7/30/2019 Basics for Selecting

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Generators are a necessity for most job sites. Portable power units can power everythingfrom small tools to portable offices. Selecting the right generator for the job doesnttake rocket science, but it does take planning. It is important to understand thatcustomers are asking for reliable, temporary power for their application when they renta generator. Sometimes the customer knows his/her power requirements, so this is not

an issue. More often however, a customer only has a general idea of how much power isneeded. In this case, you must determine from the customer the amount and type of

power required and where and for how long that power is needed in order to pick theright size generator for the job.

Three factors play a part in sizing a generator for the job: type of load, total watts to beconnected and power of the generator.

Type of load

The first factor to consider when sizing a generator is the type of load being connected.There are two types of electrical loads: resistance and induction loads.

Incandescent lights and heating units are examples of resistance loads. Mostconstruction equipment requiring generator power has induction loads for electricmotors.

There are two types of induction load motors. Universal motors, which use brushes, power hand drills, saws and concrete flex shaft vibrator motors. Capacitor motors,which are brushless, usually power submersible pumps, air compressors and table saws.

When sizing a generator to these motors, the main difference is that the capacitor motors need about a third more starting current than universal motors.

Wattage requirement .

After determining the type of load, the next step is to find the total watts of the load to be connected. The most accurate method to determine wattage is to check the nameplateon the machine. However, if this information is not available, wattage can be figured bythe simple formula:

amps x volts = watts

Load requirements to electric motors differ, but all motors require more power to startthan run. Capacitor motors require up to six times more starting power for longer

periods of time. Universal motors draw 1 to 2 times the running current for shorter periods of time.

Generator power .

Once you determine the wattage, you can choose your generator. The basic guide tofollow is if the generator has enough power to start the load its surge rating it willhave enough power to continually run it.

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There are three power ratings for generators, including maximum or nameplate rating,which has a duration of 30 minutes and 10 percent duty; continuous rating, which has a24/7 duration and 100 percent duty; and surge rating or motor starting rating, which hasa duration of two seconds and instantaneous duty. Generator designs have differentsurge capacities that vary between 1.2 to 2.5 times their continuous run rating.

Next, determine what kind of generator you have. A premium construction-gradegenerator offers 100 percent duty and with an electronic automatic voltage regulator (AVR) can surge 2 to 2.5 times its run rating. A value construction-grade generator offers 100 percent duty with capacitor or rectifier excitation and can surge 1.5 to 2 timesits run rating. A low-cost homeowner generator offers temporary duty with capacitor or rectifier excitation and can surge 1.2 to 1.5 times its run rating.

For example, the surge rating of Wacker Neusons GP5600 is equal to two times5,000W or 10,000W.

If you cannot determine the design of the generator, then assume surge capacity is equalto 1.5 x continuous rating. It is better to oversize the generator to be used by your customer.

Lets say a contractor wants to use a portable gasoline generator to power his temporary job shack. He needs to connect seven 100W light bulbs (700W), a coffee maker (850W), small refrigerator (induction, 600W to run and 2,000W to start) and a 1,500Wradiant heater.

The light bulbs, coffee maker and radiant heater would run constantly, consuming3,050W. With the refrigerator cycling between off and start/run, the total wattagedemand on the generator will vary between 3,050W and 5,050W, which can be handled

by a 5,600W portable generator.

For another example, lets say a 2-in. submersible pump requires 771W to run (6.7amps x 115V). A 2-hp flex shaft vibrator runs on 2,000W. The pump and vibrator motor require a total of 2,771W to run, however, the pump has a capacitor motor and willrequire 2,313W for starting, while the universal vibrator motor requires 2,600W. Youshould size the generator by adding the largest start load to the remaining running loads,which in this case would be 4,313W (2,313W + 2,000W). To run the pump and vibrator, a 5,600W generator will be needed. This will give the customer enough power

to also add another tool, such as rotary hammer, which needs 1,100W to run and 1,600W to start.

Another way to size a generator is to calculate amperage instead of the wattage. Youcan compare the maximum amps of the tool to the generator, the same way youcompare the wattage. If you need to calculate the maximum amps of a generator, simplydivide the maximum wattage by 120: Maximum amps = maximum wattage/120.

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