DE29717431U1 - Circuit arrangement for protecting a small electric motor of a dental drill drive against overload - Google Patents

Description

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Description

Circuit arrangement for protecting a small electric motor of a dental drill drive against overload 5

The invention relates to a circuit arrangement for protecting a small electric motor for a dental drill or similar drive against overload.

Small electric motors, such as those used in the dental sector to drive drilling, grinding or similar tools, are already significantly under-dimensioned due to the relatively small installation volumes required, i.e. sustained, heavy motor loads can only be handled by the motor, particularly by the motor winding or the mechanically or electronically designed collector, for a short time. Maximum permissible motor currents can often only be handled without damage for around 10-30 seconds, despite cooling the motor with cooling air. If the load is further, the motor will overheat and the drive will fail.

Another reason why such drive motors are relatively susceptible to higher loads is the mandatory measures to comply with EMC law, which often result in higher brush resistance. The resulting higher power loss contributes to the higher load.

The problem with such small electric motors is to set the current limit so that the motor thermally

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mechanically is not destroyed, but at the same time there is sufficient torque. Although the peak current, which is responsible for overheating, only flows for a very short time and the motor can often cool down again during average operation, the current limit that restricts the motor current is only set to a fixed value in conventional circuit technology. For a user who, due to his working style, demands a lot of torque from the motor for a long time, this can often lead to premature failure of the motor. Measures that simply switch off the motor in the event of overload are unacceptable in the dental field, since neither the dentist nor the patient can be expected to undergo a treatment process with interruptions. Just think, for example, of jawbone processing, where it is unacceptable for the milling cutter to stop.

The invention specified in claim 1 is based on the object of providing an improved and simplified solution compared to the prior art, which in particular does not result in shutdown even in the event of prolonged overload of the motor.

The invention is based on the knowledge that reliable motor protection can only be achieved if the motor temperature caused by the load is known. From the increase in the motor temperature, a conclusion can be drawn about the expected downtime and measures can be taken beforehand. According to the invention, an electronic control circuit is present which receives information about the current motor

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current is supplied. In the control circuit, the motor temperature is simulated by storing parameters that represent the motor load in an evaluator. The evaluator has the function of a variable memory, e.g. an up/down counter, which is counted up and down depending on the current motor power and whose value influences the motor output stage via a controller. The evaluator can be designed as an analog value memory, e.g. in the form of a capacitor or as a memory cell of a controller system.

Since the motor current is relatively easy to measure and no sensors are required in the drive, the motor current is used as the input value for the evaluator. The evaluator is increased depending on the load value, i.e. the increase or decrease in the motor current. Time plays a decisive role here. The evaluator then contains the load value integrated over time. At the same time, the evaluator is reduced by a relief value that reflects a cooling of the drive.

0 In the steady state of the system, the motor temperature can be determined from the evaluator reading and from this a relief value can be determined. The relief value is a value that describes the time-dependent reduction of the evaluator. The greater the temperature difference, the faster the cooling time. The relief value is therefore greater the higher the current motor temperature.

If you use a standardized value for the engine temperature, 0, which is zero at ambient temperature, at the beginning

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all values equal zero. This means that real-time behavior can be expected, in which no significant dead times occur. This circuit arrangement means that complex controllers or control algorithms are not required. By standardizing the motor temperature to the ambient temperature, no errors arise, as only the temperature difference is taken into account. The only initial value that needs to be defined is a maximum permissible ambient temperature, e.g. 40°.

In a further development of the invention, it is proposed to counteract an overload so that the motor does not become overloaded in the first place. This can be achieved by changing the current limit in good time depending on the evaluator, for example by using a current regulator which takes over the control of the motor's output stage in the event of current limitation. The setpoint for the current regulator is derived from the evaluator. With this variant, one would only notice a drop in performance during use due to the lower possible torque when the drive is overheated. This drop in performance disappears again, however, when the motor has cooled down again under lower load. It is not necessary to stop the motor completely; the evaluator also registers the cooling phases which result from temporarily lower loads. In addition, the motor is always able to deliver a certain minimum power continuously.

An embodiment of the invention is explained in more detail with reference to the drawing. It is clear from the illustration that

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the motor current of the drive motor designated M is recorded via a current measuring element 1. This information about the current motor current is fed into a control circuit 2, which has an evaluator 3, a controller 4, an operational amplifier 5 and a resistor 7. Parameters that represent the motor load are stored in the evaluator 3. The strength of the load on the motor is recorded by the operational amplifier 5, which has a non-linear characteristic curve. If the evaluator 3 reaches a level that corresponds to a specified maximum load on the motor, the motor output stage 6 is influenced via the controller 4, advantageously in such a way that the maximum permissible motor current and thus the maximum permissible torque are reduced until the motor has cooled down sufficiently. If the maximum permissible torque is exceeded, the motor current is kept constant, i.e. the motor delivers a constant torque and the speed will change depending on the load, but will never exceed the set speed, as in this case the speed controller will be active again.

When using an up/down counter, cooling is achieved by counting the evaluator or, as shown in the example, when using a capacitor, by discharging the capacitor 3 via the discharge resistor 7.

Claims (3)

GR 97 G 3756 ·"· ·&iacgr; Protection claims 1. Circuit arrangement for protecting a small electric motor of a dental drill or similar drive from overload, comprising an electronic control circuit (2) to which information about the current motor current is supplied from the motor circuit and which controls the output stage (6) of the small electric motor via a controller (4) and an evaluator (3) in which the motor temperature is simulated by storing parameters that represent the motor load, wherein the arrangement is designed in such a way that when an evaluation level determined by the maximum load is reached, the evaluator reduces the maximum permissible motor torque until a lower evaluation level can be selected due to the motor cooling down. 2. Circuit arrangement according to claim 1, in which the controller (4) is designed such that it regulates to a constant speed in the normal case and to a constant motor current in the case of current limitation. 3. Circuit arrangement according to claim 1, in which a capacitor is provided as the evaluator (3) and the cooling of the motor is represented by a discharge resistor (7).