30amp dryer on a 50amp breaker?
Your question is a bit convoluted, but the answer is very simple. The breaker size must match or be lower than the wire amp rating feeding the circuit, and appliances on that circuit should not exceed the max rating of the wire or breaker.  In a dedicated circuit for a dryer, for example, the normal size of the circuit would be 30 amps. 30 amps requires a minimum of #10 copper or #8 alu.  With this wire size, the breaker must not be larger than 30 amps.  Do not use a larger breaker then what the current rating of the wire is rated for.