Fan exhausts the hot air

Advantages

The cool air is drawn in over the output capacitors, this keeps these components cool and their lifetime is improved.

As indicated by the size of the arrows, the speed of the air entering the power supply is lower than the exit speed.  This is due to the cross sectional area of the input being twice that of the output  Lower speed air is less likely to draw in outside contaminants (dust and dirt) which could impact product lifetime.

Disadvantages

The input capacitors receive warmer air, but with an air directing baffle and perforations in the cover this could be mitigated. In general though, the input capacitors are less sensitive than the output filtering capacitors.

Higher speed airflow cannot easily be directed at hot items like magnetics.

Hot air is drawn across the fan bearings, which could affect fan life. If the fan speed is controlled according to ambient temperature, this too would be mitigated. Higher quality or higher temperature fans can also be used.

Figure 3 shows the fan blowing cool air in.

Figure 3: Fan draws in air

Advantages

Cool air is drawn across the fan bearings, increasing fan life.

Fast moving cool air creates backpressure and can be directed at hot areas, like the magnetics, reducing the overall de-rating of the power supply.

Disadvantages

The output capacitors may run hotter.  Larger capacitors can be used, which will have less internal heating and run cooler.

More contamination may be drawn into the power supply.

Many fan cooled power supplies offer “reverse fan” options on their datasheets. Often due to reduced thermal performance within the supply and heated air moving through the fan, additional derating may apply.

Power Guy

When are fuses needed in Line and Neutral for industrial applications?

Last month I wrote about why using a dual input fused medical/industrial power supply might cause an issue in some industrial applications.   Shortly after publication a customer requested an application note showing the fitting an external fuse in the Neutral connection of an industrial DIN rail power supply application.  DIN rail power supplies are rarely certified to the medical safety standards and usually only have a single input fuse.

Upon investigation, the application was in this case operating a power supply phase to phase from a 3-phase WYE configuration in North America, see Figure 1.

Figure 1: Phase to phase connection (208Vac)

Connecting phase to phase enables the equipment to be supplied with 208Vac which draws less input current than using 115Vac.  This allows the use of smaller wire gauges and connectors which saves money and are easier to install.  The power supply is connected as shown in Figure 2.

Figure 2: Power supply connected phase to phase

In the UL safety report Conditions of Acceptability, or Technical Considerations section, reference is made to all testing being performed on a protected branch circuit rated for 20A; this section also lists the items that a user should consider before applying power to the unit.

If a short or overload inside the power supply occurs across the Line and Neutral or Line and Ground, the internal fuse F1 will open.  If the short occurs from Neutral to Ground, the fuse or breaker at the distribution panel will open.

If, however, the equipment connection to the building installation wiring is made via a non-industrial plug and socket, then an external fuse or breaker (F2) has to be added in the Neutral line as shown in Figure 3.  This applies to input voltages of less than 240Vac +10%.

Figure 3: The installation of an external fuse or breaker in the Neutral

Always have your equipment installed by a qualified electrician and checked by a safety engineer for compliance to the relevant building and electrical codes!

Power Guy

When are dual fused power supplies not permitted?

For many years AC-DC power supplies for the medical market are not only safety certified to the IEC 60601-1 medical standard, but also to the industrial IEC 60950-1 / IEC 62368-1 standards.  Dual certification reduces the number of individual model numbers that have to be produced and stocked; this improves production efficiency and reduces the amount of inventory held in distribution and the supply chain.  One small cost adder is that two AC input fuses are needed in many medical applications, one in the Line side and one in the Neutral side.

Dual fusing is necessary to guarantee full protection if the polarity of the Line and Neutral wiring to the power supply should be reversed.  This might be due to a wiring error at the building’s AC socket or in the wiring feeding the power supply.

I had to say I was mildly surprised when TDK-Lambda started offering a single (Line) fuse option on their new medical/industrial power supplies.  Two protective devices surely must be better than one?  Some investigation was required, because someone else was bound to question this too!

If an industrial system is consuming more current than a regular AC socket can support, it would have to be permanently connected to a distribution panel or building wiring.  This task is normally carried out by a professional electrician, who would be aware of the potential dangers of a polarity reversal.  Figure 1 shows a power supply connected to the AC input, with the Neutral connected to the earth ground at the panel.

Figure 1: AC-DC power supply permanently installed to a distribution panel

In the event of an over-current fault condition in the power supply or fuse aging, there is a 50% chance that either F1 or F2 would open.  If F2 was to open, a service technician may believe that there is no AC power being applied to the power supply.  Inadvertent contact with the Line while touching the earth ground would result in an electrical shock.  This is even more likely if the power supply in question is of an open frame type construction.

To avoid this, the NEC, CEC, IEE Wiring Regulations and IEC 364, specifically prohibit fusing in the Neutral in this type of equipment.  To overcome this a single fuse power supply must be selected.  This can be achieved by using an industrial (single fuse) or a medical/industrial power supply that has a single fuse option.

TDK-Lambda currently offers a single fuse option on the medical and industrial certified QM modular series and CUS-M models.

Power Guy

Does it matter how an EMC/EMI filter is wired to a power supply?

An email was recently received by TDK-Lambda’s Technical Support from a technician who was fault-finding a video system.  His questions about how an EMC filter should be connected were relevant and warranted a blog article on the subject!

As indicated in Figure 1, the EMC filter is situated between the AC input to the system and the power supply providing DC voltage(s) to the system load.  The filter’s function is to reduce incoming noise from the AC input and/or outgoing noise to the AC input from the power supply.

Figure 1: System block diagram

Regarding connection to the filter, let us now look at a typical EMC filter - the 250Vac 10A rated RSEN-2010 for example (see Figure 2 and 3).

Figure 2: RSEN-2010 filter

Figure 3: RSEN schematic

Reviewing Figure 2, the nomenclature “LINE/LOAD” on the left hand and right hand side of the label indicates that either set of the terminals can be used for the input or load connection.  It can be also noted that there is no indication of where the Line or Neutral should be wired to, just numbers 1, 2, 3 & 4.  Which connections are made to these terminals is very important.

On the right hand side of the label and schematic (Figure 3), two additional capacitors, known as “Y” capacitors can be seen.  These provide a low impedance path to earth ground to reduce high frequency common-mode noise.   When the filter is used to reduce system noise from the power supply reaching the AC source, terminals 3 and 4 should be connected to the power supply.  If the filter is being used to reduce externally generated high frequency noise from entering the system then terminals 3 and 4 should be connected to the AC source input.

It is essential that if the AC line is connected to terminal “1”, then terminal “4” should be connected to the power supply Line terminal.  Likewise with terminals “2” and “3” for the neutral wiring as shown in Figure 4.

Figure 4: Connection of the filter to the AC and the power supply

If the wiring to “3” and “4” is crossed, there could be safety issues for a power supply with a single input fuse situated internally in series with the AC Line terminal.  

If the AC is correctly wired (Figure 5a), in the event the internal power supply fuse was to open due to a fault, the internal circuitry is isolated.  It is important to note that at the breaker panel, where the AC first comes into the building, the neutral input is grounded to earth.

Figure 5a: Correct AC connection to a single fuse power supply

If the Line and Neutral is reversed to the power supply due to incorrect wiring (Figure 5b), in the event of the fuse opening the internal circuit is still live.  Two issues can arise with this scenario.  A service technician fault-finding the system could receive an electrical shock, particularly with an open frame power supply without a cover.  Secondly, if an internal short was to occur in the power supply between Line and the earthed chassis, the fuse would be out of circuit and not open.  Even the simple use of a mounting screw that is too long could cause this!

Figure 5b: Incorrect AC connection to a single fuse power supply

Always ensure that adequate inspection and testing techniques are in place with AC wiring.

Power Guy

How do I balance the current between two DIN rail power supplies used in parallel?

Before economically priced DIN rail power supplies gained popularity, many 100W or higher power rated units provided users with a parallel operation function in the form of switch fitted on the front panel.  This switch allowed the user to select between “droop mode” current share between two or more supplies and single unit operation. Droop mode allowed for balancing of the output currents between the units.

If power supplies without a parallel operation function are being used and additional current is required for the system load, a second power supply could be connected in parallel, although many manufacturers do not recommend this.  Unless the output voltages of both power supplies are set to the same voltage, the unit with the greatest output voltage may deliver the majority of the current and operate in an over current condition. This will reduce the unit’s field life due to excessive internal heating.  Ideally the output current of each power supply should be routinely measured during preventative maintenance to ensure they are balanced, which is both time consuming and cumbersome.

One alternative is to use a “redundancy” module with a load balance option, like TDK-Lambda’s DRM40.  Figure 1 shows a DRM40 used to connect two 20A power supplies to deliver up to 40A and Figure 2 the DRM40 with its current balancing LED.

Figure 1: Two units connected in parallel with the DRM40

Figure 2: DRM current balance LED

If the LED is not illuminated, measure the output voltage on both power supplies. Adjust the voltage of the power supply with the lowest voltage higher until the LED turn on.  Alternatively, one can adjust the voltage of the power supply with the highest voltage lower.  The current balance LED will be illuminated when the difference between the voltages is less than 50mV and the output currents are balanced.

The DRM40 has internal MOSFETs, used to block reverse currents in the event of one power supply failing short circuit.  They also allow the measurement of each power supply’s output voltage without disconnecting any load cables.

The DRM40 can also be used in redundant power systems, where two power supplies are used in a 1+1 configuration as shown in Figure 3.  If one power supply fails the other will continue to provide current to the load.  The load balancing function can be used in same manner.

Figure 3: DRM40 used in a 1+1 redundant configuration

Power Guy

What is the difference between efficiency and average efficiency?

Efficiency

Power supply datasheets include product efficiency in a percentage format for each voltage and output power model, as a guidance to how much power is lost in wasted heat when the product is running.   As the actual operating efficiency varies with input voltage, output load, ambient temperature and component tolerance, usually there is a test condition noted. 

Phrases like “up to 95%” or “typically 93% at 230Vac input, 100% load and 25^oC ambient” are widely used.

If the selection of the power supply is being made purely on efficiency, then the manufacturer’s evaluation data has to be studied in order to determine the measured efficiency at the user’s load condition.  Figure 1 shows the efficiency vs. output current plot for TDK-Lambda’s 600W rated 24V output GXE600-24 for different input voltages.  At 60% load, 230Vac input one could expect the efficiency to be 94%.

Figure 1: GXE600-24 Efficiency vs Output Current

Average efficiency

External power supplies complying with the DoE (Department of Energy) and EU efficiency regulations will sometimes only state the standard (and its revision) they comply with.  TDK-Lambda’s DTM110PW240C8 datasheet for example, states compliance with the latest DoE Level VI & EU Tier 2 Efficiency standards and also includes that the average efficiency is >89%. The average efficiency for an external power supply rated between 49-250W has to be at least 89% to comply with the current and proposed standards.

Is “Average Efficiency” the same as “Efficiency”?  No.

Average efficiency is calculated by measuring the efficiency at 25%, 50%, 75% and 100% loads.  These four values are added together and the total is divided by four to obtain the average. Measurements are taken at 115Vac and 230Vac inputs.

Using the measurements from Figure 2 for the DTM110PW240C8, the calculated average efficiency at 115Vac is 90% and 90.5% at 230Vac.

--	Input	Output			--
Load (%)	Power (W)	Voltage (Vdc)	Current (A)	Power (W)	Efficiency (%)
Vin: 115V/50Hz
100	121.53	24.16	4.54	109.69	90.0
75	91.67	24.20	3.43	83.11	91.0
50	61.27	24.26	2.28	55.40	90.0
25	31.73	24.27	1.17	28.35	89.0
10	12.44	24.27	0.46	11.07	89.0
0	0.08	24.25	--	--	--
Vin: 230V/50Hz
100	119.33	24.16	4.52	109.07	91.0
75	92.19	24.21	3.47	83.87	91.0
50	62.11	24.27	2.30	55.83	90.0
25	30.74	24.34	1.14	27.63	90.0
10	13.26	24.35	0.49	11.82	89.0
0	0.11	24.25	--	--	--

Figure 2: DTM110PW240C8 Efficiency Measurements

Efficiency readings are also taken at 10% to check compliance to the EU Tier 2 Efficiency standard.  For a power supply rated at 49-250W it must have a minimum efficiency of 79%.  At 10% load the DTM250-D has an efficiency of 89%.

Power Guy