Frequently Asked Questions (FAQ):
What is PWM?
Let's say we have a standard DC motor and a power supply. Connected through a simple switch, you have a choice of two speeds - full speed, and stopped. For many applications, this is sufficient to get the job done. But what if full speed is too fast? How can we slow the motor down?
We could use a power supply with a lower voltage, or a motor designed to run at a slower speed. But what if we want to be able to change speeds on the fly?
One possible solution is to use a variable resistor or a rheostat connected in series with the motor:
In this scenario, voltage is dropped across the resistor and is wasted as heat. The motor runs at a slower speed due to the reduced voltage level reaching it.
This may be fine for small motors, but as the motor size increases, the amount of energy dissipated in the resistor also increases. This excess energy is wasted as heat. And more heat requires larger heat sinks and fans to prevent the resistors from burning themselves up. Not only is this expensive, but wasting electricity is not a very green thing to do.
Is there a better way?
What if we could vary the speed of the motor without using resistors whose sole purpose is to waste energy?
A switch wastes negligible amounts of energy right?
Let's go back to the motor and switch. What if the switch were replaced with a momentary pushbutton?
We could push the button to start the motor and release to let it coast to a stop. What if we did this very quickly, say 5 times a second - the motor wouldn't even have time to reach full speed when we pushed the button, and it wouldn't have time to come to a stop when we let go right? The motor would be running somewhere between stopped and full speed - and without using resistors!
If we wanted the motor to run faster, we could push the button for a longer period of time, and the motor would be energized for a longer time resulting in more power. To run slower, we could shorten the time the button is pushed and lengthen the time the button is released.
What we're doing here is a form of Pulse Width Modulation, or PWM.
We'll eventually get tired after a while pushing the switch, so we can replace the pushbutton with an electronically controlled switch, such as a transistor. Add some control electronics to turn the motor on and off very quickly (and tirelessly) and we'll get a medium speed.
By changing the time that the switch is on vs off, we can change the average power being sent to the motor, and hence the motor speed. The ratio of on time to total time is the duty cycle. At 100% duty cycle, the motor is getting power all the time, and running at full speed. At 0% duty cycle, the motor is on 0% of the time. At any percentage in between, the motor is receiving power part of the time, and runs at a medium speed.
Great! Now we have speed control without wasting power in resistors.
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What is the PWM switching frequency?
The switching frequency is the number of times per second the current is switched on and off to the load. For example, an 18 kHz switching frequency means that the controller is turning the load on and off 18,000 times per second. The ratio of the on time to the total time is what determines the average power going to the load.
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What effect does the switching frequency have?
For mechanical loads, the switching frequency has two effects - audible, and current ripple.
You may notice the sound of the switching frequency coming from the motor at low duty cycles. The sound may resonate through the mechanics. If the motor is well damped from vibration and noise is not an issue, then this is not a factor. For noise sensitive applications, you'll want a high frequency beyond the range of hearing. Controllers running at 18 kHz will not produce audible switching noise since 18 kHz is beyond the range of most people's hearing.
The high switching frequency also provides a smoother current waveform. In PWM motor control, there are two ways that the pulse width gets averaged out: mechanically, and electrically.
Mechanical averaging relates to the inertia of the motor. The motor armature and the driven load have mass, and cannot accelerate or stop instantaneously. They both take time to speed up and slow down. As the switching frequency increases, the motor starts to average out the pulses of torque and the resulting output is a smooth constant speed with little ripple. This occurs at a relatively low frequency, and depends on the system's inertia. The larger the motor, the larger the inertia, and the lower the frequency required to average out the torque.
Electrical averaging relates to the inductance of the motor windings. Inductors have a kind of "inertia" as well - in that they resist changes in current. When a voltage pulse is sent to the motor's windings, the current wants to shoot to the maximum as determined by the resistance of the windings. If it were a resistor, the current would jump to the level determined by Ohm's law. However, the motor's winding also adds inductance = and the inductance resists instantaneous changes in current. The larger the inductance, the slower the current will want to change. Thus, high inductance motors do not need as high a switching frequency to attain a smooth current wave form, while low inductance motors require a higher frequency to attain a smooth current wave form.
The current waveform affects motor efficiency. Low frequency switching relies on mechanical averaging, and has higher peak pulse currents because more time is given for the current to reach its maximum. Since:
Power Dissipation = Current2 * Resistance
The heat dissipated in the motor's winding goes up with the square of the current. Higher peak currents thus will generate more heating compared to a smoother, lower average current.
On loads with little to no inductance, such as lamps and heaters, the switching frequency will have little to no impact on the current wave form. In this case, the lower switching frequency is better.
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What kind of power supply should I use?
There are many kinds of power supplies. The main ones are linear regulated, linear unregulated, switching regulated, and a battery.
Unregulated Linear Power Supplies:
Please note that unregulated power supplies do not maintain tight control over their voltage. The output voltage rating is specified at a full load - so a 12VDC unregulated linear power supply may actually produce 15 volts when only a light load is connected. The voltage sags to its rated voltage when the load is drawing the supply's rated current. When choosing a supply, ensure that the maximum voltage the supply can ever put out does not exceed the maximum input voltage rating of the controller.
Regulated Linear Power Supplies:
These are similar to the unregulated linear supply, but also include a voltage regulator circuit. The regulator prevents the voltage from dipping under heavy loads, and also prevents the voltage from exceeding its rated output at no load. Some also add such features as short circuit protection to protect itself from shorts. These supplies are preferable in that they provide a clean source of power to the motor and the controller.
Regulated Switching Power Supplies:
These power supplies differ from the linear supplies in that they use an electronic switch and inductors to step down to a lower voltage from the AC line. These are usually more efficient than the linear supply, and can be made to produce high currents in comparatively small packages. These are preferable over linear supplies for their compactness and energy efficiency.
Whatever supply is chosen, the supply should be matched to the motor's rating. For example, a 12V motor should be run on a 12V supply. The power supply voltage should not be significantly higher than the motor's voltage rating.
Batteries:
When using batteries, ensure that the total voltage of the string does not ever exceed the maximum voltage of the controller. If the controller is connected while the batteries are being charged, please ensure that the charging voltage does not exceed the controller's rating. For example, a fully charged 12V lead acid battery is actually around 13 volts, and during charging, voltages higher than 14V may be present.
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Can I use another motor controller as a power supply?
NEVER use another motor controller as a power supply. For example, a 0-90VDC motor controller may state that the output is a variable 0-90 V DC, but in reality, the actual output is a rectified, pulse width modulated, 169 V Peak output directly from the AC Line. Doing this will destroy the controller and void your warranty.
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I have a PLC that outputs 10 volts on the analog and digital lines. Can I use this with your controller?
Yes - you will need to add a resistor divider to the output of the PLC to reduce the voltage to 5 volts.
I have two motors I want to run on the same output. Can I run them in parallel?
Sure - as long as the total current draw from the two motors combined does not exceed the controller's rating, then you can connect the motors in parallel.
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What if I have two larger motors that require separate controllers? Can I combine the pot inputs?
Sure. Just connect the pot as usual on the first controller, and run wires from the P- pin on the first to the P- on the second controller, and from the D/CN pin on the first to the D/CN pin on the second. Each controller should be hooked up to the power supply and to their respective motors. Now a single potentiometer will control both controllers.
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I am locating the potentiometer about 50 feet from the controller - is this ok?
This is possible, but ensure that the cabling connecting the pot to the controller is twisted pair or shielded. You can use two separate twisted pairs as follows:
The twisted pairs will minimize the possibility of noise being induced on the control line and affecting operation of the controller.
Using a lower value potentiometer will also help with noise immunity. A 5k or 1k potentiometer is recommended.
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The motor will be located about 20 feet from the power supply. Should I put the controller near the power supply, or near the motor?
The controller should be installed near the power supply, with twisted pair cabling running to the motor to reduce noise. Installing the controller next to the power supply reduces the need for large filter capacitors due to wiring inductance.
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My controller and motor are far from the supply. What filter capacitor should I use?
Depending on the size of the controller and the current rating of the motor, you should install a 1000 uF, 50V capacitor for controllers running at 200 Hz, and a 2200 uF, 50V low ESR capacitor on controllers running at 18 kHz, up to 30 amps. Larger currents will require larger capacitors.
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I don't have a fuse. Do I really need one?
A fuse is necessary for safe operation. The fuse will protect against short circuits and wiring errors. The fuse should be rated about 1.5x higher than the maximum normal operating current of the motor.
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Can I run a 3V motor on a 24V power supply using the controller?
The power supply voltage is much too high compared to the voltage rating of the motor. The motor's brushes and windings were not designed to take such a high voltage, even if the duty cycle is short. While this may work in the short run, it is not recommended due to possible arcing of the motor brushes, breakdown of the winding insulation due to overvoltage, and over heating of the motor.
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I have a brushless fan that seems to want to run at full speed using the controller. Why?
Brushless DC fans come with built in control electronics to commutate the motor. They sometimes come with built in filter capacitors that store energy. When they are run with a PWM signal, the capacitors will actually charge up during the ON time of the cycle, and continue supplying power to the motor during the OFF time of the waveform. When this occurs, the motor runs at full speed regardless of the PWM input. To fix this, an inductor needs to be installed between the motor and the controller to slow down the charge and discharge of the capacitor. The size of the inductor will depend on the capacitance inside the motor, the current draw, and the PWM frequency.
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I want to show the output of the PWM controller with my digital volt meter but the output voltage shown doesn't seem to correspond to the potentiometer position. Why is that?
The output of the controller is a pulse width modulated voltage. The digital volt meter is only good at measuring constant DC voltages. Any pulsing of the voltage will confuse the circuitry inside the meter, causing erroneous readings. To really verify the output, you'll need to use an oscilloscope. The oscilloscope will show the waveform clearly, and you will see that the pulse width varies linearly with the potentiometer position.
If you want your meter to show a meaningful output, a simple RC filter can be used to smooth out the pulses and produce a voltage that the meter can measure.
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