Machine Efficiency
Consider a brushless AC motor requires 10 kW of power to deliver 10 N∙m of mechanical torque. What is the machine’s efficiency if its rotational speed is 2,000 rpm?
Expand Hint
Efficiency of a machine:
$$$\eta =\frac{P_{out}}{P_{in}}$$$
where
$$P_{out}$$
is the machine’s output power and
$$P_{in}$$
is the machine’s input power.
Hint 2
Mechanical power in a rotating machine:
$$$P=T \omega_m$$$
where
$$T$$
is the mechanical torque, and
$$w_m$$
is the angular velocity.
Since angular velocity is in rad/s, let’s convert the rotational speed from the problem statement:
$$$\omega_m=\frac{2\pi}{60}n$$$
where
$$n$$
is the motor’s speed in rpm.
$$$\omega_m=\frac{2\pi}{60}\cdot 2,000=209.3\:rad/s$$$
Mechanical power in a rotating machine:
$$$P=T \omega_m$$$
where
$$T$$
is the mechanical torque, and
$$w_m$$
is the angular velocity.
$$$P= (10N\cdot m)(209.3\:rad/s)=2,093\:W$$$
Efficiency of a machine:
$$$\eta =\frac{P_{out}}{P_{in}}$$$
where
$$P_{out}$$
is the machine’s output power and
$$P_{in}$$
is the machine’s input power. For a motor,
$$P_{in}$$
is the active component of the electrical power input, and
$$P_{out}$$
is the mechanical power output. For a generator, it’s vice versa.
$$$\eta =\frac{2,093W}{10,000W}=0.21$$$
0.21
Time Analysis
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