How to check the specifications of AC motors

Checking periodically  specifications of AC motor  of air cpompressor in particular and AC motor (in general) is very important and necessary, it helps users to minimize risks such as electric shock, engine fire ... From the checked  specifications  users can  detect  of potential risks and have appropriate maintenance plans to increase the lifespan of the Engine and operating safety. In addition, the 3-phase alternating current electric motor with low voltage is  assembled,  it is necessary to check the technical parameters such as insulation resistor, and load  unload current , working speed, etc. . 

1. Check insulation by Megohmmeter 500V (or 1000V if new engine)

+ Among  phases and covers
+ Among phases together (must remove connector to separate 6 wires). Standard reaches from 0.5 Mega ohm or more is a (low voltage) motor that can be run. The good insulated motors actually  from 20Mega ohm  to infinity. If the lead meter is measured below 0.3 Mega ohm, humid motor is humid  that does not meet the technical requirements and must be dried again. The clock hand indecates  0 meaning the engine was broken to remove or repair it.

2. Check motor when unload and load by pliers-sharpened Megohmmeter operation 

The current in three phases must be equal; For unload motor , the current must not exceed the specified level  in Table 4-1. Then run the load motor measuring the load current at any phase should not exceed the rated value stated on the motor's label. If possible, check both no-load speed and rated speed when heavy load (refer to table 4-2 when the motor loses labels).

Table 4-2: Unload current of  3 phase asynchronous motor

Notes:
+In the table is the average value, the current is measured when no load is higher than this level is a good motor; if higher than the prescribed level, it may be due to wrong wrapping, worn ball , poor mechanical.

 + Special motors used in cranes; Lifting and lowering machines, the unloaded current values ​​must be 1.3-1.4 times high.

Bảng 4-2: Rotation speed of 3-phase asynchronous motor

Note: 

Normally when running out of load, rotor speed decreases to rated speed (1.5-2% in big motors, 5-6% in small 3-phase motors)when testing load network (380V motor,) every 1kw, if the ammeter indecates 2 A or less than 2A, the motor can run. For example : A 3-phase asynchronous motor 380V, output 7.5kw, recorded speed  on label  is 1450 vg / ph. Rated load current: I = 7.5 * 2 = 15A For unloading current , the current in the three phases must be equal and within the range (mentioned table 4-1). 15 * 0.45 = 6.75A (above 6.5A) For loading, use the pliers-sharpened ammeter 15A and speedometer indecates from 1450 vg/ph or more, the engine is eligible for long-term, not overheating.

                                                                                            Mr : Phu Nguyen