Protection device for electric motor driving devices and corresponding driving device

A protection device protects a driving device of an electric motor with at least three phases and at least three windings and the driving device comprises a power stage suitable for driving directly said at least three windings. The driving device is suitable for actuating a motor brake operating phase and comprises detectors suitable for detecting the currents that run in the windings of the motor. The protection device comprises selectors suitable for selecting the currents having a single direction between said detected currents and a deactuator suitable for deactuating the power stage during said motor brake operating phase when the sum of the currents having a single direction is greater than a reference current.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention refers to a protection device for driving devices for electric motors and corresponding driving device.

2. Description of the Related Art

In the prior art, driving devices for three-phase motors are known that implement the operating phase of the motor brake, i.e., the operating phase in which the three-phase motor is stopped rapidly.

FIG. 1shows a diagram of a three-phase motor provided with a driving device that implements the motor brake operating phase. The three-phase motor comprises the star-connected windings1-3(the solution is also suitable in the case of delta-connected windings) at the common terminal D and having the other terminals connected respectively at the nodes A-C with the half-bridges4-6; the latter are made with power switches such as MOS transistors or bipolar power switches or IGBTs and are the power stage for driving the windings1-3. The half-bridges4-6are connected between a supply voltage Vs and ground GND and are driven by respective terminal elements7-9forming part of a driving device10. The elements7-9are circuitally connected to a main element11that implements several control functions including the motor brake or brake operating phase. To obtain rapid braking of the electric motor the windings1-3are short-circuited to the supply voltage Vs or to ground GND; the current in the windings of the motor will reach a zero value in a period of time that depends on the features of the motor and on the application.

During actuation of the brake operating phase in the windings of the motor current overshoots occur due to the presence of the back electromotive force (BEMF) that depends on the electric constant of the motor and on the rotation speed. Said overshoots can exceed the maximum permitted current in the half-bridges4-6and can determine failure of the brake operating phase or also failure of the power stage formed by the half-bridges.

BRIEF SUMMARY OF THE INVENTION

In view of the prior art, the object of the present invention is to provide a protection device for the driving device of electric motors that enables the above drawbacks to be overcome.

According to the present invention, this object is achieved by means of a protection device for a driving device of an electric motor with at least three phases, said electric motor comprising at least three windings and said driving device comprising a power stage suitable for driving directly said at least three windings, said driving device being suitable for actuating a motor brake operating phase, said driving device comprising means suitable for detecting the currents that flow in the windings of the motor, characterized in that it comprises first means suitable for selecting the currents having a single direction between said detected currents and second means suitable for deactuating said power stage during said motor brake operating phase when the sum of said currents having a single direction is greater than a reference current.

Still according to the present invention it is possible to provide a driving device for an electric motor as defined in claim5.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2shows a three-phase electric motor with a motor driving device provided with a protection device according to the present invention; the elements inFIG. 2that are also present inFIG. 1will be indicated by the same numerical references. The three-phase motor comprises the windings1-3that are star-connected at the common terminal D and have the other terminals respectively connected at the nodes A-C with the half-bridges4-6. The latter are made with power switches such as MOS transistors or bipolar power switches or IGBTs and constitute the power stage for driving the windings1-3; in the case of the driving device100inFIG. 2the half-bridges4-6are made with respective DMOS transistors M1-M2, M3-M4and M5-M6. The half-bridges4-6are connected between a supply voltage Vs and ground GND and are driven by respective terminal elements70-90forming part of the driving device100. The elements70-90are circuitally connected to a main element110that implements numerous control functions including the motor brake or brake operating phase. To obtain rapid braking of the electric motor, the windings1-3are short-circuited to the supply voltage Vs or to ground GND; the current in the windings of the motor will reach a zero value in a period of time that depends on the features of the motor and on the application.

The driving circuit70-90comprises means suitable for detecting the current that runs in each half-bridge and in each winding of the electric motor during operation of the electric motor. Said means comprises three power or power sense transistors PS1-PS3connected to the respective half-bridges4-6. The currents I1-I3constitute portions of the currents that circulate in the windings of the motor during the operation thereof.

The driving device100also comprises a protection device200suitable for limiting the value of the maximum current that can circulate in the windings of the electric motor.

The protection device200comprises first means D1-D3suitable for selecting the currents having a single direction between said currents I1-I3that flow in the windings1-3of the motor and that are detected by the PS1-PS3and second means51suitable for deactuating said power stage4-6during said motor brake operating phase when the sum Is of said currents having a single direction is greater than a reference current Iref.

In particular, the protection device200comprises the diodes D1-D3connected to the respective transistors PS1-PS3and a summing node50; the latter thus sums only the currents exiting the nodes A-C. The sum current Is exiting the node50is sent to a comparator51that compares the sum current Is with a reference current Iref; if the current Is is greater than the reference current Iref the comparator sends a signal Dis to the control element110for deactuating the power stage4-6and switching off the electric motor.

As the windings1-3are star-connected, i.e., I1+I2+I3=0, by controlling the sum of the currents entering the node50also the sum of the exiting currents is controlled indirectly.

During the brake operating phase, i.e., when the control element sets the short circuit of the three windings1-3to Vs voltage or to ground GND respectively by commanding the switch-on of the three DMOS transistors M1, M3and M5or M2, M4and M6(in particular in the example of the mentioned embodiment, the control element sets the short circuit of the windings to the voltage Vs), if the current Is exiting the node50is greater than the reference current Iref the protection device200switches off the power stage4-6and the electric motor.

Through knowing the features of the motor it is possible to appropriately select the time in which the power stage will remain deactivated, thus optimizing motor braking time.

InFIG. 3Athe circuit ofFIG. 2is shown in an operating phase in which only the transistors M1and M4are switched on, whilst the others are switched off. During the brake operating phase (FIG. 3B) the driving device100sets the switch-on of the transistors M1, M3and M5to short-circuit the windings1-3to the supply voltage Vs. In the presence of overcurrent, i.e., when the current Is exceeds the reference current Iref, the transistors M1-M6are switched off (FIG. 3C) and the current can circulate only through the diodes coupled with the transistors M1-M6.

InFIG. 4the time diagrams of the currents I1-I3are shown. When the brake operating phase is actuated by the element110by means of a command Br, the currents I1-I3have a different time trend; if the current Is overcomes the reference current Iref, the command Dis (pulse command) intervenes that deactuates the power stage4-6.

The protection device200thus enables control to be obtained of the currents that circulate in the windings of the electric motor during implementation of the brake operating phase and to protect the power stage4-6independently of the mechanical constants of the motor and of rotation speed of the motor. Furthermore, it is possible to optimize braking time of the electric motor by a suitable selection of the reference current Iref.