BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a control block in a motor control device having a protection function according to an embodiment of the present invention;
FIG. 2 is a block diagram showing details of a temperature estimation part in the motor control device in the embodiment of the present invention;
FIG. 3 is a diagram showing an example of protection curves used by the temperature estimation part in the motor control device in the embodiment of the present invention;
FIG. 4 is a diagram showing an example of protection curves of a conventional motor; and
FIG. 5 is a block diagram showing the control block in a motor control device having the protection function by a conventional electronic thermal system.
DETAIL DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION
An embodiment of the present invention applied to a motor control device will be described with reference to FIG. 1 and FIG. 2. FIG. 1 is a block diagram showing a control block in a motor control device having the protection function according to the present invention. FIG. 2 is a block diagram showing details of a temperature estimation component.
First, operations of the motor control device having the protection function will be described with reference to FIG. 1. A speed controller 1 receives a speed feedback V calculated by a converter 6 from a position detected value of an encoder 5, which is a position detecting means for detecting the position of a motor needle, and a speed command V* as input to perform operations such as PI control and calculates a current command I*. In FIG. 1, output of the speed controller 1 is made into the current command I*, but a similar function can also be realized by using a torque command, which is proportional to the current command I*. The present invention will be described below by using the current command I*. A current controller 2 receives a current feedback I and the current command I* as input to calculate an inverter drive command. Based on the inverter drive command from the current controller 2, an inverter circuit 3 converts a drive current of a motor 4. Based on the position detected value of the encoder 5 converted by the converter 6, an operational state determination component 8 determines an operational state of the motor 4. The operational state determination component 8 determines that the motor 4 is in a locked state when the position detected value of the encoder 5 converted by the converter 6 does not change and that the motor 4 is rotating when the position detected value changes. The operational state determination component 8 outputs an operational state determination signal of the motor 4 to a temperature estimation component 7. The temperature estimation component 7 has a temperature estimation means for calculating an estimated value of temperature based on the current command I* and a threshold change means for changing the threshold based on the operational state determination signal of the motor 4 output from the operational state determination part 8. The threshold change means sets the threshold as a first threshold when it is determined that the motor 4 is rotating and sets the threshold as a second threshold, which is smaller than the first threshold, when it is determined that the motor 4 is in a locked state. Here, if the estimated temperature value reaches the threshold, the temperature estimation component 7 outputs to the current controller 2 a command to cut off passage of current to the motor 4 to stop the operation of the motor 4. In such a case, the temperature estimation component 7 outputs a command to cut off flow of current to the motor 4 to the current controller 2, but the temperature estimation component 7 can also output a command to cut off flow of current to the motor 4 to the speed controller 1 that calculates the current command I*.
Next, processing in the temperature estimation component 7 will be described in detail with reference to FIG. 2. The temperature estimation component 7 includes a calculation component 11, a determination component 12, and a data component 13. The calculation component 11 has the temperature estimation means for calculating an estimated value of temperature from the current command I*. The data component 13 has the threshold change means for changing the threshold based on the operational state determination signal output from the operational state determination part 8. The determination part 12 compares the estimated value of temperature and the threshold to output a command signal of go/no-go of operation (operation continuation or operation stop) to the current controller 2.
Here, the data component 13 will be described in detail below. The data for the protection curves shown in FIG. 3 is stored in the data component 13. Here, (a) shows a protection curve of a motor rotating state, that is, when current phases rotate and (b) shows a protection curve of a motor locked state, that is, when current phases are locked. Symbols (M3, A3, B3, T3A, and T3B) in FIG. 3 are values determined by the motor. For example, M3 is a motor current limiting value, A3 is a motor continuous rating current value, and B3 is 1/√2 of the motor continuous rating current value. When current phases are locked, a current flows as a direct current and the calorific value of a motor is proportional to the square of current and therefore, the current producing the same calorific value as that of current when current phases rotate becomes 1/√2. Thus, from the viewpoint of motor protection, the value of current that can continuously be passed when current phases are locked becomes 1/√2 of the motor continuous rating current value. Furthermore, examples of settings of T3A and T3B include T3A set prior to the temperature rising by 100° C. when the motor current limiting value is continuously passed for under the condition that the current phases are rotating, and T3B set as the time until the temperature rises by 100° C. when the motor current limiting value is continuously passed for under the condition that motor is locked.
Further, how to calculate an estimated value of temperature in the calculation part 11 will be described in detail below. By considering an equivalent circuit for an object to be protected, the electronic thermal calculation method is determined. Various methods can be considered for the calculation like the protection curve and an example will be shown below.
If the temperature rise at any given time is T[n], the last temperature rise is T[n−1], the current is i[n], and A and β are constants of proportionality, the following relation holds:
T[n]=β{A·i[n]−T[n−1]}+T[n−1] Formula (1)
where β in Formula 1 is in the range 0<β<1.
From Formula 1,
T[n]−A·i[n]=(1−β){T[n−1]−A·i[n]} holds, giving:
T[n]−A·i[n]=(1−β)n{T[0]−A·i[n]}
Here, since it is considered, based on Formula 1, that T[0]=0, the temperature rise T[n] when a constant current i continuously flows is given by:
T[n]=A{1−(1−β)n}·i Formula (2)
If, in Formula 2, n→∞, since 0<(1−β)<1
T[n]n→∞=A·i
If the current i at this point is a continuous current ic, and ic and i are represented as a ratio to a current limiting value IL, a conditional expression for a protection area by the electronic thermal system is given as:
A·ic<A{1−(1−β)n}·i=T[n] Formula (3)
Dividing both sides by A·IL gives
ic/IL <{1−(1−β)n}·i/IL=T[n]/(A·IL) Formula (4)
Substituting θ[n]=T[n]/(A·IL) and γ=ic/IL into Formula 4 gives
γ=ic/IL<A{1−(1−β)ni/IL=θ[n] Formula (5)
Formula (1) gives
θ[n]=β{i[n]/IL−θ[n−1]}+θ[n−1] Formula (6)
θ[n] in Formula 6 represents an estimated value of temperature and, from Formula 5 and Formula 6, constants to determine a protection curve based on the estimated value of temperature are γ and β. Here, γ is a constant representing a continuous current when the current limiting value is set to 1, and β means the time constant of a protection curve. If, for example, the object to be protected is a motor, the calorific value of the motor is proportional to the square of the current, and, thus, an estimated value of temperature will be calculated from the square of the current. That is, the estimated value of temperature is calculated by changing i[n]/IL in Formula (6) to (i[n]/IL)2. In other words, the estimated value of temperature must be calculated by considering what the calorific value of an object to be protected is proportional to.
In the detailed description above of the data part 13, changing the protection curve in accordance with the operational state has been described. Also, the estimated value of temperature can be changed when the operational state changes (current phases rotating ⇄ current phases locked). An example of change content will be described below.
Because the estimated value of temperature of a motor is calculated using the square of current, it is possible to consider that the estimated value of temperature is approximately equal to the squared current (≅current2). If the value of current that can continuously flow when current phases are rotating is Im, the current value that can continuously flow when current phases are locked become Im/√2. Here, if the estimated value of temperature when current phases are locked and that when current phases are rotating are TR and TL respectively, the following relational expression holds true:
T
L≅(Im/√2)2=Im2/2≅TR/2 Formula (7)
From the relational expression of Formula (7), it is clear that the estimated value of temperature is halved when the operational state changes from rotating current phases to locked current phases, and the estimated value of temperature is doubled when the operational state changes from locked current phases to rotating current phases.
Patent Application
20080054833
