Running the CM-2204-2300 motors on 4 cells can be done, but everything must be matched up perfectly, and you have to have an ESC that is capable of running at the higher speeds that this requires. Most of the ESC’s on the market cannot switch fast enough to keep up with a 14-pole motor that has a Kv value of 2300 running on 4 Li-Po cells.
A lot of the ESC’s on the market can switch up to about 220,000 to 230,000 cycles per minute. Some of the cheaper ones can only get up to about 180,000 to 200,000 cycles per minute. In a 3-phase motor, every time the ESC goes through one complete switching cycle, the motor rotates through 2 magnets. Because the Cobra CM-2204 motors use a 12-slot stator with 14 magnets, it takes 7 complete switching cycles of the ESC to make the motor go around 1 revolution. If you are running 4 cells, which put out about 15 volts under load, on a 2300 Kv motor, in a no-load situation the motor will try to spin at 15 x 2300 or 34,500 RPM. If we take this number and multiply it by 7 switching cycles per RPM, you get 241,500 switching cycles per minute.
This is why when we test the motors here in the lab, we cannot do a no-load test on 4 cells. The motor is trying to spin faster than the ESC can switch, and this leads to dropped pulses and mis-firing of the ESC. When the motor has a prop installed, it typically spins at 70% to 85% of the no-load speed, depending on the size of the prop that is used. If you have a normal size prop, the motor will try to spin at about 75% of the no-load speed, which in this case would be 25,875 RPM or about 181,000 cycles per minute at the ESC. In the air, when the motors unload, this can creep up to over 200,000 Cycles per minute, and this has you teetering right on the edge of operation for many ESC’s.
The other issue is with the size of the motor. Brushless ESC’s get their timing feedback from the motor itself. During each power pulse, Power is applied to two of the three ESC output leads, and the third lead becomes an input, waiting for a timing signal from the ESC. As the motor rotates, magnets pass over the stator poles attached to the third unused lead of the motor at that specific point in time. When this happens, a pulse gets generated and fed into the ESC. When the ESC detects this pulse, it knows that the motor has rotated far enough to switch power to the next phase pair in the power cycle.
On a small motor, like the 2204 size, there is not a lot of energy generated in the feedback pulse, so it can get overshadowed by noise or other glitches in the power cycle. When the motor is operating right up close to its maximum RPM limit, the ESC can occasionally mis-fire due to a missed feedback pulse, or noise from the motor. As motors get larger, the feedback pulses get stronger and more reliable when compared to smaller motors.
Pilots want to go fast in their 250 size FPV racing quads, so 4 cells is a great way to get extra motor speed. However, if you try to push the system too hard, you can get hiccups in your motors that can cause the quad to flip over and crash. We have addressed this issue by coming out with a new motor for the 250mm quads, the Cobra CM-2206-20, 2100Kv model. This motor is designed to run on 4 cells, with props up to the 6×4.5 size. We have had a few sets of these motors out in the field being tested by our team pilots, and they have won virtually every race they have entered with them. We should be getting our first shipment of these motors at the end of the month, and they would be an excellent choice for any 250mm racing quad.