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HomeAircraftHelicopterHelicopter Main Discussion › Current from ESC to Motor ?
05-29-2013 06:25 PM  5 years agoPost 21
JKos

rrProfessor

Redondo Beach, CA

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red_z06,
You are both saying that current enters/exits only two of the three motor leads at any given time and, at that time, the third lead does not have any current flow.

dkshema has simply made the observation that the current on the two energized leads a) must be the same, b) is the full instantenous current, and c) the same current as on the battery leads as well.

The only difference of opinion is the sizing of the phase wires. You say 2/3s and dkshema has said full current. I'm sure you are saying 2/3s because the average current on a phase wire will be 2/3s of the total average current (call it average battery current).

- John

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05-29-2013 11:14 PM  5 years agoPost 22
dkshema

rrMaster

Cedar Rapids, IA

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Simplified three-phase brushless DC motor drive. Motor is a WYE wound motor, having three phase windings, A, B, and C. The motor has three distinct phase windings, A, B, and C, they share a common connection node (the dot in the center of the motor).

The motor controller has six switches, typically some form of very low on-resistance MOSFET.

Three switches -- SHA, SHB, and SHC connect the battery voltage input to the motor phase windings. ONLY ONE IS "ON" at any time. IN the picture, SHA is ON.

Three switches, SLA, SLB, and SLC connect the battery GND (return) line to the motor phase windings. ONLY ONE IS "ON" at any time. IN the picture, SLB is ON.

Under NO circumstances is the switch control logic allowed to turn on the following pair of switches:

SHA and SLA
SHB and SLB
SHC and SLC

Any one of the three combinations above generally will cause the motor H-bridge to fail catastrophically, taking out the two MOSFETS.

The complete current path shown is --

Battery + to the top of the H-bridge. Current goes through "ON" switch SHA, to point "A" then to the motor Phase A winding input, through the motor Phase A coil, to the common junction of Phase A, B, and C. Current continues through the Phase B winding, and ends up at point B in the bridge. With SLB "ON", current goes through SLB and makes its way to the battery negative (ground/return) terminal.

If the motor requires 30 amps for the given load, 30 amps WILL flow through the path as shown. Switches SHA and SLB, as well as motor phase windings A and B ALL see 30 amps of current. The battery supplies 30 amps of current.

The input to motor Phase C from the H-bridge is disconnected, high-impedance, and for all intents and purposes, floating. (Yes whatever voltage seen at the common node of the three phase windings will be seen at the Phase C input, but as both switches SHC and SLC are OFF, NO CURRENT makes its way in or out of the phase C winding).

Legal combinations of "ON" switches for the H-bridge are:

SHA - SLB (A+, B-)
SHA - SLC (A+, C-)

SHB - SLA (B+, A-)
SHB - SLC (B+, C-)

SHC - SLA (C+, A-)
SHC - SLB (C+, B-)

With the wiring the way it happens to be, the circuit can connect each motor phase winding to + volts from the battery, or can connect that same winding to the ground/return path -- for instance, if I turn SHA "ON", the motor phase A input sees + battery voltage. On the other hand, if I turn SLA "ON", the motor phase A input sees ground/return.

SO, if I want to make the motor turn Clockwise, I would use the following sequence:

SHA on, SLC on. (A+, C-)
SHA on, SLB on. (A+, B-)
SLB on, SHC on. (B-, C+)
SLA on, SHC on. (A-, C+)
SLC on, SHB on. (C-, B+)
SHB on, SLC on. (B+, C-)

And repeat from the top of the sequence.

To make the motor turn Counter Clockwise, I simply start at the bottom of the table, and work my way up, then repeat.

(Of course, CW and CCW are relative, depending upon which end of the shaft you're looking at, but... ... reversing the sequence reverses the rotation).

No other switches are allowed to be ON during execution of the sequence or bad things happen. Usually, the FETs get smoked and blow themselves off the board.

With a little ingenuity, you can also see why swapping ANY two of the three wires reverses the motor, as well.

-----
keep saying two windings are connected.
From the picture, that is intuitively obvious.
not two with the third node in float
Well, actually, the "third node" IS NOT connected, it IS in a high impedance state with regard to its driver, and it IS floating with respect to its driving node.
There are three circuit paths
Each circuit path requires TWO windings to be involved for a complete current path. (WYE motor). Delta wind is a different animal; one phase winding required there.

-----

Capital "WRONG" turns into "splitting hairs" when you are talking RMS or average current, and I am discussing actual current through a given connected path. Comparing apples to oranges cannot be done directly, unless you are discussing fruit. The currents being discussed are related, they are not the same, however.

-----

Size the wire for at least full current. A stalled motor won't care much about "average" when told to drive. Of course, there are other things happening in the real world, current feedback is involved, the pulse width modulated voltage drive can be varied...motor control gets plenty involved and there are all sorts of protection schemes that can come into play.

-----
Dave

* Making the World Better -- One Helicopter at a time! *

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HomeAircraftHelicopterHelicopter Main Discussion › Current from ESC to Motor ?
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