Trans­formers generate waste heat when they are in oper­a­tion and oil is the coolant of choice. It trans­fers the heat through convec­tion (or supported by pump systems) to the trans­former housing, which has cooling fins or radi­a­tors similar to heat exchangers on the outside. Large high-power trans­formers require addi­tional cooling: fans that prevent damage from over­heating despite cooling surfaces that are as compact as possible (Fig. 1). The fans have to satisfy special require­ments so they can with­stand harsh outdoor deploy­ment condi­tions.

Fans turn trans­formers into energy savers

Figure 1: The fans used in the cooling systems of large trans­formers must with­stand harsh condi­tions. Here, robust tech­nology that works reli­ably for decades and does not require main­te­nance is in demand.

It isn’t enough for fans to support the required air perfor­mance. Prop­er­ties such as corro­sion resis­tance even in high humidity, no-main­te­nance oper­a­tion without lubri­ca­tion, and reli­able func­tioning over as long a service life as possible are just as impor­tant. And when loads fluc­tuate, it is also bene­fi­cial when the cooling capacity can adjust to actual demand. It does not always make sense to switch off indi­vidual fans when the trans­former is in partial-load oper­a­tion because in this state, uncooled “hot spots” can arise on the heat exchanger.

If the fans are oper­ating as effi­ciently as possible, over the years, users can save signif­i­cant amounts of money and power, which they can feed into the elec­tricity grid. This results in a posi­tive economic benefit for energy oper­a­tors. If the trans­formers are near or even in resi­den­tial areas, the noise they generate plays a role that should not be under­es­ti­mated either.

Figure 2: The trans­former fans satisfy all the require­ments for sophis­ti­cated large trans­former cooling and come in sizes ranging from 500 mm to 1,250 mm.

ebm-papst Mulfingen, the motor and fan specialist, has addressed this issue by devel­oping special trans­former fans (Fig. 2). They satisfy all the require­ments for sophis­ti­cated large trans­former cooling as per DIN EN 50216-12 “Power trans­former and reactor fittings – Part 12: Fans” and come in sizes 500 mm to 1,250 mm with air perfor­mance of up to 13 m³/s.

Their devel­op­ment is based on decades of expe­ri­ence and intense research supported by exten­sive simu­la­tions and tests. They also satisfy special require­ments with regard to long service life and salt spray resis­tance (corro­sion protec­tion class C5M as per DIN EN ISO 12944), so the fans are suit­able for deploy­ment in coastal areas with high salt contents.

Fan housing increases fan effi­ciency

Figure 3: Axial fan instal­la­tion without a fan housing means major airflow losses (left). A fan housing can signif­i­cantly increase air perfor­mance in the oper­ating range.

All the compo­nents of the plug-&-play-compatible modules – the fan housing, HyBlade® impeller, Green­Tech EC motor with inte­grated elec­tronics or AC asyn­chro­nous motor and intake-side guard grille – are perfectly harmo­nized. For example, aero­dy­namic opti­miza­tion reduces both air turbu­lence and oper­ating noise. The two motor types satisfy the require­ments of the current ecode­sign direc­tive.

The impeller has an aero­dy­nam­i­cally ideal form based on combining an aluminum frame with a covering of glass-fiber-rein­forced plastic. This reduces the noise it produces dramat­i­cally, enabling higher effi­ciency in compar­ison to tradi­tional blades. The motor mount, which also has a contact protec­tion func­tion, is installed on the intake side. On the outlet side, a guard grille is also avail­able as an acces­sory.

If the fans are oper­ating as effi­ciently as possible, over the years, users can save signif­i­cant amounts of money and power.

Both the guard grille and the fan housing are made of hot-dip galva­nized sheet steel with an extra coating. The outlet side also has an inte­grated circum­fer­en­tial flange for direct attach­ment to the radi­ator. Espe­cially when the fans func­tion with free air (as usual in the case of oil-cooled trans­formers), the posi­tive effect of this type of fan housing is excel­lent (Fig. 3). It keeps air turbu­lence to a minimum, increasing the airflow rate and there­fore, fan effi­ciency (Fig. 4).

Speed control for needs-based cooling

Figure 4: Fans with a fan housing deliver higher air perfor­mance and higher effi­ciency, which leads to signif­i­cantly better system effi­ciency (1) in compar­ison to fans without a fan housing (2).

Compa­nies that rely on proven Green­Tech EC tech­nology are taking one more step in the direc­tion of energy effi­ciency. EC motors are basi­cally synchro­nous motors with perma­nent magnet excite­ment. In them, a rotor with inte­grated perma­nent magnets synchro­nously follows the rotating field of the stator, which is gener­ated elec­tron­i­cally. The control elec­tronics enable infi­nitely adjustable air perfor­mance (linear to the fan speed), which devi­ates from the grid-synchro­nous frequency in partial-load oper­a­tion at prac­ti­cally the same high level of effi­ciency. The open-loop speed control can either be spec­i­fied analog via 0-10 VDE (e.g. oil temper­a­ture or oil pres­sure sensor) or digi­tally with a PWM or MODBUS signal. When MODBUS is used, several fans can be conve­niently inter­con­nected. This facil­i­tates the use of diag­nos­tics and moni­toring func­tions, which ulti­mately contribute to reli­able oper­a­tion. This set up means the entire system can be oper­ated more econom­i­cally and has a posi­tive effect on the life cycle costs.

Compa­nies that rely on proven Green­Tech EC tech­nology are taking one more step in the direc­tion of energy effi­ciency.

To prevent hot spots on the heat exchanger after indi­vidual fans have been switched off, ebm-papst recom­mends using all fans in partial-load oper­a­tion. The conse­quence is a more even flow through the radi­ator. A posi­tive side effect: the motor does not heat up as dramat­i­cally, which extends the fan’s service life. Another aspect of partial-load oper­a­tion is the high reduc­tion in energy consump­tion and oper­a­tion noise as a result of the phys­ical laws. The elec­trical input power is propor­tional to the fan speed to the third power (Pe ~ n³). As a result, it is only 12.5  % when the speed and in turn, the air perfor­mance, are reduced by 50  %. Loga­rith­mi­cally, this reduces the noise level by 15 dB (Fig. 5).

Figure 5: The graphics show the poten­tial energy savings and noise reduc­tion in a direct compar­ison of on/off oper­a­tion and infi­nitely vari­able speed adjust­ment.

Partial-load oper­a­tion not only saves energy and lowers oper­ating costs, but also results in less waste heat. This is espe­cially valu­able for cooling appli­ca­tions because heat that is not gener­ated does not need to be dissi­pated. Commu­ta­tion and the stator design also ensure very smooth oper­a­tion. The cycle frequen­cies are acousti­cally imper­cep­tible and noise reduc­tion drops. This makes these “stealth fans” ideal for appli­ca­tions in which the noise protec­tion regu­la­tions must be observed.

Simple commis­sioning and world­wide use

Prac­tice-oriented design details simplify trans­former fan commis­sioning. For example, it is possible to install and mount directly on the fan housing with a hori­zontal or vertical instal­la­tion posi­tion as required. The motor terminal box for supply connec­tion and control is easily acces­sible and isolated from the motor elec­tronics. The devel­op­ment team made a point of using high-quality termi­nals. The fans are perfectly designed for world­wide use. They func­tion on power supplies between 200 -240 V and 380 - 480 V (for 3-phase 50 Hz and 60 Hz grid frequen­cies), satisfy all the rele­vant stan­dards (UL, CSA, EAC and CCC) and the require­ments of degree of protec­tion IP55, and the rating label satis­fies DIN EN 50216-12 (“Power trans­former and reactor fittings” – Part 12: Fans).