A, the common asynchronous motor is designed by constant frequency and constant pressure, can never completely adapt to the requirements of the frequency control of motor speed. The following for the influence of frequency converter for motor.

1, the efficiency and the problem of temperature rise of the motor

In either in the form of a frequency converter, both in the operation of the different degree of harmonic voltage and current, make the motor in non-sinusoidal voltage, power flow operation. Rejected materials is introduced, at the current widely used sine wave type PWM inverter as an example, the basic is zero, the low-order harmonic larger than the rest of the carrier frequency times the left and right sides of higher harmonic components as: 2 u + 1 (u for modulation ratio).

Can lead to higher harmonic motor stator copper loss, rotor copper (aluminum), iron loss and the increase of the additional loss, the most significant is the rotor copper (aluminum) consumption. For asynchronous motor is close to the base wave frequency corresponds to the synchronous speed of rotation, therefore, higher harmonic voltage to a larger slip cutting rotor after conducting bar, will produce very big rotor loss. In addition, also should be considered because of skin effect produced by the additional copper loss. These losses will make motor extra heat, low efficiency, output power decreases, and such as ordinary three-phase asynchronous motor running in the inverter output non-sinusoidal power supply conditions, the temperature rise to increase 10% - 10%.

2, motor insulation strength problem

At present middle and small converter, many are using the PWM control mode. He is about several thousand to more than 10 KHZ carrier frequency, which makes the motor stator winding to withstand high voltage build-up rate, equivalent to impose great impulse voltage gradient on motor, the motor of the turn-to-turn insulation withstand ordeal. In addition, the rectangular produced by PWM inverter chopper superposition of impulse voltage on the motor running voltage, would pose a threat to ground insulation of motor, the ground under the repeated impact of high voltage insulation can accelerate ageing.

3, harmonic electromagnetic noise and vibration

Ordinary asynchronous motor by inverter power supply, can make the electromagnetic, mechanical, ventilation and other factors caused by the vibration and noise becomes more complicated. Frequency conversion power harmonic and motor electromagnetic part contains all of the time the inherent space harmonic interference each other and form all kinds of electromagnetic vibration force. When the electromagnetic wave frequency and motor of the body's natural vibration frequency is consistent or close to, will produce resonance phenomenon, so as to increase the noise. Due to the motor operating frequency range wide, wide speed change range, all kinds of the electromagnetic wave frequency is difficult to avoid the components of the motor of the inherent vibration frequency.

4, motor for frequent starting and braking ability to adapt

As a result of the inverter power supply, motor can under low frequency and voltage start, in the form of no impact current and frequency converter is available for braking modes of the brake quickly, create conditions for the realization of the frequent starting and braking, and the mechanical system and electromagnetic system of the motor is in circulation under the action of alternating force, brings to the mechanical structure and insulation structure fatigue and accelerated aging problem.

5, when low speed cooling problems

First of all, the impedance of the asynchronous motor is not ideal, when power frequency is bottom, the power loss caused by high order harmonic. Second, when ordinary asynchronous motor speed to reduce again, cooling air volume and speed of three proportional to reduce, cause the low speed of a motor cooling conditions deteriorate, a sharp increase in temperature is difficult to realize the constant torque output.

Second, the characteristics of variable frequency motor

1, the electromagnetic design

For ordinary asynchronous motor, the performance of the main consideration when the design parameter is the overload capacity, starting performance, efficiency and power factor. And frequency conversion motor, because of the critical slip inverse ratio at power frequency, you can directly start when critical slip is close to 1, therefore, overload capacity and startup performance are no longer need to think too much, and to solve the key problem is how to improve the ability to adapt the motor of the non sine wave power. Generally as follows: 1) the reduction of the stator and rotor resistance as much as possible.

Reduce the stator resistance can reduce the fundamental wave copper consumption, to compensate for higher harmonic caused by copper consumption increase

2) to suppress the higher harmonic current, should be appropriate to increase the inductance of the motor. But rotor slot leakage resistance is larger the skin effect is also big, higher harmonic copper loss also increased. Therefore, the size of the leakage reactance of the motor should give attention to two or morethings arrives the rationality of the impedance matching within the whole speed range.

3) the main magnetic circuit of frequency conversion motor generally designed to unsaturated state, one is to consider will deepen higher harmonic magnetic circuit saturation, 2 it is to consider at low frequency, in order to improve the output torque and appropriate to increase the output voltage of inverter.

2, the structure design

Structure design, main is also considering the non-sinusoidal power characteristics of variable frequency motor insulation structure, the influence of vibration and noise cooling mode, generally pay attention to the following questions:

1) the insulation class, generally for F or higher, strengthen the insulation and line turn insulation strength, especially want to consider the ability of impact resistance voltage insulation.

2) to the problem of vibration and noise of the motor, the motor should be fully considered component and overall rigidity, trying to improve its natural frequency, in order to avoid and every force wave resonance phenomenon.

3) cooling method: generally adopts the forced ventilation cooling, namely USES the independent main motor cooling fan motor driver.

4) measures to prevent shaft current, the capacity of more than 160 kw motor bearing insulation measures should be adopted. Mainly produces magnetic circuit asymmetry, also can produce shaft current, while the rest of the high frequency component generated by the current role in combination, shaft current will increase greatly, which can lead to bearing damage, so often insulation measures.

5) to the constant power variable frequency motor, when the rotation speed of more than 3000 / min, high temperature resistant special grease should be adopted, to compensate for the temperature of the bearing.

Synchronous motor:

Features:

1, the power factor in advance, generally rated power factor is 0.9, is beneficial to improve the power factor of power grid, increase capacity of the grid.

2, stable running, when suddenly fell to the power grid voltage rating of 80%, the excitation system can automatically adjust the general forced excitation, to insure the stable operation of the motor.

3, overload capacity is larger than the corresponding asynchronous motor.

4, high efficiency, especially in low speed asynchronous motor.

Second, the startup mode

1, asynchronous startup method, and most on the rotor of synchronous motor is equipped with similar to the startup winding cage asynchronous motor winding. Then concatenated excitation circuit is about 10 times exciting winding resistance value of additional resistance to form closed circuit, the synchronous motor stator directly connected to the electricity grid, according to the asynchronous motor starting, when the speed reaches the synchronous speed (95%), then resection additional resistance.

2, frequency conversion start, start in transducer, not in here.

Three, the application

Made oilfield energy-saving master knows, oil field of pumping unit motor, due to the requirements of the big start torque, engineers design of general motor design is very big, this is the "big horse-drawn cart" phenomenon, such as: 55 KW of pumping unit motor, again after basic adjusted the balance block, the actual active generally in more than a dozen KW, sometimes small. I've done such a transformation, change the pumping unit 55 kw asynchronous motor to 22 kw synchronous motor and frequency converter is used after control, of course also can according to drainage quantity or other signal automatic control. Power saving rate is 40%.

Therefore, asynchronous motor, synchronous motor, frequency conversion motor each have three characteristics, basically see you can control the working condition of environment, also according to the engineering cost, can use the asynchronous motor with asynchronous motor as far as possible.