try this, it’s work for me.
$args=array(
'posts_per_page' => 50,
'post_type' => 'my_custom_type'
'cat' => $cat_id,
);
$wp_query = new WP_Query( $args );
Category Parameters
cat (int): use category id.
category_name (string): use category slug (NOT name).
category__and (array): use category id.
category__in (array): use category id.
category__not_in (array): use category id.