Motor Shaft RPM

#110 / Electrical & Computer Engineering Easy

A 4 pole induction motor operates with a 0.05 slip, and a line voltage frequency of 30 Hz. What is the motor shaft's rotational speed (rpm)?

Expand Hint

The synchronous speed for ac motors is:

$$$n_s=120*f/p$$$

where $$f$$ is the line voltage frequency in Hz, and $$p$$ is the number of poles.

Hint 2

The slip for an induction motor is:

$$$slip=\frac{(n_s-n)}{n_s}$$$

where $$n$$ is the rotational speed.

The synchronous speed for ac motors is:

$$$n_s=120*f/p$$$

where $$f$$ is the line voltage frequency in Hz, and $$p$$ is the number of poles.

For a 4 pole motor operating on 30 Hz voltage:

$$$n_s=120*30/4=900\:rpm$$$

The slip for an induction motor is:

$$$slip=\frac{(n_s-n)}{n_s}$$$

where $$n$$ is the rotational speed.

$$$0.05=\frac{(900rpm-n)}{900rpm}$$$

Solving for $$n$$ :

$$$n=900rpm-0.05(900rpm)=900rpm-45rpm=855\:rpm$$$

855 rpm

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