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Hiện tại em đang dịch một số tài liệu về động cơ AC, nhưng do kiến thức còn non quá nên dịch rất ngây ngô. Đoạn dưới này có một số chỗ em ko dịch được, mong các thầy cô, anh chị hay bạn nào có kinh nghiệm xin giúp em với

Starting an Induction Motor

High-inertia loads put a strain on induction motors

because they prolong the starting period.  The

current is high during this interval such that

overheating is a major concern.

Rule of Thumb 1:

The heat dissipated in the rotor during start-up

(from zero to rated speed) is equal to the final

kinetic energy stored in all the revolving parts.

Assumes motor is not loaded mechanically (apart from inertia)

Braking an Induction Motor

Sometimes an induction motor (and its load) needs to be stopped

suddenly.   This can be achieved by interchanging the phase

sequence, so that the field is rotating backwards relative to the

rotor.  This is known as  plugging .

During plugging, the motor absorbs kinetic energy from the still-

rotating load and dissipates it as heat in the rotor.  However, the

motor also continues to receive electrical power from the supply,

which is also dissipated as heat in the rotor.

Rule of Thumb 2:

The heat dissipated in the rotor during plugging (from rated to zero speed)  is equal to three times the kinetic energy stored in all the revolving parts.

Abnormal Operating Conditions

1. Mechanical overload

2. Supply voltage changes

3. Frequency variation

Standardisation of Induction Motors

The frames of all industrial motors under 500hp

have standardised dimensions.

Therefore, motors (of the same frame size) can

be interchanged without changing the

mounting holes, the shaft height or the shaft

coupling.

The standards also establish limiting values for

electrical, mechanical and thermal

characteristics (such as starting torque,

locked-rotor current, overload capacity and

temperature rise).

Classifications According to

Operating Environment

1. Drip-proof motors           

2. Splash proof motors

3. Totally enclosed non-ventilated motors

4. Totally enclosed, fan-cooled 

5. Explosion-proof motors

Classifications According to

Electrical/Mechanical Properties

1. Motors with standard locked-rotor torque (NEMA B)

•Good for fans, centrifugal pumps, machine tools…

 2. High-starting torque motors (NEMA C)

•Good for starting under load – hydraulic pumps and

piston-type compressors

3. High-slip motors (NEMA D)

 • Good for starting high-inertia loads

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