Breaker feeding an electric heater trips after one minute
As noted by isherwood, you are pulling too much power. 7,500 W /240 V = 31.25 A on a 30 A circuit. Circuit breakers are typically designed to handle a somewhat higher than rated current for a short time - e.g., motor startup, a really high current (e.g., hundreds of Amps on a 30 A circuit) for an extremely short time, and a moderate overage for a moderate amount of time. This actually matches well with another rule: continuous loads should not exceed 80% of the rated circuit. In the case of 30 A, that means your continuous load should be no more than 24 A = 7,200 W.

So this is actually normal operation.

You need either a slightly smaller heater or a larger breaker with matching appropriately sized wiring.