Rates of Depericiation

no, you have to claim depreciation at the rate specified in the act. you can't claim depreciation at lower rates

give the refrence where it is mentioned

it is nowhere mentioned that the rates of depreciation are maximum rates then how could we assume so. also if it is allowed to charge Dep at lower rates people will start abusing it by claiming lower dep in years of losses.

and what in the years of profit they have to pay higher tax if they charge at lower rates

After many Tax plannings by Ambani Sir, govt understood the importance of depreciation. One need to charge depreciation mandatorily at the rate specified in act. Sir Dhiru Bhai Ambani have done many tax plannings on depreciation and that's why, it's mandatory to charge depreciation. you escape from it and not even charge at lower rate.
In case of lower profit MAT will apply. Depreciation will carried forward for infinite period bin case of loss. In case of higher profit also you need to charge depreciation at same rates.

dep rates are fixed and it is compulsory it is mention in a circular issues by CBDT that if any assessee does not claim dep then A.O shall at the time of assessment charge the dep