# DC Motor - PWM vs Voltage

When controlling a DC motor that is rated at 5V theoretically I can drive the motor in these ways:

1. 5V with 100% duty cycle PWM.
2. 10V with 50% duty cycle PWM.
3. 20V with 25% duty cycle PWM.

Is there a difference between the two? Will using option 2/3 shorten the life of the motor? Would changing the PWM frequency improve the motor lifespan? The reason for doing this is that it is possible to drive the motor much slower with higher voltage.

• I'm not sure I follow the last statement of why you think you can drive the motor slower with a higher voltage? Aug 15 '13 at 13:04
• 25% of the time you will be applying 4 times the rated voltage. You will have larger current spikes along with the higher voltage and both could damage your motor even if they are only being applied 25% of the time. Aug 15 '13 at 13:27
• You might want to consider what's really happening - voltage is just a means to push current through your motor, the product of the two being the actual power. Controlling voltage is a very vague way of controlling a DC servo motor. Aug 15 '13 at 13:33
• I think its also possible you might have more torque ripple as a result too. Aug 15 '13 at 13:33
• What's your duty frequency? There's more impact if it were 1Hz, rather than 1kH :-) Aug 15 '13 at 14:30

Yes, this is very different. PWM drive and voltage drive separately don't really mean that much, but when combined in this way you will get significantly different performance.

Electrically, you can model a motor as an inductor (windings), resistor and a voltage source (the EMF, proportional to the motor speed). When you apply a lower voltage compared to a higher voltage, you will:

• get a slower $dI/dt$ through the motor windings, reducing torque
• have a lower maximum speed because of back EMF

With PWM and a higher voltage, you will be able to achieve higher peak speeds and often much higher torque at equal speeds.

There is no real reason that a motor will damage when applying higher voltages. Motor damage is caused by:

• Bearing overload (e.g. pushing sideways on the shaft of a thrust bearing axle)
• Bearing overspeed (depends mostly on the oil used)
• Brush arcing (caused by high speeds and to a much, much lesser extent also higher current)
• deformation and delamination of internal structures due to overheating

Also, overheating or running a motor at very high currents will cause a (significant) reduction in torque because of magnetic saturation.

If you can guarantee that you keep your motor within speed, torque and force limits as well as properly cool it, there is no downside to running it at higher voltages with PWM.

• Exactly how does a slower $dI/dt$ reduce torque? Aug 15 '13 at 19:35
• @PhilFrost I should have said: lower $dI/dt$ combined with PWM. A slower slope means a lower mean value of the current over the duration of the PWM cycle. This is most true for DCM operation (which is most often the case with PWM motor control, as you're using extremely low PWM frequencies) but also holds true to some extent for CCM, although the effect there is much smaller. Aug 16 '13 at 7:49
• Are you sure DCM is the norm for (well designed) motor control? Wikipedia says PWM motor control frequencies range from "from few kilohertz (kHz) to tens of kHz for a motor drive"; this is not what I'd call "extremely low", and with most motors I've worked with, well into the CCM region of operation. Aug 16 '13 at 11:36
• This completely depends on your application. I kind of assumed typical small DC motor applications - hobby stuff, RC planes, etc. For that domain, DCM is the norm. They are usually operated at at most a couple kHz, with very low motor inductances (often <1mH). As current and physical size goes up, it's hard to do DCM at all and CCM is all that people do. And by extremely low frequency, I mean audible region. I should have quantified that. Aug 16 '13 at 11:42
• Thanks for a detailed answer. My goal is to runt he motor as slow as possible and I already have a 1:1000 gear on it. I'm using s small motor. The motor runs cool but I'm concerned on its lifespan. Aug 18 '13 at 7:08

A PWM driven motor is a buck converter. To the extent that your circuit and motor make a good buck converter, there is no difference between each of your options.

Some things to keep in mind:

At some much higher voltage, maybe around $1000V$, the insulation in the windings will fail. $20V$ is far too low for this to be a concern.

If your PWM frequency is too low for the winding inductance of the motor, the current over each PWM cycle will change significantly, you will have high torque ripple, and you no longer have a good buck converter. Performance and efficiency will suffer. In the extreme case, you may not even get the motor to spin.

Just as with a buck converter, a PWM drive involves transistors and other circuitry which is necessarily introduces additional losses. PWM drive is also more complex to design, harder to get working properly, is more expensive to implement, etc. Some of these losses (for example, hysteresis losses) are inside the motor, and will result in a higher motor temperature, which is usually the limiting parameter for motor performance, and a higher temperature will overall reduce the motor's lifetime. However, you would require a very poor design to make these losses approach losses you already have at DC, such as resistive losses in the windings, so the difference between PWM and DC drive efficiency is not much.

There are a couple things you need to watch out for when you use PWM.

First, the PWM voltage that your motor will see will have high frequency harmonics. These harmonics will introduce losses in your motor that wouldn't be there if you were using DC voltage. What this means is that your motor will get hotter when with PWM vs. DC (all other things being equal). How this affects the life of your motor depends on a lot of things. There is a very crude rule of thumb in the motor world that says for every 10 °C increase in motor coil temperature, you reduce the life of the motor's insulation by half. You would have to do a ratings test on the motor with the PWM drive you are using to either derate the motor or to tell you how much hotter it would get using PWM at the nameplate rating. Most people are really equipped to do this test, though.

Second, assuming you are talking about low voltages like your example (5V-20V), the increased voltage itself probably won't have much of an effect on insulation life. Obviously it depends on the motor, but generally the insulation in motors can withstand voltages up to at least 1000 V for short periods of time. Given voltage spikes due to the PWM, this means you don't really have to worry about reduction in insulation life until you are talking about motors that are rated ~ 400V-600V or above. The higher voltages with PWM could have an effect on brush life but its really hard to say without testing.