def rotational_speed(np, slip, hz):
    """Calculate rotational speed at different number of poles of an AC motor.

    :param np (int): number of poles of an AC motor
    :param slip (int): slip factor [%]
    :param hz (int): utility frequency [Hz]
    :return rpm (float): rotational speed [rpm]
    """
    rpm = 120 * hz / np * (1 - slip / 100)

    return rpm