DC motor torque vs DC motor speed

This blog post article is intended to show you the relation between DC motor torque and DC motor speed for the provided PWM signal and frequency. It is covered by three different video clips for three different PWM modulation signals. This is initial version of the blog post and it will be changed in time. In the near future more video clips will be presented with different types of motors in order to show you how DC motor torque and speed depends on each other regarding provided PWM modulation including DC motor current consumption.

Pulse Width Modulation (in short: PWM) is control method with possibility to apply different voltage levels to the DC motor wire coils. Here we are dealing with DC motors with only two wires controlled in these examples by SERPENT I DC motor controller and VF driver (variable frequency driver - new zilsel-invent product). VF driver PCB is built as a prototype version with the thermal transfer method. It is not intended for professional PCB manufacturing since it is a revision number one and documentation is available for free. Anyway, VF driver is an adjustable variable frequency driver with possibility to find the right frequency for DC motor including PWM modulation capabilities.

VF Driver Rev#1 - prototype version

VF Driver frequency measurements with digital multimeter. This video clip is intended to show frequency measurement related to the VF driver with a digital multimeter. 20KHz is maximum measurement limitation. 11KHz is starting generated frequency, by placing a jumper at 104 ceramic multilayer capacitors, the total capacitance of the VF driver is increased, so that is the reason why frequency is decreased to above 6KHz. When a jumper is removed, frequency goes back to the 11KHz. Set of four different capacitance, in this case, two 104 ceramic multilayer and two 2n2 disk ceramic capacitors are responsible for frequency generation including POT configured to work as a variable resistor. By changing resistance and total capacitance, VF driver generates different frequencies. As you can see from the video clip, generated frequency is stable. More capacitance and greater resistance generate lower frequencies and vice versa.

As you will see from provided videos, it is possible to control DC motor with low level impulses which brings threshold voltage Vth to the DC motor wires just to start the slowest possible rotation. In provided figure #1, diagram presents the relation between a DC motor torque and DC motor speed for complete set of different PWM duty cycles starting from the slowest rotation related to Vth and ending to the highest speed defined by 100% PWM duty cycle. By changing PWM signal, we are actually changing position of the DC motor torque vs speed curve (bold dotted line) and position changes are done in parallel between minimum curve and maximum curve as it is shown in the diagram. What does that mean? It means that it is not possible to change DC motor speed and not to change DC motor torque for providing PWM duty cycle. By PWM duty cycle changes the torque and the speed are changed accordingly, both at the same time, which is the reason why we noted that changes are made in parallel between minimum and maximum DC motor vs speed curve.

Figure #1: DC motor torque vs DC motor speed

Video #1 - Threshold voltage (Vth) and slowest possible speed

This first video is a demonstration about the relationship between DC motor torque and speed for the threshold voltage Vth generated by VF driver. We already noted that it is not possible to change DC motor speed without torque changes, both values are changed at the same time. Rotation of the DC motor shaft is too slow, a very small applied load is good enough to stop DC motor shaft rotation since DC motor torque is too small in this case. As you can see, with naked fingers it is possible to stop DC motor shaft rotation very easily. We have to pay attention here, without load, DC motor torque does not exist, and it is a zero. When load is applied to the DC motor shaft, DC motor torque start to react in the opposite direction with tendencies to continue with shaft rotation. The load is external applied force, it could be a finger like in this example, or it could be some mechanical part like wheel, for example, but anyway external load to the DC motor shaft has tendencies to stop DC motor shaft rotation, while DC motor torque works in the opposite direction. 

Another, very important parameter is current consumption related to different PWM duty cycles. In this case as digital multimeter has been measured, about 0.01 amperes is current consumption regarding PWM duty cycle and threshold voltage (Vth), even in case when external load is applied to the DC motor shaft. That is very small current consumption with very small power losses, but provided DC motor speed as well as torque are useless since it is not possible to drive anything heavy.

Video #2 - DC motor torque vs speed and current consumption

Video #2 is related to higher speed in comparison to the video #1. The PWM duty cycle is above DC motor voltage threshold and it rotates with higher speed – RPM rotation per minute. As we can see from the provided video the current consumption regarding free running mode (without externally applied load) is a little bit more in comparison with the first video example, it is 0.02 amperes. Now, pay attention to the current consumption in case when external load is applied to the DC motor shaft. In some cases, digital multimeter has been measured around 0.07 amperes. It is not too much current, but there is a certain difference in comparison with video #1, the current consumption is higher and it has been increased when external load has been applied to the motor shaft. Now we will increase PWM duty cycle up to 100% and see what’s happening, so pay attention on the third video clip.

Video #3 - DC motor torque vs speed and current consumption

Video #3 is related to the maximum voltage applied across the DC motor wire coils since the PWM duty cycle is set to 100% presently. Since, RPM rotation per minute has a maximum value, protect your naked fingers with insulation tape since friction between the fingers and the motor shaft is too high and you will feel the heat, so do not do that without naked fingers. It is very hard to stop DC motor shaft rotation in this case since RPM has maximum value, including DC motor torque, which acts in the opposite direction immediately when you touch motor shaft with fingers. The RPM is maximum and we can see more clearly how DC motor consumes much more current when it is loaded with externally applied force (load). In some cases DC motor shaft rotation is stopped completely and digital multimeter has been measured above 0.5 amperes, in some cases it was above 0.6 amperes. If external load is increasingly applied to the shaft, current consumption increases as well, and it reaches a maximum in case when motor shaft is completely stopped. When the DC motor shaft is completely stopped while it is in running mode, it is called a stall mode. The DC motor stall mode is not very well welcomed, since the DC motor could be damaged permanently with high DC currents, including DC motor controller/driver.

YES, it is very bad when your DC motor stall in free running mode, higher current flows through DC motor, since there are no back electromotive force which works against applied electrical power, more worse, it gets heated which led to overheat since DC motor has ESR or equivalent serial resistance. But that is not all, DC motor driver starts to heat, which lead to overheating, in a worst case scenario, DC motor driver/controller could be melted with very high DC current, so heatsinks and thermal cooling plays important part in the DC motor design. Yes, it is very bad when the DC motor stalls in running mode, no rotation at all, and high current flows through DC motor coils as well as DC motor controller. But anyway, there are protections against a DC motor stall, where you actually measure current consumption with shunt resistor and switch off complete electronics (H-Bridge) when current consumption becomes critical, with simple reason to avoid any kind of damage etc.

Video #4 - DC motor torque vs speed and current consumption

In this example different DC motor is used, the specification is not available, but it is more powerful DC motor in comparison with DC motor regarding first three videos. Anyway: by increasing the applied external load, DC current consumption also increases, and vice versa. In this case at DC motor stall, current consumption is over 1 amperes, much more in comparison to the first three examples.

Conclusion

Let’s make conclusions with these four video examples. First, DC motor current consumption is small when DC motor is running without externally applied force – free running mode. Current consumption increases when external load is applied to the DC motor shaft. If the DC motor shaft is stalled, with too high externally applied load, not able to be overcome with DC motor torque, the highest current flows through DC motor coils and DC motor controller as well, and permanent damage is not excluded. Heatsink design plays an important rule in DC motor controller/driver design, including to choose right power MOSFET able to handle maximum rated DC motor current consumption. It is not possible to change DC motor speed rotation without DC motor torque changes. Both values are changed at the same time and values depends on the provided PWM duty cycle generated by the VF driver.

In the near future, this blog post will be updated with more features, videos, diagrams, etc. All changes under the blog post will be shared under the social networking media.

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zilsel-invent assumes no responsibility or liability for any errors or inaccuracies that may appear in the present document.
Specification and information contained in the present schematics are subject to change at any time without notice.