thumb|Power transformer overexcitation condition caused by decreased frequency; flux (green), iron core's magnetic characteristics (red) and magnetizing current (blue). Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current. At a lower frequency, the magnetizing current will increase. Operation of a large transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. Transformers may require protective relays to protect the transformer from overvoltage at higher than rated frequency.
thumb|Power transformer overexcitation condition caused by decreased frequency; flux (green) iron core's magnetic characteristics (red) and magnetizing current (blue) Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current At a lower frequency the magnetizing current will increase Operation of a large transformer at other than its design frequency may require assessment of voltages losses and cooling to establish if safe operation is practical Transformers may require protective relays to protect the transformer from overvoltage at higher than rated frequency