Motor control unit provided with anti-burning device

A motor control unit having an anti-burning device is provided which includes a rotation detection device for detecting whether the rotational speed of a given motor is below a predetermined level, a power supply limiting device for limiting the time of supplying current to the motor according to its rotational speed when the speed has been detected by the rotation speed detection device to be below the predetermined level. Thereby, the motor is prevented from burning, deteriorating in performance or shortening in life when the motor is overloaded. Also, the rotation detection device detects the condition when the motor is not rotating, and a cyclic power supply device for supplying current at a given cycle when the motor has been found to be not rotating by the aforementioned rotation detection device. Thereby, if the motor has been locked under an overload condition the motor can be readily restarted automatically when the load has been relieved.

FIELD OF THE INVENTION 
The present invention relates to a motor control unit in particular, to a 
motor control unit provided with an anti-burning device to ensure against 
burning of the motor coil or a similar element due to an overcurrent 
resulting from overloading of the motor. 
BACKGROUND OF THE INVENTION 
In conventional motor control units, for instance, as disclosed in Japanese 
Laid-Open Patent Application No. 280751/'86, a unit which generates a 
rotating magnetic field through repetitive energizations of the exciting 
coil and interruptions thereof by timing a transistor ON/OFF is known for 
starting or otherwise controlling a motor. 
When a motor is overloaded, its speed will be lowered until it eventually 
locks. Then, the exciting coil of the motor is subjected to a current 
approximately three times greater than the current supplied when the motor 
is running under a normal load. This results in undue heating of the motor 
which causes the motor to burn, deteriorate, shorten in life and other 
similar malfunctions. 
Therefore, a motor control unit provided with an anti-burning device has 
been used which, for instance, detects the temperatures of semiconductors 
constituting a control circuit. When the temperature of such 
semiconductors have risen above a preset temperature, the supply of 
current to the motor is stopped. 
Furthermore, various motors used as power source have different resistance 
values of the exciting coil according to the rated voltage and the torque 
generated. Hence, the amount of current flowing through the exciting coil 
is different. Even with motors having the same amount of current flowing 
under an overloaded condition varies according to differences in the 
resistances of the exciting coils, environmental conditions, etc. 
However, a motor control unit provided with a conventional anti-burning 
device, as mentioned above, is arranged so that the current supply to the 
motor is stopped upon detecting a rise in temperature of the semiconductor 
elements constituting the control circuit. Hence, with a motor having a 
relatively high amount of working current, it is possible for the 
anti-burning device to stop the supply of current to the motor even if the 
load on the motor is not extremely large. 
Meanwhile, in a motor having a relatively low amount of current flowing 
even when it is overloaded, it is possible that the temperature of the 
semiconductor elements constituting the control circuit may not reach the 
preset temperature even if the motor is overloaded. In such a case, the 
anti-burning device may fail to act or act too late which may result in a 
failure to ensure the motor against burning, deteriorating, shortening in 
working life, etc. 
To cope with this problem it is possible to modify the detection 
sensitivity and detect the rise in temperature of the control circuit 
according to the type of the motor to be controlled, but this is bound to 
result in the loss of the general purpose features of such a motor control 
unit. 
In another function required for this kind of motor control unit, after the 
current supply to the motor is stopped by actuation of the anti-burning 
device, it is necessary to resume the current supply when the load on the 
motor has been relieved. In such a case, as mentioned above, it is 
possible that an anti-burning device for stopping the current supply to 
the motor through the detection of a rise in temperature of the 
semiconductors constituting the control circuit. It is conceivable to 
resume the current supply to the motor after stopping the current supply 
to the motor, the temperatures of the semiconductor drop below the 
predetermined temperature. 
However, even if the load is relieved immediately after stopping the power 
supply to the motor, a relatively long amount of time is required before 
the temperature of the control circuit is lowered and the current supply 
is resumed. Also, the amount of time until the power supply resumes will 
vary depending on the environmental conditions such as the atmospheric 
temperature. Even when power supply is resumed, if the overload condition 
remains, it means that the overcurrent will continue to blow until the 
temperature of the control circuit increases and actuates the anti-burning 
device, which may cause a deterioration in performance and a shortening in 
life of the motor. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide a motor control 
unit having an anti-burning device which prevents the motor from heating 
by an overcurrent condition when the motor is overloaded, so that burning, 
deteriorating, shortening in life and similar malfunction of the motor do 
not occur. A particular object of the invention is to provide a motor 
control unit having an anti-burning device capable of preventing the motor 
from burning through a secure detection of the overloaded condition even 
when the motors controlled thereby have different exciting coil 
resistances or different amounts of current flowing through the exciting 
coil when the motors are overloaded. 
Another object of the present invention is to provide a motor control unit 
having an anti-burning device which is capable of automatically and 
readily restarting after the load has been relieved even when the motor 
controlled has been locked as a result of the overloading. 
Still another object of the present invention is to provide a compact and 
highly reliable motor control unit of a simplified composition having a 
few elements and an anti-burning device where the important elements are 
integrated on a semiconductor chip. 
In order to accomplish the aforementioned objects, the motor control unit 
of the present invention includes a rotation detection device for 
detecting whether the rotational speed of a given motor is below a 
predetermined level and when the motion is not rotation a power supply 
limiting device for limiting the time in which current is supplied to the 
motor according to its rotational speed when the rotation detection device 
detects that the speed of the motor is below the predetermined level, and 
a cyclic power supply device for supplying current at a given cycle when 
the motor has been detected as not rotating by the aforementioned rotation 
stop detection device. 
The control unit having the aforementioned anti-burning device for the 
motor includes a power supply control circuit combining the functions of 
the aforementioned rotation detection device, the power supply limiting 
device, and the cyclic power supply device. This power supply control 
circuit may, for instance, include a capacitor, a current supply device 
which supplies current to charge the capacitor, a rotational speed 
detection device for outputting negative logical pulse signals at a timing 
which corresponds to the rotation cycle of the magnet rotor for the motor, 
a discharging circuit which discharges the electric charge accumulated in 
the capacitor when the difference between the output potential difference 
of the rotational speed detection device and the potential difference 
across the capacitor becomes greater than the predetermined value, and a 
comparator which causes the power supply for the exciting coil of the 
motor to be stopped upon the detection of the potential difference between 
both ends of the capacitor becomes greater than the predetermined value. 
The rotational speed detection device having the aforementioned power 
supply control circuit may, for instance, include a rotational position 
detection device which outputs high level and/or low level signals 
according to the rotational position of the magnet rotor for the motor, 
and a pulse generation device which outputs negative logical pulse signals 
when the output level of the rotational position detecting device has been 
varied. 
The rotational position detection device having the aforementioned 
rotational speed detection device may as also include a hall element so 
that the rotational speed of the motor can be detected through detection 
of the magnetic force of the magnet rotor for the motor by the 
aforementioned hall element. 
Also, the rotational position detection device having the aforementioned 
rotational speed detection device may also include a detection coil so 
that the detection of the rotational speed of the motor can be detected 
through detection of the magnetic force of the magnetic rotor for the 
motor by the aforementioned detection coil. 
The rotational position detection device having the aforementioned 
rotational speed detection device may also include a voltage detection 
device so that the rotational speed of the motor can be detected through 
detection of the kick-back voltage of the stator exciting coil for the 
motor by the aforementioned voltage detection device. 
Meanwhile, a pulse generating device having the aforementioned rotational 
speed detection device may also include a NOT circuit which inverts the 
level of the output signal from the rotational position detection device 
and delays the output signal at the same time by a predetermined length of 
time and an OR circuit which determines the logical sum of the output 
signal from the NOT circuit and the output signal of the aforementioned 
rotational position detecting device. 
Furthermore, the current supply device having the aforementioned power 
supply control circuit may also include a constant current circuit or may 
be arranged so that the maximum current is limited by resistors connected 
in series to the power source.

DESCRIPTION OF THE EMBODIMENTS 
An example of a control unit having an anti-burning device for controlling 
a 2-phase half-wave motor is described below for an embodiment of the 
present invention with reference being made to FIGS. 1-4. 
A 2-phase half-wave motor 11, as shown in FIG. 1, includes a freely 
rotatable magnet rotor 12 with a plurality of magnetic poles formed along 
its outer periphery, and stator exciting coils 13 and 14 arranged on both 
sides of the magnet rotor 12. 
A hall element 21 for detecting the rotational position of the magnet rotor 
12 by the aid of the magnetic force of the aforementioned magnet rotor 12 
has two input terminals one input terminal is connected to a power source 
+V through a constant voltage circuit 22 and the other input terminal is 
grounded. The one output terminal of the hall element 21 is connected to 
the input terminal of the positive side of an operational amplifier 23, 
and the other input terminal is connected to the input terminal on the 
minus side thereof. 
The output terminal of the operational amplifier 23 is connected directly 
to a NOR circuit 25 and to another NOR circuit 26 through a NOT circuit 
24. The aforementioned operational amplifier 23 and the NOT circuit 24 are 
also connected to an OR circuit 27. 
The output terminal of the OR circuit 27 is grounded through a resistor 
R.sub.1 and another resistor R.sub.2 so that the voltage divided by the 
resistors R.sub.1 and R.sub.2 is applied to the base of the transistor 
Tr.sub.1. The aforementioned hall element 21, the operational amplifier 
23, the NOT circuit 24, the OR circuit 27 and the resistors R.sub.1, and 
R.sub.2 constitute a rotation detection device which outputs negative 
logical pulses for a timing which corresponds to the rotation period of 
the magnet rotor 12 of the 2-phase half-wave motor 11. 
The hall element 21 acts as a rotational position detecting device for 
outputting high level and low level signals according to the rotational 
position of the magnet rotor 12 of the 2-phase half-wave motor 11. Also, a 
pulse generating device is formed with the operational amplifier 23, the 
NOT circuit 24, the OR circuit 27 and the resistors R.sub.1, and R.sub.2 
for putting negative logical pulse signals when the output level of the 
rotational position detection device varies. 
The base of the transistor Tr.sub.1, which is connected to both resistors 
R.sub.1 and R.sub.2, is grounded through the collector and the emitter of 
another transistor Tr.sub.2, while the collector of the transistor 
Tr.sub.1 is grounded through the collector and the emitter of still 
another transistor Tr.sub.3. The base of the transistor Tr.sub.2 is 
connected with the base of the transistor Tr.sub.3, which are also 
connected with the collector of the transistor Tr.sub.1. 
The emitter of the aforementioned transistor Tr.sub.1 has one end of the 
capacitor C.sub.1 connected thereto and the other end grounded. A 
discharge circuit is formed by the aforementioned transistors Tr.sub.1 
-Tr.sub.3, and the electric charge accumulated in the capacitor C.sub.1 is 
discharged when the potential of the ungrounded end of the capacitor 
C.sub.1 exceeds the potential at the connection of the resistors R.sub.1 
and R.sub.2 by the predetermined level. 
The ungrounded end of the capacitor C.sub.1 is also connected to a constant 
voltage circuit 22 through a constant current circuit 28 which serves as a 
current supply device so that supplying current to the capacitor C.sub.1 
for an electric charge may be built up. Furthermore, the ungrounded end of 
the capacitor C.sub.1 is connected to the input terminal on the positive 
side of a comparator 29 which stops the supply of power to the stator 
exciting coils 13 and 14 of the 2-phase half-wave motor 11 after a rise in 
the potential of the ungrounded end is detected to be above the 
predetermined level. On the negative terminal of the comparator 29, the 
voltage resulting from a division by resistors R.sub.3 and R.sub.4 from 
the output voltage of a constant voltage circuit 22 is applied as a 
reference voltage V.sub.S. 
The output terminal of the comparator 29 is connected to NOR circuits 25 
and 26 as well as the aforementioned operational amplifier 23 and NOR 
circuit 24. The output terminals of the NOR terminals 25 and 26 are 
connected to the base of transistors Tr.sub.4 and Tr.sub.5. The emitters 
of the transistors Tr.sub.4 and Tr.sub.5 are both grounded, while their 
collectors are connected to one end of the stator exciting coils 13 and 
14. The other ends of the stator exciting coils 13 and 14 are connected to 
the power source +V. 
The aforementioned hall element 21, operational amplifier 23, the NOT 
circuit 24, the OR circuit 27, the constant current circuit 28, the 
comparator 29, the resistors R.sub.1 -R.sub.4, the transistors Tr.sub.1 
-Tr.sub.3 and the capacitor C.sub.1 comprise a power supply control 
circuit 30 for the 2-phase half-wave motor 11. This power supply control 
circuit 30 outputs power supply control signals for generating rotating 
magnetic fields in the stator exciting coils 13 and 14 which also acts as 
the rotation detecting device for detecting the rotational speed and the 
stoppage of the 2-phase half-wave motor 11. A power supply limiting device 
limits the duration of the power supply to the 2-phase half-wave motor 11 
according to the rotational speed when the rotational speed of the motor 
has been detected to be less than the predetermined limit, and a cyclic 
power supply device supplies power at a given cycle when a stoppage of the 
rotation for the 2-phase half-wave motor 11 by the rotation detection 
device has been detected, so that the devices act as an anti-burning 
device. 
In the composition described above, the hall element 21 is subjected to an 
alternate magnetic field, when the magnet rotor 12 of the 2-phase 
half-wave motor 11 is rotating. Hence, as shown in FIG. 2(a), the 
operational amplifier 23 outputs signals for alternatively and 
periodically repeats high level and low level signals. Meanwhile, the NOT 
circuit 24, as shown in FIG. 2(b), generates the output signal from the 
operational amplifier 23 in an inverted form to be outputted and delayed 
by the delay time t.sub.D. 
As shown in FIG. 2(c), the negative logical pulses 41 are synchronized with 
the timing for the switching of the output signal from the operational 
amplifier 23 from the high level to the low level. The levels of the 
negative logical pulses 41 become lower by a value equivalent to the delay 
time t.sub.D of the aforementioned NOT circuit 24 outputted from the OR 
circuit 27. 
With the voltage of the resistors R.sub.1 and R.sub.2 when the logical 
pulses 41 are not outputted from the OR circuit 27 as V.sub.H, the 
diffusion potential between the emitter and the base of the transistor 
Tr.sub.1 as V.sub.BE1 and the voltage across the capacitor C.sub.1 as 
V.sub.C, current does not flow from the emitter to the base of the 
transistor Tr.sub.1 when: 
EQU V.sub.H +V.sub.BE1 &gt;V.sub.C, 
hence the transistor Tr.sub.1 is turned OFF along with the transistors 
Tr.sub.2 and Tr.sub.3. Thereby, the capacitor C.sub.1 is charged by the 
current from the constant current circuit 28 and, as shown in FIG. 2(d), 
the voltage V.sub.C across the capacitor C.sub.1 rises gradually. 
Meanwhile, when the negative pulses 41 from the OR circuit 27 are outputted 
and 
EQU V.sub.L +V.sub.BE1 &lt;V.sub.C 
(where V.sub.L is the voltage of the resistors R.sub.1 and R.sub.2), the 
transistor Tr.sub.1 turns ON. Since the current, which flows from the 
emitter to the collector of the transistor Tr.sub.1, also flows from the 
base to the emitter of the transistors Tr.sub.2 and Tr.sub.3, the 
transistors Tr.sub.2 and Tr.sub.3 are both turned ON. In this case, even 
if the output signal from the OR circuit 27 should be restored to the high 
level, current continues flow from the emitter to the base of the 
transistor Tr.sub.1, and from the collector through the emitter of the 
transistor Tr.sub.2, so that transistors Tr.sub.1 -Tr.sub.3 are all turned 
ON. Thereby, the electric charge accumulated in the capacitor C.sub.1 is 
discharged through the emitter to the collector of the transistor Tr.sub.1 
and the collector to the emitter of the transistor Tr.sub.3. 
As the voltage across the capacitor C.sub.1 becomes lower as a result of 
the aforementioned discharge, the transistors Tr.sub.1 -Tr.sub.3 are both 
turned OFF, and the capacitor C.sub.1 again starts to charge. 
Thus, the negative logical pulses 41 from the OR circuit 27 are 
synchronized with a repetition of the discharging from and the recharging 
of the capacitor C.sub.1. The maximum voltage occurring across the 
capacitor C.sub.1, that is, the voltage immediately before the discharge 
start from the capacitor C.sub.1, is determined by the cycle of the 
negative logical pulses 41, namely the rotational speed of the 2-phase 
half-wave motor 11. 
If the reference voltage V.sub.S applied to the negative side terminal of 
the comparator 29 is set to be higher than the maximum voltage generated 
across the capacitor C.sub.1 when the 2-phase half-wave motor 11 is 
operating under a proper load, the comparator 29 continues outputting low 
level signals as shown in FIG. 2(e). 
As the comparator 29 continues outputting the low level signals, the NOR 
circuits 25 and 26 alternately output the high level signals each time the 
low level signal is transmitted from the operational amplifier 23 and the 
NOT circuit 24. By the high level signals from the NOR circuits 25 and 26, 
the transistors Tr.sub.4 and Tr.sub.5 are turned ON and since the stator 
exciting coils 13 and 14 are energized alternately to generate a rotating 
magnetic field, the magnet rotor 12 continues rotating. 
As the 2-phase half-wave motor 11 is overloaded to lower the rotational 
speed of the half-wave motor 11, the cycle of the negative logical pulses 
41 and the cycle of the discharge from the capacitor C.sub.1 become longer 
and the maximum voltage across the capacitor C.sub.1 becomes higher. 
When, the reference voltage V.sub.S is so set that, as shown in FIG. 3(a), 
for instance, the maximum voltage built up across the capacitor C.sub.1 is 
higher than the reference voltage V.sub.S applied to the negative terminal 
of the comparator 29. When the 2-phase half-wave motor 11 is overloaded to 
lower the rotational speed, the comparator 29 outputs the high level 
signals during the length of time according to the degree of overload, as 
shown in FIG. 3(b). 
While the high level signals are being outputted from the comparator 29, 
the NOR circuits 25 and 26 both continue outputting low level signals 
regardless of the levels of output signals from the operational amplifier 
23 and the NOT circuit 24. The transistors Tr.sub.4 and Tr.sub.5 are, 
therefore, turned off OFF, and the power supply to the exciting coils 13 
and 14 of the stator is limited. 
That is, when the 2-phase half-wave motor 11 is overloaded to cause 
lowering or the rotational speed of the 2-phase half-wave motor 11 to such 
an extent that an overcurrent flows through the stator exciting coils 13 
and 14. The power supply to the stator exciting coils 13 and 14 is limited 
according to the degree that the rotational speed is lowered. Thereby, 
heating of the 2-phase half-wave motor 11 is prevented and burning of the 
exciting coils 13, 14 of the stator and shortening of the life of the core 
(not shown) due to deterioration, changing in quality and other 
malfunctions do not occur. 
Meanwhile, when the 2-phase half-wave motor 11 is subjected to a more 
excessive load to be eventually locked, the output signal from the hall 
element 21 ceases to change, and either of the output signals from the 
operational amplifier 23 or the NOT circuit 24 will always be kept at a 
high level. Therefore, like when the negative logical pulses 41 are not 
outputted from the OR circuit 27 with the aforementioned 2-phase half-wave 
motor 11 running and the relationship among the voltage V.sub.H of the 
resistors R.sub.1 and R.sub.2, the diffusion potential V.sub.BE1 between 
the emitter and the base of the transistor Tr.sub.1, and the voltage 
V.sub.C across the capacitor C.sub.1 is represented by the formula 
EQU V.sub.H +V.sub.BE1 &gt;V.sub.C, 
the transistors Tr.sub.1 -Tr.sub.3 are turned OFF, the capacitor C.sub.1 is 
charged with the current flowing from the constant current circuit 28, and 
the voltage across C.sub.1 rises gradually, as shown in FIG. 4(a). 
With the maximum voltage which the constant current circuit 28 can output 
as V.sub.O, the condition represented by the formula 
EQU V.sub.H +V.sub.BE1 &lt;V.sub.C 
is reached in time with progressive charging of the capacitor C.sub.1, when 
the resistance values of resistors R.sub.1 and R.sub.2 set to satisfy the 
formula 
EQU V.sub.H +V.sub.BE1 &lt;V.sub.O. 
Then, in the case where the negative logical pulses 41 are outputted from 
the aforementioned OR circuit 27, the transistors Tr.sub.1 -Tr.sub.3 are 
turned ON, and the electric charge accumulated in the capacitor C.sub.1 is 
discharged. As the voltage V.sub.C across the capacitor C.sub.1 becomes 
lower with progressive discharging, the transistors Tr.sub.1 -Tr.sub.3 are 
turned OFF and recharging of the capacitor C.sub.1 is started. 
The cycle of such charging and discharging of the capacitor C.sub.1 can be 
set freely through adjustments of the capacitor C.sub.1 as well as the 
amount of current from the constant current circuit 28. 
While the voltage V.sub.C across the capacitor C.sub.1 after discharge of 
the capacitor C.sub.1 remains lower than the reference voltage V.sub.S 
applied to the negative of the comparator 29, the comparator 29 continues 
outputting low level signals, as shown in FIG. 4(b). Therefore, either of 
the NOR circuits 25 or 26 outputs high level signals according to the 
output signals from the operational amplifier 23 or the NOT circuit 24, 
and the stator exciting coils 13 or 14 is energized thereby. 
If the excessive load applied to the 2-phase half-wave motor 11 has by then 
been relieved, the magnet rotor 12 resumes rotation due to the rotating 
force created by the magnetic force generated by the stator exciting coil 
13 or 14. 
When, on the other hand, the excessive load applied to the 2-phase 
half-wave motor 11 remains unaltered, the power supply to the stator 
exciting coil 13 and 14 is stopped when the voltage V.sub.C across the 
capacitor C.sub.1 has in time exceeded the reference voltage V.sub.S of 
the comparator 29. This reference voltage V.sub.S can be set freely by 
resistors R.sub.3 R.sub.4. Accordingly, the duration of the power supply 
to the 2-phase half-wave motor 11 can also be set freely. 
If the duration of the power supply to the 2-phase half-wave motor 11 
should be properly set, heating of the 2-phase half-wave motor 11 can be 
prevented or controlled securely. Therefore, an effective way is provided 
for preventing burning of the stator exciting coil 13 and 14 or shortening 
of the life due to deterioration, change in quality, and other 
malfunctions of the core (not shown). 
Also, in the composition described above the entire circuit except the 
capacitor C.sub.1 can be easily integrated in a semiconductor chip. 
Thereby, a motor control unit provided with a compact anti-burning device 
may be obtained. 
Although in this embodiment, an example is provided for detecting the 
rotational position of the magnet rotor 12 through detection of the 
magnetic force of the magnet rotor 12 by the hall element 21. This causes 
no limitation and it is also possible to use a detection coil instead of 
the hall element and the kick back voltage of 13 and 14 may also be 
utilized. 
As to the current for use in charging the capacitor C.sub.1, a constant 
current is desired to be supplied through the constant circuit 28. However 
alternatives such as limitations in the maximum amount of current from the 
resistors or similar devices are acceptable. 
As described above, the motor control unit of the present invention having 
the anti-burning device allows the speed of the motor to be lowered with 
an increase of the current flowing through the exciting coil when the 
motor is subjected to a large load. When the speed of the motor is further 
lowered such that an overcurrent starts flowing through the exciting coil, 
the condition of the speed of the motor being below the predetermined 
lower limit is detected by the rotation detection device and the power 
supply time to the motor is limited according to the speed by the power 
supply limiting device. Thus, undue heating from an overcurrent can be 
prevented as well as burning or deterioration of the motor which will 
shortening of its life. 
Thus, since the power supply control device limits the duration of the 
power supply to the motor when its speed has been detected by the rotation 
detection device to be below the predetermined lower limit, any overloaded 
condition can be detected even when motors having different resistances of 
the exciting coil or being subjected to fluctuations in the amount of 
current flowing through the exciting coil when the motor is overloaded. 
Thereby the burning of the motor or similar malfunctions can be prevented. 
Meanwhile, when the motor has been locked due to an excessive load, the 
condition is detected by the rotation detection device. Then, the cyclic 
power supply device suspends the power supply and after relieving the 
overload power supply, resumes at the given cycle. 
Since there is no risk in continuing the overcurrent flow even if the motor 
is subjected to an undue load, burning or deterioration of the motor, 
shortening of its life or the similar malfunctions can be prevented 
without fail. Moreover, the power supply to the motor is done at the given 
cycle, so that the motor can be automatically and readily restarted when 
such a undue load has been relieved. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.