Electric motors are by far the most widely used rotating equipment. With the development and popularization of frequency converters, an increasing number of electric motors are required to operate in conjunction with them.

However, numerous issues inevitably arise during the combined operation of frequency converters and motors, and these emerging problems merit in-depth consideration and professional discussion.

Q1: Can motor soft starters save energy?

Soft starters deliver limited energy-saving effects. Nevertheless, they reduce the impact of motor startup on the power grid, enable smooth startup, and protect motor windings.

In accordance with the law of conservation of energy, the additional complex control circuits of soft starters result in higher overall power consumption rather than energy savings. Even so, they effectively limit the starting current of the circuit and provide reliable motor protection.

Q2: What are the starting current and starting torque of a motor driven by a frequency converter?

When operated via a frequency converter, the frequency and voltage rise gradually as the motor accelerates. The starting current is restricted to below 150% of the rated current (ranging from 125% to 200% depending on the model). By contrast, direct online startup with industrial frequency power generates a starting current 6 to 7 times the rated value, causing severe mechanical and electrical impact.

Frequency converter drive enables smooth motor startup with an extended acceleration time. Its starting current is 1.2 to 1.5 times the rated current, and the starting torque ranges from 70% to 120% of the rated torque. For frequency converters equipped with automatic torque boost function, the starting torque can exceed 100%, supporting full-load startup.

Q3: Is there a correlation between motor overload and short circuit?

Motor overload falls into two categories:

1. Mechanical overload: Caused by excessive load or jamming in the transmission system, which has no connection with short circuits.

2. Normal mechanical load with electrical current overload: This may result from local ground faults or turn-to-turn short circuits in motor windings.

Q4: What are the application scenarios and advantages of variable frequency speed regulation?

It is applicable to all rotating machinery requiring speed regulation.

Before the practical application of variable frequency speed regulation (the theory was established long before its widespread implementation following the invention of power electronic devices), traditional speed regulation relied on DC motors. The main drawbacks of DC speed regulation are as follows:

1. Complex structure and high maintenance costs of DC motors.

2. The commutator limits the maximum power output of DC motors.

Accordingly, the core advantages of variable frequency speed regulation are summarized below:

1. It endows AC motors with speed regulation performance equivalent to that of DC speed regulation systems.

2. Squirrel-cage asynchronous AC motors feature simple structure and low maintenance requirements.

3. The power output of AC motors is not restricted by commutators.

Q5: Is a 100 kVA transformer sufficient to supply electrical equipment with a total power of 100 kW (maximum single-unit power: 37 kW)?

The load capacity of a 100 kVA transformer can be calculated with the formula:P=Capacity×Power Factor×80%=100×0.9×0.8=72 kW

A 20% overload operation for one hour is generally permitted, so the transformer is sufficient for the above load.

The key assessment indicator is the total operating current. For a 100 kVA transformer, the high-voltage rated current is 5.8 A and the low-voltage rated current is 150 A. Occasional minor overload is acceptable, provided that the temperature rise does not exceed 55°C. Temperature rise refers to the difference between the actual equipment temperature and the ambient temperature.

Q6: How to measure the insulation resistance of a motor?

For three-phase AC motors: Measure the inter-phase insulation resistance and winding-to-ground insulation resistance of the three-phase windings.

For DC motors: Measure the insulation resistance of armature windings to ground, series excitation windings to ground, shunt excitation windings to ground, and between series excitation and shunt excitation windings. Select a megohmmeter matching the motor’s voltage class for testing.

Measurement Procedures:

1. Cut off the power supply completely.

2. Discharge all residual charges to the ground.

3. Disconnect the neutral point of three-phase AC motors (if accessible).

4. Lift the carbon brushes of DC motors.

5. Use a megohmmeter to test inter-phase and winding-to-ground insulation resistance respectively.

6. Discharge residual charges again after testing.

7. Restore all circuits to their original state.

8. Record the measured insulation resistance value and ambient temperature for filing.

Q7: What is a brushless and ring-free starter?

A brushless and ring-free starter is a starting device that eliminates the defects of wound rotor asynchronous motors, such as slip rings, carbon brushes and complicated starting systems, while retaining their strengths of low starting current and high starting torque.

It serves as an ideal replacement for traditional starting equipment including resistance starters, reactors, frequency-sensitive rheostats, liquid rheostat starters and soft starters. It is suitable for three-phase wound rotor asynchronous motors of models JR, JZR, YR and YZR (excluding variable-speed motors and those equipped with phase advancers).

Q8: What are the two types of capacitor starting modes for motors?

There are two mainstream capacitor starting modes:

1. Capacitor start only: The starting capacitor is disconnected automatically once the motor reaches normal operation.

2. Capacitor start and run: The capacitor functions in both the starting phase and continuous operating phase of the motor.