Separately excited motors

Are there any features that you would like to add to the VESC?
drenehtsral
Posts: 3
Joined: 07 Jun 2017, 14:59
Location: Seattle, WA, USA

Separately excited motors

Postby drenehtsral » 25 Oct 2017, 15:59

I have been experimenting with driving a separately excited motor from the VESC as of late, and it has proven very promising. I modified a garden veriety automotive alternator (whatever the cheapest one the auto parts store around the corner had on hand, nominally rated (as an alternator) for ~69A @12V).

What this looks like is a three phase motor with one extra pair of leads which, through a pair of brushes (which make continuous contact with smooth rings this wear slowly and don't cause as much sparking and noise as a brushed dc motor's brushes do) power the field coils in the rotor, which are wrapped around a core with alternating "teeth" on top and bottom to form a 14-pole rotor.

The interesting thing about this configuration is that while at any fixed field current you can drive it just like a normal permanent magnet 3-phase motor, by varying the field current within its linear region (below where you saturated the rotor core) you can vary the torque vs. speed characteristic of the motor.

The particular motor I've got, for instance, has an effective useful range of ~50 rpm/V at ~2.5A field current up to ~200 RPM/V at ~400mA field current.

This has some interesting control implications because you can perform regeneration over a wider speed range (that's effectively what the regulator that I ripped out of it when doing the alternator to motor conversion was doing all the time if you want to think of it that way) but it also would allow on-the-fly tuning of speed vs. torque (off the line you could crank up the current and then wind it down towards your cruising target while increasing duty cycle more slowly).

Since the field winding is relatively low current and all it wouldn't take a very beefy MOSFET to drive it from an auxiliary PWM.

Had anybody else played with control strategies for a separately excited motor? I have seen rheostats used in the field windings of separately excited motors for analog manual speed control but it seems that with a microcontroller sequencing the armature phases and all a much more flexible system is possible, although I could also see using a couple fixed settings as a high/low range on a go-kart or similar fun EV.

ThierryGTLTS
Posts: 55
Joined: 09 Aug 2017, 11:10

Re: Separately excited motors

Postby ThierryGTLTS » 25 Oct 2017, 16:51

Could be also interesting for implementing field weakening better than with permanent magnets.

But it would be better to build a "3rd party" hardware.

Thierry

pf26
Posts: 263
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: Separately excited motors

Postby pf26 » 26 Oct 2017, 09:30

While this looks interesting for the price and extra control capabilities, I fear the total efficiency will remain lower than a comparable brushless motor. But this needs to be checked, and the exitation could be controlled to optimize the motor efficiency.
Do you have some figures about no load power requirement vs RPM for different exitation currents ? And winding resistance ?


pf26
Posts: 263
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: Separately excited motors

Postby pf26 » 27 Oct 2017, 10:48

My efficiency concern was only regarding the automotive alternator.

drenehtsral
Posts: 3
Joined: 07 Jun 2017, 14:59
Location: Seattle, WA, USA

Re: Separately excited motors

Postby drenehtsral » 06 Nov 2017, 02:35

I don't currently have a good dummy mechanical load to put it under (I'm still tinkering with that part) although, being designed to crank out up to 69A for hours on end in its original role as an alternator it does have a fairly beefy fan on the output shaft which means that I essentially am always testing it with a low to moderate load just from it sucking air through past the stator windings and blowing papers and other clutter around my workbench.

When I get home I can post my recent "no load" (fan only) data (eRPM, steady state current, mechanical RPM, duty cycle) at various different field currents as well as the phase-to-phase resistance. The resistance of the field coil is 3.6 Ohms and from poking around at it it would appear that the useful range in terms of field current is about 500mA to about 2000mA.

If there's anybody who has a good setup for measuring mechanical output for the purpose of measuring efficiency I'll gladly lend or give you the motor (if I can find a cost-effective way to mail it to you, or of you're in the Seattle, WA metro area) and/or take some pictures and write up some instructions for converting one (I just winged it without instructions and it only took a couple hours (there are some jerks online selling an instruction booklet and measurements as if this were some sort of secret and my geek pride wouldn't allow me to shell out for a how-to pamphlet for something as trivial as ripping out one IC, one resistor, and one capacitor and replacing them with nylon spacers...)).


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