Low Power Extended Range DC Motor Controller: Design and Implementation, Study Guides, Projects, Research of Electrical and Electronics Engineering

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Low Power Extended Range DC Motor Controller

ECE 445

Final Paper

May 3, 2005

Team Members: Chris Groesch Chris Swanson TA: Bryan Dobbs Abstract

As power electronics become more widely used and understood, as well as cheaper in cost, the use of DC motors in everyday devices will increase. This is why we chose to build a DC motor controller for our project. Our goal was to show that power electronics can be used in a cost effective manner to control machines that cannot be connected to the AC power grid because of their mobility. Our circuit was implemented using to sub-circuits controlled by two IC chips. The first circuit is a boost converter with its switching proscribed by the UC3825 chip. We designed the converter to increase the input from a lead-acid battery to a constant 15V. The second half of our circuit is an H- Bridge with a LS7260 chip to control the gates of the four MOSFET and therefore the output voltage to the motor. In this manner, the LS7260 is maintaining a user specified output voltage; the motor speed will also be determined by the user.

1. Introduction

Title: Low power extended range dc motor controller The motivation of this selected project is to accommodate smaller remote motors with sufficient power and control supplied from a conventional 12V lead-acid battery. In particular, motors found in items such as handicapped wheel chairs, luggage carriers, golf bags, or any other type of small scale transportation based motors will now have the capability to run on a simple 12V dc supply. In addition, the converter will allow for speed control by enabling the user to define a maximum speed dependent on the load and retain full adjustability up to that maximum. Unfortunately circuits currently on the market that performs these tasks are fairly expensive which is why they are not often utilized in projects pertaining to small scale transportation. This dc motor controller will open the market to a more cost feasible option. 1.1 Objectives The primary objective of this project is to convert a 12Vdc input to an output up to 15Vdc while allowing speed control defined by the user. The user will be able to set a maximum amount of output voltage and be able to adjust the output voltage similar to a throttle. The design will provide control for continuous 250W loads and temporary 500W loads. In order to extend the duration of the 12V battery, the highest possible efficiency will be sought and the cost to the consumer will be kept at a minimum. 1.2 Benefits  Easy and convenient transportation  Cost effective  Opens the market for other various remote motorized designs  User friendly 1.3 Features  Supplies 250W continuous loads  Can withstand 500W loads up to a minute  Allows adjustable speed defined by user  Adjustable current limit  Efficient to prolong battery life

 Circuit Breaker protection

2. System Block Diagram

Figure 1. DC Motor Controller Block Diagram 2.1 12V Battery The input source is taken directly from a conventional 12V lead-acid battery. The reserve capacity of the battery is fully dependent on the quality and cranking amps. The ouput to the boost converter is 10-15 Vdc with a current range of 0-56 amps depending on the speed setting. 2.2 Boost Converter The boost converter, or step-up converter, is a switching dc/dc converter that produces an output voltage greater than the source(ranging from 10-15Vdc). The general layout of a boost converter is shown in Figure 2. For the worst case scenario, a 10V input voltage will have to be boosted to the maximum output of 15V. The design of the boost converter entails four main parts: Inductor, MOSFET, MOSFET Control, and a Diode. Figure 2. Basic Boost Converter Design. 2.2a Inductor One of the major aspects of the boost converter is the design of the inductor. Special consideration must be taken in choosing a core in order to accommodate at least

Wb A A turns Wb N

R

i c^ 71. 24 30

4 7 

  (^) (6) Figure 3. BH Curve for Inductor Core. In the worst case scenario the wire used in wrapping the inductor would need to carry about 60A taking losses into account. According to the National Electric Code (NEC), 8awg can carry a maximum of 70A in free air. Since the wire is pvc coated, it can withstand approximately another 5A above the free air rating. This will be the gauge of wire used throughout the circuit wherever full current can flow. Due to the fact that 8 gauge is rather difficult to wrap around the window area of 44.18cm^2 , a flat braided copper was chosen for its flexibility and ease of wrapping. The flat braided copper implemented has a width of a quarter of an inch which is equivalent in cross sectional area to that of an 8 gauge wire. The flat braided copper had to be insulated since the wire is not coated. Since the wire used was not a typical round gauge wire, the number of turns determined may be off by a bit due to the wire being braided within itself. The core was wrapped so the surface area was completely covered which took 30 turns. The inductance resulted in 90μH which is good since at least 60μH was needed. The next component modeled in the boost converter is the MOSFET which is used to vary the duty cycle. 2.2b MOSFET

An n-channel MOSFET with current ratings exceeding 70A was sought, but tough to find in the T0-220 package. After searching through various manufacturers, a STB200NF04 by STM was utilized. The STB200NF04 is contained in a T0-220 package with ratings of 120A, 40V and a minimal on state resistance of 3.7mΩ. The NFET has a duty cycle ranging up to approximately 33% that was determined by the Unitrode gate drive control chip, UC3825. 2.2c MOSFET Control The UC3825 is a high speed PWM controller that can be used for either voltage or current mode topologies. The basic pin layout is shown in Figure 4. Figure 4. UC3825 Pin Layout. For this circuit, a voltage topology was employed. The controller provides protection circuitry such as a current limit comparator with a 1V threshold, a TTL compatible shutdown port, and a soft start pin which will double as a maximum duty cycle clamp. It also features totem pole outputs designed to source and sink high peak current from capacitive loads and has fully latched logic to provide jitter free operation. The UC3825 provided closed loop operation through a proportional integral negative feedback from the output bus voltage to provide output voltage regulation. Figure 5 and equation (7) outlines how the voltage divider feedback functions in the boost circuit except in a simplistic form. R 1 and R 2 were chosen such that Vout would be 5.1V. V 1 is the dc bus voltage that is wanted at the boost output. To get an output of 15V, R 1 and R 2 were chosen to be 750kΩ and 390kΩ that will output 5.13V. The UC3825 compares the output voltage from the voltage divider to the internal reference of 5.1V and adjusts the duty cycle accordingly to maintain the 5.1V. As stated previously the frequency of the PWM controller was set to 30kHz through a resistor and capacitor combination. The

2.3 H-Bridge The H-Bridge has the ability to change the direction of the motor and stop it completely. The control unit adjusts the inputs to the MOSFETs to accommodate the user defined action. 2.3a MOSFET The MOSFETs for the H-Bridge needed to be able to handle more than 70 A, so the STB200NF04 was used for the NFETs and the STB80PF55 was used for the PFETs.. Both were contained in a T0-220 package. The NFETs had ratings of 120A and 40V. The PFETs have ratings of 80A, 55V, and a minimal on state resistance of 1.8mΩ. 2.3b MOSFET Control The control portion of the circuit is primarily responsible for ultimately regulating the input voltage to the motor. The control unit also adjusts the inputs to the MOSFETs of the H-Bridge to vary the motor direction and has built in dead time to assure that all four MOSFETs are not on at the same time within the H-Bridge. Figure 6 shows the pin layout of the LS7260. Figure 6. LS7260 Pin Layout. The LS7260 was used to control the MOSFETs in the H-Bridge and there by the voltage output and the motor speed. The Commutation Select capability of the LS was not needed for the H-Bridge application. The gates of the MOSFETs were driven using four of the six outputs on the chip. Out 1-3 were specified to drive PFETs, and Out 4-6 were to drive NFETs. By using this chip, the H-Bridge can be disabled using the Enable or Brake switch. This would cause the voltage to the motor to vanish and therefore stop it. It also allows for either forward or reverse operation of the motor

through the use of a switch. The Sense inputs were not used for our application. The Oscillator determined what the switching frequency would be given a specific resistor and capacitor combination. The V Trip was connected to a potentiometer to allow the user to set the voltage across the output and therefore the speed as well. The Overcurrent function was an added safety feature to give the motor some protection from large currents. The Overcurrent could also be set using a potentiometer based on how much current the applied motor can handle. 2.4 Motor A basic dc motor, brush type or brushless, supplied by the user, is used for loads up to a peak of 500W for one minute with a continuous amount of 250W. This was not available in the testing stage but the circuit should properly function based on specifications and component ratings. 2.5 Performance Requirements

1. Input Voltage ranging from 10Vdc to 15Vdc.
2. Output voltage ranging from 0 Vdc to 15 Vdc specified by user.
3. Motor loads ranging up to a continuous 250W or 500W for one minute.
4. Efficiency greater than 85% (excluding motor).
5. Voltage ripple less than ^2 %.

was due to a varying 5% tolerance level in the resistors and also since the voltage divider was calculated to output 5.13V. Regardless, the output was still within specification. To verify that the boost converter would output 15V with a normal battery, an Optima deep cycle battery was utilized. The battery had a measured voltage of 11.6V. The boost output did indeed remain at approximately 15.2V as with the adjustable Kenwood power supply. Figure 8. Boost Converter Output Regulation The voltage ripple was also of concern since the specification stated that it had to be less than  2 %. The plot shown in Figure 9 is of a 10V input where the output voltage ripple is approximately .25V which is about a 1.67% ripple. A 10V input is the worst case scenario since the duty ratio is the highest in order to maintain a 15V output. This was also within specification since there is a peak to peak ripple allowance of 4%.

Figure 9. Boost Converter Voltage Ripple with 10Vin. To verify that the H-bridge functioned correctly at lower inputs, the voltage potentiometer was adjusted to output approximately 5V across the motor terminals with a small load of 8.33Ω. The resulting waveforms for the forward bias are shown in Figure

10. For reference channel 1 is the boost converter NFET switching signal taken from drain to source, channel 2 is the output voltage, channel 3 is the output current and channel 4 is the boost converter output. As shown, the boost output maintained approximately a 15V output. The motor output follows a sawtooth waveform with a 5V output. The current also follows a similar sawtooth shape with an output of about 1.2A. Channel 1 shows that the NFET is switching on and off to accommodate the set reference voltage. Also worth noting is the fact that there is slight ringing in the on-state of the NFET. This ringing is due to no load present at that particular instant. This could be suppressed by placing a small resistor in parallel with the output boost capacitor to dissipate any ringing or oscillations. The noise spikes seen in the switching voltage and output voltage are brought upon by the current switching on and off. This is acceptable and expected.

To ensure the circuit was working efficiently, a 10 and 8.33 load was used on the output. Unfortunately, a motor was not obtainable so the circuit could not be completely tested. In place of a motor, numerous 500Ω resistors were put in parallel to reduce the resistance and outline a motor equivalent. The 500Ω resistor boxes used from the power lab have a minimal amount of inductance within them so the output across the load terminals is primarily resistive with a small amount of series inductance. The input was varied and input/output power values were recorded. Figure 12 and 13 show the efficiency curves for the 10Ω and 8.33Ω loads. The efficiency for the smaller load had an average efficiency of about 92% while an increased load resulted in approximately 93%. This is well within specification where the efficiency had to be greater than 85%. This circuit is anticipated to have slightly larger efficiency at the larger power levels where the voltage drops are at a minimum. Figure 12. Efficiency for 10Ω Load.

Figure 13. Efficiency for 8.33Ω Load.

6. Conclusion

The purpose of this project was to design and build a low cost and low power DC motor controller. There were also other specifications on efficiency, output voltage, and voltage ripple. All of these were met at low power which can be seen in the previous figures. However, this circuit is meant to be applied at higher power levels. This leaves some doubt as to whether it will work as designed, but given the limitation of not having a load to test at high power, the low power testing is all the assurance that can be given at this time. The project was overall a success given that the controller meets its specifications and is much cheaper than current productions.