GB2406004A - Pump motor start-up circuit - Google Patents

Abstract

A portable water pump assembly comprising a sealed housing (1), a rechargeable battery pack (9) and one or more pump motors (12, 13). The motor or motors are synchronous motors with a low voltage coil and controlled to run above the nominal start frequency. In a start-up procedure the motor speed is progressively increased to a speed marginally below the stall point by using current monitoring to determine the speed control point. If motor stall occurs then the motor is restarted and the speed increased again. The assembly has buoyancy such that it floats in water with a centre of gravity such that the inlet is submerged. Weighted filter assemblies may be attached to adjust buoyancy, centre of gravity or to sink the assembly.

Description

WATER PUMP

This invention relates a water pump, and in particular but not exclusively to a water pump operable at low voltage from a rechargeable source. s

There are various situations in which it is desirable to have a portable pump with an integral power source. For example when it is desired to use water collected in a water butt, it is laborious to fill watering cans in order to convey the water to flower beds or other places. In other instances, it may be desirable to be able to pump water from some kind of reservoir, examples of which may be a child's paddling pool or a washing machine with a failed pump.

In such situations it is important that the pump be safe and waterproof and durable.

The present invention is directed towards providing a safe, portable water pump.

According to the invention there is provided a portable water pump assembly comprising a sealed housing, a rechargeable battery pack and at least one pump motor, the motor being a synchronous motor with a low voltage coil and controlled to run above the nominal start frequency.

In a preferred embodiment, in a start up procedure the motor speed is progressively increased to a speed marginally below the stall point by using current monitoring to determine the speed control point. If motor stall occurs then the motor is restarted and the speed increased again.

The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of an assembly according to the invention; Figure 2 is a schematic functional diagram of a motor drive control; - 2 Figure 3 is a switch and voltage diagram at lower than 50Hz for operation of a motor in accordance with the invention; Figure 4 shows voltage and signal timing at frequencies greater than 50Hz; Figure 5 shows voltage and current timing for a dual motor system; Figure 6 is a circuit diagram for a single motor drive and DC input control; Figure 7 is a circuit diagram for a system showing functional blocks; and Figure 8 is a graph of motor pump pressure versus flow at different frequencies.

Referring to Figure 1 a portable water pump assembly is illustrated. The pump has a waterproof housing I with an inlet port 11 to which a hose connector (Figure la) or a filter (Figure lb) unit may be coupled externally. A hose outlet connection port 7 is provided on the opposite side of the housing to inlet 11, with one or more pump units, in this embodiment two pump units 12 and 13, and a solenoid valve 6 interconnected between the inlet and outlet. A rechargeable battery 9 and control circuitry 5 are also located within the housing. For recharging, or allowing mains operable function, a waterproof power connector 14 is provided. The housing also has an integrally formed handle 30 and o-ring seals 8 and 10 seal connecting ports within the assembly.

On the outside of the housing there is a mode selector switch 2, indicator 3 and display 4.

A pump assembly of the general construction described may be used, for example, to pump water from a water butt for which purpose the water inlet 11 will be attached via a suitable connector to a tap or other outlet on the water butt. The outlet 7 of the pump assembly will be attached to a hose which could be used for filling other vessels such as watering cans, or for direct watering. Timing mechanisms may also be employed to provide an automated watering system.

Another use, which illustrates other functions, is emptying paddling pools or ponds. In this instance the unit may be placed in the water so that water from the pool or pond directly enters the inlet 11, although a filter will usually be used as well. The unit is lightweight and will float on water, its centre of gravity and buoyancy being designed to orient the unit with the inlet port 11 submerged in the water. However, in some applications it may be desirable for the unit to be totally submerged, in which case a weight is attached, which could for example be made as an integral feature of the inlet filter. Weighted filters may also be used to adjust buoyancy.

Providing a durable, safe pump system at a reasonable cost for domestic use is a challenge, which is solved in the invention by modifications and operation as I S described below.

The motor pumps used in the preferred embodiment are 240 Volt 50 Hertz 34 Watt permanent magnet synchronous motor pumps having a single rotor impeller totally enclosed within the fluid partition of the pump. Pumps of this general type are commonly used as drain pumps on domestic washing machines.

The above type of pump is modified for operation at a lower voltage and higher frequency than standard mains, so as to enable safe and battery operation. A suitable modification is to replace some of the light gauge wire (estimated as about 3,100 turns of 0.22 gauge wire) with 180 turns of 1.0 mm enamelled copper wire which gives a low resistance winding with enough turns to avoid magnetic saturation of the motor's core, when run in conjunction with the drive control described later. Running the motor at a higher frequency than the baseline design of 50 Hertz enables the pump to provide increased fluid pressure. An AC brushless motor of the type described provides highly efficient pumping and durability to allow continuous operation. Figure 8 shows typical performance of the pump assembly at SO Hertz and 70 Hertz. - 4

This low resistance winding minimises losses and also allows for efficient transfer of unused energy, stored in the motor's inductance, back to the power source, storage capacitor (capacitor C11 in Figure 7) and to the second motor. When two motors are driven, they are driven with a timing phrase shift of a quarter period to take advantage of the reconciliation of energy.

The magnetic core material, usually a laminated iron assembly, ideally has a high relative permeability and low magnetic hysteresis losses in order to achieve high rotor speeds. Low magnetic hysteresis losses will improve the energy efficiency of the motor especially when operating at frequencies above 60 Hertz.

Figure 2 shows a schematic functional layout for the motor drive control circuitry of the assembly of Figure 1. The motor drive used is a single phase, full wave bridge drive. Current is monitored via a resistive shunt, then rectified, amplified and filtered and used to provide a variety of functions such as current limiting, detection of a short drive failure or, as will be seen is particularly relevant, control and a stalled motor.

Short circuit protection is incorporated by checking if the current through the drive bridge exceeds the worst case operating levels. In this embodiment, if the processed current signal exceeds 4.5 V all drive outputs and the unit are switched off.

Short circuit detection may be incorporated by a set level or other level and time dependent algorithm to activate means for isolating the short circuit.

A solenoid valve provides a shut-off to prevent flow when the motor pump is off in order to curtail flow due to siphoning.

Operation of the motor utilises the maximum operating motor speed being greater than the maximum frequency at which the motor will start. The motor drive starts the motor and increases the drive frequency providing that the monitoring current signal is below a predetermined level, to effectively ensure a maximum motor speed marginally below the stall point. If motor stall occurs then the motor is restarted and the speed increased again. This is now described in more detail with reference to Figures 3 to 7.

In Figure 7, which is a two motor system, functional blocks are shown in dotted outline. These are, on the left, DC input voltage monitor 41, temperature input 42, ] 2 volt battery 43, crystal oscillator 44, DC power input 45 and on/off and mode switch 48.

Figure 6 shows a single motor pump drive with DC input FET circuits, with detail for understanding the control principles at a drive bridge and DC input PET.

Each motor has an associated full wave bridge converter 46, current shunt 47, voltage follower (rectifier) 49, filter 50 and signal amplifier 51. There are also shown LED indicator 52, battery charger monitor 53 and 5 volt regulator and reference signal 54.

Quartz crystal oscillator 44 (Figure 7) in conjunction with a microcontroller timer is used to derive a 125 microsecond program minor cycle time that is used for all the event timing applications, for example switch de-bounce and software filters. When in off mode all the output switches are set to off, and when on the switches for each bridge are switched on and off for predefined times. At 40Hz (100 counts), the half period is set to 12.5ms (i.e. 50 minor cycle counts) and the offtime to 4.9ms as shown in Figure 3, giving a 61% duty cycle. As the halfperiod is reduced, the off time is also reduced until the frequency exceeds approximately 55Hz, thereafter the off period is set to 1 count to allow time for device switching. This allows for ruMing efficiently at lower motor speeds. At lower motor speeds (lower frequencies), the motor's magnetic core will saturate such that the motor no longer behaves as an inductive load but as a resistive loss for some of the cycle. By limiting the ON time for the switches, energy losses in the motor are reduced.

The ac waveform supplied to the motor is a quasi square wave voltage, as shown in Figure 3, with switches S1 to S4 shown in Figure 6 being driven and producing the output as shown in Figure 3. The maximum output frequency reference is set to 75.5Hz (53 counts) and the motor is started at 45Hz (89 counts). The cycle is repeated with the half period and off count for each motor decreasing every 200ms providing that the monitored current signals is below approximately 1.6V. This threshold allows sufficient margin for an imminent stall condition that would occur above approximately 1.8V (see Table 1 below) and will allow the motor to run up to a maximum sustainable speed for the ambient pressure and flow conditions. Once the motor drive frequency is greater than 60Hz, increase of frequency will only be permitted if the conditioned current signal is less than approximately 1.6V. this ensures a maximum motor speed marginally below the stall point for each specific application, thus allowing higher motor speeds where for higher pressure drops. If a stall condition is detected for either motor, then both motors and restarted from 45 Hz.

If low supply voltage is detected or off mode selected, then both motors are switched off.

With a nominal supply voltage of 12.5V d.c., the current drawn is limited by varying the drive duty cycle when the drive frequency is below 50Hz, so that the current is approximately 1.1 Amps at zero fluid flow. As the motor supply frequency is increased the duty cycle allows more energy to be delivered to the motor. For a dual motor system each motor runs between approximately 1A at zero flow to 3A at maximum flow rate. The total maximum power consumed by the motors is approximately 70W (12.5V x 2.8A x 2=70W) at maximum output and at minimum power approximately 25W.

The following table shows how the typical conditioned current signal varies with drive supply frequency at no flow, maximum flow and stall conditions for each motor.

Table 1

Drive Zero Max Stall Current Current Current Frequency fluid fluid signal signal signal stall Hz flow flow Zero max Current Current Current flow flow

A A A V V V

1.1 2.7 N/A 1.0 1.86 N/A 1.15 2.8 1 0.89 0.95 2.0 3.72 1.1 2.8 0.94 1.0 2.0 3.36 1.33 2.65 2.0 1 1.1 1.8 2.9 During normal operation of the motor, the current ramps up during the first half drive cycle until the motor rotor starts moving and doing work. Figure 4 shows typical waveforms for start run and stall conditions.

At the second and further half drive cycles, some of the energy stored in the inductance is fed back to the source (e.g. battery or a capacitor or second motor) as indicated by the negative current shown in Figure 3 and Figure 4. Feeding back the inductive energy is possible due to the low resistive winding and low motor impedance and the inherent diodes associated with the MOSFET drivers and their low on state resistance. By running the motors at the same frequency, but with the second motor drive delayed by a quarter of a full cycle, the smoothest operation for the pumps is achieved with minimal interaction between the motors and the peak current demands from the source evenly distributed. See Figure 5.

During a stall condition the motor behaves as an inductor (motor is doing a minimal amount of work by vibrating in lieu of rotating). In addition the average current is lower than during a synchronous rotation and energy stored in the inductance is in most part returned to the source, however, the peak current is higher for a stall - 8 condition. See Figure 4. Thus by monitoring the peak current a stall condition can be differentiated from a normal running condition.

The current is converted to a proportional voltage signal and rectified via a perfect rectifier, in this case utilising a voltage follower single rail op-amp. This voltage signal now only contains the drive current element with the negative currency (energy fed back to the source) omitted. The signal is filtered and scaled for measurement by the microcontroller analogue to digital converter. See Figure 4. The signal may alternatively be fed to a peak detection circuit.

In order to prevent erroneous stall detection during motor start up which may take several cycles, the motor is started at a frequency low enough, approximately 45Hz, to ensure an immediate start to synchronous operation. In addition stall detection is disabled in the software code until the drive frequency has passed approximately 55Hz. In order to prevent erroneous stall detection, the software currently acknowledges a stall if the average drive current (excludes reverse current) is above approximately 2.4V for greater than 0.4 seconds.

The hardware is designed so that minimal power consumption occurs when the unit is switched off or if DC input is present. An indicator provides visual means for on mode-indicator on continuously, battery on charge rapid flash, fully charged indication off, and trickle charge - slow flash. When the battery is fully charged the unit will enter a low power sleep mode until the batter voltage has fallen by approximately 10%, then the unit will effectively trickle charge the battery to maintain the battery at maximum capacity. This is achieved by controlling the duty cycle for the DC input FET so that the battery voltage is maintained at a level approximately 12% below the fast charge switch off level (or at a level to suit the battery technology).

Removal of DC input will invoke sleep mode in the unit if it is not in on mode.

A temperature monitor is provided to give additional protection against abuse of the pump, for example using with hot fluids or running the pump without fluid present for extended periods (approximately 20 minutes dry running allowed). Figure 7 shows a - 9 - temperate sensing termistor (Rth). The temperature sensor is placed adjacent to one of the motor windings and if the temperature exceeds 85 C, then the power is removed from the motors until the temperature is less than 85 C when the motors will be driven again. Each motor may have independent sensors but without significant improvement in the thermal cut-out feature.

The software utilises inputs from switches and controls both motors' full bridge drives, shut off valve if fitted, display and indicator outputs. Voltages at the power input connector, battery and processed current signals are monitored.

As part of the growth of the water butt pump to an intelligent watering system, provisions for a solenoid valve output, display and auxiliary input switches may be provided. r -

Claims (8)

1. A portable water pump assembly comprising a sealed housing (1), a rechargeable battery pack (9) and at least one pump motor (12, 13), the motor being a synchronous motor with a low voltage coil and controlled to run above the nominal start frequency.

2. A portable water pump assembly according to claim l including a startup control that progressively increases the motor speed to run at a predetermined margin below the stall point.

3. A portable water pump assembly according to claim 2 in which should a stall condition occur, it is detected and then the motor is restarted according to claim 2.

l 5

4. A portable water pump assembly according to claim 3 in which the motor is a SOHz motor and is run at a frequency in the range of 60 to 75Hz.

5. A portable water pump assembly according to any preceding claim further comprising a second motor driven with a timing phase shift of a quarter period.

6. A portable water pump assembly according to any preceding claim in which the assembly floats in water and has a centre of gravity such that when floating the inlet is submerged.

7. A portable water pump assembly according to any preceding claim including a hose connector point at the inlet.

8. A portable water pump assembly according to claim 7 further comprising weighted filter assembly connectable to the connector point.