Motors

By Paul Williams.
Article posted 7th March 2009.

Types Of Motor

There are two types of motor used in Fast Electric model powerboats, these being brushed and brushless. An electric motor produces power and torque by using two sets of magnetic fields that are switched on and off in a precisely controlled way, such that they oppose one another and cause the motor shaft to rotate. In model electric motors, one of the magnetic fields is supplied by a set of permanent magnets, the other field is created by passing current through a set of wires wound around an iron core.

Brushed Motors

In a conventional brushed motor, the magnets are fixed to the inside of the motor can, and the windings and iron core are mounted to the rotor. Some mechanism is needed to pass current into the windings, and in a brushed motor this is done via a set of carbon blocks that wipe, or brush, against the commutator. The commutator can be thought of as mechanical switch that controls when current is passed into the motor windings, by virtue of the angle at which the brushes are set relative to the magnets. This is known as motor timing.

Brushed motors have a number of serious disadvantages. In order to minimise arcing, the brushes must be pressed hard against the commutator, so right away the motor must expend energy to overcome this ever present braking force. Carbon brushes wear down quickly and fill the boat with a fine black dust. At high currents in low wind, 3-pole 540 motors, the commutator wears and burns very quickly due to arcing, fast enough to require frequent skimming on a special miniature lathe to remove the burned and scorched areas and restore the commutator's shape and surface finish. The need for frequent skimming means an armature (the rotor of a brushed motor that contains the windings, iron core and commutator) has a limited lifespan - it can only be skimmed so many times before the commutator has no more material left on it.

If a brushed motor is subject to very high rpm, the windings can move on the armature, which destroys the balance of the rotor. Pushed too hard, the windings can actually detach from the armature with spectacular results. Brushed motors are therefore quite fragile compared to brushless, to say nothing of the amount of maintenance they require in cleaning, comm skimming, new brushes and springs etc. etc.

Another disadvantage of brushed motors is the radio interference caused by the arcing on the commutator. This requires the motor to be fitted with a suppression kit of capacitors and a diode. The biggest problem with brushed motors though is heat. No motor is 100% efficient. All motors waste some of the energy they consume from the cellpack as heat. In a brushed motor, the difficulty is trying to remove waste heat from the armarture and windings. Fitting a water cooling coil or jacket to the motor case only really cools the magnets - the heat trapped in the windings cannot escape across the air gap between the armature and the magnets, and as the windings heat up, their resistance increases proportionately. The more resistance, the more waste heat.

Brushless Motors

Brushless motors solve a lot of the problems inherent in brushed motors by reversing the arrangement of magnets and windings. In a brushless motor, the windings are fixed inside the motor can, with the magnets on the rotor. Motor commutation and timing are achieved electronically in the speed controller, so there is no brushgear - a major source of friction and electrical interference is thus removed completely, making a brushless motor very much more efficient. Brushless motors run cooler, and are easier to effectively cool with the windings located on the outside. After a 5 minute race, a brushless motor should be only lukewarm to the touch - a brushed motor will almost be hot enough to burn skin.

Mega 22/20/1E motor stripped down for new bearings. The endcap is pinned to the can with "tamper-proof" dome-headed fasteners. Tamper-proof, that is, unless you own a Dremel. When you've dremelled out the original fixings, tap the holes M2.5 and use stainless socket cap screws to hold the endcap in place.

Mega 22/20/1E rotor. The rotor is wound with kevlar to retain the magnets at high rpms.

Mega 22/20/1E motor windings and iron cores.

Choosing A Motor

When it comes to choosing a motor, the question of whether to choose brushed or brushless is pointless; brushless motors are superior in almost every way that matters, so the simple advice is to forget that brushed motors even exist. Unless you plan to race in a controlled, restricted class (for example SWAMBCs R2R "ready to race" formula) then there is no point even thinking about buying brushed motors.

Motors are one of the least interesting components of a fast electric boat. Obviously, you have to have one, but the eternal question is "which one?". Take a look on any of the various FE boating forums and there will always be at least one guy asking "I have such-and-such a boat, what motor should I buy?". So, how do you choose a motor for a particular application?

There are a number of variables you must quantify before you can answer this question. For example:

• Mono, Cat or Hydroplane?
• Cells - 2S, 4S, 6S, 8S?
• Purpose - racing or fun or SAWs?
• Budget

Let's look at each of these in turn.

Hydroplanes have the least drag of the three hull types, and therefore can use the hottest (ie., highest revving) motors. A 2S lipo-powered Hydro 1 class boat would probably use a motor of around 4500 - 5000 Kv or more. A 2S Mono 1, in contrast, would be more likely to use a motor of 4000 Kv or so. So, a basic "rule of thumb" is the higher the weight/drag of the hull, the lower the Kv motor needed.

The higher the voltage you run with, the lower the Kv motor you will need. Taking a motor with a high Kv intended for, say, Hydro 1, and running it in a monohull on four lipo cells instead of two will result in very high motor rpm and very impressive speeds from your boat - for a few seconds before the motor explodes. Unless you are into speed record attempts and are willing to sacrifice motors in the pursuit of ultimate speed, then you need to be sensible about matching cell pack voltage to motor Kv.

If you just want a cheap motor for fun running at moderate speeds, then there is a vast range of motors available to you. You're not concerned with having to meet a specific run time, and are probably not too bothered if your cheap motor isn't the most efficient. For fun running, you can pick up cheap motors for around £20 from ebay or from places like Hobby City. It definitely pays to shop around though - it's not unknown for UK retailers to buy cheap motors from the far east and put on a 500% mark up, so do your homework and you can save money buying direct and cut out the middleman.

I mentioned forums just now. These can be a good place to ask the question "which motor?". If you want to race in a particular class, there will be a small range of motors that other racers have tried and which are known to work well. In the UK, try the Astec Models forum. In the USA, try the Offshore Electrics forum.

If you are racing, then you need to decide whether spending more money on a slightly more efficient motor to gain a few extra rpm or a a few more seconds run time is really worth it. For example, Mega motors are solid, mid-range motors that fill the gap between the really cheap chinese stuff and the top-end items from the likes of Lehner and Plettenberg. You can buy a Hydro 1 Mega motor for about £65; a Lehner 1920 series motor is €189, which at current exchange rates is near enough £189 - almost three times the price. You won't get three times the performance from the Lehner, despite the excellent build quality and higher efficiency.

Before you take the plunge and splash the cash, be honest with yourself. Have you totally optimised every other aspect of your boat? The drive system, the wiring, the propeller, the hull - unless all of these things are running at peak efficiency, buying premium motors is a waste of money as you will not see the benefit. Instead of spending the money on a motor, look at your boat. Can you reduce the amount of wiring in the power circuit? Is the propeller really sharp and well balanced? Is the drive system running freely?

A good guide to how your boat is running is the noise it makes at speed on the water. If you can hear an obvious vibration or resonance, then something is out of balance. When you rev the boat out of the water (not to maximum throttle!) the drive should spin without any kind of vibration or noise. Try it with and without a propeller fitted and this will show where any vibration is coming from - most of the time, it's the propeller that's not balanced. Fix this first, before thinking about a better motor.

A wiredrive will resonate badly a certain rpm, due to the natural harmonic frequency of the free wire lengths. As long as this resonance point does not coincide with the on-load full throttle rpm of the boat when running at speed, don't worry about it - it only happens as you throttle up and down and will be appear much worse free running on the bench than when the boat is running on the water where the drive is damped.

So, assuming you have tuned all the other parts of the boat, should you spend perhaps as much as £200 or more on a premium German motor? Only you can answer that question. If you have the money, and you're getting beaten but only just, then maybe a few extra percentage points of efficiency could be the margin you need. Or maybe you need to learn how to drive better? Again, be honest with yourself. Are you driving the best, shortest route around the course? If not, practice more. Does your boat allow you to run close to the turn marker bouys, or is it too unpredictable? Sorting out the handling of your boat to allow you to really shave those turns could be all you need.

With the current global turmoil in the currency markets, the Pound has collapsed in value against the Euro, making goods from the Eurozone extremely expensive. The price of German motors has rocketed, putting them far beyond the reach of all but the most competitive (and wealthy) racers. When you could almost build an entire boat for the price of a single motor, you need to ask yourself if your boat, and your driving skill, would do justice to that kind of expenditure, and whether you can find more speed elsewhere for free.

Brushless Motor Maintenance

As there is no brushgear on a brushless motor, maintenance is little more than oiling the bearings. With no carbon dust to clean up, no brushes and springs to replace and no commutator to skim, life is easy. Some of the cheaper motors come with low quality ball-race bearings that probably won't last too long once they've got wet for the first time. Replacing the bearings is probably the biggest job you will do on a brushless motor.

Motor Cooling

Effective motor cooling can be achieved by using a simple aluminium coil. On my motors, I thermally couple the coil to the motor can using a heat sink compound that ensures heat is conducted from the motor case to the cooling water far more effectively by eliminating any insulating air gaps between coil and motor.

Lehner motors intended for marine use have an integral cooling jacket. You can purchase similar plastic cooling jackets which are sealed to the motor can with rubber O-rings. I'm not a fan of cooling jackets for several reasons. Fitting them can be a nightmare, they can leak at high speeds as the water pressure exceeds the sealing capability of the O-rings, and air pockets and turbulence can leave part of the motor can uncooled. Also, they can be quite expensive. A simple aluminium coil works fine at a fraction of the cost, especially if you make your own.

© Copyright Paul Williams and www.fastelectrics.net, 2010.

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Last modified: 08th July 2010 @ 09:06