In product quality supervision and inspection at all levels, unqualified input power items of electric heating appliances based on resistance wire heating principle often occur, such as common products such as hair dryer, electric heater, electric kettle, electric water heater, electric cooker, electric blanket, etc. The common causes of disqualification can be summarized as follows:

1. In order to reduce the cost, the thin wire is selected as the heating element

In order to reduce the cost, some enterprises choose thin and short resistance wire to wind the heating element. Although the power is the same, the temperature rise of the heating element will be higher if the current of the same size is passed on the thinner resistance wire. For example, some hair dryers can see the resistance wire reddening visually when they are working. In order to ensure that the temperature rise of the product is qualified and avoid the danger caused by local overheating of the product, it is necessary to further ensure that the input power of the product is near the lower deviation.

2. When calculating the resistance of the heating wire, the change of the resistance of the heating wire in the heating state is not considered

The resistance wire usually has a positive temperature coefficient characteristic. In the working process of electric heating products, with the gradual rise of temperature, the resistance value of heating wire will gradually increase. Some enterprises only design according to the resistivity of the resistance wire at room temperature. With the increase of the temperature of the heating wire, the resistance increases. According to Ohm’s law, the input power of the product decreases naturally, which will inevitably lead to the unqualified input power of the whole electric heating appliance under working condition.

3. In the design verification stage, the number of samples is insufficient

The input power of the product is normally distributed. In order to obtain the objective distribution state, there should be enough sample size for testing in small batch production. For small household appliances, it is more appropriate to have nearly 100 samples. Some enterprises complete the acceptance of product design and development projects only through the monitoring data of several prototypes, which is difficult to ensure that the mass-produced products are qualified.

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