Damper for power train

Abstract

A damper for a power train, comprising a piezoelectric transformer and a load element connected across the output of the piezoelectric transformer.

Claims

1. A power train for a high impedance load, comprising: an input EMC filter for connection to a power supply; a pulse width modulation (PWM) converter connected to an output of the input EMC filter and configured to provide a PWM output to drive the high impedance load; and a damper connected between an output of the PWM converter and terminals of the high impedance load, wherein the damper comprises: a piezoelectric transformer; and a load element connected across an output of the piezoelectric transformer.

2. A power train according to claim 1 wherein the damper further includes: a piezoelectric transformer for each of one or more phase lines of the power train, each piezoelectric transformer having a respective load element connected across its output.

3. A power train according to claim 2, for a three-phase power train, wherein the damper has three piezoelectric transformers, one associated with each phase line, and three load elements, one for each transformer.

4. A power train according to claim 2, for a two-phase power train, wherein the damper has two piezoelectric transformers, one associated with each phase line, and two load elements, one for each transformer.

5. A power train according to claim 2, for a single-phase power train, having a single piezoelectric transformer, and a single load element.

6. A power train according to claim 1, for a three-phase power train, wherein the damper comprises three piezoelectric transformers, one associated with each phase line, and a single load common to all piezoelectric transformers.

7. A power train according to claim 1, wherein the load element is a resistor.

8. A power train according to claim 1, wherein the load element is a power converter configured to regenerate switching energy.

9. The power train of claim 1, further comprising the power supply.

10. The power train of claim 1, further comprising the high impedance load, wherein the high impedance load is connected to an output of the damper.

11. The power train of claim 10, wherein the high impedance load is a motor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of the components of a typical power train for a motor.

(2) FIG. 2a shows a piezoelectric transformer for use in a damper according to this disclosure.

(3) FIG. 2b is an equivalent circuit of the piezoelectric transformer of FIG. 2a.

(4) FIG. 3 is a schematic circuit diagram of a damper according to this disclosure.

(5) FIG. 4 is a single phase equivalent circuit of a damper according to this disclosure.

(6) FIG. 5a shows an ideal transfer function for a conventional RC damper.

(7) FIG. 5b shows an ideal transfer function of a damper according to this disclosure.

(8) FIG. 6 is a power train incorporating a damper according to a three-phase embodiment of this disclosure.

(9) FIG. 7 is a power train incorporating a damper according to a single phase embodiment of this disclosure.

(10) FIG. 8 is a power train incorporating a damper according to another three-phase embodiment of this disclosure with a common load.

(11) FIG. 9 is a power train incorporating a damper according to another embodiment of this disclosure.

(12) FIG. 10 is a power train incorporating a damper according to another embodiment of this disclosure.

DETAILED DESCRIPTION

(13) The described embodiments are by way of example only. The scope of this disclosure is limited only by the claims.

(14) A typical power train for a motor is described with reference to FIG. 1. Power is provided from a power supply 1 to a motor 2 along a power train 3. The power from the power supply 1 passes through a converter which comprises, here, an input EMC filter 5 to reduce high frequency electronic noise that may cause interference with other devices, and a main converter 6. An output filter 7 is then generally provided to mitigate transmissions line effects as described above. The converter and input and output filters are mounted to a system chassis, e.g. a copper plate.

(15) As described above, various solution have been proposed to address transmission line effects including those in CM mode. The damper of the present disclosure aims to address transmission line effects without the use of capacitors.

(16) The present disclosure makes use of a piezoelectric transformer (PZT) to recreate the effect of an RC damper but without the use of a capacitor.

(17) Piezoelectric materials have found an increasing number of applications in recent times due to their characteristics that enable electrical energy to be generated due to compressing or lengthening the piezoelectric component.

(18) PZTs are solid state devices made up of two piezoelectric materials. One generates voltage when compressed, the other lengthens when a voltage is applied. By appropriate selection of the piezoelectric materials, such PZTs can be used as step up or step down transformers.

(19) FIGS. 2a and 2b show a typical PZT (FIG. 2a) and the circuit equivalent of a PZT (FIG. 2b). From the equivalent circuit it can be seen that if the input (C1) and output (C2) capacitances are removed, or set to zero, the PZT has the equivalent structure of an RLC circuit. This can be used to function as an RC damper. In addition, a load needs to be added to the output of the PZT to either passively control the overshoot (e.g. a resistive element) or to actively recycle energy (if the load is a converter) and to control the overshoot.

(20) FIG. 3 is a circuit diagram showing how such a loaded PZT can be connected to the motor terminal for a three phase system. For each phase line 10a, 10b, 10c there is provided a respective PZT 11a, 11b, 11c each loaded with a respective resistive element or converter 12a, 12b, 12c. The same principle can be applied to a single or other multi-phase system. For the sake of simplicity, the structure for a single phase system will be used for the following description.

(21) FIG. 4 shows the equivalent circuit for a single phase system. The PZT is represented as shown in FIG. 2b to which a resistive load RL is added. ZM is the motor impedance.

(22) The transfer function for a conventional RC damper is represented as:

(23) $Z_1\left(s\right)=\frac{s{R}_M{C}_M+1}{s{C}_M}$

(24) The inventors performed testing to determine if the same damping effect can be obtained using a PZT damper, i.e. by obtaining a similar transfer function.

(25) Setting C2 to zero (which is acceptable because C2 only causes effects in a high frequency range that is not of interest in this context), and considering R.sub.L and R.sub.r as a single entity, the transfer function of the proposed damper can be represented as

(26) $Z_1\left(s\right)=\frac{s^2{L}_r{C}_r+s{C}_r{R}_L+1}{s\left(C_r+C_1\right)\left(s^2{L}_r{C}_{eq}+s{C}_{eq}{R}_L+1\right)}$

(27) Where C.sub.eq can be defined as

(28) $C_{eq}=\frac{C_1{C}_r}{C_r+C_1}$

(29) FIGS. 5a and 5b compare the ideal transfer function for a conventional RC damper (FIG. 5a) with that for the proposed PZT damper (FIG. 5b).

(30) As can be seen, the performances in the lower frequency ranges are essentially identical. There is a difference at higher frequencies but such high frequencies are not relevant here.

(31) Further, tests have shown that the optimum point in terms of overshoots depends on the selected value for the resistive load RL connected to the output of the PZT. The higher the overshoot reduction required the higher the losses are going to be on the resistor or converter.

(32) If, instead of a resistor load, the PZT is loaded with a converter, this value RL can be controlled to enable recycling of energy and also to adapt the power within a certain range.

(33) The load can also be adjusted to work for cables of different lengths.

(34) FIGS. 6 to 10 show some alternative ways, as examples only, of how the concept of this disclosure can be implemented in a power train.

(35) All of FIGS. 6 to 10 show, schematically, a damper 20 according to the disclosure connected at the terminals of a load—i.e. here a motor 21. The motor 21 is connected to a power source 22 (here a PWM-based power source) via cables 23 which can be very long. The damper 20 is located at the motor terminals.

(36) FIG. 6 shows a three phase system in which the structure of the damper 20 is the same as shown and described in relation to FIG. 3. The same reference numerals are used for corresponding components in FIG. 6.

(37) FIG. 7 shows a single phase system, where the damper 20′ comprises a single PZT 11′ loaded with a resistive element 12′ or a converter.

(38) FIG. 8 shows a three phase system similar to that of FIG. 6 but all three PZTs 11a, 11b, 11b share the same load 12″.

(39) FIG. 9 shows an alternative three phase system where the damper structures for each phase (here a block 15a, 15b, 15c, 15d) representing a loaded PZT as previously described, are arranged in a star configuration 20″ where the star point may be connected to the system chassis directly or via an additional damper 5d. In such an arrangement, some of the loads may be passive and some active.

(40) FIG. 10 shows the damper structures arranged in a ‘wye’ configuration 20′″ referenced to the system chassis either directly or via additional dampers 15e, 15f, 15g.

(41) The damper can be used in a power train with a PWM based source to manage transmission line effects. The damper can also reduce dv/dt at the motor terminals, reduced common mode currents and reduce stress on the motor windings.

(42) The description is of preferred embodiments only. The scope of protection is defined by the claims.