How can those little motors replace a great big diesel on A dive Catamaran or even an Ice breaker?
An 18 kw motor by San Diego Boa electric is 17 inches long (excluding the shaft), 15 inches wide and weighs 169 pounds. Yet it can easily turn a 30 inch propeller that would choke a 50 hp diesel. How is that possible?
The answer is that this SDBE configured off the shelve dual motor provide’s usable, prop-turning torque where it's needed most. Greg Moore the engineer say’s “at the low-rpm speeds at which displacement yachts usually motor at my system using electric motors produce as much torque at low rpm as they do at high. They can turn as big a prop at 10 rpm as they can at 1,000.
That's why when people ask us the horsepower ratings of our motors, we tell them instead to focus on what size propeller their boat requires and the maximum rpm it needs to turn. Our motors generally take a larger prop with a greater pitch than the prop turned by a standard-equipment diesel with a much higher horsepower rating.
Horsepower is a misleading term, especially in marine engines. A diesel's horsepower rating is not a measured quantity, but a calculated one. It only applies at a specific rpm, usually around 2500-2800 for a typical marine diesel. At the low rpm used during most sailboat operations, actual diesel horsepower is much less than advertised. More importantly, diesel torque is much less as well.
Torque is what turns the propeller, not horsepower. Torque is the twisting or rotary force applied to a shaft. It's measured as a force pushing at a fixed distance on a lever attached at right angles to the motor shaft. In the English system, force is expressed in pounds and the distance is a foot, so the torque unit is the pound-foot (not foot-pound, which is prop work.) In the metric system, force is in Newtons and the distance is one meter, so the torque unit is the Newton-meter FYI.
“When an engine is tested in the laboratory on a dynamometer, torque is the quantity actually measured. Horsepower is then calculated by multiplying the torque in pound-feet by the rpm and then dividing by 5,252 As per SAE” Greg Moore adds.
Because both horsepower and torque in a diesel drop sharply at low rpm, the propeller turned by a diesel engine must be undersized. A large prop that a diesel would in theory have enough torque to turn at high rpm would stall the engine at low rpm, when the torque is much less. And of course, below a diesel's idle speed, typically from 600-1,000 rpm, torque and horsepower disappear completely because the engine stops running.
That doesn't happen with SDBE electric motors. Their torque curve is flat. Torque doesn't drop off at low rpm, and there's no minimum idle speed. An SDBE motor spins as slowly as you want it to and still turns as big a prop as it can at its maximum speed of 1,000 rpm.
Location of energy source is key difference Why does torque drop precipitously in a diesel at low rpm and not in an E motion electric motor? The key difference is the location of the energy source that produces the torque. In electric motors - as in steam engines - the energy source is external to the device that turns the shaft. In a diesel or any other internal combustion engine the source is internal - a series of explosions inside the engine's cylinders.
Consider an old-time piston steam engine. Water is turned into steam in a boiler and conducted by pipes to the pistons that turn the output shaft. Even if the shaft is rigidly locked in place and can't turn at all - say by a chain wrapped around a ship's propeller - the steam pressure still applies the full amount of torque to the shaft. In other words, a steam engine can have maximum torque at zero rpm.
An SDBE electric motor is essentially the same. The energy source also is external - the battery pack. Chemical reactions there produce negatively charged electrons that mutually repel each other and are attracted to the batteries' opposite, positive poles. The electrons flow from the batteries through a cable to the motor controller, just like steam flows to a turbine through a pipe.
The controller chops the direct current into pulses that flow through the motor stator windings. The pulsing current produces expanding and collapsing magnetic fields. They push against the magnetic fields from permanent magnets arrayed around the motor rotor and make it turn. As long as electricity is pulsing through the stator windings, the magnetic fields push against the rotor fields, and torque is applied to the shaft. (See Pulse-width Modulation.)
That means that the rotor - and the boat's propeller - can turn very slowly and still have the full amount of torque applied to the shaft. In fact, if the propeller were unable to turn at all - if it were locked in ice, for example - the full amount of torque could still be applied to the shaft as long as electricity is pulsing through the stator windings. Resistance on the output shaft of an electric motor - or a steam engine - does not affect the external source of energy.
That's one reason icebreakers are powered by electric motors. They can break the prop free from ice with an extra inrush of electricity. The inrush greatly increases the magnetic field strength in the motor and provides extra torque well above the normal maximum.
Contrast that with what happens in a marine diesel engine with its internal source of energy. Exploding fuel in the cylinders pushes on pistons that push - apply torque to - the crankshaft. Between explosions, a heavy flywheel at the end of the crankshaft keeps the speed up - and the momentum moving.
At low rpm, there are fewer explosions, a longer period of time between them and less momentum from the flywheel. [Momentum equals mv2/2, so "V" (velocity) is a much more important factor than "M" (mass) is.] If something - such as a prop that's too big - restrains the shaft from turning, the exploding fuel can't expand, the fires pushing the pistons go out and the engine stalls. Unlike the external energy source of a steam engine or an electric motor, the internal energy source of a diesel - or any internal combustion engine - is strongly affected by resistance on the output shaft.
That's why an automobile has a transmission. It lets the engine build up speed first so it can produce the torque necessary to move a car. A DC electric motor doesn't have that problem, as anyone knows who has tried to start a manual-transmission car in gear. The tiny starter motor that can be held in one hand easily lurches a 3,000 pound vehicle forward as soon as it begins turning. Even if the car is in third gear, the starter motor can move it. But try to move the car in third gear with its 300 hp internal combustion engine. As soon as the clutch is let out, it That's also the reason modern diesel locomotives are actually diesel-electric. A steam locomotive has no trouble powering its drive wheels directly to set a 100-car coal train in motion from a full stop. Its energy source, the fire in the boiler, is external and unaffected by the load on the drive pistons. But the diesel locomotive's energy source is combustion inside the cylinders, which couldn't take place if the engine were directly connected to the drive wheels. Even multiple 3,200 hp locomotive diesel engines connected directly to the drive wheels would never be able to move the train. Unless it is first running at a high enough rpm, the huge diesel engine in a locomotive can't produce the necessary torque. The multi-gear transmission required for such a massive load would be impossibly large.
So a diesel locomotive transmits torque to the drive wheels through electric motors. The diesel engine spins an alternator to produce electricity for the motors, and they turn the wheels. Unlike the locomotive's diesel engine, the electric motors can apply full torque to the wheels even when they're held at zero rpm by 10,000 tons of coal.
And unlike a marine diesel, an E motion Hybrids motor can apply full torque to the shaft at zero rpm even when it's being resisted by a large, three-bladed propeller.
End complied using e-motion data tables. Reprint by SDBE.
Props by SDBE
SDBE Hybrid motors are built to turn big propellers. We install 3-bladed props with our systems, and they are generally one-two inches larger in diameter and pitch than those turned by a diesel engine sized for the same boat.
A big, slow-turning, 3-bladed prop pushes more water more efficiently than a small, fast-turning one. There's less slippage and less thrust lost from water spinning off the propeller tips. For the same reasons, a big prop regenerates electricity more efficiently, producing more electrical power under sail to recharge the batteries.
A large propeller also provides superior low-speed maneuverability. Especially when docking, a big prop pushing a lot of water provides immediate response - including instantaneous reverse - at any speed from near zero rpm to our motors' maximum of 1000 rpm.
What about the drag? Sailors spend a lot of time and money trying to squeeze the last 1/8th knot out of their boats. Many people don't understand how we can justify adding something that apparently increases drag.
But there are several ways to minimize drag with our system:
1. With a fixed blade prop, keep the motor running at a very low speed so it turns slowly while sailing. This is the recommended option for regeneration. The prop essentially corkscrews through the water and minimizes drag, while consuming only a small amount of electricity. When the wind picks up or the boat slides down a wave, the prop is forced to spin faster by the water rushing by it. The prop turns the motor rotor and produces electricity to recharge the batteries.
2. Install a feathering prop. The blades automatically align themselves parallel to the water flow and greatly reduce prop drag. Feathering props are somewhat less efficient than fixed-blade props for both propulsion and regeneration and considerably more expensive, but they will cut drag to a minimum. Regeneration is still possible by switching the motor briefly into reverse to lock the blades open. The prop will then rotate under sail, turn the motor rotor and produce electricity to recharge the batteries.
3. Install our SDBE motor with a retractable outboard drive leg such as the Sillette Sonic, if the boat configuration allows. The big advantage is that the prop can simply be swung out of the water under sail, and prop drag is eliminated. But there also are a couple of disadvantages. Regeneration is reduced because of resistance in the drive leg's bevel gears and U-joint. And the largest allowable prop diameter in the manufacturer's current line is 16 inches, which is smaller than the 18-22 inch props used with our 9 kw and 16 kw motors. The smaller diameter can be compensated for to some extent by increasing the pitch, for example, by using a 16 x 20 prop. Drive legs that accept larger props can be custom-built to order, but at greater expense.
4. Install a folding prop. Probably the most common form of prop drag reducer, the folding prop is the least desirable for our system because it eliminates regeneration. The blades simply fold back as they're designed to do when water flows by them under sail. There's no way to lock them open as can be done with a feathering prop. But in certain circumstances a folding prop may make sense. For example, a racing skipper uninterested in regeneration or efficient motoring could choose a simple two-blade folding prop to minimize drag.
Pulse width Modulation for marine by Greg Moore of SDBE
SDBE (sandiegoboaelectric) hybrid motors use three-phase, permanent-magnet, pulse-width-modulated motors. The rotor is fixed to a shaft supported by a tapered roller-thrust bearing at each end.
A ring of powerful permanent magnets is arrayed around the rim of the rotor. The magnets are an advanced alloy of neodymium, iron and boron (NdFeB). In a conventional motor, the rotor is wound with coils of copper wire, which must be energized with electricity to create a magnetic field. In a permanent magnet motor, the only windings are those that make up the stator, which sits inside the motor's external casing.
A digital controller which is an off the shelve common device used by electrical engineers for years turns the continuous DC from the batteries on and off, creating a series of pulses that pass through the stator windings. The pulses create expanding and collapsing magnetic fields. The magnetic fields alternatively attract and repel the poles of the permanent magnets on the rotor, causing it to spin. Greg Moore’s SDBE system synches props in dual prop applications for ultra smooth water flow and less vibration than conventional disels synched.
The controller regulates the frequency and duration of the pulses to control the amount of current passing through the stator coils. Each pulse gives the rotor a push. The longer the pulse lasts, the longer the push lasts and the more current passes through.
Represented on a graph, the electrical pulses form a series of rectangular peaks. Voltage is on the Y axis, and time is on the X axis. The peak of each rectangle is at the output voltage of the battery pack, 144 volts. The width is the length of time each pulse lasts.
Since the controller regulates the width, i.e., the duration, of the pulses, the control technology is known as pulse-width-modulation, or PWM. Although voltage from the battery pack stays relatively constant at 144 VDC, the average effective voltage applied to the stator windings varies with the pulse duration.
When the pulses are on 25% of the time – known as a 25% duty cycle - the motor is effectively receiving 25% average voltage. If the pulses are on a 75% duty cycle, the effective average voltage is 75%.
"Hall-effect" sensors in the stator windings provide feedback to the controller to regulate motor speed. As the electromagnets pass by the sensors during each revolution, they send a signal back to the controller. The controller compares the sensor signal rate to the rate of the pulses it sent through the stator windings. If it's not the same, the pulse duration, or width, is increased or decreased to keep the rotation at the desired speed.
E motion PWM motors are very efficient because electricity flows only during the pulse. When the pulse is completed, there's no electricity consumed. With narrow pulses, the electricity is off more than it's on. Speed control is very precise because the digital circuitry can make minute changes in pulse duration, effectively creating a smoothly varying speed from minimum rpm to maximum. There's no more electricity wasted at low speeds than at high, since electricity is simply turned off for a longer period of time for low speed operation.
By contrast, a conventional AC motor is much less efficient because it's regulated by controlling the voltage to the windings with resistors. To reduce rotational speed in a conventional AC motor, more resistance is put into the circuit. Voltage to the windings is reduced, but the same amount of electricity is consumed. The resistance has simply converted part of it to useless heat, which is very wasteful.
Speed control in a conventional AC motor, such as the low, medium and high settings in a window fan, is extremely crude. Since voltage is reduced by switching resistors into the circuit, the speeds available are limited to the voltage variations possible from different resistors or resistor combinations. Not good. End
It produces direct current at144 volts - not alternating current like most marine generators - and recharges the battery bank directly.
When battery voltage in the main bank declines to a pre-set level, the generator automatically starts up to recharge them. The boat can continue motoring as the generator brings the batteries up to full charge. When the batteries are fully charged, the generator automatically shuts off.
A controller on the generator monitors current going into the battery bank and current going out to the load.
Besides automatic start and stop, the controller provides 0. A trickle charge safety test for shorted batteries 0. A bulk charge rapid charge capability 0. An equalizer charge for battery conditioning for longer life. 0. A float charge for battery maintenance while accessories are operated 0. Charge shutdown on sensing battery over-temperature 0. Remote battery voltage sensing. The Polar DC generator is powered by a Volvo Penta series D1 or D2 diesel engine, depending on kw output. The engine turns an alternator that produces electricity.
All alternators create alternating current in the stator windings. But in a DC generator, a series of diodes – one-way pathways for electricity - convert the AC into DC. The diodes are mounted separately, on a large heat sink away from the alternator, to protect them from heat.
The alternator width in cross section (left) varies in three increments for increasing output ranges up to 32 kw.
Like our propulsion motors, the rotor consists of a ring of neodymium-iron-boron permanent magnets.
The alternator stator is held in place by a yoke that fastens to the engine bell housing. The alternator rotor is attached to the flywheel.
DC-6-A DC-10-A DC-15-A DC-22-A DC-32-A kw output (cont. 24 hrs) 6.5 10 15 22 32 Volvo Penta engine model # D1-13 D1-20 D1-30 D2-40 D2-55 # cylinders 2 3 3 4 4 rpm (max) 3200 3200 3200 3200 3000 rpm (cont. 24 hrs) 2700 2700 2700 2700 2500 weight lb. (kg) [excl. cover] 302 (137) 353 (160) 379 (172) 430 (195) 684 (265) length - in. (mm) 23.6 (600) 24.8 (630) 26.2 (665) 29.6 (751) 35.4 (900) width - in. (mm) 18.7 (476) 18.5 (471) 19.0 (482) 19.0 (482) 20.5 (520) height - in. (mm) 20.4 (518) 21.1 (535) 21.3 (541) 22.6 (575) 32.7 (830)
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