Electric Trolley Speed Controller
Ian Hooper, 27 July 2007

Requirements

I was recently asked by our neighbours to replace the speed controller in an old electric trolley used by a local plant nursery. The existing speed controller was a system of contacts and resistors, offering just three discrete speeds, and after 20 years of use wasn't working so well anymore. The trolley has a 24V electrical system powered by four 6V Trojan T105 deep cycle lead acid batteries. The motor is a Baldor 0.7hp (~500W) series DC type motor. There are many off-the-shelf electronic speed controllers on the market which could have been used, but I decided it would be more interesting to design and build one myself.

Circuit Diagram

So here's the circuit diagram of the design I came up with. In short, it uses a microprocessor to control a FET driver, which switches two large MOSFETs. Ultrafast diodes are used for current freewheeling during the FET off-time, and a bank of 10 capacitors provide up to 20 amps of ripple current. For more information about how motor speed controllers work, click here.

Main Components

Atmel ATMEGA8L Microprocessor I used this microprocessor primarily because it would do the job and they are oh-so-easy to work with. I'm using a few of its 10-bit ADC inputs for throttle, battery voltage monitoring and internal temperature sensing, and just the one PWM output signal to the FET driver. CodeVision AVR software was used to program the microprocessor. [Click here to view the datasheet] [Click here to view the C code]
IXYS IXDD414PI FET Driver These are a standard DIP8 FET driver package, but offering one of the highest current ratings at 14 amps - great for running several large MOSFETs in parallel. They also have an Enable pin which can be used to gracefully shut down the FETs in a failure condition, though here I just pull the input pin low to disable output. [Click here to view the datasheet]
Panasonic FC-A 220uF Capacitors These are fairly standard 50V 220uF rated capacitors, used to supply ripple current to smooth the power flow coming from the batteries. Panasonic capacitors were used due to the good value and reasonably good ripple current capabilities. These offer about 2 amps of ripple current each at 20kHz, 70°C. [Click here to view the datasheet]
STTF6002C Ultrafast Power Diode One of the most important features of the freewheel diode in a motor speed controller (or buck converter) is the reverse recovery time - in general, the faster the better. These ST diodes are rated to 200V, 60A and have a very fast trr of just 22 nanoseconds. Specifications: Voltage rating, VRRM= 200V Current rating, IF(AV)= 2x 30A Voltage drop, VF (typ) = 0.75V Reverse recovery time, trr (typ) = 22ns [Click here to view the datasheet]
IRFP2907Z Power MOSFET This FET was used due to the extremely low on-resistance of 4.5mΩ and a fast reverse recovery time of 41ns typical. They are available in TO-247 package (like the diodes) which is very common, allowing easy interchange with different FETs on the same board layout. Specifications: Voltage, VDSS= 75V On resistance, RDS(ON) = 4.5 mΩ Max current, ID= 90A [Click here to view the datasheet]

Future improvements

One particular omission with this speed controller was current sensing and limiting. In this case the speed controller is rated to about 5x the continuous power rating of the motor, so hopefully it will never approach the limits of the power electronics. Nevertheless it would be nice to guarantee the controller could handle overcurrent situations gracefully, if it ever came up. Something for me to work on.

The same design could also be used as a controller in higher voltage systems. IXYS make a great 200V rated MOSFET, available in a 247-sized package, the IXFK120N20. They have a 17mΩ on resistance - very good for such a high voltage FET. The linear voltage regulators used would have to be the high input voltage type (which I believe are available up to 120 volts), though it may be better to use a small DC-DC converter for the logic power supply because they are much more efficient.

Pictures

View of the circuit board, complete minus throttle and thermistor leads	Top view of circuit board	Fitted to heat sink and front panel
Underside of circuit board. Note heavy duty wires between fets and along capacitor banks	All boxed up and ready to go	Another view of the complete unit

Oscilloscope trace of FET gate voltages (lower) and FET drain voltage i.e motor negative (upper)

Installation in the electric trolley

[FEB 09 UPDATE] Using the design for higher voltages/currents

A common question I'm asked is how the design could be modified to work with higher voltages and/or currents. The first change you'd have to make to have it work with higher voltages would be to replace the 7812 linear voltage regulator with something which will work up to your new voltage - so either a high voltage linear regulator, or an appropriate DC/DC converter with 12V output. (100mA is enough current to run the logic side.) You may also need to use higher voltage rated power devices - that is, the FETs, diodes and capacitors. Typically you should "overrate" their voltage by about a third, e.g use 100V components for a 72V controller.

Higher current capabilities can be achieved by paralleling up more FETs, diodes and capacitors. Be wary that you don't exceed the current rating of your FET driver when it has to turn many more devices on/off, and watch your physical layout e.g keep devices as close together as possible, and paths from FET driver to FET gates as short as possible!

One caveat, current limiting becomes pretty important as power levels go up. I'd highly recommend adding a current sensor (check out the Allegro or Tamura hall effect sensors), monitored by your microcontroller to avoid overcurrent and consequent damage to your power devices.

As a parting note, controller development is a reasonably steep learning curve (even if you have something to work from!) and in truth unless you're doing it for the technical challenge/enjoyment, it's usually much easier to buy a controller! The low voltage ones in particular are relatively inexpensive and usually work much more reliably than custom-made ones.

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