Speed Controller Failure: Degradation

Speed controller failure can happen in many forms.  The electronic speed controller inTypical Electronic Speed Controller your golf cart or electric vehicle is the most complicated component in the vehicle and is often considered the brain of the electrical system.  Although a book could be written on all of the various failures, for the purpose of this article, we’ll stick to simply performance degradation over time and use.

If you’ve ever opened up an electronic speed controller, you would see numerous printed circuit boards, resistors, diodes, MOSFETs, etc.  Without a background in electronics, it may look a bit overwhelming. Through this discussion, we’ll show several pictures of the speed controller internal components for identification purposes.

Speed Controller Circuit Boards

Due to a recent situation at hand, we’ve learned something new this week, despite over 40 years in the golf car and electric vehicle industry. A speed controller can absolutely experience performance degradation over time, whether it is just sitting on the shelf unused or in operation within a vehicle system. We have confirmed with several speed controller manufacturers that if it is left sitting on the shelf for significant lengths of time, likely years, without use, the capacitors will actually dry up and go bad or fail when used. The capacitors are shown in blue in the picture below.

Speed Controller Capacitors Under Circuit Boards

Another speed controller failure which we learned of this week relates to issues with controller performance as it ages.  For example, a brand new, fully operational controller should be capable of an output current (amps) near or in excess of its current limit.  A 300 amp speed controller should be able to output a current of 300 A or more to the motor when brand new.

However, over time and use, internal parts of the speed controller begin to degrade.  When heat is applied to any metal or conductor in the proper duration and temperature, it can cause any metal based product to become brittle and less conductive.  The same is true of the MOSFETs inside these speed controllers.   A MOSFET stands for Metal Oxide Semiconductor Field-Effect Transistor, and is basically a transistor for amplifying or switching electronic signals.  MOSFETs are commonly used in modern electronic speed controllers for this very purpose.  However, they are a common component to both fail in the unit under extreme current loads, as well as a common component to degrade over time.  The MOSFETs are shown in the picture below.  They are the black squares screwed in all down the inside of this controller.  There is another complete row on the other side as well.

Speed Controller MOSFET

As the speed controller is used throughout its life in any application, lots of current repeatedly runs through the MOSFETs and causing them to become more brittle due to the heat.  It can also cause their output over time to decrease. A 300 amp controller which used to output 300 A+, can sometimes only output 200 A as degradation occurs over time to these MOSFETS.

There are generally numerous MOSFETs in any given speed controller.  If one or two fail, the speed controller may continue to function but at a greatly reduce level due to this reduced current output to the motor.

This discussion and tutorial was initially started due to a recent issue we experienced with a customer.  We sold a customer in Florida a relatively stock aftermarket motor for his Club Car. Once it was installed, the speed was fine, which is a function of voltage. But, the torque, which is a function of amperage was greatly lacking.  There are very few hills in Florida and it could barely climb them. After much research with both motor and speed controller experts we discovered the issues reported above.  Long story short, there was nothing wrong with the motor we sent him.  His controller was very old (early 90’s) and had experienced serious degradation of the output and MOSFETs.  The controller which was supposed to output 300 A was only outputting around 200 A.  The motor was expecting much more current to perform to its full potential. Without enough current, the motor had a serious lack of torque which is consistent with this case.  Once the speed controller was repaired internally, the issue was resolved and the cart now functions normally.

To remedy these types of speed controller degradation issues, you can either replace the speed controller or have your controller rebuilt.

For more specific information related to MOSFETs, please visit http://en.wikipedia.org/wiki/MOSFET.

Written by Michael Williams

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