Hello, I am wondering is there any way of testing a three phase motors load without knowing the motors power factor or efficiency ratings?
I am refering to the following formula.
Watts = volts X amps X p.f X 1.74 than divide
this by 746 to get Input HP
Than to get Output HP take the efficiency rating X Input HP.
Does this formula look right? And what does the factor 1.74 refer to?
I am asking this question because most older motors do not list p.f or eff. ratings on there nameplates.
Is there another way to test this?
Thanks Ebean

Yes, the formula is right.

The factor of 1.74 (actually 1.73) is the sqrt(3) which arises in 3-phase power formulas because of the fact that the voltage in the formula is line-to-line voltage which is sqrt(3) more than line-to-neutral voltage.

You can infer the product of full-load power factor and full-load efficiency from the nameplate as follows:
pf * efficiency = (Pnp /[Sqrt(3) * FLA*Vnp]) * (745 watts/hp)
where
Pnp = nameplate power in horsepower
FLA = nameplate full load amps
Vnp = nameplate voltage
(745 watts/hp) = conversion

This is really the same formula you used, where we have assumed the HP is known and solved for power factor and efficiency. If you combine your formula and my formula together you get:

Actual Horsepower = Nameplate Horsepower * (Actual Current / Nameplate Current)
This assumes the actual voltage is equal to nameplate and more importantly assumes that power factor and efficiency haven't changed from nameplate values. Unfortunately at low load this is a bad assumption. So unless actual amps is above maybe 75% of FLA you won't get a very good prediction with this approach.

Another parameter you can measure is slip. Roughly speaking, slip should be proprtional to output power. If the nameplate of a 100HP motor says 3550 rpm and you measure 3575rpm you can very roughly estimate roughly 50hp output. Variations in line frequency, voltage, and rotor temperature (affects rotor resistance) can cause errors in this approach. Also motor manufacturers may round the nameplate speed to the nearest 5rpm introducing more errors to this approach.

There are other approaches - IEEE112 provides a lot of info related to determining motor performance.

quote:

Originally posted by bean440:
Hello, I am wondering is there any way of testing a three phase motors load without knowing the motors power factor or efficiency ratings?
I am refering to the following formula.
Watts = volts X amps X p.f X 1.74 than divide
this by 746 to get Input HP
Than to get Output HP take the efficiency rating X Input HP.
Does this formula look right? And what does the factor 1.74 refer to?
I am asking this question because most older motors do not list p.f or eff. ratings on there nameplates.
Is there another way to test this?
Thanks Ebean

bean440,

I hope I am not missing something here but Baker Instrument's Explorer can calculate the motor torque, load, efficiency and Pf from just the current and voltage signals from the MCC. When creating the data base the only information needed is the name plate speed, Hp, Volts and FLA. The equipment is very accurate when loaded to 50% or grater it is typically with in .5 % of the dyno readings, when less then 50% it is with in 1% of the actual readings of the dyno.

Please see the attached document for a comparison of the Explorer readings to the dyno.

Comparison_Explorer_MSRF_100hp_motor.pdf (12 Kb, 25 downloads)