Does alternator work as dynamometer?

  • #1
does alternator work as dynamometer??

i'm doing small engine test rig to analyze it performance characteristic. then,i connect the pulley to alternator.thing that i need to considered is power that can produced from alternator must be larger that small engine.from engine specification, it can produces maximum power around 810Watt.so i choose alternator 12V/75A so i can get the maximum value of power from engine, by using P=IV

my question is it's true that power,P=IV that i calculated is true value of power from this small engine?
 

Answers and Replies

  • #2
FredGarvin
Science Advisor
5,067
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You will always have some losses in your systems. The use of alternators/generators as loads for engines is a well established practice.
 
  • #3


thanks...so that why there are 2 type of power.brake power and indicated power.

but,it's right with my method to replace dynamo meter with alternator??
 
  • #4
803
9


Are you asking whether electical power P=IV is the same as brake power? Brake power simply refers to a method of testing...so the answer, as far as I know, is yes.
 
  • #5


Are you asking whether electical power P=IV is the same as brake power? Brake power simply refers to a method of testing...so the answer, as far as I know, is yes.

this is what i mean.lol..!!! sorry,really bad in english.thanks Lsos for helping..
 
  • #6
stewartcs
Science Advisor
2,177
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I'm not sure what you mean by "brake power", but the shaft power of an electric motor and the input power (P = IV) are not the same thing.

The shaft power is reduced by the efficiency (and power factor if it's an AC motor) of the electric machine.

CS
 
  • #7


what i'm understand is brake power mean that power produced from engine and some losses occur during power transmitted to it shaft.

btw,how could i know the power factor for this AC motor?
 
  • #8
stewartcs
Science Advisor
2,177
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what i'm understand is brake power mean that power produced from engine and some losses occur during power transmitted to it shaft.

btw,how could i know the power factor for this AC motor?

Power factor should be on the nameplate of the motor.

CS
 
  • #9
stewartcs
Science Advisor
2,177
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what i'm understand is brake power mean that power produced from engine and some losses occur during power transmitted to it shaft.

The power produced from the reciprocating engine at the crankshaft/flywheel is essentially the same as the shaft power of an electric motor (some energy source - fuel or electricity - being transformed into mechanical energy). Thus the input power (apparent power for electric motor and equivalent energy of the fuel) will not equal the shaft/brake power. The irreversibilities in the conversion process reduce it as previously noted.

CS
 
  • #10
803
9


They're the same thing, as in the same physical concept and the same units: they're both measures of energy/ time. But, as I was hoping someone would clarify (as stewartcs did) it's important to realize that this "energy/ time" is measured at different places.

P=IV is taken at the terminals. You hope most of it gets from the terminals to the shaft, (or from the shaft to the terminals, as in the alternator's case) but whether it's actually 90% or 0% is a different story. Brake power is, as the name implies, the power that a brake attached to the engine shaft measures. It's important to take all this into consideration....
 
  • #11
4,662
5


Here are definitions of brake and indicated HP from Wikipedia:

Indicated horsepower (ihp)
Indicated horsepower (ihp) is the theoretical power of a reciprocating engine if it is completely frictionless in converting the expanding gas energy (piston pressure × displacement) in the cylinders. It is calculated from the pressures developed in the cylinders, measured by a device called an engine indicator—hence indicated horsepower. As the piston advances throughout its stroke, the pressure against the piston generally decreases, and the indicator device usually generates a graph of pressure vs stroke within the working cylinder. From this graph the amount of work performed during the piston stroke may be calculated. It was the figure normally used for steam engines in the 19th century but is misleading because the mechanical efficiency of an engine means that the actual power output may only be 70% to 90% of the indicated horsepower.

[edit] Brake horsepower (bhp)
Brake horsepower is the measure of an engine's horsepower without the loss in power caused by the gearbox, alternator, differential, water pump, and other auxiliary components such as power steering pump, muffled exhaust system, etc. Brake refers to a device which was used to load an engine and hold it at a desired RPM. During testing, the output torque and rotational speed were measured to determine the brake horsepower. Horsepower was originally measured and calculated by use of the indicator (a James Watt invention of the late 18th century), and later by means of a De Prony brake connected to the engine's output shaft. More recently, an engine dynamometer is used instead of a De Prony brake. The output delivered to the driving wheels is less than that obtainable at the engine's crankshaft.

Bob S
 
  • #12


hurmm ... let say i run my engine with speed around 3000rpm.then i put a bulb as load for 100Watt.then i increase the load to 200Watt.what happen to power while i maintain the speed(still 3000rpm,where it will decrease when load is increase).. the power will increase will increase or decrease ??
 
  • #13
7
0


what if the engine direct with alternator using shaft?is that ok?
 
  • #14
brewnog
Science Advisor
Gold Member
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Should be fine, just make sure it's all mounted correctly and that you aren't going to overspeed the alternator.
 
  • #15
7
0


what kind of load suitable we use if using alternator as dyno?

how its operate?
 
  • #16


i used bulb as load...but this experiment not got what i need.it not easy to maintain the rpm of small engine.lol~
 
  • #17
7
0


how much your engine horsepower? and how many watts you used?
 
  • #18


i using grass cutter engine...very low hp.and i use variable load(bulbs)
 
  • #19
7
0


how you running the test? procedure of testing....
can u expain it...
you using AC or DC current?
 
Last edited:
  • #20
brewnog
Science Advisor
Gold Member
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Bulbs and resistive heating elements should be fine. Speed control can be tricky, does the engine have a governor?
 
  • #21
7
0


the engine have a govenor...
but how the procedure of the testing?
 
  • #22
brewnog
Science Advisor
Gold Member
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I have a list of around 80 different test procedures for an engine on a dynamometer. Just asking for a test procedure, without any indication as to what you want to establish isn't going to get you very far!
 
  • #24
brewnog
Science Advisor
Gold Member
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Either the engine wasn't running under the same conditions in each test, your calibration (dyno or alternator) is wrong, or you've slipped up. There's no inherent reason you should be seeing such different results.

Is the plot the engine's power curve? In the alternator case, is torque measured or calculated from power? Is it a DC alternator?
 
  • #25
7
0


yes...it is the plot engine power curve...

the torque for alternator is calculted from power. and it is ac alernator...
 

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