WO2009043373A1

WO2009043373A1 - A drive unit for an electric motor

Info

Publication number: WO2009043373A1
Application number: PCT/EP2007/060452
Authority: WO
Inventors: Mats Karlsson
Original assignee: Abb Technology Ab
Priority date: 2007-10-02
Filing date: 2007-10-02
Publication date: 2009-04-09

Abstract

A drive unit for at least one electric motor, the drive unit comprising: a power source (1 ) producing direct current to the motor, an energy storage (C1 ) arranged at the output of the power source for storing energy recovered during braking of the motor, a discharge circuit for discharging the energy stored in said energy storage if the voltage across the energy storage becomes too high, wherein the discharge circuit includes a resistor (R) adapted to discharge the energy stored in the energy storage. The drive unit is adapted, during start-up of the drive unit, to lead an inrush current through said resistor and during normal operation to discharge the energy storage by means of said resistor when the voltage across the energy storage becomes too high.

Description

Reference: 400707PCT/AMR Applicant: ABB Technology AB

A DRIVE UNIT FOR AN ELECTRIC MOTOR

FIELD OF THE INVENTION

The present invention relates to a drive unit for an electric motor comprising a power source producing direct current to the motor, an energy storage arranged at an output of the power source for storing energy recovered during braking of the motor and a discharge circuit for discharging the energy stored in the energy storage if the voltage across the energy storage be- comes too high, and the discharge circuit includes a resistor adapted to discharge the energy stored in the energy storage. The invention is useful for any type of drive unit in which an inrush current is generated upon start-up of the power source, for example a drive unit for an industrial robot or a drive unit for an electric screwdriver.

PRIOR ART

In a motor drive unit there is normally a large bleeder resistor arranged in order to take care of energy generated in the motor when the speed of the motor is reduced. Further, the motor drive unit is also normally provided with a capacitor bank, which must be charged with a limited current upon start-up of the drive unit. This is, for example, the case in a drive unit for an industrial ro- bot. When the motors of the robot are electrically braked, energy from the motors are recovered and fed back to the capacitor bank. If this energy is not consumed in another motor, the voltage across the capacitor is increased. In order to protect the capacitor and other components, it is necessary to discharge the capacitor if the voltage across the capacitor becomes too high. Therefore, the voltage across the capacitor is supervised, and if the voltage rises above a limit value, the bleeder resistor is connected to the capacitor so that the resistor discharges energy of the capacitor. When the voltage across the capacitor is below a limit value the resistor is disconnected.

A problem upon start-up of the drive unit is that a large inrush current is generated when the power supply is turned on. In order to protect the capacitor bank and the electronics connected to the DC output, the inrush current must be limited. Other rea- sons for limiting the inrush current are to avoid blowing of fuses arranged at the input of the drive unit and to prevent diodes of a rectifier from being damaged by the inrush current. To prevent the drive unit from high inrush currents, it is required to have some kind of charge circuit for the energy storage, or a current limiting circuit for the input current. Examples of traditional solutions are a switch regulator, a serial resistor, or a serial inductor on the AC main supply of the rectifier.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to provide a more cost- effective solution for limiting the inrush current in a drive unit for an electric motor.

This object is achieved by a drive unit as defined in claim 1 .

Such a drive unit is characterized in that the drive unit is adapted, during start-up of the drive unit, to lead an inrush current, generated during start-up of the drive unit, through said resistor and during normal operation to discharge the energy storage by means of said resistor when the voltage across the energy storage becomes too high.

According to the invention, the same resistor is used to take care of the inrush current during start-up of the drive unit, and to discharge the energy storage when the voltage across the en- ergy storage becomes too high during normal operation. A drive unit according to the present invention requires fewer components than the traditional solutions, and accordingly provides lower costs, higher reliability and less space consumption.

By "during start-up of the drive unit" is meant from the point in time the power to the drive unit is turned on until the energy storage is fully or almost fully charged. The invention is useful for both low- and high-voltage power sources. The power source is, for example, a rectifier converting alternating current to direct current. However, the power source can also be a battery or a DC generator.

According to an embodiment of the invention, the drive unit comprises a first switch adapted to switch, upon command, between a first connection in which the inrush current is led through the resistor and a second connection in which the resistor is arranged to discharge the energy stored in the energy storage if the voltage across the energy storage becomes too high.

According to an embodiment of the invention, in the first connection the resistor is arranged in series with the energy storage, and in said second connection the resistor is arranged in paral- IeI with the energy storage via a second switch adapted, upon command, to connect and disconnect the resistor to the energy storage.

According to an embodiment of the invention, the first switch is adapted to stay in the first connection during start-up of the power source, and the drive unit is adapted to command the first switch to switch to the second connection during normal operation, and to command the second switch to connect the resistor to the energy storage if the voltage across the energy storage exceeds an upper limit and to disconnect the resistor from the energy storage if the voltage across the energy storage is below a lower limit.

According to an embodiment of the invention, the common node of the first switch is electrically connected to the output of the power source, the first connection of the first switch is electrically connected to a first end of the resistor, and the second connection of the first switch is electrically connected to a second end of the resistor.

The power source has a positive and a negative DC output. According to an embodiment of the invention, the energy storage is a capacitor, a first node of the capacitor is connected to one of the DC outputs of the power source, and a second node of the capacitor is connected to the other DC output of the power source via the first switch.

According to an embodiment of the invention, the first end of the resistor is electrically connected to the first node of the capaci- tor via the second switch, and the second end of the resistor is electrically connected to the second node of the capacitor.

According to an embodiment of the invention, the first end of the resistor is connected to the power source via a second capacitor adapted to clamp the voltage across the second switch due to inductances in the resistor and the cables of the discharged circuit. When the second switch is turned off, there might be energy stored in stray inductances in the resistor and in its cables. This energy is transferred to the second capacitor, in order to limit the peak voltage applied to the switch when it is turned off. A further advantage with the second capacitor is that it contributes to the EMC filtering.

According to an embodiment of the invention, the first end of the resistor is connected to one of the DC outputs of the power source via a diode adapted to handle currents due to induct- ances in cables of the discharged circuit, and the output of the diode is electrically connected to one of the DC outputs of the power source via the second capacitor. The energy stored in stray inductances in the resistor and in its cables is transferred via the diode to the second capacitor, in order to limit the peak voltage applied to the switch when it is turned off. The diode prevents the rectifier from being short-circuited when the second switch is closed.

According to an embodiment of the invention the drive unit is a drive unit for an industrial robot and the DC source comprises a rectifier converting alternating current into direct current for at least one of the motors of the robots. The invention is particularly useful for a drive unit of an industrial robot.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

Fig. 1 shows a drive unit according to a first embodiment of the invention.

Fig. 2 shows a drive unit according to a second embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 shows a principal circuit scheme for a drive unit according to a first embodiment of the invention. It is to be understood that the circuit shown in figure 1 is only a part of the drive unit. For example, the drive unit may further comprise a fre- quency converter for converting the DC current to a variable alternating current in dependence on reference values generated based on a control program. The variable alternating current from the frequency converter is then supplied to the motor. The drive unit according to the first embodiment comprises a rectifier 1 adapted to convert alternating current into direct current. In this embodiment the rectifier is a three-phase bridge that converts three phases of alternating current into direct current. However, the invention is also applicable to a rectifier converting one or two phases into direct current. The rectifier 1 has a positive and a negative DC output. In another embodiment, the rectifier could be a DC power source.

The drive unit is provided with a capacitor bank in the form of a first capacitor Ci having a positive node connected to the positive DC output of the rectifier via a switch 2. The negative node of the capacitor Ci is connected to the negative DC output of the rectifier. The DC output from the rectifier is directly or indirectly supplied to a motor (not shown). When the motor is electrically braked, energy from the motor is recovered and fed back to the capacitor Ci . If the energy stored in the capacitor Ci is not con- sumed, the voltage across the capacitor is increasing and if the voltage becomes too high, the capacitor can be damaged. In an alternative embodiment the capacitor Ci can be changed to another component that stores electrical energy, such as a rechargeable battery, for instance a lead accumulator.

In order to avoid damaging of the capacitor, a discharge circuit is provided for discharging the capacitor if the voltage across the capacitor becomes too high. The discharge circuit includes a resistor R, a so-called bleeder resistor, and a switch 4.

In this embodiment the switch 4 is a break chopper. Alternatively, the switch 4 can be a relay or any other type of controllable electric switch. The resistor R is arranged in parallel with the capacitor Ci via the break chopper 4. When the break chopper 4 is turned on, the resistor R is connected in parallel with the capacitor Ci and the capacitor is discharged. When the break chopper 4 is turned off the resistor R is disconnected from the capacitor Ci and no discharging can take place trough the resistor.

A control unit (not shown) controls the turning on and off of the break chopper 4. The voltage across the capacitor Ci is measured and the measured values are sent to the control unit, which compares the received voltage with a lower and an upper limit value. If it is detected that the voltage across the capacitor ex- ceeds the upper limit, the break chopper is turned on and thereby the resistor R is connected to the capacitor Ci and the capacitor is discharged. When the control unit detects that the voltage across the capacitor is below the lower limit, the break chopper is turned off and thereby the resistor R is disconnected from the capacitor Ci .

The switch 2 comprises a common node 6 connected to the positive DC output of the rectifier, a first connection 7 connected to a first side of the resistor R and to the input of the break chopper, and a second connection 8 connected to a second side of the resistor and to the positive node of the capacitor Ci . The drive unit is provided with a control circuit (not shown) controlling the position of the switch 2.

The first end of the resistor R is connected to the negative DC output of the rectifier via a diode 10 and a second capacitor C₂. The diode 10 is adapted to handle currents due to inductances in the cables of the discharge unit. The second capacitor C₂ is adapted to clamp the voltage across the break chopper 4 due to inductances in the cables of the discharge circuit. The positive and negative node of the capacitor is, for example, connected to a frequency converter.

The key components are the first switch 2, the resistor R, the second switch 4, and the first capacitor Ci . The second capacitor C₂ and the diode 10 are present only to take care of indue- tance in the resistor R and its cables. The size of the resistor R depends on how much energy must be recovered from the motors during breaking. The resistor must be selected such that it can take care of all the recovered energy. The resistor is, for example, 1 -50Ω. The capacitor Ci is normally a large aluminum electrolytic capacitor. The second capacitor C₂ is preferably a smaller plastic capacitor. The capacitor C₂ is, for example, 1 -10 mF and the capacitor Ci is, for example, 1 -10 μF. The second switch 4 can be several types of electric valves, for example, a MOSFET transistor or a relay, typically an IGBT. The first switch 2 is, for example, a relay or a manual switch. If the switch 2 is a relay, it is suitable that the first connection 7 is normally closed and the second connection 8 is normally open. This means that the first connection is closed and the second connection is opened when the relay is without current. This is an advantage since during start-up of the drive unit the relay coil is without current and then the inrush current is led through the resistor R, which limits the inrush current.

The switch 2 is movable between a first and a second state. In the first state, the common node 6 is connected to the first connection 7. In the first state the positive DC output from the rectifier is connected to the first end of the resistor R and the other end of the resistor is connected to the positive node of the ca- pacitor Ci . When the switch 2 is in the first state, it is important that the second switch 4 is opened, i.e. the break chopper is turned off, in order to avoid a short circuit between the positive and negative DC output of the rectifier. During a start-up phase, the switch 2 is in its first state and the resistor R limits the cur- rent to the capacitor Ci and thereby protects fuses, the rectifier diodes, and the electronics connected to a DC output. The switch 2 can further be switched to a second state in which the common node 6 is connected to the second connection 8. In the second state the positive DC output of the rectifier is directly connected to the positive node of the capacitor Ci and the negative DC output of the rectifier is connected to the negative node of the capacitor Ci . Further, in the second state the resistor R is connected in parallel with the capacitor Ci via the break chopper 4. Accordingly, when the break chopper 4 is turned on, the resistor discharges the capacitor d .

It must be noted that the direction of the current through the resistor R is reversed when the switch 2 switches between the first and the second state.

During start-up of the drive unit the switch 2 is in its first state. When the voltage is supplied to the AC input of the rectifier, the capacitor Ci is charged via the resistor R with a limited current determined by the resistance of the resistor R. When the capacitor Ci is fully charged, the switch 2 is operated by a control cir- cuit (not shown) or manually by an operator and switched into its second state. Now the rectified input voltage is applied directly to the capacitor Ci , and the main load connected to the capacitor Ci can start working. If the load feeds back energy to the capacitor Ci , its voltage will rise. The voltage must be limited to a certain level, and if the voltage reaches this level the break chopper 4 turns on and the resistor R consumes the surplus energy.

When the break chopper 4 is turned off, there might be energy stored in stray inductances in the bleeder resistor R and in its cables. This energy is transferred via the diode 10 to the capacitor C₂, in order to limit the peak voltage applied to the break chopper 4 when it is turned off. The capacitor C₂ also contributes to the EMI suppression.

Figure 2 shows an alternative embodiment of a drive unit according to the invention. In this embodiment, the rectifier 12 is a single-phase bridge. The positive and negative DC outputs are switched relative to the drive unit shown in figure 1 . The diode 10 is switched so that it leads to current in the opposite direc- tion compared to figure 1 . Also, the direction of the break chopper 4 is switched, unless the break chopper is bidirectional.

Claims

1 . A drive unit for at least one electric motor, the drive unit comprising: a power source (1 ) producing direct current to the motor, an energy storage (C₁) arranged at the output of the power source for storing energy recovered during braking of the motor, a discharge circuit for discharging the energy stored in said energy storage if the voltage across the energy storage becomes too high, wherein the discharge circuit includes a resistor (R) adapted to discharge the energy stored in the energy storage, characterized in that the drive unit is adapted, during startup of the drive unit, to lead an inrush current through said resis- tor and during normal operation to discharge the energy storage by means of said resistor when the voltage across the energy storage becomes too high.

2. The drive unit according to claim 1 , wherein the drive unit comprises a first switch (2) adapted to switch, upon command, between a first connection (7) in which the inrush current is led through the resistor (R) and a second connection (8) in which the resistor is arranged to discharge the energy stored in the energy storage (Ci ) if the voltage across the energy storage be- comes too high.

3. The drive unit according to claim 2, wherein in said first connection (7) the resistor (R) is arranged in series with the energy storage (Ci) and in said second connection (8) the resistor is arranged in parallel with the energy storage via a second switch (4) adapted, upon command, to connect and disconnect the resistor to the energy storage.

4. The drive unit according to claim 2 or 3, wherein the first switch (2) is adapted to stay in the first connection (7) during start-up of the power source, and the drive unit is adapted to command the first switch (2) to switch to the second connection (8) during normal operation, and to command the second switch (4) to connect the resistor (R) to the energy storage (Ci) if the voltage across the energy storage (Ci) exceeds an upper limit and to disconnect the resistor from the energy storage (Ci) if the voltage across the energy storage (Ci ) is below a lower limit.

5. The drive unit according to any of claims 2-4, wherein a common node (6) of the first switch (2) is electrically connected to the output of the power source (1 ), the first connection (7) of the first switch is electrically connected to a first end of the resistor (R), and the second connection (8) of the first switch is electrically connected to a second end of the resistor.

6. The drive unit according to any of claims 2-5, wherein the energy storage is a capacitor (Ci), a first node of the capacitor is connected to one of the dc outputs of the power source (1 ), and a second node of the capacitor is connected to the other dc output of the power source via said first switch.

7. The drive unit according to claim 6, wherein the first end of the resistor (R) is electrically connected to the first node of the capacitor (C₁) via said second switch (4), and the second end of the resistor is electrically connected to the second node of the capacitor (C₁).

8. The drive unit according to claim 7, wherein the first end of the resistor is connected to the power source (1 ) via a second capacitor (C₂) adapted to clamp the voltage across the second switch (4) due to inductances in the resistor and the cables of the discharge circuit.

9. The drive unit according to claim 8, wherein the first end of the resistor (R) is connected to one of the dc outputs of the power source (1 ) via a diode (10) adapted to handle currents due to inductances in the resistor and cables of the discharge circuit, and the output of said diode is electrically connected to one of the dc outputs of the power source via said second capacitor (C₂).

10. The drive unit according to any of the previous claims, wherein said drive unit is a drive unit for an industrial robot, and said dc source is a rectifier converting alternating current into direct current for at least one of the motors of the robot.