10.1 POWER UNITS
Power is defined as the rate of
using (or providing) energy and, in the electrical world, is measured in the
unit ‘watt’ (W).
In the mechanical world of
hydraulics, power is the product of pressure and volume flow. In modern SI units pressure is measured in
newtons per square metre (N/m2) and volume flow in cubic metres per
second. The product of these two
N
|
x
|
m3
|
=
|
Nm
|
=
|
J
|
=
|
W
|
m2
|
s
|
s
|
s
|
The electrical equivalent of
pressure is the volt, and the electrical equivalent of hydraulic flow is the
ampere, so that the power in watts is the product of voltage and current - that
is
W = V
x I
where V is measured in volts and / in amperes; W is then in watts.
Applying Ohm’s Law
V
|
=
|
IR
|
or
|
I
|
=
|
V
|
to this formula:
|
|||||
R
|
||||||||||||
W
|
=
|
IR
|
x
|
I
|
=
|
I2R
|
||||||
or
|
W
|
=
|
V
|
x
|
V
|
=
|
V2
|
|||||
R
|
R
|
|||||||||||
These are two alternative forms for
power when only I and R, or when only V and R, are known.
In earlier years the power output
of electric motors was measured in ‘horsepower’ to align them with mechanical engine
practice. Many motor nameplates are
still marked in ‘hp’, but more and more are now being marked in kilowatts (kW).
The kilowatt used in this case is
the equivalent of the mechanical power output of the motor. Horsepower and
kilowatts are directly related:
The horsepower in this case is
British horsepower. (Some Continental
countries use a slightly different unit, known as the ‘metric horsepower’,
which is equivalent to 0.735kW, a difference of about 1½%.)
Special care is needed when
referring to the power of motors. Motors
are rated by their mechanical output (hp or kW), but, because no motor is 100%
efficient, its electrical power input, also measured in kW, is always greater
than its mechanical power output.
Because both may be measured in kilowatts, confusion can easily
arise. When it is desired to distinguish
between mechanical output and electrical input, suffixes ‘m’ and ‘e’ are often
used: thus output is kWm and input kWe. Their ratio is
the efficiency of the motor, thus:
kWm
|
=
|
efficiency
|
kWe
|
Power - that is, the rate of
producing, absorbing or transmitting energy - is in the SI system always
measured in watts or, more usually, kilowatts.
It occurs in fields other than electricity and mechanics. For example energy can be produced thermally
in boilers or reactors, or chemically in batteries or by burning fuel. The power being developed is still measured
in kilowatts and would be distinguished by suffixes ‘kWth’ or ‘kWch’.
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