Sphinx wildcard match on realtime index

I have created realtime index:

index berni_filter2
{
    rt_mem_limit    = 512M
    type            = rt
    path            = /var/lib/sphinxsearch/data/berni_filter

    rt_attr_uint    = product_id
    rt_attr_uint    = store_id
    rt_field        = product_name
    rt_attr_json    = attributes

    prefix_fields   = product_name
    expand_keywords = 1
    min_infix_len   = 2
    dict            = keywords
}

Then I trying to get data by product name field over SphinxQL:

SELECT product_id FROM berni_filter2 WHERE store_id = 0 AND MATCH ('@product_name mothercare')

This query work fine, but I need to find partically words, for example "mother" must return products with "mothercare". I have tried:

SELECT product_id FROM berni_filter2 WHERE store_id = 0 AND MATCH ('@product_name mother')
SELECT product_id FROM berni_filter2 WHERE store_id = 0 AND MATCH ('@product_name mother*')
SELECT product_id FROM berni_filter2 WHERE store_id = 0 AND MATCH ('@product_name *mother*')

I tried change min_infix_len to min_prefix_len. Nothing work.

My mistake was, that I don't delete data files from filesystem, when change config, this is final working config:

index berni_filter
{
    rt_mem_limit    = 512M
    type            = rt
    path            = /var/lib/sphinxsearch/data/berni_filter

    rt_attr_uint    = product_id
    rt_attr_uint    = store_id
    rt_field        = product_name
    rt_attr_json    = attributes

    index_exact_words = 1
    expand_keywords   = 1
    min_prefix_len    = 3
    min_word_len      = 2

    morphology        = stem_enru
    dict              = keywords
}