CN104953903B - A kind of ac synchronous motor and its overload protection method - Google Patents

Abstract

The present invention relates to a kind of ac synchronous motor and its overload protection method, judge whether motor overloads, by current loop control limiting motor rotating speed if overload, if overload startup amount is more than zero, overload protection electric current, which is drawn, determines electric current；If overloading startup amount is not less than zero, overload protection speed takes the smaller value of verification electric current and given electric current；Wherein, overload protection electric current is the given value of current value calculated through overload protection method；Verification electric current is produced according to the verification of rated current, maximum current and current effective value；By the way that the motor feels hot, real-time calculate of model obtains overload startup amount.When limiting motor speed, verification speed is also calculated, it is to avoid improperly setting causes burn-down of electric motor.

Description

An AC synchronous motor and its overload protection method

technical field

The invention relates to an AC synchronous motor and an overload protection method thereof.

Background technique

With the development of society, motors, especially AC synchronous motors, are used more and more widely as irreplaceable power and driving devices in industrial production and people's life. Occasionally, the motor burns out due to overload.

The existing overload protection method mainly includes: firstly judge whether the motor is overloaded or will be overloaded, and protect the motor by stopping, reducing the rotating speed or output power when judging the overload.

For the method of judging that the motor is or will be overloaded, the prior art mainly includes:

① Thermal relay protection method. Thermal relays are developed using the principle of bimetal expansion. However, the discreteness of the metal sheet is large, and the metal sheet needs to be calibrated frequently.

②The method of directly measuring temperature by embedding a temperature sensor. This method requires the cooperation of the motor factory. At present, some large motors have buried temperature sensors, but most of the small and medium-sized motors have not buried temperature sensors.

③Establish the thermal model of the motor. This method is a more practical method, which can realize heat accumulation. At present, most of the first-order or second-order thermal models are used in motor thermal protectors. The models are relatively simple, but it is relatively difficult to obtain key parameters in the motor thermal model.

④ Use inverse time overcurrent to realize overcurrent protection. This method realizes the thermal overload protection by setting the limit line and judging the I-t characteristic curve, but this method does not reflect the heat accumulation effect.

⑤ Use the magnitude of the motor current to judge, and collecting the motor current will cause the calculated value to lag.

In addition, after judging that the overload has occurred or will occur, the speed needs to be limited. If the speed is not set properly, it will still burn out.

Therefore, the defects of the existing methods mainly lie in three aspects: 1. When current sampling is performed, there is a hysteresis in collecting the motor current; 2. When judging the overload, it is difficult to obtain the thermal model of the motor involved; 3. When limiting the motor speed, it is difficult to set the speed Improper still cause the motor to burn out.

Contents of the invention

The purpose of the present invention is to provide an overload protection method for a synchronous motor, which is used to solve the problem of motor burnout caused by improper setting of the rotating speed when the rotating speed of the motor is limited. At the same time, the present invention also provides a synchronous motor using the above method.

To achieve the above object, the solution of the present invention includes:

AC synchronous motor overload protection method, judge whether the motor is overloaded, if it is overloaded, the motor speed will be limited by current loop control, if the overload starting value is less than zero, the overload protection current will take the given current; if the overload starting value is not less than zero, the overload protection The current takes the smaller value of the verification current and the given current; among them, the overload protection current is the current given value calculated by the load protection method; the verification current is generated according to the verification of the rated current, maximum current and current effective value The overload starting value is calculated in real time through the motor heating model.

Further, any two-phase current and rotor angle are collected for calculation,

ia, ib are phase a and b currents, θ is the rotor angle of the motor, and iq is the q-axis current, that is, the effective value of the current.

The present invention also provides an AC synchronous motor. The AC synchronous motor includes a controller. The controller determines the overload protection current according to the overload protection method, and the overload protection current is a current given value calculated by the overload protection method.

When limiting the motor speed, the calibration speed is also calculated to avoid motor burnout caused by improper settings.

Description of drawings

Fig. 1 is a block diagram of a synchronous motor control system embodiment of the overload protection method of the present invention;

Fig. 2 is the schematic diagram of the overload protection method of the embodiment of the overload protection method of the present invention;

Fig. 3 is an overload protection logical structure diagram of an embodiment of the overload protection method of the present invention;

Fig. 4 is a flow chart of the overload protection program of the embodiment of the overload protection method of the present invention.

detailed description

The present invention will be described in further detail below in conjunction with the accompanying drawings.

Embodiment of Overload Protection Method

The calculation method of the current effective value of the AC synchronous motor uses two-phase real-time current for calculation, and only needs to collect any two-phase current when in use. For example, phases a and b, the current RMS calculation part is realized by the following formula

Among them, i _a and i _b are phase a and b currents, θ is the rotor angle of the motor, and iq is the q-axis current, which is the effective value of the current.

The above is a calculation method for obtaining the effective current, and other calculation methods in the prior art may also be used.

As shown in Figure 1, collect any two-phase current, calculate current effective value iq (as above embodiment); Other detection values), to judge whether it is overloaded, if overloaded, limit the motor speed.

Since the overload protection first needs to detect the effective value of the current to carry out subsequent judgment and processing, the above calculation method can quickly detect the effective value of the current to facilitate subsequent processing, and avoid overload protection due to the hysteresis of current collection and cause the motor to burn out in time.

According to the different synchronous control methods, there are different specific forms of limiting the motor speed. For example, the synchronous motor adopts the control method of the voltage outer loop and the current inner loop, and the limitation of the motor speed is finally converted into the control of the current. For example, the control method of the outer loop of the speed and the inner loop of the current, the limitation of the motor speed is finally transformed into the control of the current.

As shown in Figure 2, according to the overload start-up condition, if the overload start-up condition is met, the overload protection current takes the smaller value of the verification current and the given current, and if the overload start-up condition is not met, the overload protection current takes the given current. Among them, the given current is the given current provided by the outer ring, and the overload protection current is the current given value determined by the overload protection method. As shown in Figure 1, the given current generated by the outer ring of the speed is transformed into an overload by the overload protection protection current.

The main scheme of the present invention is: when judging the overload, if it is overloaded, the motor speed is limited by the current loop control; if the overload starting value is less than zero, the overload protection current takes a given current; Take the smaller value of the verification current and the given current; among them, the overload protection current is the current given value calculated by the load protection method; the verification current is generated according to the verification of the rated current, maximum current and current effective value ; The overload start-up amount is calculated in real time through the motor heating model, and the overload start-up amount can be an overload start-up criterion in the prior art.

As shown in Figures 2, 3, and 4, a preferred method is provided. In the above two aspects-the method of judging the overload and the method of limiting the speed of the motor, an optimized method that is different from the prior art is adopted: in When judging the overload (i.e. the overload starting condition), the heat accumulation effect is reflected, and the motor parameters (parameters other than the effective value of the current) that are difficult to obtain in the existing method are not needed, which is reflected in the overload starting condition in Fig. 2; When limiting the motor speed, calculate the calibration speed to avoid burning the motor due to improper settings, which is reflected in the comparison between the calibration current and the given current in Figure 2.

Specifically, the overload protection method for an AC synchronous motor includes four parts: a q-axis current conversion part, a target current selection part 1, a PID controller 2, and an overload protection starting part 3.

The current RMS calculation part is realized by the following formula

Among them, ia and ib are phase a and b currents, and θ is the rotor angle of the motor.

The target current selection section 1 includes a pass difference calculator 11 , a zero comparator 12 , trigger timers 13 and 14 , an AND logic 15 , and a truth selector 16 . The q-axis current and the maximum current are input into the difference calculator 11 . The output of the difference comparator 11 is input to a zero comparator 12 . The output of the zero comparator 12 is fed to the trigger timers 13 and 14 on the one hand and to the AND logic 15 together with the output of the trigger timers 13 and 14 on the other hand. The output of AND logic 15 is used as a criterion input to the truth selector 16, when the criterion is true, the truth selector 16 outputs the maximum current, and when it is false, it outputs the rated current.

The function realized by the target current selection part 1 is: the target current takes the maximum current when the q-axis current is greater than the maximum current and the duration is less than 2s, and takes the rated current when the duration is greater than or equal to 2s.

The PID controller 2 includes a subtracter 21 and a PID calculation section 22 . The subtractor 21 calculates the difference between the target current and the q-axis current, and then the PID operation part 22 calculates the verification current through proportionality, integration and differentiation.

Overload protection starting part 3 includes multipliers 31, 32 and 33, subtractors 34, 35, 36 and 39, zero lower limit integrator 37, gain 38, comparators 310 and 311, positive value selector 312; rated current and maximum current The heating action value is calculated by multipliers 32 , 33 , subtractor 36 and gain 38 .

The q-axis current and the rated current are calculated by the multipliers 31, 32, the subtractor 35, and the zero-lower limit integrator 37 to calculate the calorific value of the motor. The heat generation value of the motor and the heat generation action value are input to the subtractor 39 to output the heat generation deviation amount. The q-axis current and the maximum current are input to the subtractor 34 to output the current deviation amount. The heat generation deviation and the current deviation output the overload starting value through the comparator 310 . The comparator 312 determines the calculated current by the overload starting value. When the overload starting value is non-negative, the calculated current takes the smaller value of the verification current and the given current; when the overload starting value is negative, the calculated current directly takes the given current.

That is to say: the starting part of overload protection first integrates i _q ² -I ² _at the zero lower limit, and calculates the difference between the integral result and the heating action value; at the same time, calculates _the difference between i _q and I; compares the above two differences The larger one is the overload starting value; if the overload starting value is not less than zero, the motor speed is limited. Wherein, the heating action value is k (I ² _most -I ² _amount ), k is a setting coefficient (in this embodiment, k is 2, as other implementations, other values can also be used), and I _amount is the rated value of the motor Current, I _is the maximum current of the motor.

The zero-lower limit integration of i ² -I ² _is used above, and the integration is only performed when the effective value of the current is greater than the rated value, and not integrated when it is less than the rated value, which can sensitively reflect the heat accumulation effect. And compared with k(I ² _most -I ² _amount ), taking k(I _2most ^{-I 2 amount} ₎ as the heating action value can reflect the tolerable degree of heat generation. Since the difference between I and I _is also considered, the overload starting value takes into account both heat generation and current overrun.

The motor speed is limited by current loop control. If the overload starting value is less than zero, the overload protection current takes the given current; if the overload starting value is not less than zero, the overload protection current takes the smaller value of the verification current and the given current.

Figure 4 shows the specific protection process.

Step 1 executes step 2 after specifying "current effective value = [sin(θ-4π/3)-sinθ]*ia+[sin(θ-4π/3)-sin(θ-2π/3)]*ib".

In step 2, execute step 3 after setting "difference value = the square of the q-axis current - the square of the rated current".

In step 3, execute step 4 after setting "motor heating value = motor heating value + difference".

Step 4 judges whether the "motor calorific value" is less than or equal to zero. If it is less than or equal to zero, go to step 5; otherwise, go to step 6.

In step 5, execute step 8 after setting "motor calorific value = 0, overload flag = 0".

Step 6: Determine whether the "motor heating value" is greater than or equal to the "hysteresis value". If greater than or equal to, go to step 7; otherwise, go to step 8.

In step 7, execute step 8 after setting "motor heating value = hysteresis amount".

Step 8 judges whether the "overload flag" is equal to zero. If equal, go to step 9; otherwise, go to step 13.

Step 13 judges whether "overload trigger" is equal to 2 and whether "counter" is less than "setting time". When the "overload trigger" is equal to 2 and the "counter" is less than the "set time", go to step 14; otherwise, go to step 15;

Step 14 executes step 16 after adding 1 to the "counter";

Step 15 executes step 16 after ordering "target current = rated current";

Step 16 executes step 17 after ordering "current difference=target current-rated current";

Step 17 calculates "current 1" through PID and executes step 18;

Step 18 judges whether "current 1" is greater than or equal to a given current. If greater than or equal to, go to step 19; otherwise go to step 20;

Step 19 makes "calculated current=current 1" and returns;

Step 20 returns after ordering "calculated current = given current";

Step 9 judges whether the "actual current" is greater than the "maximum current". If greater, go to step 10; otherwise, go to step 11.

Step 10 sets "motor heating value = heating action value, overload flag = 2, target current = maximum current, counter = 0" and returns.

Step 11 judges whether the "motor heating value" is greater than or equal to the "heating action value". If greater than or equal to, execute step 12; otherwise, return.

Step 14 sets "overload flag = 1" and then returns.

Synchronous motor embodiment

The structure of the synchronous motor in this embodiment is the same as the synchronous motor in the prior art, the difference is that the overload protection method applies the method in the above embodiments.

Claims (3)

1. The AC synchronous motor overload protection method is characterized in that it judges whether the motor is overloaded, and if it is overloaded, the motor speed is limited by current loop control, If the overload starting value is less than zero, the overload protection current takes the given current; If the overload starting value is not less than zero, the overload protection current takes the smaller value of the verification current and the given current; Among them, the given current is the given current provided by the external environment; the verification current is generated according to the verification of the rated current, maximum current and current effective value; the overload starting amount is calculated in real time through the motor heating model; The verification current is generated by PID adjustment of the difference between the target current and the effective value of the current; wherein, the target current takes the maximum current when the q-axis current is greater than the maximum current and the duration is less than the set time. When it is greater than or equal to the set time, take the rated current. 2. the AC synchronous motor overload protection method according to claim 1, is characterized in that, gather any two-phase current and rotor angle, calculate, ia, ib are phase a and b currents, θ is the rotor angle of the motor, and iq is the q-axis current, that is, the effective value of the current. 3. The AC synchronous motor applying the overload protection method according to any one of claims 1-2, characterized in that the AC synchronous motor includes a controller, and the controller determines the overload protection current according to the overload protection method.