Radio Control: Electrics
Bob Kopski 25 West End Dr. Lansdale, PA 19446
I'm starting off the new year with the introduction of two new column features in this January issue. The first is a column header that will list the column's topical content—a table-of-topics sort—so you'll eventually be able to search past columns at a glance for items of interest. This was prompted by reader comments of the form, "I know I read it in your column somewhere..." My hope is that this monthly prefix will ease the search for past topics, since you won't have to scan entire columns to find an item of interest.
This month's topics:
- New column features.
- BEC discussion: Futaba electric receiver, reader reaction.
- Speed control discussion—more on high-rate and frame-rate.
- Motor electrical noise.
Electric Connection Service
The second new column feature will be an Electric Connection service. A pattern in my incoming mail is letters that say, essentially, "I'm the only person in my club (or area) who flies electric..." Many readers lament the difficulty of finding local flyers who share their interest. If you want to connect with other electrics in your area, I'll print your name and address and invite nearby readers to contact you.
To start:
- Jim Jansen, 6525 43rd Ave., Kenosha, WI 53142 — would like to hear from others nearby.
If you would like to be listed, send your name and address. We'll try this for a few months; if it proves worthwhile I'll continue. "Worthwhile" will be determined by how many of you respond and use the service.
Reader reaction and BEC follow-up
Reader reaction to the September 1989 column has been strong, particularly about Battery Eliminator Circuits (BEC) and Futaba's electric-dedicated receiver. A few points and follow-ups based on readers' questions and my field observations:
What is a BEC?
- BECs use the motor battery to power the receiver system, saving the weight of a separate receiver battery.
- They're designed to shut the motor off as the battery runs out of usable charge, but before receiver operation is compromised.
Concerns raised by readers:
- How much receiver time remains after a BEC-initiated motor shutdown?
- In my experience flying several planes equipped with the Futaba receiver, I have not seen loss of receiver power following automatic motor shutdown. Flights after shutdown have included long thermal flights (around five minutes) with no power loss observed.
- How much motor duration is lost because power is reserved for the receiver?
- Very little. Receivers and servos draw comparatively tiny power relative to motors; on-field experience suggests the motor is still getting nearly all available energy. Any loss is likely a few seconds and not practically significant for most flights.
- Is the battery fully discharged after BEC shutdown (i.e., safe to recharge)?
- Based on observed pack behavior, packs recharged normally by timed chargers and peak-detect chargers with no problems. After BEC shutdown the pack is generally very nearly dead—ready to accept a full charge—but not so low as to cause unusual charging issues.
- Electrical noise from the motor affecting receivers and servos?
- I have observed no problems with motor electrical noise on Futaba-equipped aircraft, so the receiver/system seems well-shielded or designed.
- Cell-count limitations?
- Futaba documentation indicates limitations. While the manual may show up to 12 cells, Futaba recommends fewer (I recall about seven). Operating outside recommended limits is at your own risk.
Fusing and BECs
- A reader (Harding Orem) raised a critical point applicable to any BEC installation: where to fuse the power system. Fusing close to the battery is standard practice, but if the fuse blows in flight and the BEC is between the fuse and the receiver, the motor and receiver could both lose power (a radio-less free flight). This is a serious consideration—plan fusing and wiring so that a single blown fuse doesn't render the receiver dead, or accept the risk and use redundant protection schemes.
Personal preference
- I prefer to decide when to shut down the motor rather than have a monitor (BEC) do it automatically. I developed this preference after an event where a BEC shut down someone else's plane while it was far and low. In any case, keep good situational awareness—use a stopwatch and learn how long your motor will run so you can stay close to the field when needed (BEC or not).
MOSFET overheating anecdote
- The Futaba receiver/speed-control system is designed to shut down under overload; apparently the shutdown is triggered by MOSFET heat. One reported case: a model running a cobalt .05 heavily caused the MOSFETs to overheat and the motor to shut down prematurely. The fix was simple—add a small air scoop near the MOSFETs with an exit to create airflow. That relieved the heat and eliminated the shutdowns.
Speed control: high-rate vs frame-rate
As promised in the December column, here's why power-system efficiency can differ dramatically between frame-rate (low switching rate) and high-rate (audio-rate) speed controls. We'll approach this from the frame-rate (lower-efficiency) end.
Start-up and current peaks
- When a motor is at rest, application of voltage causes a very large initial current (stall/start current) because total circuit resistance is low. This large current is momentary—motor speed rises and current falls to normal running values—but while it lasts the I^2R losses are enormous.
- If you repeatedly turn the motor on and off, the average motor current will be higher than if a reduced constant voltage were applied. Repeated rapid reapplication of full voltage to a slowed motor produces repeated high current pulses, increasing average losses.
Frame-rate switching
- Frame-rate controls switch at a relatively low rate (around 50 Hz). Although faster than manual switching, 50 Hz still allows significant motor speed fluctuation between pulses. That fluctuation leads to noticeable current peaks when voltage is reapplied, producing higher average losses and lower overall efficiency.
High-rate (audio-rate) switching
- High-rate controls switch at much higher frequencies (audio rates). Switching is so fast that motor speed does not have much chance to drop between pulses, so the current peaks from reapplying full voltage are much smaller.
- Additionally, each motor armature winding is an inductance. Inductors resist changes in current, tending to smooth current flow. At high switching frequencies this inductive "electrical flywheel" effect becomes significant, further smoothing current peaks and reducing I^2R losses.
- The combination of minimal speed fluctuation between pulses and inductive smoothing leads to markedly improved power-system efficiency with high-rate controls.
Context and credit
- The inductive-effect explanation was discussed previously by Roland Boucher ("The Quiet Revolution," RCM, 1970s). The performance differences between control types are real and can be dramatic, which is why many readers have shown interest.
Motor electrical noise and optocouplers
Motor noise questions prompted requests for where to get optocouplers and how to install them. Important points:
- Optocouplers are not simple add-ons. Speed controls that use optocouplers are designed with the peripheral circuitry and layout required to support them.
- You cannot reliably add an optocoupler to a speed control that was not designed for it and expect correct operation. If electrical isolation is needed, use a speed control that explicitly supports an optocoupler or other isolation method.
Closing
I'll investigate whether the Futaba-built speed control is a high-rate device and report next month. Meanwhile, keep writing with your observations and questions—especially if you want to be listed in the Electric Connection service.
Transcribed from original scans by AI. Minor OCR errors may remain.
