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DC Motor Controller Explained
The DC motor controller function is simple: to control the motor. If you are like me, you need to figure out how it works. In the case of an electric car conversion, the DC motor controller takes the throttle position input and sends the required power to the DC motor.
On internal combustion engines the style of engine control has evolved over time. There are two main engine control devices that are used: the carburetor, and the fuel injector through an electrical control unit. Both take the throttle position input and adjust the engine by adjusting the fuel flow into the intake manifold. The motor controllers used on electric vehicle conversions are functional replacements of the ECU (engine control unit).
Can I hook up the throttle directly to the motor?
The short answer is: no. The main reason for not having the throttle connected directly to the motor is throttle position response. When you remove your foot from the throttle there would be no power going to the motor. When the vehicle is slowing down for a stoplight in a city, then it would make sense that the motor is shut off.
Now think of another situation you are passing a slower moving vehicle on the highway when you remove your foot from the throttle. The motor would shut off causing excessive resistance similar to an engine stall. This would not be the expected result of slowing down for a couple seconds to decrease your speed slightly.
High Current DC Motor Starts
A higher amount of current is required to get the electric motor turning. This is the same situation with an internal combustion engine, which is why the starter motor is geared to provide higher torque to overcome the engine’s inertia. With electric motors, inertia is overcome with a higher current.
The inertia is due to Newton’s first law, also known as the law of inertia:
An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
The electric motor does not want to motor from its resting position, which now requires more energy to get it moving. Once it is rotating the motor will want to continue to rotate and all of the energy that is spent rotating the motor is used to propelling the vehicle, and other losses like friction.
The purpose of the pre-charger is to store additional energy required to start the electric motor. The typical flow of energy moves from the battery pack to the fuse, the contactor, the controller then the motor. The pre-charger and ignition key would be wired to the contactor. This is called a soft start as it protects the components from a hard start, which is a high current start.
Without a pre-charger, there is a chance that the current would be high enough across the main contactor to weld the two points together. This would render the main contactor useless and would require replacement.
How does the controller send power to the DC motor?
The controller uses a type of transistor to amplify an electrical signal. The signal being the logical supply, or control voltage, from the potentiometer. This type of transistor is called a metal-oxide-semiconductor field-effect transistor, or MOSFET for short.
By using a MOSFET to control the electric current, it can be adjusted with the control voltage through the accelerator. The logic control supply will operate a gate that is normally open, the farthest away from the source and drain. The source is the battery pack side and the drain is the motor side of the MOSFET.
By using a 12VDC logic control supply to send the desired throttle position voltage the wire size can be much smaller.
Pulse Width Modulation (PWM)
The requested power has been set and is ready to be sent to the motor. To control the DC electric motor the controller uses a technique called pulse width modulation, or PWM. This allows the amount of power going to the motor to be varied from an analog signal to a digital signal. So when there is a request for 25% of throttle by the driver the output from the controller will not be 25% of the voltage and current. The controller actually sends out the system power at a 25% duty cycle which is a different way to describe PWM.
At 25% duty cycle, there would be 25% of power sent with a 75% off cycle. In terms of time, there would be 1ms of power sent followed by 3ms of no transmission of power. This would total 4ms for a complete cycle with 1ms of duty. Keep in mind these times are for clarity as the operation speed or frequency is much faster. Most DC controllers operate around 16 KHz, which is 16,000 cycles per second.
Motor Controller Generates Loads of Heat
With high current flowing through the controller, heat is generated. Overheating the controller from the internal components is a major concern, and there are three methods to remove the heat from the controller.
The controller enclosure is made out of aluminum which dissipates heat quickly, but it also heats up quickly. If placed in a position where there is good air flow then this would be suitable. Adding a heat sink may also be required to increase the surface area exposed to the airflow in order to get proper cooling.
Using radiant air flow to cool with the addition of a secondary source of power to run the cooling fan across the enclosure or heat sink. This would be required if the controller is placed in a position where there is limited or inadequate airflow to properly cool.
Coolant plumbed into the controller is requested by some manufacturers. Typically, this would be when the controller has been put into a small sized enclosure which would not be able to radiate the excessive heat through the enclosure alone. This will force the purchase of a small coolant pump and reservoir. Not a big issue if you are already planning to use a motor that needs to be liquid cooled.
If the controller does get too hot, it will trigger a thermal protection event. This will override the throttle position that the driver indicates and reduce the throttle to slow down the generation of heat. This can be anywhere from 55°C for Zilla controllers and 80°C for Curtis controllers. Check the controller specification for the full thermal shutdown. Once that temperature is hit you will be stranded until the temperature is lowered.
Communication with Other Devices
Most controllers only use the potentiometer (pot box) as the only input. There are some motor controllers however; that do have the ability for CAN bus integration. This would allow the central processing unit (CPU) to automate some functions.
Even if the controller states that it is able to integrate into a CAN bus system, it can still work without that communication.
CAN Bus Ready for Integration
Some of the possible features that could be added by using a CAN bus system include:
Battery usage with data recorder
Vehicle range optimization with transmission coordination
Here are a couple electric car motor controllers worth looking into:
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Here are some videos for more information.