Sunday, January 20, 2013

Motor Basics

I think it is time to clear the air surrounding the DC motor in my car. All this talk of armature controllers and field controllers must be confusing to many, so I will do my best to explain these things further.

First off: electric motor basics. Take a nail and wrap some wire around it to make a coil. Hook the wire up to a power source, and you have turned the nail into an electromagnet. Now, take another electromagnet, put it end to end with the first one and power it up. The two nails will pull themselves together. Reverse the polarity (direction of current flow) in one of the electromagnets and they will push themselves away from each other. If you hold one electromagnet stationary (stator) and fasten the other to something that will rotate (armature), you have an electric motor.

Here is a drawing of an electric motor:

The commutator is a ring on the armature that spins with the armature. The brushes press against the commutator so current can flow through them to and from the armature windings. The commutator also acts as a switch that constantly changes the polarity of each pole of armature windings so that as they spin, the magnetized armature poles are always pulling towards one field magnet and pushing away from the other.

Simple, right? To complicated it a little, there are several types of DC motors. The most common type of motor in EV conversions is a series-wound motor. This means that current goes into armature POS, out armature NEG, straight into field POS and out field NEG. The field windings are made of heavy gauge wire designed to handle the same amount of current as the armature. Therefore, only one current control device is necessary.

The motor in the Electric Booger is a sepex (separately excited) DC motor. This means that the field windings have much smaller wires, making many more revolutions around the stator poles so that relatively little current is required for the electromagnets to have the same effect. Since the armature is identical to (and requires the same amount of current as) a series-wound motor, two current control devices are required - one that controls a large amount of current (armature) and one that controls a smaller amount of current (field).

A proper sepex controller is actually two controllers in one. These controllers generally have the ability to output field current in a non-linear manner, relative to armature current, giving a much broader power curve than a series-wound motor. Unfortunately, a proper sepex controller is double the price of a series wound controller, and since I had already purchased a used series controller on eBay, I had to improvise.

The separate field controller that I ended up purchasing is a small controller meant for vehicles like electric scooters. It is rated for 100A and 72V. Its throttle input is provided by an amp transducer that measures armature current. I have it set so that whenever the key is on, this controller outputs a 7A "idle current" to the field so there is absolutely no chance of powering the armature with no field current present, as this would cause all sorts of nastiness with the brushes.

Here is a graph showing the relationship between my field current and armature current:
Unlike a series-wound motor, the relationship between field current and armature current can be adjusted in a sepex motor. The relationship between the two is rather interesting:
-A higher amount of field current provides more torque but a lower max RPM
-A lower amount of field current provides less torque but a higher max RPM

Like I mentioned above, a properly set up sepex controller has the ability to achieve the best of both scenarios by implementing a non-linear relationship between armature and field like this:
Right now I am going with 50A of field current which is a nice balance of torque and max RPM. This allows me to start off in second gear most of the time and top out at 60 km/h (4600 RPM) in second, although I usually shift into third at 50-55 km/h.

When I first drove the Electric Booger, I was powering the field with a 12V battery, which equated to 12A of current. With such a small amount of current it barely had any torque and first gear starts were mandatory, but it would pull all the way up to 30 km/h in first which is 6000 RPM. I even shifted into second at 40 km/h a few times which is 8000 RPM! I guess it was a good test of resilience for my motor to transmission adapter.

Anyway, this is all to show how unique a sepex motor is. Hopefully I have simplified things for a few people so when I throw a bunch of terms out there you know what I am talking about!

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