There was a motor manufacturing company where employees were highly sensitive to quality, stemming from their years of persistent efforts in quality control . A senior technician in their electrical workshop recounted an incident: a batch of newly fired stator leads were all cracked, despite the customer's requirement for JG500V special leads. To ensure safety in future use, they conducted a complete inspection of the entire batch of motor leads. The inspection revealed that even the uncracked lead sheaths were essentially brittle, necessitating the rework of the entire batch of motor stators.
After consulting with experts in the lead wire industry, the direct cause of the cracking in this batch of motor leads was identified as a quality issue with the lead wire insulation material. Ms. Sante would like to share the basic requirements that these leads should meet.
Overview of actual usage
Motor lead wires account for a very small percentage of the total material of a complete machine, but they play a very important role. A large part of the problems between the motor and ground or between phases are caused by the lead wires. However, how to solve this problem is a headache for many motor manufacturers.
Currently, many motor manufacturers use vacuum pressure impregnation for their varnishing processes. However, this has led to a problem: the stator leads harden and crack after impregnation with leads attached. If impregnation is performed with leads attached, there are two ways the varnish can enter the leads: one is through the solder joint between the lead and the main lead, and the other is through the lead terminal. To address this, many motor manufacturers use silicone rubber self-adhesive tape to seal both ends, achieving noticeable results.
In response to the fact that the insulation of the JG lead cracked after impregnation and drying, a comparative test simulating stator drying was conducted on the lead with and without impregnation. The insulation of the lead without impregnation did not change significantly, but the insulation of the lead after impregnation became very brittle, and the insulation broke completely with slight bending.
For this type of problem, lead wire manufacturers should be aware of the special requirements of motor leads and must meet the requirements of the motor lead wire impregnation process.
JB/ T6213.4 standard, "Flexible Cables and Wires for Motor Winding Leads - Part 4: Flexible Cables and Wires with a Maximum Conductor Temperature of 180°C for Continuous Operation," clearly states that the cable should be able to withstand the varnish impregnation test. The standard stipulates that after the finished cable undergoes the specified varnish impregnation test, the insulation and sheath should not exhibit any swelling, cracking, or other damage visible to the naked eye, and the withstand voltage test after bending should not be broken down.
Immersion resistance test of motor lead cable
●Sample preparation
Three samples, each about 1 meter long, were cut from the finished cable. About 20 mm of insulation was stripped from one end to expose the conductor for connection to high voltage.
●Test Procedure
(1) Place the sample in an oven at a specified temperature for 24 hours to preheat. Then remove it from the oven and immerse it in an impregnation varnish at room temperature for 1 hour within 2 minutes. Remove the sample from the impregnation varnish, hang it vertically to drip dry for 30 minutes, and then put the sample into the oven for baking. The baking temperature and baking time of the oven are specified or agreed upon by the supplier and the buyer.
(2) Take out the sample and wait for it to cool down. Then, slowly bend one section of the sample 180° on a round bar of a specified diameter.
(3) Immerse the bent portion of the U-shaped sample in water at room temperature to a depth of at least 100 mm. Then, apply the specified voltage between the conductor and the water and maintain it for 1 minute.
(4) When the buyer does not specify the brand of impregnation varnish, bisphenol A epoxy solvent-free impregnation varnish shall be used, and the viscosity of the varnish at (20±1)C shall be 30 seconds (Coat 4 cup method).
The specific test parameters involved in this content are specified in JB/T6213.1 .
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