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Speed controllers

If you want to know how an electronic speed controller works then I’d refer you to the article on the ACTion website as it’s far too long to reproduce here. Please see: http://www.action-electronics.co.uk/pdfs/ESCs.pdf
In the bad old days we used to use mechanical variable-resistance speed controllers, operated by a standard servo, which controlled the speed of the motor by 'bleeding off' unwanted power. A contact on the end of a wiper arm moved across a coil of resistive wire like a rheostat, or a miniature electric fire. This was very wasteful in terms of battery power and the advent of the Electronic Speed Controller (ESC) has made these almost extinct. Okay, users of ‘Bob’s Boards’ will object to that statement, but matching the correct resistive board to the motor was always critical to operating the things and few users seemed to manage to do that successfully. The contacts also could work loose in operation, would only handle relatively small currents, and required regular maintenance.

The modern electronic speed controller comes in two distinct types, depending upon the motor, that’s brushless and brushed. We’ll concentrate on the latter here, as most of what applies to brushed types also applies to brushless.

There are two distinct parts to the inside of an ESC; the logic circuitry and the power circuitry, Figure 18. The logic components are the ones which are connected to the receiver via the 3-wire lead with a plug on the end. Its operating voltage is dictated by the receiver and is generally of the order of 4.8v, but you don’t need to make a conscious decision about that as the receiver will take care of it. The purpose of the logic circuitry is to detect and decode the signal coming from the receiver and to switch the high-speed, high-current semiconductors in the power circuit which control the speed and direction of the motor. The ESC manufacturer will state in the technical information the range of main motor battery voltages with which the ESC will cope, along with what should be a value for the maximum motor current (in Amps) which the ESC will handle under continuous operation. You should be guided by those two values when choosing your ESC, after first ascertaining the working voltage of the motor and its current consumption under load as described earlier.

I’ve emphasised the words 'under continuous operation', because the current rating often causes confusion and I suspect, involves a little skullduggery on the part of some sellers. When you switch on an ESC and bang open the throttle there will be a sudden inrush of current from the battery to the motor to get it spinning. This will always be a larger value than that consumed when the motor is running at full speed, i.e. continuously. Modern switching semiconductors therefore have two current rating values; one for continuous and one for 'inrush' or pulsed currents. The value we should be concerned with is the continuous current rating. Because it is a much more impressive figure, I have long suspected that certain oriental ESCs are rated with their inrush current instead. Indeed, we have handled supposedly 50A ESC's from such sources which failed consistently at 15A. So, the moral is clear; buy a known unit from a reliable source with a good reputation and a clearly stated service/warranty policy. Popular UK-manufactured ESC's are by ACTion, Electronize and Mtroniks, while other popular and reliable imports carry the Graupner or Robbe labels. Make sure also that the ESC has both forward and reverse if you are fitting out a workboat like a tug. Many of the cheaper ones are forward-only or have limited reversing capability. Incidentally it’s quite okay to run a low-current motor on an ESC which has a much higher rating, but not the other way around.

The power circuit connections of an ESC will always comprise a pair of thick battery cables. These are usually made in red and black, for positive and negative connections respectively. There will also be a pair of cables to connect with the motor terminals, often in blue and yellow, but be careful and check your instruction leaflet first. Brushless motor ESC's have three motor wires. It doesn’t matter which way round you connect the motor wires to the motor, BUT the battery connections should NEVER be reversed. If you wish to reverse the direction in which the motor is rotating then just swap over the two wires from the ESC to the motor. For brushless motors swap over any two of the three motor wires.

Setting up the ESC to operate from the radio can either be a real pain or none at all, depending on the type chosen. Some require you to move the transmitter stick to its neutral and extreme positions while you push a small button on the ESC in response to flashing coloured LED's. Others have rotary trimmers and slide switches which can adjust the working frequency and the neutral and peak motor speeds. Others, you just switch on, and they ‘Autoset’ themselves! Brushless ESC's are a law unto themselves and frequently require a separate programming card.

The only other variable is whether or not the ESC contains a battery eliminator circuit (BEC – see earlier). If it does, then you must NOT connect an additional power supply to the receiver. Some BEC-equipped ESC's have a small slide-switch on a pair of thin wire leads. This usually controls the onward power from the internal BEC voltage regulator to the rest of the radio system and therefore serves as a radio ON/OFF switch. It does not control the power supply from the battery to the ESC, so I would recommend also fitting an addition high-current switch in the positive lead from the main battery to the ESC unless you are in the habit of disconnecting the battery physically each time you remove the model from the water. If you do disable the BEC for any reason, then leave the little switch in its ON position or you may find that the ESC doesn’t work; again, this should be covered in the ESC Instructions.

Modellers have complained about the high-pitched whine that some speed controllers appear to emit at low motor speeds. In fact it’s the motor which is making the noise and it’s caused by the very high frequency at which the motor is switched on and off. This is usually only audible at very low speeds. If it annoys you, then the only solution is to change to a speed controller which operates at a lower frequency, whereupon that whine becomes a low-frequency buzz. Some ESC's are switchable between high and low frequency, but that is more because high frequency ESC's are more effective at low speed control.

Whichever type of ESC you buy, read the instruction manual before you do anything with it. Of those units which were returned to us 'for repair', at least 50% had nothing wrong with them; it was usually a case of the user thinking he knew more about how the ESC should work than the guy who wrote the instructions. If ever you hear an electronics engineer muttering about 'Hardware-User Interface Conflict' then you’ll now know exactly what it means!

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