Multiple motor & controller approach?
Posted: Tue Aug 19, 2008 5:28 pm
The thread on high voltage versus low voltage, plus Malcolm's timely reminder on the IoM thread about the power to weight ratio of brushless model motors, has got me thinking.
We're used to thinking in terms of a single big motor, or perhaps a set of wheel motors, as being the preferred approach for an EV. This used to the the preferred approach in other areas, like computing, but we've gradually come to realise that a distributed architecture can provide some benefits, like better performance, a measure of redundancy or lower overall cost.
We're also used to the idea that high voltage means lower losses and better efficiency. However, that comes at a price - any vehicle that has a power supply that's greater than 50V is supposed to comply with some pretty stringent safety requirements, to reduce the risk of death or injury from electric shock, both in use and in the event of an accident. Clearly there is some safety (and potentially regulatory compliance demonstration) benefit from opting for a low voltage (sub-50V) system, if only the efficiency issue could be adequately resolved.
One way to reduce the effect of losses is to look at a multiple motor/controller solution. What if an array of small, efficient, brushless motors were connected together mechanically, with each run by it's own controller, and used to provide a multi-motor that could be treated like a single, bigger motor?
The obvious questions revolve around technical feasibility, efficiency, cooling, weight and cost.
Technical feasibility
Brushless, sensorless motors will happily work together if mechanically locked and driven by individual controllers. This has been proven by modellers who have experimented with motors on a common shaft.
Motor starting is now good, with even budget controllers having good start algorithms. These motors can be simply reversed, either by the controller or by simply swapping two phase wires. Not all motors would need to be reversed to provide a reverse gear, as reverse wouldn't require full power. There are no good technical reasons as to why such a configuration shouldn't work well, provided that the mechanical installation is adequate.
Efficiency
Brushless motors in the 1 - 5kW range can be around 90% efficient, with fairly broad and flat efficiency curves. Controllers are around 98 to 99% efficient, even at fairly high power outputs and operating currents up around 80 to 100A. There would be some additional loss caused by the combining transmission, but this could, perhaps, be as low as 3% or so. Overall motor/controller system efficiency of a multiple motor system could be over 85%, with good design and careful component selection.
To reduce battery losses, and perhaps add redundancy, separate battery sub-packs for each motor would seem a good idea.
Cooling
This is an area that needs careful attention, because small motors have a low thermal mass, so will overheat quickly if overloaded. On the other hand, a multiple motor array has a high surface area to volume ratio, so should cool more effectively than a single large motor. Because of the mechanical configuration it's likely that some form of forced air cooling would be needed.
Weight
This is where the multiple motor/controller approach starts to look attractive. A typical brushless motor that will deliver a couple of kW continuously (maybe 3kW peak) will weigh less than 1kg, complete with it's controller. Even allowing for the mechanical components needed to both couple the motors together and reduce the output shaft speed to a more useful rpm will still result in a motor and controller combination that is much lighter than an equivalent power single motor and controller.
Cost
Another advantage is the significant cost benefit from the multiple motor/controller approach. I've compared the watts per pound for some typical brushless motor/controller combinations with my own Mars ME0709/Alltrax 4834 system. Brushless motor and controller combinations working at around 30V, delivering continuous power outputs of about 1500 to 1800 watts, deliver around 35 to 40 watts per £. The mechanical and transmission parts will add to the cost, but I would be surprised if the this reduced the above figure below about 30 watts per £. For comparison, the Mars/Alltrax combination delivers around 12 to 15 watts per £, so it looks as if the multiple motor/controller could be around half the price for any given power output.
Obviously there are some snags with this approach, the most significant being the added mechanical complexity and potential reliability issues this may bring. If the coupling transmission used toothed belts, then it could use standard, off-the-shelf, components, with good reliability and efficiency, plus the advantage of low noise levels. If CNC machined kits were made available, consisting of a pair of alloy sandwich plates to mount the motors, plus accurately machined spacers, bearing housings and an output shaft, then such a system would be relatively easy to assemble.
The sandwich plates could incorporate the cooling system, if the sides were enclosed and the space between fed by a high pressure fan, feeding air to each motor via it's perforated end bell, to exit via the spinning rotor end vents that would project out from the plates. The controllers could be mounted inside the space between the plates, to produce an integrated unit that only needs power supply and throttle connections.
To sum up, what I'm proposing is a multiple motor/controller unit that could be substituted for a single large motor and controller, but at a lower cost and with a lower weight. I would like to encourage a sensible and constructive debate on the pros and cons of this sort of approach, rather than start a contentious debate on what's intrinsically "right" or "wrong" - I fully appreciate that it's a novel and unusual approach that might offend the traditionalists!
Jeremy
PS: If anyone wants an idea of the sort of motor and controller that I'm considering in this (lengthy!) discussion starter on low voltage, multi motor, systems, then take a look here:
Motor: http://tiny.cc/xPlns
Controller: http://tiny.cc/eavKS
We're used to thinking in terms of a single big motor, or perhaps a set of wheel motors, as being the preferred approach for an EV. This used to the the preferred approach in other areas, like computing, but we've gradually come to realise that a distributed architecture can provide some benefits, like better performance, a measure of redundancy or lower overall cost.
We're also used to the idea that high voltage means lower losses and better efficiency. However, that comes at a price - any vehicle that has a power supply that's greater than 50V is supposed to comply with some pretty stringent safety requirements, to reduce the risk of death or injury from electric shock, both in use and in the event of an accident. Clearly there is some safety (and potentially regulatory compliance demonstration) benefit from opting for a low voltage (sub-50V) system, if only the efficiency issue could be adequately resolved.
One way to reduce the effect of losses is to look at a multiple motor/controller solution. What if an array of small, efficient, brushless motors were connected together mechanically, with each run by it's own controller, and used to provide a multi-motor that could be treated like a single, bigger motor?
The obvious questions revolve around technical feasibility, efficiency, cooling, weight and cost.
Technical feasibility
Brushless, sensorless motors will happily work together if mechanically locked and driven by individual controllers. This has been proven by modellers who have experimented with motors on a common shaft.
Motor starting is now good, with even budget controllers having good start algorithms. These motors can be simply reversed, either by the controller or by simply swapping two phase wires. Not all motors would need to be reversed to provide a reverse gear, as reverse wouldn't require full power. There are no good technical reasons as to why such a configuration shouldn't work well, provided that the mechanical installation is adequate.
Efficiency
Brushless motors in the 1 - 5kW range can be around 90% efficient, with fairly broad and flat efficiency curves. Controllers are around 98 to 99% efficient, even at fairly high power outputs and operating currents up around 80 to 100A. There would be some additional loss caused by the combining transmission, but this could, perhaps, be as low as 3% or so. Overall motor/controller system efficiency of a multiple motor system could be over 85%, with good design and careful component selection.
To reduce battery losses, and perhaps add redundancy, separate battery sub-packs for each motor would seem a good idea.
Cooling
This is an area that needs careful attention, because small motors have a low thermal mass, so will overheat quickly if overloaded. On the other hand, a multiple motor array has a high surface area to volume ratio, so should cool more effectively than a single large motor. Because of the mechanical configuration it's likely that some form of forced air cooling would be needed.
Weight
This is where the multiple motor/controller approach starts to look attractive. A typical brushless motor that will deliver a couple of kW continuously (maybe 3kW peak) will weigh less than 1kg, complete with it's controller. Even allowing for the mechanical components needed to both couple the motors together and reduce the output shaft speed to a more useful rpm will still result in a motor and controller combination that is much lighter than an equivalent power single motor and controller.
Cost
Another advantage is the significant cost benefit from the multiple motor/controller approach. I've compared the watts per pound for some typical brushless motor/controller combinations with my own Mars ME0709/Alltrax 4834 system. Brushless motor and controller combinations working at around 30V, delivering continuous power outputs of about 1500 to 1800 watts, deliver around 35 to 40 watts per £. The mechanical and transmission parts will add to the cost, but I would be surprised if the this reduced the above figure below about 30 watts per £. For comparison, the Mars/Alltrax combination delivers around 12 to 15 watts per £, so it looks as if the multiple motor/controller could be around half the price for any given power output.
Obviously there are some snags with this approach, the most significant being the added mechanical complexity and potential reliability issues this may bring. If the coupling transmission used toothed belts, then it could use standard, off-the-shelf, components, with good reliability and efficiency, plus the advantage of low noise levels. If CNC machined kits were made available, consisting of a pair of alloy sandwich plates to mount the motors, plus accurately machined spacers, bearing housings and an output shaft, then such a system would be relatively easy to assemble.
The sandwich plates could incorporate the cooling system, if the sides were enclosed and the space between fed by a high pressure fan, feeding air to each motor via it's perforated end bell, to exit via the spinning rotor end vents that would project out from the plates. The controllers could be mounted inside the space between the plates, to produce an integrated unit that only needs power supply and throttle connections.
To sum up, what I'm proposing is a multiple motor/controller unit that could be substituted for a single large motor and controller, but at a lower cost and with a lower weight. I would like to encourage a sensible and constructive debate on the pros and cons of this sort of approach, rather than start a contentious debate on what's intrinsically "right" or "wrong" - I fully appreciate that it's a novel and unusual approach that might offend the traditionalists!
Jeremy
PS: If anyone wants an idea of the sort of motor and controller that I'm considering in this (lengthy!) discussion starter on low voltage, multi motor, systems, then take a look here:
Motor: http://tiny.cc/xPlns
Controller: http://tiny.cc/eavKS