Question is pretty much in the title. I know it's definitely possible to use certain DC brushless motor as a generator, but is it always the case?
Thanks a lot in advance.
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asked Aug 4, 2017 at 17:48
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\$\begingroup\$ What different types of BLDC do you know? \$\endgroup\$– Eugene Sh.Commented Aug 4, 2017 at 17:49
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\$\begingroup\$ generate what? ac or dc? \$\endgroup\$– user3528438Commented Aug 4, 2017 at 17:56
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\$\begingroup\$ @EugeneSh. No I mean could some motors have some added features specially added to stop the motor from being used as a generator or something similar... \$\endgroup\$– Cyril_engineeringSECommented Aug 4, 2017 at 18:06
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\$\begingroup\$ @user3528438 I want to use the motor in combination with a speed controller to recharge a battery (I'm working on a regenerative breaking project), I hope this answers your question \$\endgroup\$– Cyril_engineeringSECommented Aug 4, 2017 at 18:08
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\$\begingroup\$ Of course there could be some extras on the motors, and only the imagination of the manufacturer is the limit. It can be mechanical restriction, such as a one-way gear (worm gear). It can be electrical, like a bunch of diodes or similar, allowing one way power transmission only. \$\endgroup\$– Eugene Sh.Commented Aug 4, 2017 at 18:27
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4 Answers
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Brushless DC motors are usually motors that have permanent magnet rotors. It would be extremely unusual to find any other type of motor described as a brushless DC motor. All such motors can be used as generators, but some designs are easier to use as generators than others. A major example of a difficult motor is a BLDC fan motor found in a computer. Those have electronic circuitry built into them that must be removed or disconnected in order to use the motor as a generator. You might find some other design described as a BLDC motor that would be difficult to use as a generator, but most of them only require the shaft to be turned to produce AC at the terminals and a rectifier added if you want DC.
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\$\begingroup\$ Exactly. A simple permanent magnet DC motor, like the ones that used to be common in battery-operated toys and model kits, work as generators without trouble. But a DC brushless motor has MOSFETs controlled by a microcircuit to switch power to the stator coils. The microcircuit can only be powered from the outside. \$\endgroup\$ Commented Dec 6, 2018 at 6:20
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\$\begingroup\$ well there are brushless motors without permanent magnets which use induction to make a iron core magnetic, however you won't see them in any compact motors or general brushless motors, that said if you salvage motors from big old industry mashines you might encounter one. so they do exist, even though I don't know how common they are also since ones with magnets tend to be more efficient. but magnetizing a iron core with induction is cheaper to produce for big motors, but if it is actually used a lot outside of scientific testing or very rare cases, I don't know. \$\endgroup\$ Commented Sep 26 at 13:31
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Permanent magnet BLDC motors can be used as generators.
There is a type of brushless motor called a Reluctance Motor, which has soft-iron core, so will generate nominally zero voltage when spun.
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3\$\begingroup\$ An easy way to check if any unknown motor would work as a generator or not would be to rotate the shaft with the terminals open, then short the terminals and rotate the shaft again and compare if it's more difficult to rotate with shorted terminals or not. \$\endgroup\$ Commented Aug 5, 2017 at 6:25
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\$\begingroup\$ Actually, a switched reluctance motor can be used as a generator. It just needs a little encouragement to start. \$\endgroup\$– JonCommented Dec 20, 2018 at 10:11
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\$\begingroup\$ that encouragement however only kind of works if you plan to constantly use it under high enough load, as it is very hard to keep stable when not generating a lot of power and when not generating enough power it will rapidly cost more to encourage it than it generates. it is possible indeed, but I doubt many home hobbyists will have much succes pulling that off properly especially since the effect itself is hardly ever discussed or explained properly due to most people not understanding how it's working as generator can even be possible, or straight up rejecting it despite it working. \$\endgroup\$ Commented Sep 26 at 13:34
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Two examples where it is desirable for a motor to function as a generator are:
Regenerative braking in a vehicle so at least some of the energy is recovered rather than being totally wasted in a mechanical friction brake.
Using the generator function to hold the motor in a stopped condition by switching a resistor or a short across its terminals.
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A Brush-less Direct Current Motor, also known as Permanent Magnet BLDC motor can be used as a generator. It is a common application in case of regenerative braking. In case of regenerative braking, the accelerator is not being used, but the rotor of the motor which is connected to the wheel via the shaft continues rotating. This causes the motor to act as a generator and feed power back into the batteries of the electric vehicle. When the brakes are applied, a sudden pulse of electrical energy is sent into the batteries. It is also possible to manually rotate the wheel of an electric vehicle and charge the battery. However, this is not practical and achievable only in case of very low performance E-Bikes.
Besides, permanent magnet AC motors used in microwave ovens can also generate electricity when turned manually. However, one must be careful as these motors can give high voltages at a very low turning torque and pose an electrocution risk.
The use of motors as a generator has these following points:-
Only permanent magnet motors will generate electricity when rotated and this will not work with the Induction type motors which will have no permanent magnets.
Motors will supply a mix of AC and DC and not pure current. Thus use a full bridge rectifier that will output DC and fix a capacitor to ensure a purer amount of DC. Use a DC to DC converter to regulate the output.
Ensure the KV rating of the motor. The KV rating of the motor is the rating, which when multiplied to the input voltage value will give the value of the torque in RPM. E.g. If a motor has a value of 5KV is given a 12 Volt supply, then 5KV = 5000 V and 5000*12 = 60000 RPM. Thus if you manually rotate the motor's load shaft at 60000 RPM then you will get an output voltage of 12 Volts. The best thing would be to use a motor with a low torque rating so that you can get a usable voltage at very low spinning speed. The best example for this is the microwave oven motor which can give 110-220 Volts while being spun at just 2-4 RPM. However, this is a high voltage that poses an electrocution risk, so be careful when you use this motor.
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\$\begingroup\$ Regarding point 3: (1) As far as I can make out the K of 'Kv' is for "konstant", not 'kilo' so a motor with a rating of 5 Kv would only run at 60 RPM on a 12 V supply. The 'v' (lower case to avoid confusion with V, the symbol for 'volt' (also lower case when spelt out) stands for 'velocity', not voltage. See Motor constants for details. (2) RPM is not a value of torque. You can Edit your answer to improve it. Welcome to EE.SE. \$\endgroup\$ Commented Sep 12, 2020 at 13:54
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\$\begingroup\$ See electronics.stackexchange.com/questions/262106/… for another question on the topic. Bruce Abbott's answer is worth a read. \$\endgroup\$ Commented Sep 12, 2020 at 13:55