Magmotor Break-in, Repair, Adjustments, and Battery Choice

Break-in
Each motor is tested before it leaves the factory, but it is a good idea to make sure that the motor is fully broken in before using it under high loads. Just run the motor at 12 Volts for about 10 minutes with no load. Let it cool and do the same in the opposite direction. You might also want to run it a few minutes at 24 Volts with no load.

Repair
Here are some tips if you ever decide to take the motor apart: (1). Remove the brushes and mark them for location and orientation. (2). Scribe a line across the rear end-bell and the black magnet housing. When you reassemble the motor use this scribed line to realign the rear end-bell and the housing in exactly the same orientation as it was originally. (3). When you reassemble the motor put each brush back into the slot from which it came and make sure to orient them the same as they were originally. If you get new brushes or if you change the orientation of the old ones, you should break the motor in again. (4). Tighten the screws very well. The extreme levels of torque possible with these motors can twist the magnet housing relative to the mounting plate if the screws are not tightened properly. While you have the motor apart you can check for damage in the brush assembly. Check to see if the spring is still "springy" and the brush slides easily in the slot. You should also make sure that the little felt insulating washers are in place under the brush cap. If you have pressurized air you can blow the carbon dust out of the bush holes. If the brushes don't slide easily you might be able to loosen them up by filing the inside of the brass brush tube. You should take the whole motor apart if you file the tubes.

Timing Adjustment
You can adjust the timing of the motor by rotating the magnet housing relative to the rear end bell. Your scribed line, (from step 2 above) will be for neutral timing. Advancing the timing will cause the motor to speed up, retarding the timing will cause it to slow down and run rough. If you plan to use the motor in just one direction you might want to try advancing the timing to get two to three hundred extra RPM. Do not operate the motor with retarded timing. You can fine-tune your motors using the Astroflight Whatt Meter. For neutral timing, rotate the magnet housing so that the motor draws the least amount of current. The no-load current should be about 3.5 Amps (C40), 3.4 Amps (S28-150), and 4.5 Amps (S28-400). When using the Whatt Meter make sure it is connected to the battery and "booted up" before making the connection to the motor.

Spinner Tips
BattleBots requires that all spinning weapons be designed such that they can be stopped within 45 seconds of powering them down. This can be accomplished by shorting the motor leads together after you shut off the power. Braking a flywheel by shorting the motor can be very hard on the motor. The current passing through the windings and the brushes can momentarily be much higher than the current required to spin it up. This is because the internal resistance of the battery is no longer in the circuit. You can reduce the wear and tear on the motor by shorting it through a resistor. You will need a high-power resistor in the range of about .1 to .5 Ohms. You can make your own resistor from a coil of wire. Fifty feet of 20 gauge will give you .5 Ohm and weighs just a few ounces. Make sure you use wire with high-temperature insulation (Teflon or silicone). Start with more wire than you need and test the spin-down time, then you can trim the wire to the length required to get to your desired spin-down time. The shorter the wire - the faster the spindown. Make sure that the resistor is not in the circuit when you are powering up the flywheel or the spin-up time will be increased.

Some people use a "V" belt to power their spinner because it allows some slippage and reduces the current draw and heating of the motor. If there is excessive slippage the friction on the motor pulley will cause heat build-up. A better solution is to use a torque limiter rather than relying on belt slippage. Here are some nice torque limiters: Dalton, McMaster Carr (bottom of page).

Overvolting
We don't recommend running the Magmotors at more than 24V but many people have done it with mostly good results. More voltage is not really a problem for the motors - it's the higher current that causes motor heating, and using higher voltage will cause a motor to try to draw more current. If you are careful about limiting the current or the duty cycle you can get away with higher voltages without damaging the motors. You can limit the current by using a battery that can't supply supper-high current, or you can use a current-limiting speed controller, or you can use one of the torque limiters mentioned above.

For example: The best power to weight ratio for a spinning weapon in a lightweight robot is probably going to be a single Magmotor S28-150 with a single 36 volt battery pack. At 36V the motor develops 3.2 horsepower at 80 Amps. You probably won't need that much horsepower to keep the blade spinning unless it has very poor aerodynamics or a lot of friction. The battery is 6 pounds and the motor is 3.8 pounds for a total of less than 10 pounds. The RPM at 36V and 80A will be about 7800 so you need to use an appropriate gear ratio. You probably can't overheat the motor with a single Battlepack, but if you try 36V with two or more Battlepacks (or Hawkers), you run the risk of overheating the motor. In that case we recommend either torque, current, or duty-cycle limiting. If you don't use any form of current limiting, you should consider using the S28-400 motor instead, (it will handle higher current for longer than the S28-150. For even higher current handling, consider using the C40-300).

Another example: Which is "better", running the C40-300 Magmotor at 24V and high current, or 36V and higher gear reduction in a spinning weapon?

Case 1:
Three 24V Battlepacks, assuming 80A current output = 240 A limit.
Stall Torque: 1905 oz-in
Top Speed: 4000 rpm
Maximum Efficiency: 83.7% @ 3685 RPM
Current draw at Max Eff: 41 Amps
Maximum Horsepower at 240A: 3.8
Maximum Horsepower at unlimited current: 3.8

Case 2:
Two 36V Battlepacks (same number of cells as above) = 160 A limit.
Stall Torque: 1260 oz-in
Top Speed: 6000 RPM
Maximum Efficiency: 86.5% @ 5620 RPM
Current draw at Max. Eff: 50 Amps
Maximum Horsepower at 160A: 5.87
Maximum Horsepower at unlimited current: 8.6 (momentary only).

As you can see from the above numbers, you are better off using 36V at 160A rather than 24V at 240A. Using 36V would also result in less motor heating, (as long as your average current stays below the average current of the 24V set-up).

The C40-300 will handle 160A and generate 5.87 horsepower for a full 3 minutes but you can expect to fuse it very quickly at 8.6 horsepower.

Battery Choice
Just as every motor has a maximum horsepower figure, every battery can also be thought of as having a maximum possible "horsepower". Batteries that have very low resistance, (like the Hawkers) can supply nearly all the current the Magmotors can draw. Other batteries have a lower limit to the amount of current they can produce.

Two 12V Hawker PC680s in series have a resistance of about .014 Ohms. The popular 24V, 3.6Ah NiCd Battlepacks have an internal resistance of about .08 Ohms. In theory it would take almost six Battlepacks in parallel to get the same peak horsepower as one set of Hawkers.

The performance of motor/battery systems can be calculated by adding the battery resistance to the motor resistance in the standard motor performance formulas. Doing this, we find that for the C40 and S28-400 Magmotors, using two Battlepacks per motor gives about 47% more peak power than using just one, and using three per motor gives about 19% more power than two. The S28-150 draws less current so using two Battlepacks per motor gives about 40% more peak power than using just one and using three per motor gives about 15% more power than two. This shows the advantages of using multiple Battlepacks in parallel. If you are limited to using just one pack, the Magmotors will still produce more power from that one pack than any other small PMDC brush motor available, (because of the high efficiency).

Comparing Hawkers with NiCds is a little like comparing apples with oranges. The Hawkers can produce extremely high current but they are heavy. The NiCds give great run-time, but at the cost of higher resistance. This may require the use of several Battlepacks in parallel even though the run time might be adequate when using just a single set. At 28 pounds, one set of Hawkers weighs as much as seven Battlepacks and can produce the same peak current as about six Battlepacks, so the BattlePacks have a small edge in power to weight ratio. Six Battlepacks cost over $1000 while one set of Hawkers cost about $172.

The above figures are for peak horsepower. To compare the run times of the Hawkers and NiCds you must consider the reduction in capacity of the Hawker batteries when being discharged at very high rates. When a set of Hawkers is discharged quickly it yields about the same Ah capacity as two Battlepacks in parallel. When discharged more slowly, it yields about the same capacity as four Battlepacks. This indicates that the NiCds have a large edge in Ah capacity to weight ratio. Another thing to consider: Hawkers can be recharged at 20 Amps or more, while the NiCds are limited to 4-5 Amps.

Note: Several different sizes of Hawkers and NiCds are available, each of which has a unique performance profile. All the above figures are based on a comparison of the Hawker PC680 and the Sanyo 24V 3.6Ah NiCd Battlepacks.

Generally, you should choose a battery that has low internal resistance per weight, and use enough of them in parallel to give sufficient run-time. If the resistance is still too high you can reduce the internal resistance of the system even further by adding more batteries in parallel until you achieve performance that is satisfactory. For each configuration, you should check that neither the cost nor the weight go over budget. You should also make sure that the maximum current ratings of the batteries are not exceeded for anything more than brief periods of time.

For the best prices on Hawker batteries click here. For the best Hawker chargers click here. For custom Battery Packs click here.

Technical Specifications

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