Getting to Know Your Electric Starter By James Goss      I can remember when I first saw an electric starter being used to crank a model engine. I think it was in 1968, or in that general time frame. It truly amazed me when the guy placed the starter against the spinner and the engine cranked with no effort at all. At that time I had been around model engines for some time and had several myself on small control line planes. Flipping it by hand was the only way that I new to crank these little engines. Well, I did see two guys trying to crank a small model engine using a bicycle turned upside down with one of the guys turning the pedals. With the wheel turning at high speed, the second guy placed the engine shaft against the wheel. He was holding the engine in his hand and the engine didn't have a prop on it, nor did he have a battery to heat the glow plug. They were also using regular gasoline for fuel. As I think back, it must have been a little .049 engine with a self-contained fuel tank.  They cranked and cranked but the engine just would not hit a lick.  Obviously they didn't know anything about model engines at all. I was around twelve years old at that time and when I told them “you have got to have a battery to crank that engine” they said, “yea right, sure you do, we have been working on engines before you were born”. When I walked away and looked back, the fools were still pedaling away. These two guys had to be the stupidest fellows I have ever met to this day.      A lot of modelers still like to hand prop an engine for starting. It does look nice to see an engine start with one or two flips of the prop, especially when it starts on the first flip. When I hand prop my engines I will use a chicken stick most of the time. I don't like to get hit on the finger, particularly in cold weather. Nowadays it is quite common to see modelers using an electric starter to get things going. They have become a tool of our trade, so to speak. Electric starters may not be as good for your engine, but they sure do get it running in a hurry. The reason I say it may not be good for your engine is that during starting you are applying a lot of force to the crankshaft of the engine. How much? Who knows, but all that force is transferred directly to the ball bearings. This will cause wear on the balls and race, but for no longer than we keep it attached to the shaft the engine will last for years. Not so for bushed engines though. Those bronze bushings will wear more than the steel ball bearings will. The smaller the bushed-engine is, the more effect there will be on its life span, but you still get many years of service from the little guy. It's always best if you apply your starter to the engine in a short burst, and not in a long run. A lot of times the engine will crank and the modeler will not realize it, he will continue to hold the starter to the spinner creating undue stress on the engine.      One thing is for sure; using an electric starter to crank your engine is the safest way to start it. You can still have an accident while starting your engine if the drive cup on the starter does not have a rubber cup that matches your prop drive nut or spinner. I have seen this quite a few times and have even been guilty of this myself. Starting an engine that only has a hex nut, not a spinner nut, will allow the cup to jump off the nut, the starter and your hand may go into the prop. Ouch! I guess everyone has got his finger hit by the prop at one time or another if they have been in this hobby for a while. By and large, you will get hit a fewer number of times when using an electric starter. If you have been in this hobby for at least three years and have never got your finger hit by a prop, please let me know and I will publish this unusual fact in our newsletter. We all know one of our club members that even got his finger cut off with a sixty-size engine. Lets try and be more careful with these engines, they can really do some damage in a hurry.        Our electric starters are pretty reliable in there operation. We use them for years and years, while they are treated pretty rough along the way. They are one of the most reliable components we have in the hobby. If the manufacture would just put a better on off stitch on them because this is where a lot of our trouble comes from.  It's not unusual to see a modeler fling his starter several feet through the air after the engine has been started. I guess they get excited like a baseball player slinging his bat after a hit. Here is something we see a lot of at the field. A guy is in the process of starting his engine and realizes he forgot to get the starter. Instead of getting up off the ground, he grabs the line cord and pulls the starter across the ground at about 50 miles an hour. Also you may see that same guy carrying his starter by the line cord with the starter motor swinging like a pocket watch on a chain. I guess I have been guilty of this at one time or another, but I have been using the same starter for twenty-five years and it is still going strong. The electric starter is not very expensive, but I still like to take care of it. What can you do to take better care of your starter? If you have a good understanding of how the starter operates and what is inside the case, then you will know how to take care of it.      As you already know, our starters are direct current motors (dc), not unlike the ones in our cars, just smaller. The starter motors for autos have two windings in them, whereas our starters only have one. The big starters in our cars have an armature winding and a field winding. The armature is the part that rotates and the fields are stationary. Remember that it takes two magnetic fields to create torque. Torque is the twisting power that the shaft develops when two magnetic fields are present. The armature and field windings are wound, in most cases, with copper wire known as magnetic wire. Magnetic wire has a thin enamel coating for its insulation and can be wound very close to its neighboring windings. The field winding is an electromagnet that depends on current flow through it for its strength, the same as the armature winding does. The more current, the more strength the fields will have, thus producing more torque.      The difference between our little starters and an automobile starter is that our starter does not have field windings. You may find some very old starters, or a starter that someone made from an old wiper motor that still has field windings. Starters with field windings will draw more current than ones without field windings. You may wonder how a dc motor can run without field windings because I just stated that it takes two magnetic fields to have torque. Instead of using windings, our starters have permanent magnets for the fields. This works very well and the motors are more compact in size. Now on some really big hand starters you may still fine windings for the fields. It is easy to test your starter to see if you have field windings or field magnets. If you change the polarity of the dc input to a permanent magnet motor, it will reverse its direction of rotation. If you change the polarity of the input on a motor with field windings, it will still run in the same direction. On any dc motor, except the permanent magnet field motor, you must change the polarity of the field windings in respect to the armature windings, or change the armature polarity in respect to the field windings, to reverse its rotation. I am sure you have accidentally plugged your starter leads in reverse polarity, at one time or another, and your starter ran in the wrong direction. This indicates you have a permanent magnet motor (pm motor).  Another way to tell if you have a permanent magnet motor is to rotate the drive cup by hand; if it is a permanent magnet motor you will feel the armature try to stop at many different locations as you rotate it.        The only wire in this motor is on the armature. The two line leads go in and make contact to the armature through carbon brushes. You have two brushes, one for the negative input and one for the positive input; this makes it a two-pole motor. The brushes ride on some copper segments called a commutator. The job of the commutator is to set the timing of the armature coils so they are in the right position when the brushes connect dc voltage to them. This insures the shaft develops maximum torque.     From time to time you may need to go inside your starter and give it a cleaning. Even though these starters are sealed, they get a lot of contamination from within the starter itself. They are very simple to take apart for servicing and cleaning. Be sure to mark the end bells with a mark that will not fade away so you can get good alignment when you reassemble the motor. When inside you will notice that a black residue has formed inside your starter if it is a few years old. This comes from the carbon brushes and can be a problem if it gets into the slots between the commutator segments. These tiny segments are insulated from each other by mica insulation and should not have any thing that is conductive between them. You can clean these slots by using a sharp object to scrape them clean and cut the mica down if it is higher than the copper segments. This is called undercutting the commutator and is typically done in electric motor shops with a tiny saw. Next clean the commutator so the brushes will make good electrical contact. You can clean it by using some 400-grit paper. Now would be a good time to replace the brushes with some new ones if the old ones are worn. Also at this time you should check the solder connections where the armature wire connects to the commutator, some may need to be soldered again. There is a lot of centrifugal force exerted on the wire at this location.      The smaller starters will always use bushings instead of ball bearings. The bushings are made of bronze and are of the self-aligning type. This means that they are round in shape and can swivel around in a socket. This prevents excess wear on them because they will always be aligned with the armature shaft. Basically you will never need to oil these bushings because most of them are impregnated with graphite as they are being manufactured. As the bushing (sometimes called a sleeve bearing) wears, it gets new lubrication while the motor is in operation. A tiny amount of oil would not hurt anything, but excess oil will get on the armature windings and damage the insulation. Best not to add any oil. If the bushings wear too much, it will allow the armature shaft to touch the magnets as it rotates around, this is called polling out, and the motor will be sluggish when running or will not run at all. Time to replace the bushings, a couple of dollars, or get a new starter.  Most would get a new starter. This is normally what happens when the starter in your auto begins to fail. It will start dragging and lock up, you hear the solenoid click when you turn the ignition key, but nothing happens. So you think you have a weak battery or alternator problems. After you check things out you get a new starter when all you need is a set of bushings that cost about $2.00. It takes about 15 minutes to replace the bushings.      One thing of interest that I need to mention about our little starters when trying to crank a larger engine is its voltage rating. The horsepower varies directly with the torque and speed; applied voltage determines the amount of current through the armature so voltage determines torque. It we increase applied voltage; the output power will also increase. Even though the starter is rated for 12 volts, it will handle 24 volts for short periods of time. If you really want to spin your engine try the 24 volts instead of 12 volts. Remember to not run it for long periods of time. To obtain the 24 volts, connect two 12-volt batteries in series aiding. If you connect them in series opposing, you will get 0 volts. Our small starters are rated for about 200 in oz of torque. If you double the voltage the torque will also double, as will speed. Heavy-duty starters that are designed to run of 24 volts will produce 600 to 800 in oz of torque. Most of these will draw a locked rotor current of 60 to 80 amps and will crank engines up to about 2.4 cubic inches (40 cc). Still heaver starters, rated at 100 amps, will produce up to 1200 in oz of torque on 24 volts. These are the big double handle jobs that will crank a 130 to 150 cc gas engine. They have a lower rpm, about 5600 no load rpm on 24 volts, which along with the higher current draw will produce the greatest starting torque.      Not having field windings means that our starter is more reliable and less likely to become defective. The magnets are of the ferrite type and the motor will contain two of them. This will be called a two-pole motor. Unlike an ac motor, a dc motor's speed is not determined by the number of magnetic poles, but rather by the amount of voltage applied to the motor. With 12 volts applied to our starters they will run around 5000 rpm with no load. With a load such as a .46 or .61 engine offers, the speed will drop to around 3000 rpm. When the starter is running at 5000 rpm with no load, it will pull very little current from your battery. This is because as the motor is running as a motor, it is also running as a generator. A voltage is developed across the armature that is of opposite polarity to the input voltage. This voltage is called Counter Electromotive Force or CEMF for short. This is how it works. When your starter is at standstill, no CEMF is present to buck the source; this allows maximum current to flow. So every time you apply voltage to your starter, you have max current flowing until the armature starts to move. This may be anywhere from 2 to 30 amps, depending on the mechanical load on the starter. As the starter gains speed, CEMF will build up and current will decrease for any given load. So the faster your starter is turning the less current it will be pulling, thus the armature windings will be cooler. Not having any CEMF when the armature is at a standstill explains why we have such a large surge current when you first close the switch to this type of dc motor. The surge current may be as large as 10 times the normal full load current. This can drain our battery down pretty fast if our engine does not want to start.      How strong are these electric starters? Lets say the engine you are starting is an os .46 FX which is rated at 1.62 bhp @ 16,000 rpm.  Horsepower is equal to the rpm of the engine times torque divided by 5252. So you can see that hp is directly proportional to the rpm of the engine. This hp rating is for the output of the engine, but it works both ways. If you were turning the engine at 16,000 rpm it would take 1.62 hp to turn it at that speed, due to the compression and friction inside the engine. Depending on the charge-state of your battery, the starter will be turning the engine at about 3,000 rpm. This is only 18.75% of the 16,000rpm. Since hp is directly proportional to the speed, the hp needed to turn the engine would be about .3 hp. Now take the os .91 FX engine that is rated at 2.8 bhp at 15,000 rpm. If your starter would turn this engine at 3,000 rpm, this would be 20% of 15,000 so 20% of 2.8 will be .56 bhp. You are not going to get much more than 1/4 to 1/2 hp out of these starters without having locked rotor. This horsepower is a developed horsepower, which means that it will not start under a ½ hp load; it develops more horsepower as it builds up speed. If the starter were to produce ½ hp at 12 volts, it will be drawing about 30 amps of current. This is a lot of current to be flowing on the small gauge wire that is placed on the starters. One horsepower will require about 60 amps of current.       Most average starters will come with 16 awg copper wire. The ampacity rating for number 16 awg is about 10 amps and 14 awg is 15 amps. When your starter stalls and will not turn the load, we call it locked rotor, it will be pulling as much as 40 amps of dc current from your battery. Yes your 8-amp battery can supply as much as 40 to 60 amps for a short while. With the armature not moving it looks like a short circuit to the battery because there is no CEMF being developed by the armature. This is when your armature really begins to heat up and will shorten the life of the starter motor. Heat is the number one enemy for insulation material, so if your starter locks up, remove it from the spinner as fast as you can.      If you replaced the number 16-gauge wire on your starter with some number 12-gauge wire, you would see some real difference in its power to start a larger engine. At its upper limits when high current is flowing, you are getting a pretty large voltage drop, about 1.5 to 2 volts, due to the conductors and connectors that you are using. This voltage drop is reducing the maximum output power that the starter can now develop. Larger wire will reduce this voltage drop by a good percentage. The 12-gauge wire can withstand 20 amps forever and 30 amps for quite some time. For the same load it will only have about .5 to .7 volts dropped by the supply conductors. It's not the copper wire that can't withstand the high current; it's the insulation. The 12-gauge copper itself can withstand 100 amps for about a minute or two before it destroys itself, but the insulation will be smoking in about 10 seconds. It's the resistance of the copper wire that creates the voltage drop. All that can be done to increase the rating of our starters by the manufacturer is to wind the armature with larger wire, use stronger field magnets, and have larger supply conductors. There is not much that can be changed about the armature core; its basic design has been the same for years.      Sometimes there is some confusion about hp (horsepower), torque, and bhp (brake horsepower). One horsepower is referenced to a mechanical device lifting 550 foot-pounds per second. For electrical applications this value is converted to 746 watts. For example if an electric motor is rated at 1 hp @120 volts, it will require 6.2 amps of current from its supply. Simply divide 746 by 120. This is assuming the motor is 100% efficient. Most motors are around 80% efficient, so you would multiply the 120 by .8, which now produces 7.7 amps. It works the same way with our starters; they are about 80% efficient so that accounts for some of the higher currents. Bhp or brake horsepower, sometimes called shaft horsepower, is the amount of power available for useful work. This is why all our model engines are rated in bhp. Bhp is less than indicated horsepower because of the various power losses that occur inside an engine. Bhp has the sum total of all mechanical losses deducted from the total horsepower. Indicated horsepower measures the power produced inside the cylinder. This is arrived at by the average pressure on a piston, the area of the piston, the length of the stroke of the piston, and the number of power strokes per minute. So it is the bhp that us modelers are interested in. It is also interesting to note that an os 120 four-stroke engine only has a rating or 1.9 bhp  @11,000 rpm compared to the .91 as having 2.8 bhp @ 15,000 rpm. This is where torque comes into the picture. Torque is inversely proportional to the rpm of the engine. That is to say that as the speed on our engines decrease, the torque (or the twisting power of the shaft) will go up. By the four-stroke engine running at a lower rpm it has the torque to swing a much bigger prop that produces more thrust. If you ran the .91 engine at 11,000 rpm you would develop about the same 1.9 bhp rating that the four-stroke has, but it would not be moving as much air past the prop as the four-stroke will with its bigger prop. The two stroke depends a lot on the inertia of the prop for developed hp and must turn fast to achieve it. It is the same with our starters, the slower they turn the more torque they develop.      Brake horsepower got its name because of the way hp is measured on a shaft. A force is developed to place a load on the shaft by pressing a device, called a brake, against the shaft while the shaft is rotating. Thus the name “brake horsepower”. If you don't get anything worthwhile from this article, maybe you will at least remember this tip in the future. The next time you are having trouble starting your engine with your electric starter, just throw your starter about five feet across the ground and get a bicycle.