Monday, March 2, 2015

Motor Controller Packaging

Quick note: This post is a work in progress. Some sections are incomplete or missing.

1. Objectives

The motor controller used in the MY2015 will be based off of the Unitek Bamocar D3 modules used in the 2014 car. The D3 controller meets the electrical requirements for our tractive system (voltage, power, control scheme) but there is an opportunity to make significant improvements for use in our application by reducing size, weight, and installation complexity. To make the most of a redesigned housing, both control modules and will be integrated into a single package.

2. Architecture

A general approach to laying out the parts was decided by comparing several different configurations. Benefits and weaknesses of each design were compared. Eventually, the “wide” configuration shown in Figure 3 was chosen. Its overall shape fits well into the frame mounted just above the batteries. Like the configuration shown in Figure 4, there are only two coolant connections, but because it requires a new cooling plate we can tailor the heat dissipation capabilities. The cooling subteam determined that the Figure 4 configuration would provide insufficient cooling for the two power electronics units. Figure 5 shows the finalized CAD of the new motor control module.
Figure 1: A single inverter module in the original Bamocar D3 layout.

Figure 2: A "tall" configuration of two modules stacked vertically. 

Figure 3: A "wide" configuration of two modules placed side by side on a single, larger cooling plate (bus capacitors not shown).

Figure 4: A "tall" configuration of two modules. Both power sections are mounted to a single cooling plate from the original unit.

Figure 5: The finalized configuration of the inverter modules.

3. Connectors

The original high voltage power connections are of high quality, but are expensive (≈$100 each) and bulky. Additionally, they are not compatible with the Choroplast shielded cables we intend to use. This cable was selected based on EV 4.5.7 of the FSAE rule book.

EV4.5.7 All tractive system wiring that runs outside of electrical enclosures must either be enclosed in separate orange non-conductive conduit or use an orange shielded cable. Except in the case where the tractive system wiring runs in a fully enclosed container, the conduit or shielded cable must be securely anchored at least at each end so that it can withstand a force of 200N without straining the cable end crimp, and must be located out of the way of possible snagging or damage. NOTE: body work is not sufficient to meet this enclosure requirement. Any shielded cable must have the shield grounded.

Using shielded cable will reduce interference from the AC power that drives the motors. The connectors on the motor side of the controllers will be replaced with cable glands that have shield connections built in. The terminology for these parts is a bit strange; EMC, EMI, grounded, and shield compatible are all used to describe the feature from different suppliers. The connectors on the input side of the controller will be the original removable plugs. The ratings for the connector are high enough to allow the power for both controllers to flow though a single pair of plugs. The cable on this (DC) side of the controller will be shielded from the opposite end of the cable at the unit housing the high voltage disconnect (HVD) and energy meter.

4. DC Link Capacitors

The DC link capacitors make up a majority of the height in the original Bamocar controller. These are also commonly called bus capacitors. The capacitors used are high quality film caps with low ESR and ESL. They are connected directly across the high voltage DC rails and serve as power storage. When fast throttle changes are requested by the vehicle computer the inductance of the cabling between the controller and battery can be a problem since dI/dt will be limited. Using a sufficient amount of bus capacitance allows dI/dt to very high on the controller side despite the inductance in the cable.

Keeping the cabling between the battery and controllers short allows for a reduction in the stock amount of DC link capacitance. We have removed one bank of capacitors, which halves the total capacitance, after speaking with the company and confirming the controllers will still operate as intended. A new bus bar configuration had to be designed to connect the capacitors’ power rails to each IGBT module.

5. Cooling

During operation some of the power passing through the motor controller is converted to heat. The IGBT modules are mounted to an aluminum liquid cooling plate both in the standard Unitek packaging and in the redesigned configuration. Putting all the heat generating components on a single cooling plate reduces the number of connections in the cooling system (from four to two) and allows the overall package to be smaller. According to its datasheet, the Bamocar D3 is approximately 97% efficient and needs to dissipate a maximum of 3000 watts per module. Doubling the 3kW power figure gives a starting point of what specification the larger two module plate will need to have. Collaboration with the cooling sub-team has allowed for simulation of the heat sink’s performance in context of the entire cooling system.

The internal coolant path of the new heat sink is shown below in Figure 6. The double spiral layout of the coolant channels keeps the temperature of the plate as homogenous as possible by placing warmer and cooler paths next to each other. This is somewhat analogous to a counter flow heat exchanger, but with just one fluid path instead of two.

Figure 6: Cooling plate internal fluid path. Colors are for visualization of double spiral layout and are not direct results of flow or heat simulation.

6. Enclosure and Waterproofing

The casing and lid will be made from bent and welded aluminum sheet. The parts were designed using sheet metal tools in SolidWorks and all connector and mounting cutouts were added “in context” with the full assembly which allows automatic updating of hole positions in the model. Once the design was finalized, the data was sent to a company called Rapid Sheet Metal. They offer an add-in for SolidWorks that analyzes the sheet metal features and gives real time quotes. It will even parse PEM hardware that is included in the assembly.
Figure 7: Sealing screw.
A method to keep water from entering the enclosure will be needed around the enclosure parting line and all inlet/outlet ports. All of the electrical connectors being used have a compressive gasket or o-ring to keep liquid out. Similarly, the screws that go through the sheet metal case will be a special sealing type with a groove cut under the head for an o-ring to seat in. A compressible foam gasket will be used around the interface of the lid and main body of the casing. 

7. HV Distribution

New bus bars must be made to make the high voltage, high power connections between the capacitors and IGBT modules. New bus bars were designed as sheet metal in SolidWorks and sent out to a fabricator (with the case sheet metal) to be cut and bent. The positive and negative rails are mechanically separated in space by their geometry and mounting points.
Figure 8: DC distribution bus bars.
The electronics sub-team suggested that all high voltage distribution be done inside the motor controller enclosure instead of creating a separate box for those components to save weight and complexity. The IMD, TSAL driver, pre-charge circuit, discharge circuit, and TSMP (tractive system measurement point) resistors are all located inside the controller housing. Each of these components requires an HV connection and some require a GLV (grounded low voltage) connection. The HV connections come off of the bus bars and the GLV signals are all connected though bulkhead panel connectors to external cabling. 

Figure 9: Power distribution within the motor controller.

Look out for the build post for this project. Coming soon.

Until next time.