Final answer:
The correct method to use a thermal overload unit for a range of motor full-load currents is by changing the heater element. Heater elements in thermal overload relays are key components that respond to motor current and can be adjusted to match the motor's current range.
Step-by-step explanation:
The thermal overload unit in electric motors is designed to protect against overheating when the motor draws excessive current due to mechanical overload or other issues. In response to the student's question, the correct option that can be used for a range of motor full-load currents is d) Changing the heater element. The heater element in a thermal overload relay is responsible for heating up in response to the motor current. By changing the heater elements, you can match the thermal overload relay to the motor's full-load current range.
When a motor begins to operate or is under a heavy load, as in the case of an electric wheelchair climbing a hill, it may draw a higher current than its normal operating current. This is also known as the inrush current. The correct thermal overload protection is crucial to prevent the motor from overheating or sustaining damage due to the excess current. Devices like fuses and circuit breakers may be designed to withstand brief periods of high current, but for extended durations of high current, a device such as a thermal overload relay with the appropriate heater element is necessary to provide adequate protection.