Why do motors generate shaft currents?
The current in the shaft - bearing housing - base circuit of a motor is called shaft current.
Reasons for shaft current generation:
1. Asymmetry of the magnetic field;
2. Harmonics in the supply current;
3. Poor manufacture and installation, uneven air gap due to rotor eccentricity;
4. There is a gap between the two semicircles of the detachable stator core;
5. There are sectors stacked into the stator core of the number of pieces of the patchwork is not the right choice.
Hazard: the motor bearing surface or ball erosion, shape process point microporous, so that the bearing running performance deterioration, friction loss and heat increase, and ultimately cause bearing burnt.
Why can't general motors be used in high altitude areas?
Altitude has an adverse effect on motor temperature rise, motor corona (high voltage motors) and DC motor commutation.
The following three aspects should be noted:
1. Higher altitude, the greater the motor temperature rise, the lower the output power. However, when the temperature with the increase in altitude and reduce enough to compensate for the effect of altitude on the temperature rise, the rated output power of the motor can be unchanged;
2. High-voltage motors should be used in the plateau to take anti-corona measures;
3. Altitude is unfavorable to the DC motor commutation, pay attention to the selection of carbon brush material.
Why should motors not be operated at light loads?
When the motor is running at light load, it causes:
1. low motor power factor;
2. low motor efficiency.
It will cause equipment waste and uneconomical operation.
What are the causes of motor overheating?
1. Excessive load;
2. lack of phase;
3. blockage of air ducts;
4. Low-speed running time is too long;
5. Excessive power supply harmonics.
What needs to be done before putting the motor into use for a long time?
1. Measurement of stator, winding insulation resistance between phases and winding to ground.
Insulation resistance R should meet the following formula:
R＞Un/(1000+P/1000)(MΩ)
Un: motor winding rated voltage (V)
P: motor power (KW)
For the motor with Un = 380V, R＞0.38MΩ.
If insulation resistance is low:
a: Dry the motor by running no-load for 2~3h;
b: Use low-voltage alternating current of 10% rated voltage to pass into the winding or connect the three-phase winding in series and then dry it with direct current, keeping the current at 50% of the rated current;
c: use the fan to send in hot air or heating element heating.
2. Clean the motor.
3. Replace bearing grease.
Why can't I arbitrarily start a motor in a cold environment?
A motor left in a cold environment for too long will:
1. crack motor insulation;
2. freezing of bearing grease;
3. Solder pulverization of wire joints.
Therefore, motors should be heated and stored in cold environments, and the windings and bearings should be inspected before operation.
What are the causes of motor three-phase current imbalance?
1. Three-phase voltage imbalance;
2. Poor welding or bad contact of a phase branch circuit inside the motor;
3. Motor winding turn-to-turn short circuit or short circuit to ground or phase-to-phase;
4. Wiring error.
Why can't a 60Hz motor be connected to a 50Hz power supply?
The motor design generally makes the silicon steel sheet work in the saturation area of the magnetization curve, when the power supply voltage is certain, reducing the frequency will make the magnetic flux increase, the excitation current increases, resulting in an increase in the motor current, copper consumption increases, and ultimately leads to the motor temperature rise, and in serious cases, may be due to the overheating of the coil and burned out the motor.
What are the causes of phase loss in motors?
Power supply:
1. Poor switch contact;
2. Transformer or line disconnection;
3. insurance fuse.
For the motor:
1. poor contact with loose screws in the motor junction box;
2. Poor welding of internal wiring;
3. Motor winding broken wire.
What are the causes of abnormal motor vibration and sound?
Mechanical:
1. poor bearing lubrication, bearing wear;
2. Loose fastening screws;
3. Debris in the motor.
Electromagnetic aspects:
1. Motor overload operation;
2. three-phase current imbalance;
3. lack of phase;
4. Stator, rotor winding short circuit failure;
5. Cage rotor welding part open welding caused by broken bars.
What needs to be done before starting the motor?
1. Measure the insulation resistance (should not be less than 0.5MΩ for low voltage motors);
2. Measure the power supply voltage. Check whether the motor wiring is correct and whether the power supply voltage 3 meets the requirements;
3. Check whether the starting equipment is good;
4. Check whether the fuse is appropriate;
5. Check whether the motor grounding, zeroing is good;
6. Check whether the transmission device is defective;
7. Check whether the environment of the motor is suitable, remove flammable products and other debris.
What are the causes of motor bearing overheating?
The motor itself:
1. Bearing inner and outer rings fit too tightly;
2. Problems with the shape and position tolerances of the parts, such as bad coaxiality of the seat, end cover, shaft and other parts;
3. Improper selection of bearings;
4. poor bearing lubrication or bearing cleaning is not clean, grease with debris;
5. Shaft current.
Use:
1. Improper installation of the unit, such as the motor shaft and the dragged device shaft coaxiality of one with the requirements;
2. Pulley pulling too tight;
3. Poor maintenance of bearings, grease is insufficient or more than the use period, dry and deteriorated.
What are the reasons for low insulation resistance of motors?
1. Moisture or water intrusion into the windings;
2. Accumulation of dust or oil on the windings;
3. Aging of insulation;
4. Damage to the insulation of the motor leads or terminal board.