JPH02174594A - Motor control method - Google Patents

Abstract

PURPOSE:To conduct a stable change-over of the operation of a motor by issuing a connection command in advance before the number of revolutions of the motor corresponds to the maximum output frequency of a variable-frequency power supply. CONSTITUTION:When a contactor 16 is first turned OFF to stop a feed at the time of a change-over of the operation of a motor 17 by a commercial power supply 11 to that by a variable-frequency power supply 14, the number of revolutions of the motor 17 lowers gradually. The lowering of the number of revolutions is monitored, the time for the number of revolutions to become the number corresponding to the maximum output frequency of the variable- frequency power supply 14 is forecast, and the closing command of a switch 15 is issued from a change-over control circuit 19 at the time by the operating period of time of the switch 15 before the time. When the switch 15 is closed and the variable-frequency power supply 14 is connected with the motor 17, the number of revolutions of the motor 17 corresponds to the maximum output frequency of the variable-frequency power supply 14 so that the change-over is conducted smoothly.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a motor control method for operating an electric motor by switching the connection between a commercial power source and a variable frequency power source.

[Conventional technology]

FIG. 4 is a circuit configuration diagram to which a conventional motor control method disclosed in, for example, Japanese Patent Publication No. 56-36675 is applied, and FIG. 5 is an explanatory diagram showing the relationship between rotational speed and torque according to the motor control method.

In FIG. 4, 11 is a commercial power source, 12 and ]-3 are input circuit breakers, 14 is a variable frequency power source, 15 is a contactor that is turned on when operating the motor 17 with the output of the variable frequency power source, and 16 is a commercial power source. 11 is a contactor that is turned on when operating the electric motor 17, 18 is a rotation speed command generator, and 19 is a switching control circuit.

Also, in FIG. 5, τ. is the load torque, τ1,
are the generateable torque at 50% and 40% rotation speed, respectively, and Δτ and 4τ are the generateable torque and load torque τ at 50% and 40% rotation speed, respectively. shows the difference between

Next, the operation will be explained. In FIG. 4, when the input breaker 13 and the contactor 16 are closed, the motor 17 is connected to the commercial power source 1.
1, and when the input breaker 12 and contactor 15 are closed, the motor 17 is operated with the variable frequency power supply 14.

The selection of closing the contactors 15 and 16 is determined via the switching control circuit 19 depending on whether the command signal from the rotation speed command generator 18 is below a certain level or above a certain level, and when it is below a certain level, the contactor 15 is closed;
When it is above a certain level, the contactor 16 is closed.

Note that when the contactor 15 is closed, a command signal from the rotation speed command generator 18 is given to the variable frequency power supply 14, and an output voltage of a predetermined frequency is given to the electric motor 17.

With this configuration, when the load on the electric motor 17 is a fan or pump, the load torque is proportional to the square of the rotation speed n,
Since the required electric power KW is proportional to the cube of the rotation speed n, the capacity of the variable frequency power supply 14 can be significantly saved compared to the capacity KW of the electric motor 17, and it is also possible to obtain a large energy saving effect. be.

For example, if the variable frequency power supply 14 is operated up to 50% of the rotation speed n, its capacity will be 100% the capacity of the electric motor 17.
%, 0.53=12.5% is sufficient.

FIG. 5 is a diagram illustrating the relationship between generated torque and load torque when the variable frequency power supply 14 is operated up to 50% rotation speed. The variable frequency power supply 14 is used up to 50% speed, so for overcurrent protection, when using a current type power supply, the current is limited at its limit value (described below as 125% of the 50% speed base). However, when using a voltage type power supply, it is necessary to activate the overcurrent protection at 125% of the limit value, and in any case, the maximum output current of the variable frequency power supply 14 is 125% of the limit value (actually due to the time-limited characteristic). (which will be ignored for the sake of brevity).

According to FIG. 5, since the maximum output current is about 125%, the difference between the generated torque τ1 and the load torque at 50% speed, τ, is only about 10 to 15%. Due to output current ripples, output voltage fluctuations, etc., the generated torque τ1 is the load torque. When the contactor 15 is turned on after detecting the 50% speed point, the rotational speed of the motor 17 decreases during the operating time of the contactor 16, and the rotational speed difference recovers. In some cases, a current greater than the maximum output current may be required to achieve this.

Therefore, if the variable frequency power source 14 is connected to the motor 17 at a value lower than the maximum rotation speed of 50% of the variable frequency power source 14 (hereinafter explained as 40%), the maximum output current of the variable frequency power source 14 will be 50% of the speed. The same as when the load torque τ. is small, so the generable torque τ2 and the load torque τ. The difference 4τ between the two is sufficiently larger than the difference 4τ, and the switching operation to the variable frequency power supply 14 is performed stably.

[Problem to be solved by the invention]

Since the conventional motor control method is performed as described above, it is necessary to operate on commercial power until the load is reduced to 40% of the rotation speed, and energy saving is achieved in the rotation speed range of 40% to 50%. There was a problem in that the effectiveness had to be sacrificed.

This invention was made to solve the above-mentioned problems, and it is possible to stably switch from a commercial power source to a variable frequency power source, and it is also possible to stably switch from a commercial power source to a variable frequency power source. The purpose of this invention is to obtain a motor control method that can switch to a variable frequency power source at a speed close to % rotation speed).

[Means to solve the problem]

The motor control method according to the invention of claim 1 is such that a connection command is issued in advance so as to connect the variable frequency power source when the rotational speed of the motor reaches the maximum rotational speed of the variable frequency power source. .

In the motor control method according to the invention of claim 2, when a variable frequency power source is connected to the electric motor instead of the commercial power source, a command signal from a rotation speed command generator to the variable frequency power source and a rotation speed signal from the electric motor are connected to the variable frequency power source. While the difference between the two is greater than a predetermined value, a rotation speed command value equal to the motor rotation speed plus a predetermined value is given to the variable frequency power supply, and the timing to turn on the variable frequency power supply is set near the upper limit frequency of the variable frequency power supply. This is the selected one.

[Effect]

The electric motor control method in the invention of claim 1 is such that the variable frequency power source is connected to the electric motor when the rotational speed of the electric motor reaches the maximum rotational speed of the variable frequency power source according to the preceding command. , 1 ~ rk (3rd lτ
) is minimized and stably switched to a variable frequency power supply.

In the motor control method according to the invention of claim 2, the electric motor that is switched from the commercial power source to the variable frequency power source is connected to the variable frequency power source near the upper limit frequency of the variable frequency power source, thereby eliminating operational limitations on energy saving. Also,
While the difference between the command signal from the rotation speed command generator and the rotation speed signal from the motor is greater than or equal to a predetermined value, a rotation speed command value equal to the motor rotation speed plus a predetermined value is given to the variable frequency power supply to control the rotation speed as required. The acceleration torque is minimized, and therefore the transient overcurrent is minimized, and the variable frequency power source is stably connected to the motor.

〔Example〕

Hereinafter, one embodiment of the present invention will be described with reference to the drawings. 1st
The figure is a diagram for explaining the method of the invention according to claim 1, and the circuit configuration to which the method of the invention is applied is the same as that shown in FIG. 4 above.

In the figure, the horizontal axis is the time axis, and the vertical axis is the current of the motor 17.
, is the actual rotational speed N of the electric motor 17.

In FIG. 1, T1 is the point at which the contactor 16 is opened, Ic and NC are the current value and rotational speed of the motor 17 operated by the commercial power supply 11, T3 is the point at which the contactor 15 is closed, and Nv is the point at which the variable frequency power supply 14 is operated. The maximum rotation speed, IV is the steady current value when operating at rotation speed NV, and T4 is the rotation speed N and current ■ after time T3.
The time point when becomes steady state, /JN and aI are respectively time points T
The transient change in rotation speed and current value between T3 and T4, T,
is the time point preceding time point T3, and N2 is the rotational speed of the electric motor 17 at time point T2.

Next, the operation will be explained. At time T, the electric motor 17 was previously driven by the commercial power source 11 and rotated at the rated rotation speed Nc, and the motor rotation speed was at a steady value. It is.

If the contactor 16 opens at time T, then the motor 1
7 becomes no voltage, and the motor 1 is turned off like the curve of rotation speed N.
7 is a natural deceleration and the electrical current becomes 0.

At the rotation speed N2 preceding the time T3 when the rotation speed N decelerates to the maximum rotation speed NV of the variable frequency power supply 14, that is, at the time T2, a signal is issued to close the contactor 15.
Contactor 15 actually closes at time T, after an operating time T of 5 (time T3T 2 ).

The current of the motor 17 starts to increase from time T3, but it gradually increases due to the reactor of the motor 17, so at the beginning of the increase, the torque generated by this current is the load torque τ. Although the deceleration speed of the electric motor 17 becomes smaller, the deceleration still continues for a while. 17 starts accelerating after the torque generated by the increasing motor current {circle around (2)} becomes larger than the load torque, and at time T, the speed returns to the maximum rotational speed NV of the variable frequency power source and enters a steady state.

If the drop in the rotational speed of the motor 17 (4N) is large between time T3 and T4, this is recovered (Δ
The current change 4■ due to N→O) becomes large and does not fall within the limit value of the maximum output current of the variable frequency power supply 14, but as shown in FIG. Since the contactor 15 was closed at time T3 when the rotation speed of the variable frequency power supply 14 reached the maximum, the change amount 4N was minimized, and therefore the motor current including the current change Δ
can be kept within the allowable maximum output current of the variable frequency power supply 14.

For example, the following method is used to issue the preceding command. The rate of change (dNV/dt) of the motor 17 at time T3 in FIG. 1 can be estimated at the design stage or through actual measurements, so multiply this value by the operating time of the contactor 15 to determine the preceding time (T, -T2). Calculate. In other words, the time point T when the advance command is issued is the maximum output rotation speed N of the variable frequency power supply.
There is an idea that the rotational speed is the point in time when a predetermined rotational speed is added to v.

Alternatively, the following method may be used.

Since it is expected that the rotational speed rate (d Nv/d t ) of the electric motor 17 will change slightly depending on the load, (d N/d t)
XTc+N=Nv Here, Tc is the point in time when the motor rotation speed N that satisfies the operating time of the contactor 15 is the point in time T2. That is, T2 is the point in time at which the rotation speed N satisfies the condition that the sum of the rotation speed N of the motor and the value obtained by multiplying the rate of change (dN/dt) by a predetermined time is equal to the maximum output rotation speed NV of the variable frequency power supply. shall be.

The second is a circuit configuration diagram to which the method of the invention of claim 2 is applied, and the same parts as in FIG. In FIG. 2, 20 is a comparator that compares the command signal from the rotation speed command generator 18 and the rotation speed signal from the electric motor 1-7.

Next, the operation will be explained. When the difference between the input command signal from the rotation speed command generator 18 and the rotation speed signal from the electric motor 17 is less than a predetermined value, the comparator 20 transmits the command signal from the rotation speed command generator 18 to the switching control circuit 19. When the difference between both signals is greater than a predetermined value, the rotation speed signal 21 is output.
A value obtained by adding a predetermined value to the switching control circuit 19 is output to the switching control circuit 19.

The operation of other parts is the same as that shown in FIG.
The value of the rotation speed command generator 18 at the time when it is connected to the fourth
In the figure, it is 40%, but in FIG. 1 it is close to 50% (the upper limit of the variable frequency power supply 14).

Under the above circumstances, the inventive method of claim 2 will be explained with reference to FIG. 3, in which the same or corresponding parts as in FIG. 5 are given the same reference numerals.

In FIG. 3, 20a is the output of the comparator 20, and between time TS and time T6, the difference between the command signal from the rotation speed command generator 18 and the rotation speed signal from the electric motor 17 is greater than a predetermined value. It is shown that.

The electric motor 17 was operated by the commercial power supply 11 before time T, the electric motor 17 rotates at the rated rotational speed Nc, and the motor current ■ is a steady value ■. It is.

If the contactor 16 opens at time T□, then the motor 1
7 becomes no voltage, and the motor 1 is turned off like the curve of rotation speed N.
7, the speed naturally decelerates, and the current I becomes O.

At the rotation speed N2 preceding the time T3 when the rotation speed N decelerates to the maximum rotation speed Nv of the variable frequency power supply 14, that is, at the time T2, a signal is issued to close the contactor 15, and the contactor 1
The contactor 15 actually closes at time T3 after an operating time T of 5 (time T3T, ).

The current of the electric motor ]-7 starts to increase from time T:l, but it gradually increases due to the reactor of the electric motor 17, so at the beginning of the increase, the torque generated by this current is the load torque. Although the deceleration speed of the electric motor 17 is smaller, the deceleration will continue for a while.

The electric motor 17 starts accelerating after the torque generated by the increasing motor current {circle around (2)} becomes larger than the load torque, and at time T4, it recovers to the maximum rotational speed Nv of the variable frequency power source and enters a steady state.

Between time points T5 and T6, the rotation speed N of the electric motor 17 drops significantly, and the difference between the rotation speed signal 21 obtained by measuring this value and the command signal from the rotation speed command generator 18 becomes large, and the comparator 20 During this period, the value becomes the rotation speed signal 21 plus a predetermined value.

The command signal from the rotation speed command generator 18 is transmitted at time TS-T.
, is corrected to a small value as described above, so that the current that generates the acceleration torque to compensate for the frequency difference is
This is smaller than when there is no current, and the current change lI from the rated value can be made smaller.

Furthermore, if the drop change IN in the rotational speed of the motor 17 is large between the above time T3 and T4, the current change B to recover (dN-)O) will be large, making it variable. Although the maximum output current of the frequency power supply 14 does not fall within the limit value, as shown in FIG. Since it is closed, the change is 4N
Therefore, the motor current (2) including the current variation a can be kept within the allowable maximum output current of the variable frequency power supply 14 as a total effect in combination with the action of the comparator 20.

Note that each of the above control methods is particularly effective in the case of a voltage type variable frequency power supply in which the ripple of the output current is much smaller than that of a current type power supply.

A voltage-type variable frequency power supply is an output voltage control type, which means that there is essentially no fluctuation in the output voltage and the ripple in the output current is small. This fact, combined with the effect of the comparator 20 and the ability to eliminate the reduction in the rotational speed of the motor 17 during the operating time of the motor 15, reduces the load torque τ even if the generated torque of the motor is small when switching to the variable frequency power supply 14. .

If it exceeds , it can be stably drawn into a variable frequency power supply.

The method of eliminating the influence of the operating time of the contactor 15 by the preceding command and the effect of the comparator 20 are also preferable in the case of a current type variable frequency power supply, and the effects can be expected.

Further, in the above-mentioned embodiment of the invention of claim 2, the advance turning-on command of the variable frequency power source 14 is issued by aiming at the point in time when the rotation speed N of the electric motor 17 becomes equal to the upper limit value of the variable frequency power source 14. As explained above, the effect of the comparator 20 is that the variable frequency power supply can be turned on at any time, including when the variable frequency power supply is turned on when the rotation speed N of the electric motor 17 matches the upper limit frequency of the variable frequency power supply 14 or a value in the vicinity thereof. It works effectively.

〔Effect of the invention〕

As described above, according to the invention of claim 1, when a motor driven by a commercial power source is switched to a variable frequency power source and driven, the rotational speed of the motor is a rotational speed corresponding to the maximum output frequency of the variable frequency power source. The configuration is configured so that a variable frequency power supply connection command is issued in advance before reaching the maximum output frequency, and the motor is connected at a rotation speed close to the maximum output frequency, so that the transient current when the variable frequency power supply is connected can be reduced. and switching is performed stably.

Further, since switching can be performed near the maximum capacity of the variable frequency power supply, there is no operational restriction on switching, and energy saving effects can be effectively obtained.

According to the invention of claim 2, since the output frequency of the variable frequency power supply is controlled by the comparator to a value that is equal to or lower than the rotational speed of the motor and a predetermined value, there is an effect of suppressing the transient current (ii) of the electric motor. This has the effect of stably switching the motor from a commercial power source to a variable frequency power source within the allowable current range of the variable frequency power source.

Furthermore, since the variable frequency power source is connected near its upper limit frequency, the energy saving effect can be effectively exhibited.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram for explaining the inventive method of claim 1, FIG. 2 is a circuit configuration diagram to which the inventive method of claim 2 is applied, and FIG. 3 is an explanatory diagram for explaining the inventive method of claim 2. FIG. 4 is a circuit configuration diagram to which the conventional method is applied, and FIG. 5 is an explanatory diagram showing the relationship between rotation speed and torque according to the conventional method. 11 is a commercial power supply, 14 is a variable frequency power supply, 17 is an electric motor, 18 is a rotation speed command generator, 19 is a switching control circuit, 20
is a comparator. In addition, in the figures, the same reference numerals indicate the same or corresponding parts. Patent Applicant: Mitsubishi Electric Co., Ltd. 〒L Bunshin Pe Procedure Amendment (Voluntary) 4 +”'J”flos

Claims (2)

[Claims] (1) In a motor control method in which the motor is driven by a variable frequency power supply in a range below a predetermined rotational speed, and driven by a commercial power supply above it, after the commercial power supply is disconnected, the rotational speed of the motor is changed to the variable frequency power supply. A command to connect the variable frequency power source to the electric motor prior to reaching this connection point so that the variable frequency power source is connected to the motor near the point when the rotation speed corresponds to the maximum output frequency of the frequency power source is reached. A method for controlling an electric motor, characterized by: (2) In a motor control method in which an electric motor is driven by a variable frequency power source in a range below a predetermined rotational speed, and is driven by a commercial power source above a predetermined rotation speed, the motor being driven by the commercial power source is driven by the variable frequency power source. When switching to the power source, if the difference between the command signal from the rotation speed command generator and the rotation speed signal from the electric motor is greater than or equal to a predetermined value, the rotation speed command is obtained by adding a predetermined value to the rotation speed signal from the electric motor. A motor control method, characterized in that a value is given to the variable frequency power supply.