Determining DC Motor Horsepower Ratings
Say you have new 24-volt DC motor that came with no manual;
it also has no data plate. It only has “Made in China”
stamped on one end of the chassis. You need to rate this
motor for UL approval. Your company actually needs a generic
testing procedure to determine the horsepower rating. How
ican this be done?
Here are some ideas:
Decide upon your load as per your requirement. You already
know the voltage rating; now, depending on your load, use a
current meter to measure the current it is drawing. Then you
can have its KW rating according to its power factor. Then,
convert KW rating to HP rating:
1 HP = 746 KW (British ratings)
1 HP = 736 KW (Asian ratings)
Here’s another rough way to do it, assuming that the motor
is Shunt-wound or permanent-magnet type.
This method assumes that the motor operates at around 80% of
its maximum efficiency on rated load. Which is often a good
approximation. Again, it is not 80% efficiency — it is 80%
of the MAXIMUM efficiency. For example, if you discover (see
below) the maximum efficiency as, say, 75%, at full load
operating point, approximate efficiency is: 80 % of 75% = 60%.
Load test to find max efficiency:
Keep the supply voltage constant at 24 VDC. Measure:
i) the input current;
ii) the mechanical load applied on the motor shaft by
standard shaft loading methods.
1. Run the motor on no-load. P_out = Load = 0 hp. Find the
input power as
P_in = input amps x supply power / 746 -> hp
Efficiency= P_out * 100/ P_in = 0%.
2. Increase the load in small steps. Find the mechanical
load on the motor as P_in. The input current will increase.
Find the new P_in and then, the new efficiency. You should
get non-zero values now.
3. Continue step 2. The efficiency increases with load up to
a point. Then it starts decreasing. Get a couple of points
on the decreasing side also. Make sure that you don’t run
the motor for long in this load.
Plot the variation of efficiency with load. Find the rated
load as indicated in paragraph 2 above.
NOTES:
i) If the motor heats up excessively when you increase the
load, this trick may not work. The method assumes a normal
shunt/permanent magnet DC motor.
ii) In “80% of maximum efficiency”, you could refer to
standard motor manuals of similar capacity to come up with a
better value for the factor. Nevertheless, 80% is a fairly
good value.