Move your little hands and stay away from annoying motor failures?

Move your little hands and stay away from annoying motor failures?

1. The motor cannot be started

1. The motor does not turn and there is no sound. The reason is that there is a two-phase or three-phase open circuit in the motor power supply or winding. First check for supply voltage. If there is no voltage in three phases, the fault is in the circuit; if the three-phase voltages are balanced, the fault is in the motor itself. At this time, the resistance of the three-phase windings of the motor can be measured to find out the windings with open phase.

2. The motor does not turn, but there is a “humming” sound. Measure the motor terminal, if the three-phase voltage is balanced and the rated value can be judged as severe overload.

The inspection steps are: first remove the load, if the speed and sound of the motor are normal, it can be judged that the overload or the mechanical part of the load is faulty. If it still does not turn, you can turn the motor shaft by hand. If it is very tight or cannot turn, measure the three-phase current. If the three-phase current is balanced, but it is larger than the rated value, it may be that the mechanical part of the motor is stuck and the motor Lack of oil, bearing rust or serious damage, the end cover or oil cover is installed too obliquely, the rotor and the inner bore collide (also called sweeping). If it is difficult to turn the motor shaft by hand to a certain angle or if you hear a periodic “chacha” sound, it can be judged as a sweep.

The reasons are:

(1) The gap between the inner and outer rings of the bearing is too large, and the bearing needs to be replaced

(2) The bearing chamber (bearing hole) is too large, and the inner hole diameter is too large due to long-term wear. The emergency measure is to electroplate a layer of metal or add a sleeve, or punch some small points on the wall of the bearing chamber.

(3) The shaft is bent and the end cover is worn.

3. The motor rotates slowly and is accompanied by a “humming” sound, and the shaft vibrates. If the measured current of one phase is zero, and the current of the other two phases greatly exceeds the rated current, it means that it is two-phase operation. The reason is that one phase of the circuit or power supply is open or one phase of the motor winding is open.

When one phase of the small motor is open, it can be checked with a megohmmeter, a multimeter or a school lamp. When checking the motor with star or delta connection, the joints of the three-phase windings must be disassembled, and each phase must be measured for open circuit. Most of the windings of medium-capacity motors use multiple wires and are connected in parallel around multiple branches. It is more complicated to check if several wires are broken or a parallel branch is disconnected. The three-phase current balance method and the resistance method are often used. Generally, when the difference between the three-phase current (or resistance) values is more than 5%, the phase with small current (or large resistance) is the open circuit phase.

Practice has proved that the open-circuit fault of the motor mostly occurs at the end of the winding, the joint or the lead.

2. The fuse is blown or the thermal relay is disconnected when starting

1. Troubleshooting steps. Check whether the fuse capacity is appropriate, if it is too small, replace it with a suitable one and try again. If the fuse continues to blow, check whether the drive belt is too tight or the load is too large, whether there is a short circuit in the circuit, and whether the motor itself is short circuited or grounded.

2. Ground fault checking method. Use a megohmmeter to measure the insulation resistance of the motor winding to ground. When the insulation resistance is lower than 0.2MΩ, it means that the winding is seriously damp and should be dried. If the resistance is zero or the calibration lamp is close to normal brightness, the phase is grounded. Winding grounding generally occurs at the outlet of the motor, the inlet hole of the power line or the winding extension slot. For the latter case, if it is found that the ground fault is not serious, the bamboo or insulating paper can be inserted between the stator core and the winding. After confirming that there is no grounding, it can be wrapped, painted with insulating paint and dried, and continue to be used after passing the inspection.

3. Inspection method for winding short-circuit fault. Use a megohmmeter or a multimeter to measure the insulation resistance between any two phases at the separate connecting lines. If it is even close to zero below 0.2Mf, it means that it is a short circuit between phases. Measure the currents of the three windings respectively, the phase with the largest current is the short-circuit phase, and the short-circuit detector can also be used to check the interphase and inter-turn short circuits of the windings.

4. Judgment method of stator winding head and tail. When repairing and checking the motor, it is necessary to re-evaluate the head and tail of the stator winding of the motor when the outlet is disassembled and forgotten to be labeled or the original label is lost. Generally, the cutting residual magnetism inspection method, the induction inspection method, the diode indication method and the direct verification method of the change line can be used. The first several methods all require certain instruments, and the measurer must have certain practical experience. The direct verification rule of changing the thread head is relatively simple, and it is safe, reliable and intuitive. Use the ohm block of the multimeter to measure which two wire ends are one phase, and then arbitrarily mark the head and tail of the stator winding. The three heads (or three tails) of the marked numbers are connected to the circuit respectively, and the remaining three tails (or three heads) are connected together. Start the motor with no load. If the starting is very slow and the noise is very loud, it means that the head and tail of one phase winding are reversed. At this time, the power should be cut off immediately, the position of the connector of one of the phases should be reversed, and then the power should be turned on. If it is still the same, it means that the switching phase is not reversed. Reverse the head and tail of this phase, and switch the other two phases in turn in the same way until the starting sound of the motor is normal. This method is simple, but it should only be used on small and medium motors that allow direct starting. This method cannot be used for motors with large capacity that do not allow direct starting.

Post time: Apr-01-2022