Sepex Motors explained for dummies (like me) by JohnnieB

[DaveTM]

JohnnieB.....what the heck is a MOSFET and what does it do? (Pls explain in 3rd grade terms!!)

[georgia088]

Originally Posted by DaveTM

JohnnieB.....what the heck is a MOSFET and what does it do? (Pls explain in 3rd grade terms!!)

I don't know that I can understand in third grade terms..... Maybe Kindergarten? lol

[cgtech]

A mosfet is a "electronic switch" that can carry a good amount of power, can be turned on & off thousands of times a second, and has no moving parts. It comes in a small black package, and has 3 terminals on it, input from power source (source), signal wire (gate), and output to load (drain).

[DaveTM]

Great explanation cgtech. I get that. I don't need to know the "how" I just wanted to understand "what" that they do.

Thanks!

[JohnnieB]

Originally Posted by georgia088

Thanks JB!

The problem with embedding questions in a quote of someone else's post is they don't carry forward when re-quoted.
It is better to add reference numbers to the quoted text and then use those numbers to ask the questions or make statements pertaining to those referenced areas.

Here is a cut and paste of your quote of my post with reference numbers added to your added questions.

Originally Posted by JohnnieB View Post
SepEx stands for Separately Excited and means the stationary windings and the moveable windings are excited (powered) from separate power sources.

Remember, the difference between a speed and torque motor is the relative strength of the magnetic fields, so the speed/torque curve can be changed electrically by altering the voltage applied (which in turn alters amp flow through) to the stationary and moveable windings.

The motors themselves are wound for speed or torque or something in between at the factory, but once installed in the cart and connected to a sepex controller, the speed/torque curve can be adjusted in either direction by altering the voltage applies to the stationary winding in a process known as field mapping. (Stationary winding is typically called the field winding in sepex motors)

Basically, when the cart is in a situation that needs high torque, the voltage to the field windings is increased and the the sepex motor becomes more of a torque motor and in a situation when more speed is called for the voltage to the field winding is decreases to create more of a speed motor. This is done during operation and is more or less unnoticeable by the operator.

1. So this process is done within the the controller? How does the controller recognize the need for speed or torque? Is it a drop in amps (for torque) and volts (for speed) within the controller?

The field windings in a sepex motor are composed of a high number of turns on relatively thin wire so the actual amp flow through the field is only about 1/10th the amp flow through the armature and a sepex controller has two outputs, a high power one passing amps to the armature and a low power (bi-directional) one passing amps to the field windings.

2. I'm a little confused here. I thought the field windings were the "stationary windings" in a sepex motor. But, the sepex controller has two field outputs and one passes amps to the armature. Does this mean passes controller sends to the field windings which passes them through the armature (turning the armature)? If this is the case how are the amps passed from the field output on the controller to the field windings to change the direction?
These questions don't make sense I am sure. I just don't understand how the field outputs on the controller can pass amps to the field windings and cause two things to happen? Do the two field outputs happen simultaneously? or separately?

The attached drawing showing the outputs of a generic sepex controller.

Q1 represents a number of MOSFETs in parallel capable of passing up to 500A (depending on controller) to the armature and Q2 thru Q5 form an H-bridge that pass up to about 50A to the field windings.

3. So an H-Bridge switches the polarity to change the direction of the motors rotation.
I assume the H-Bridge is "chosen" (Q2 and Q5 for forward and Q3 and Q4 for reverse) based on the f/r selection switch?

When Q2 and Q5 are used, field amps flow from left to right and when Q3 and Q4 are used, it flows from right to left, reversing the direction the motor rotates.

1. In all honesty, I'm not exactly sure how a sepex controller without a motor RPM sensor (called a speed sensor) knows to increase the amps through the field windings for more torque or decrease them for more speed. I suspect the decision is based on amps being drawn at the time. Sepex controllers with speed sensor have motor RPM and that plays a role in the decision process in the more recent designs.

The information is probably out there, but I've never sought it out, since I've never felt the need to write my own field maps. On the other hand, I do believe tire height and some other factors could be used to better tailor the field mapping for a specific cart. (Inputted via Toolkit or whatever programming tool used)

2. A sepex controller has separate two outputs. One for the armature and one for the stationary (field) windings. The armature output will pass several hundreds of amps. The field will only pass up to about 50A or less. (I've never seen the field amps go above 25A on a data log from my XCT48400-PDS)

The armature output is controlled by the throttle. The field output is controller by the field map.

3. Yes. The pair of "electronic valves" that are used are chosen by the F/R switch. When F1 is connected to B+, the motor spins in one direction and when F2 is connected to B+, it spins in the other direction.

[georgia088]

Sorry about the quoting issue.

So the field windings either receive f1 (+) and f2 (-) or f2 (+) and f2 (-) ?

Does the armature always receive positive voltage to the same terminal and (-) to the other regardless of direction?

Again, I know I am on the preschool level of understanding this.

[JohnnieB]

Originally Posted by georgia088

Sorry about the quoting issue.

1. So the field windings either receive f1 (+) and f2 (-) or f2 (+) and f2 (-) ?

2. Does the armature always receive positive voltage to the same terminal and (-) to the other regardless of direction?

Again, I know I am on the preschool level of understanding this.

1. Yes.

2. Yes.

[Sergio]

As far as how the Controller mitigates Torque/Speed without the use of a speed sensor, it simply relies on the Field Curve provided by the Motor manufacturer.

Those graphs define the "safe" relationships between Field and Armature currents.

So if you are driving at high speed and start to go up a hill, the motor slows down which causes the Armature current to increase (due to a reduction in back emf).

The controller then raises the field current to make sure the relationship between the two currents stays on the left side of the graph.

If a speed sensor is used the Controller can make better decisions like allowing the field current to get lower "field weakening" as long as the Motor speed continues to increase.

Some Controllers like Curtis don't have field maps that You can import (like Alltrax) so a trained technician needs to manually and dynamically create the curve parameters by driving the cart under different conditions and observing what happens to the Armature current at different speeds and torque conditions.

Below is an example of a Motor curve for the ADC motor used in Club Car Precedents.

[JohnnieB]

Thanks Sergio.

Some of the field maps available play a little loose with the Field Weakening Limitations Maps.

[georgia088]

Originally Posted by JohnnieB

Thanks Sergio.

Some of the field maps available play a little loose with the Field Weakening Limitations Maps.

Does this mean (in your opinion) these “general” field maps for carts with no speed sensor could weaken the current sent to the field windings in order for the cart to gain more speed?