Performance and application research of Danfoss FC302 variable frequency drive

introduction

Fiber drafting in the chemical fiber industry and finished product transmission in the glass manufacturing industry both require very high speed control accuracy. They are generally driven by permanent magnet synchronous motors and use frequency converters as open-loop speed control drives. Danfoss frequency converters have a long history of application in these two industries due to their stable performance and high reliability. In order to further improve the reliability of equipment operation and improve the output performance of permanent magnet synchronous motors at low speeds, we analyzed the basic characteristics of permanent magnet synchronous motors and proposed some useful methods.

1. Structural characteristics of permanent magnet synchronous motor:

The structure of a common permanent magnet synchronous motor is shown in Figure 1. The stator is made of common silicon steel sheets with good magnetic conductivity. The surface of the rotor is covered with permanent magnets, and the inside of the rotor is still made of iron with good magnetic conductivity. The air gap between the rotor and the stator is about 0.2 mm, which is similar to that of a common asynchronous motor.

Figure 1. Structure of a permanent magnet synchronous motor

Most modern mainstream permanent magnet synchronous motors use rare earth permanent magnet materials as permanent magnets. Since rare earth permanent magnet materials have very large intrinsic coercive force, their magnetization characteristic curve is close to a rectangle. Below the coercive point, the magnetic susceptibility is almost the same as that of air. Therefore, when performing magnetic circuit analysis, rare earth permanent magnets can be treated as air gaps. Therefore, the internal equivalent air gap of permanent magnet synchronous motors is much larger than that of asynchronous motors.

Based on the differences in electrical parameters of d-axis inductance Xd and q-axis inductance Xq, rare earth permanent magnet synchronous motors can be divided into two types: hidden pole synchronous motors and salient pole synchronous motors.

The main feature of the hidden pole permanent magnet synchronous motor is that the permanent magnet is attached to the uniform rotor, and the air gap magnetic potential is ensured to be a sine wave by modifying the surface shape of the permanent magnet. The air gaps on the d-axis and q-axis of the magnetic circuit are the same, so: Xd = Xq

The main feature of the salient pole permanent magnet synchronous motor is that the actual air gap of the motor is uniform, but the d-axis magnetic circuit air gap is equal to the motor air gap plus the thickness of the permanent magnet, and the q-axis magnetic circuit air gap is equal to the motor air gap, so usually: Xd < Xq

2. Precautions for inverter open-loop driving of permanent magnet synchronous motor:

1. Impact of motor ripple current

Since the internal equivalent air gap of a permanent magnet synchronous motor is much larger than that of an asynchronous motor, the permanent magnet synchronous motor has a much smaller main inductance and electrical time constant than an asynchronous motor, which has the advantage of fast current response in control. However, frequency converters are usually designed to drive asynchronous motors with large inductance. If used to drive a permanent magnet synchronous motor, the output current ripple of the frequency converter will be much larger than the current ripple when driving an asynchronous motor.

Under such conditions, many inverters that are not specially designed will fail or have their service life greatly shortened. Danfoss' FC302 inverter is a drive designed specifically for driving permanent magnet synchronous and asynchronous servo motors. The design of the inverter has taken into account the impact of higher carrier ripple current on the inverter's hardware and control software; at the same time, the voltage mutation rate of the inverter's output PWM square wave is subject to special restrictions. After sufficient internal testing in the factory and long-term practical application tests by a large number of users, the long-term reliability of the FC302 drive in driving permanent magnet synchronous motors has been proven.

2. Motor control mode of inverter

When the inverter is open-loop controlled, there are generally several control modes, such as VVCplus control, flux vector control without speed feedback, and V/F control (scalar control). Among them, VVCplus control and flux vector control without speed feedback are control modes for asynchronous motors and cannot be used for synchronous motor control. Therefore, only V/F control can be used.

The V/F control mode of the Danfoss FC302 drive allows users to customize 6 points (frequency, voltage), which has greater flexibility in application and is particularly suitable for open-loop control of permanent magnet synchronous motors.

Figure 5, V/F point definition of FC302

3. Driver power selection

When selecting the power of the driver that matches the permanent magnet synchronous motor, the starting current of the permanent magnet synchronous motor should be the main consideration. This current is generally higher than the rated current, and the rated current of the driver should be greater than or equal to the starting current of the permanent magnet synchronous motor. When the permanent magnet synchronous motor is overloaded, it may generate a step-out current of more than 10 times the rated current, which is far more than the stall current of the asynchronous motor by 5-7 times. In addition, the driver cannot provide effective torque limit protection during open-loop control. Therefore, the user must be responsible for ensuring that the load will not be overloaded. In situations where tripping is not allowed, the driver power selection must have a large enough margin. The open-loop automatic torque limit and overvoltage limit functions of general frequency converters are for asynchronous motors, and are not suitable for permanent magnet synchronous motors. When driving permanent magnet synchronous motors in open loop, it is required to turn off all these intelligent protection functions.

3. Operating characteristics of permanent magnet synchronous motor:

After neglecting the stator resistance and leakage reactance, the simplified vector diagram of the permanent magnet synchronous motor can be obtained as shown in Figure 6.

Among them: 0 is the induced potential generated by the rotor permanent magnet, and the angle is strictly consistent with the rotor position.

Power angle characteristics of permanent magnet synchronous motor:

◆ The power angle characteristic refers to the relationship curve between the electromagnetic power PM and the power angle θ, PM = f (θ).

When no-load, θ is 0, that is, the space voltage vector is consistent with the rotor angle; when the space voltage vector angle leads the rotor by 90°, the output torque of the permanent magnet synchronous motor reaches its maximum value. The load torque cannot exceed TMmax at any time, otherwise it will cause a loss of step.

◆ Power angle characteristics of hidden pole permanent magnet synchronous motor

Due to the influence of salient pole reaction torque, the power angle characteristics of the non-salient pole permanent magnet synchronous motor deteriorate.

4. Ways to improve output performance when driving permanent magnet synchronous motor in open loop:

1. Since the angle of attack characteristics of the salient pole permanent magnet synchronous motor are worse than those of the non-salient pole permanent magnet synchronous motor, it is recommended to use the non-salient pole permanent magnet synchronous motor for applications with higher performance requirements.

2. For a non-salient pole permanent magnet synchronous motor, according to formula [4], TM ∝ U·sinθ, it can be seen that the load capacity of the non-salient pole permanent magnet synchronous motor is proportional to the input voltage. Therefore, increasing the open-loop output voltage of the inverter can increase the load capacity of the permanent magnet synchronous motor and prevent loss of steps. However, if the open-loop output voltage of the inverter is increased too high, the output current power factor will be reduced, and even the excitation current will be overcurrent, which requires debugging according to the actual load conditions on site.

3. Speed ​​fluctuation caused by torque fluctuation during open-loop control.

Therefore, it can be seen that when U is larger, the mechanical characteristics of the permanent magnet synchronous motor are also harder, and the angle (speed) fluctuation caused by the same torque fluctuation is relatively small.

4. How to adjust the V/F characteristic curve.

At low frequency, the starting voltage is increased, mainly considering the voltage compensation for the resistance and leakage inductance of the stator and conductor;

At medium and low frequencies, there is also consideration of artificial increase in holdings to improve the open-loop operation performance of the synchronous motor;

At high frequencies, due to the smoothing effect of inertia, adjustment is relatively simple, just set 50Hz to correspond to 380V.