G8MNY > TECH 09.07.18 10:33l 103 Lines 4362 Bytes #999 (0) @ WW
BID : 28250_GB7CIP
Subj: Measuring Motor Efficiency
Sent: 180709/0806Z @:GB7CIP.#32.GBR.EURO #:28250 [Caterham Surrey GBR]
To : TECH@WW
By G8MNY (New Feb 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
To determine the efficiency of an electric motor 2 things are needed, input &
Two items to be measured & multiplied together, Voltage & Current.
With DC this is relatively easy, but pulse currents can mean "DC RMS" not an
"average meter" is needed, but for an AC motor not so easy, as there is power
factor as well as any pulse power (core saturation) to take into account.
DC motor AC Motor
│ | | | │ |~| <-Saturation (3+5th harmonic)
│~-─~│~'─'│~~~~│ │~ │ / \ /
│ │ │ │ │ ┼ / \ /
└────┴────┴────┴─┴─ │/ \ / Current lagging
Brush Commutation noise │ |_| Voltage by < 90°
DC Motor AC Motor
Voltage Voltage.-. .
├────────────────── │/ \ /
│ ┼ \ /
└────────────────── │ \ /
Power = Volts x Amps │ '─' Power = instantaneous V x I
With modern true RMS meters & true AC power meter this can be can be accurately
measured otherwise you are guessing!
Using Si units you end up with everything in Watts. Again two items, Force
(Newtons) over a Distance (m) for linear measurement, or Torque times RPM for a
shaft power. The torque can be measured from 1st principles with a simple
============˙============= ==Ů beam
Balance │Belt │ │ ▒
beam │ │ \│/ Belt ▒
│ _ │ Weights ▒ Flat
│/~_~\│ 1kg=9.81N /~~~~~\ ├─┤ Pulley
BREAK ║ (˙) ║ │ │=Á Water
\_~_/ Centre hole in \Motor/ ├─┤
~ pulley for water /~~~~~~\
The 2 belt positions to the beam are at the pulley diameter apart, but with the
central pivot slightly offset, this gives is self tensioning action as the
weights are put on. The flat pulley has 2 side disk that insure the belt can't
come off. For powers of over a few 10s of Watts water cooling is used inside
the spinning pulley & allowed to boil off. The pivot's hight is set to keep the
With this set up the force (N) on the balance beam equals the torque & is set
by the weights at the distance d (m) on the beam to pivot.
e.g. 0.1kg x 9.81 @ 1m = 0.981 Nm
The RPM can be done from the motor shaft with several types of pick up (e.g. on
the pulley) or even on a small DC brush motor from the commutator noise.
==== ┌──────────┐ ________
DC>─((()┬───┤├─┤Triggered ├──┤Counter │
│ │ Monstable│ │or timer│
3pole ~~~~~~~~~~ ~~~~~~~~
Here the monstable is set to just wider than the commutator break time, so that
counter sees cleaned up pulses of 6 per Rev due to the 3 breaks per brush. The
inductor needed to see the current pulses across it, can often be just that of
the connecting wires. So say with a simple 3 pole motor 120Hz on the counter
equals 20 Rev/s & 1200RPM.
With say a torque of 0.981 Nm & 20 Rev/s = 19.62 Watts
Power Watts = Torque (Force x Distance) x Revs per second.
With AC synchronous motors the shaft RPM is a few Revs per second below the
mains field speed, called slip. e.g. a 4 pole motor on 50Hz may be @ 1400RPM or
3.33 Rev/s slow seen strobing with a lamp.
With everything in Watts it is straight forward to do calculations
Efficiency % = ------ X 100
The motor losses are of course mainly in heat, windage, bearings & noise/
vibration. Very good ones will be >80%, but some light weight motors (e.g. for
carrying like hand tools) may be very poor @ 50%, & rely very heavily on fan
cooling to allow that power rating. Faulty motors will show up below 20%.
For a series of results to find say the best efficiency, put all the results on
a spread sheet & plot graphs etc.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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