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Why use DC Generators for Telecommunications, Solar, UPS, Backup, Prime Power.

There is not a single universal reason why all applications should choose a DC generator over an AC generator. There are, however, specific reasons for each project or a combination of reasons that favor the DC generator. Some of which include:

 Applications in General (including solar/wind hybrid, prime power, and backup):

  1. The DC generator system is designed and optimized to deliver the high currents at low voltages required for battery charging and operating DC loads.  No battery chargers or power supplies are required.

  2. DC generators do not require a transfer switch. Transfer switches lower system reliability.

  3. In prime power applications the DC generator lowers the overall cost of the system.

  4. Certain AC generators and switch mode power supplies are incompatible.  These AC generators have voltage regulators that cannot regulate voltage due to the current pulsing load of the switch mode power supplies.  Polar’s DC generators when connected to a battery do not suffer this incompatibility.

  5. DC generators are more fuel-efficient. Site operators want the longest run time with the least amount of fuel on site.

  6. Polar’s DC generators are simpler in design, have considerably less maintenance and are more reliable than AC generators. Propane carburetion and electronic speed governors require frequent calibration and testing. If the propane carburetion, ignition system, or governor speed control should develop a problem, alternator voltage regulation and frequency control will fail. Some equipment powered by the generator will be damaged, other equipment may survive. How many of your AC loads can tolerate a wide swing in frequency?

  7.  Polar’s DC generators have a current limit control to prevent the alternator from overheating and the engine from stalling during shorts or overloads. This feature is extremely important in battery charging because a battery in a low state of charge can demand more power than the generator or battery charger can manage. Polar’s DC generators will continues to supply power under current limit control, allowing the battery to increase its charge and drop its current demand. The AC generator uses a fuse or circuit breaker to protect against shorts and over current so the battery fails to get charged if it is overly discharged. The additional problem is that for remote sites a person is required to visit the site to replace or reset the fuse or circuit breaker and devise a means where the batteries can be brought to a state of charge where the batteries can take over.

  8. Polar’s DC generators can be connected in parallel and load share. Paralleling these small AC generators is not practical. In many projects there are concerns about future site expansion and given load estimates are sometimes understated.

  9. Maintainability. DC generators use smaller engines that can be lifted by hand and transported to a shop for repair. The high-level generator mechanics are not required, or their expensive travel time to the site.

  10. AC generators are typically oversized to handle starting currents of motors and to provide light enough engine loads to facilitate speed regulation. Engines that are lightly loaded build up carbon around the valves and exhaust lines (wet stacking) this creates additional engine maintenance.

  11. DC generators are considerably smaller in size and use smaller horsepower engines. This facilitates both roof mount installations and installations inside the small shelters, as well as the required permit process.

  12. DC generator lowers the costs to install and operate.  It is smaller in size and requires less site support.  Transport to the site and installation is facilitated because: smaller shelter and concrete pads can be used, elevator transport to roof rather than crane, smaller helicopters or vehicles for transport. Fuel consumption is lower, so cost to transport fuel to the site is lower (transportation of the fuel to the site can be very expensive).

  13. Engines need a warm up period before they are able to provide full power, for an AC generator this translates to providing a stable frequency and voltage. Polar’s DC generator can deliver power at a reduced level immediately after starting, then switch to full power after warn up. Also a cold engine can exhibit speed fluctuations, which has no effect on Polar’s DC generators.


UPS Systems:

  1. Polar’s DC generators connect direct to the battery bank so there are fewer problems with on site permitting.  Many inspectors and utilities are concerned with isolation of the AC generator from the utility grid.  Using a DC generator there should be no questions on isolation from the inspectors.

  2. Polar’s DC generator can provide regulated charging to the battery and is unaffected by the pulse currents of the switch mode inverter.

Comparison Chart for Telecommunications Applications

No Matter Which Way You Look At It, DC Sets Are Superior!



For N+1 redundancy, peak load demand, and site expansion, multiple DC generators can be operated at the same time and power outputs can be paralleled.

It is impractical to parallel AC generators for these applications; therefore they must be over-sized for peak load demands and future expansion. Small loads can cause the engine to "wet stack”.

DC generator backs up both utility power and the rectifier bank.

AC generators back up only utility power. If the rectifier fails, the site is down.

Backing up power board reduces N+1 redundancy demand on the number of rectifier modules. This lowers the purchase and operational costs.

N+1 redundancy of rectifier modules within a power board increases site capital and operating cost.

The battery charger or rectifier and the transfer switch for both prime power and solar hybrid applications can be eliminated. Fuel consumption is reduced, and reliability is greatly increased.

Rectifiers, transfer switch and controls add considerably to the purchase, installation, and maintenance costs.

Variable engine speed option automatically adjusts to meet battery charging and load requirements, providing up to 40% savings in fuel. Engine speeds can vary without affecting voltage regulation.

Engine must run at fixed speeds in order to maintain 60 or 50Hz frequency. Engine speed control provisions add to maintenance and reliability problems. Running engine without load shortens engine life.

Continuously running the DC generator during long power outages can be avoided in order to conserve fuel and reduce maintenance. The generator can charge the battery, then shut down and begin cycle again.

The cumbersome design of an AC Generator/ transfer switch/rectifier system make frequent cycling undesirable.

Polar’s Airgen system consists of a DC generator with belt-driven air conditioner. The Airgen system provides the most reliable and cost effective means for supplying both DC power to the load and air conditioning the site in either prime power or back up applications.

A simple engine or alternator problem will affect frequency or voltage regulation, which can damage the air conditioner and rectifier.

The lightweight and compact size of Polar’s DC generator and Airgen systems are ideal for rooftop and mountain top installations.

AC generator systems are more difficult to install due to their very large weight and size. Reinforcing structures is commonly required.

With Polar’s high efficiency alternator we are able to use smaller horsepower engines, which produce lower acoustic noise. Smaller engines also facilitate installation permits on sites where the regulations are based on horsepower and noise.

Combined with the rectifier power losses and the rectifier and air conditioning current surges during startup, the engines and alternators on the AC sets require over sizing. Engines are also oversized to assist in output frequency regulation.

Polar’s electrical quality is higher than either an AC ferroresonant or switch mode rectifier.


A DC generator can "come on line faster than an AC generator. This allows a reduction in the size of the battery bank. Reducing the battery bank size lowers procurement and replacement costs. 

From experience, larger battery banks are required for AC generator backup systems


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Revised: December 13, 1999 .