This is a bit technical engineering stuff but I hope this could be a good reference for you.
Some features of electric motors
Industrial electric motors can be classified, for example, as,
- as being built to work with alternate or direct current, and
- as being built to work with just a single phase or three phases.
This information is provided by the manufacturer in the motor name plate as shown in Fig. 1.
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| Fig. 1 Electric motor nameplate with key data for power consumption estimations |
In other cases, the information is not available because the equipment has not been bought yet! Fortunately, some manufacturers have made available datasheets of its equipments on their official internet site. This usually occurs when a process plant is the design stage but this info shall be paramount for energy operation costs estimations.
A third important case occurs when the electric motor is already installed, has been working for a long period (years, may be) and its namplate rated features may be different from its actual operating conditions. In this case power consumption has to be measured in the motor directly.
From here on we will focuse on AC motors only since the calculation for DC motors is barely the same for single phase AC motors.
Some details on power consumption estimations
As you may see, power in the motor nameplate is usually given in hp (horse power) as shown in Fig. 1 above. For cost of power consumption the unit for power of common use all over the world is: kW h (kilo Watts per hour).
Next, if the power data is in hp, just use the following conversion factor to get kW:
$\dfrac{1\; hp}{0.7457 \; kW}$
It is not obvious but the power provided in the nameplate is given per hour. In other words, what you see in the nameplate is in fact: kW h. This can be confusing but take it as it is.
Another element you should not forget is the motor efficiency. Efficiency is different in several conditions:
- if the motor is new or not,
- if proper maintenance has been performed,
- the design of the motor (for example, squirrel cage is a type of design),
- according to the load conditions,
- if it is a high efficiency (and more expensive) motor or not.
For short, the efficiency of an electric motor shall lye between 70 and 90% according to a combination of factors as those listed above. The less efficient a motor is, the more energy this one will need to develop the work. In other words, an electric motor with low efficiency will in fact need more kW h than those specified in the nameplate.
A final comment. Power consumption for single or three phases motors is different.
Any question? Write in the comments and I shall try to help.
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