Known drive systems for such vehicles have employed a commutator type DC motor with the supply of power to the motor from the battery being controlled by contactors which provide directional control and in conjunction with resistors, speed control. Electronic control by regulators using the"chopper"principle have replaced the resistors and associated contactors but some contactors are still required for the purpose of directional control and to provide regenerative braking. The motors being of the commutator type, have to be accessible for maintenance purposes and the cost of such motors is high.

A cheaper form of motor is an AC cage type induction motor and such motors in conjunction with variable frequency inverters, have replaced DC motors and the attendant speed controllers in machine tool applications.

In said machine tool applications power is supplie to the inverters by rectifying the mains supply so that, for example, with a mains supply of 220 volts AC, the supply voltage to the inverters will be of the order of 310 volts DC. Due to the high demand the combination of the AC induction motor and the inverter can be produced more cheaply than the DC motor and its controller and as a result the AC motor and inverter have become the norm in the industry.

It is possible to construct an AC motor and the associated inverter to operate at the much lower voltages available in a battery powered vehicle but the cost is high and in the absence of a high demand, significant cost saving is not possible.

The object of the present invention is to provide a drive system for a battery powered vehicle in a simple and convenient form.

According to the invention a drive system for battery powered vehicle comprises an AC motor, an inverter associated with the motor, a DC powered drive circuit for controlling the speed of the motor and a DC-DC converter for providing a DC voltage supply to the inverter higher than the battery voltage.

According to a further feature of the invention the DC-DC converter includes means for delivering power to the battery from the motor for the purpose of regenerative braking of the vehicle.

The drive system may include a battery. The battery may have an output voltage less than 100 volts DC, a voltage of 24 or 80 volts being preferred. The motor may be of a kind operating at 220/240 volts AC or higher.

In the accompanying circuit diagrams:- Figure 1 shows a basic converter circuit with the motor and inverter; Figure 2 shows the basic circuit of Figure 1 modified to provide for regeneration; Figure 3 shows a modified converter circuit which has a DC output voltage of opposite polarity; and Figure 4 shows another modified circuit for providing a high DC output voltage.

With reference to the circuit shown in Figure 1, an AC cage type induction motor is shown at 10 and is supplied with power by means of a DC powered drive circuit in the form of a variable frequency inverter 11. The inverter 11 is of any convenient type and can itself be controlled in order to provide for speed and directional control of the motor. The power supply to the inverter is derived from a DC-DC converter which includes a capacitor C which is charged from the vehicle battery by controlling the operation of a solid state switch Q1. The switch, when closed, causes current to flow from the battery into an inductor L and when the switch is opened, the flux collapse in the inductor induces a high voltage which is added to that of the battery to charge the capacitor by way of the diode D1. The diode prevents discharge of the capacitor when the switch is reclosed.

The switch, which in this case is shown as an IGBT, is subject to mark/space ratio control and the voltage Vc at the capacitor terminals is related to the battery voltage Vb by the equation:- Vc = Vb. Tp [/Toff where Tp is the sum of the on and off times of the switch and Toff is the off time of the switch.

It will be noted that as Toff tends to zero Vc tends to infinity. In practice boost ratios of 5-6 to one are attainable.

Figure 2 shows a modification to the circuit of Figure 1 in order to pass energy from the capacitor to the battery. This is achieved using the solid state switch Q2 which is connected in parallel with the diode D1 and a further diode D2 is connected in parallel with the switch Q1. By controlling the operation of the switch Q2 energy can be passed to the battery, thus providing regenerative braking of the motor.

Figure 3 shows a modified circuit which produces an inverted output voltage. A further inductor L2 and a further capacitor C2 are introduced into the circuit. When switch Q1 is closed the flux builds up in the inductor L1 and when the switch is opened capacitor C2 is charged as in the example of Figure 1.

When the switch is reclosed the charge on capacitor C2 is transferred to capacitor C1 by way of the second inductor.

The circuit can operate in the reverse direction by operating the switch Q3, for the purpose of transferring energy back to the battery and providing regenerative braking.

In the circuit shown in Figure 4 an upward voltage conversation is effected in two stages. L1, C1, Q1 and the diode of Q2 form a well known boost converter circuit. The switching of Q1 will produce a voltage in excess of that of BT1 across C1 as a function of the switching mark to space ratio. Whilst it is theoretically possible to utilise this stage to achieve the full step up range (in the case shown 80 to 560 volts) in practice a step up exceeding 5: 1 becomes inefficient and a second stage consisting of Q3, Q4, L2 and C2 is provided. Q3 and Q4 are switched on a 50: 50 basis with the result that a voltage across C2 is produced equal in magnitude to that across C1, as shown in the diagram. The resulting 560 volt bus feeds the motor drive inverter.

Under energy recovery conditions, the charge on C1 is transferred to the battery via L1, Q2 and the diode of Q1 acting in the well known buck converter configuration, where energy from a high voltage source can be transferred to a low voltage load.

The action of C2, Q3 and Q4 corresponds to the above described actions, ensuring that the potentials across C1 and C2 remain constant and equal.

Using the circuits described it is possible to power volume produced high voltage motor/inverter units from the low voltage batteries fitted on battery powered vehicles, thereby enabling such vehicles as fork lift trucks, to be manufactured more cheaply.

More particularly, the invention facilitates the provision of a trransformerless bi-directional DC-DC voltage converter wherein a three phase variable frequency AC motor drive inverter, designed for operation from a high voltage, rectified AC source, can be powered by a low voltage DC source such as a battery. The bi-directional feature of the DC-DC converter allows the recovery of kinetic and potential energy associated with movement of a load on a fork lift truck when braked by an AC motor acting as a generator, this energy being fed, at least in part, into the low voltage DC source.

A further and advantageous aspect of the invention is that it allows the use of a battery of lower output voltage, and thus a smaller number of cells, than is commonplace for many applications. In consequence it is possible to recharge the battery using only one or a small number of low voltage chargers, and more accurate charging can be achieved with a reduced risk of damage by over-charging.