Suppose i am using my motor as generator, snd the motor is a BLDC motor. My doubt is under what conditions the generated voltage is maximum. That is for the same speed a rotor (permanent magnet) with high magnetism will generate more voltage compared to rotor with low magnetism. Am i correct?

 

The best way to tell is to try spinning it with a meter on it to read the output voltage. Personally, I would have to guess that a BLDC motor won't act as a generator under any circumstances. But what do I know ! ? !

 

It will generate. Basically, voltage is induced by a changing magnetic field. The faster the field changes, the more voltage generated.
So more speed = more voltage. Stronger magnets = more voltage. At very high speeds the inductance of the windings have an effect and may limit the voltage and current output.

A simple test to see if a motor will make a good generator is to short the terminals together (all 3 in the case of a BL motor) and try turning it by hand. Should feel completely different (much stiffer) compared with the terminals open.

 

It will generate. Basically, voltage is induced by a changing magnetic field. The faster the field changes, the more voltage generated.
So more speed = more voltage. Stronger magnets = more voltage. At very high speeds the inductance of the windings have an effect and may limit the voltage and current output.

A simple test to see if a motor will make a good generator is to short the terminals together (all 3 in the case of a BL motor) and try turning it by hand. Should feel completely different (much stiffer) compared with the terminals open.

A brushless DC motor, has an electronic circuit that that does the commutation. mostly these are not bilateral, meaning that the motor will not work as a generator. So you would need to do some rearrangement to use it as s generator, but then it would not work as a motor. One or the other, not both.

 

You also have this thread on about the same subject matter. Questions and threads like these normally get better response and you will get more help when you only use one thread rather than a few addressing the same subject matter. MisterBill2 has covered things well in both threads in that a BLDC motor is just that, a motor. Not practical to try and use as a generator as they are not designed as such.

Ron

 

A brushless DC motor, has an electronic circuit that that does the commutation. mostly these are not bilateral, meaning that the motor will not work as a generator. So you would need to do some rearrangement to use it as s generator, but then it would not work as a motor. One or the other, not both.

MisterBill2 has covered things well in both threads in that a BLDC motor is just that, a motor.

I take some disagreement with those statements. While they can't be used like a DC car type generator can as both a motor and a generator, like some small garden tractors did for many many years, using the generator as a starter motor. They can be use as an 'alternator' by doing away with the motor drive circuit. They will when driven by another motor or other means, put out three phase DC.

To say they don't/can't would be news to many people in the alternative energy movement. There are just to many people using the Fisher PayKel type BLDC motors to make electricity for their off grid type life. It just takes a three way rectifier to change it into DC. The same three way rectifiers used in car alternators to do it.

 

I agree shortbus but I guess I question if the end justifies the means. So while a BLDC motor can be modified to work is it just easier to use a generator designed to be a generator? I guess if all you have is a BLDC motor it won't hurt to try and modify it. So I concede.

Ron

 

A brushless DC motor, has an electronic circuit that that does the commutation. mostly these are not bilateral, meaning that the motor will not work as a generator. So you would need to do some rearrangement to use it as s generator, but then it would not work as a motor. One or the other, not both.

I have to disagree. A brushless motor will make a good generator. Generates AC of course, so you will need a 3-phase bridge rectifier to make DC.

A brushless motor can be a motor and a generator. The motor is usually driven using a 3-phase H-bridge and the recirculation diodes (in reverse across each transistor) are the 3-phase bridge rectifier. This is why decelerating servo motors, for example, diving high inertia loads can feed power back to the supply (and cause an over-voltage situation unless braking resistors are used to dump the excess power).

 

I take some disagreement with those statements. While they can't be used like a DC car type generator can as both a motor and a generator, like some small garden tractors did for many many years, using the generator as a starter motor. They can be use as an 'alternator' by doing away with the motor drive circuit. They will when driven by another motor or other means, put out three phase DC.

To say they don't/can't would be news to many people in the alternative energy movement. There are just to many people using the Fisher PayKel type BLDC motors to make electricity for their off grid type life. It just takes a three way rectifier to change it into DC. The same three way rectifiers used in car alternators to do it.

I am familiar with the garden tractor type of generator/starter motor, and my observation is that NONE OF THEM are the brushless DC type of device. My comment is only about those brushless DC motors with the internal electronics package. AND, my remark addresses MOST of the motors of the internal electronics type. Certainly there are a small number of products intended to do both functions, but those were not mentioned in the original post. None of the BLDC motors that I have will show any terminal voltage when spun by hand. BUT the several permanent magnet ,brush type, motors that I have will generate enough voltage to spin a second motor connected to their terminals with just a hand spin. Adding a diode bridge array to a BLDC motor may allow it to serve as a generator but that is seldom found on standard ones.
If the BLDC motor in the discussion were one that utilized a separate electronics package, and, had that been stated, my answer would have been different.

 

A brushless DC motor, has an electronic circuit that that does the commutation. mostly these are not bilateral, meaning that the motor will not work as a generator. So you would need to do some rearrangement to use it as s generator, but then it would not work as a motor. One or the other, not both.

That is the first time I have heard that, I use BLDC motors all the time and the best way of aligning the hall sensors or tracks is by back-feeding/rotating the motor and use a double beam scope.
Also All you have to do is short the three winding connections and try and spin the motor, this is also the way you count the No. of poles.
It is a 3ph generator.
Max.

 

I agree shortbus but I guess I question if the end justifies the means. So while a BLDC motor can be modified to work is it just easier to use a generator designed to be a generator? I guess if all you have is a BLDC motor it won't hurt to try and modify it. So I concede.

Ron

Ron, no modification needed with a P.M. BLDC motor.
Max.

 

That is the first time I have heard that, I use BLDC motors all the time and the best way of aligning the hall sensors or tracks is by back-feeding/rotating the motor and use a double beam scope.
Also All you have to do is short the three winding connections and try and spin the motor, this is also the way you count the No. of poles.
It is a 3ph generator.
Max.

Both of the actions that MAX described are simple with a motor that is separate from it's electronic control module, both would be a bit complex for an experimenter playing with a smaller motor like those in computer fans. Also, most folks playing with motors who are asking questions like that may not have access to any sort of scope, let alone a dual beam or dual trace one.

 

. Also, most folks playing with motors who are asking questions like that may not have access to any sort of scope, let alone a dual beam or dual trace one.

I was using this as an example of the use of the generated BEMF which generally all BLDC posses.
The OP does not indicate the actual type of motor he is using.
Max.

 

The BLDC motor I initially though of was that of what is found in a hard drive, where its electronics package is housed within the motor itself. Now, I'm certainly no expert on these things, but I haven't seen a BLDC motor (with internal electronics) act like a generator. Just personal observation. And just because I've not seen EVERY waterfall, doesn't mean other waterfalls don't exist outside of the ones I've seen. Same with electric motors.

I know the traction motors that drive a locomotive (train) don't have permanent magnets in them but they DO have a way to apply "Dynamic Braking" to those motors. In short, they energize the field winding and (with resistance) short the motor to act as a brake. I have some treadmill motors (with Perm Mag's) that CAN generate electricity. I once tried spinning one with a gasoline engine and found that at best it would produce around 70 VDC. The problem is that the gasoline motor spins max at around 3500 RPM whereas the treadmill motor was designed to spin higher. I don't recall the numbers at present, but my conclusion was that I could not spin the "Generator" fast enough using just the pulley's I had on hand. Certainly if I had a 4 inch pulley on the motor and a 2 inch pulley on the generator I would have doubled the RPM's. Oh, and the treadmill motor is NOT BLDC. Just thought I ought to throw that factoid in there.

 

When I hear someone talking about a BLDC motor, computer fans and hard drives is the last thing I think about. Most every thing you see referencing BLDC is either model/hobby related, or off grid power generation.

 

Most every thing you see referencing BLDC is either model/hobby related, or off grid power generation.

Never said I have been well exposed to the wonderful world of motors. I can sit here and tell you what I know but I can't tell you what I don't know. How could I?

 

My main experience is with Servo's (motion control), also the ECM, now used in HVAC air circulation, together with washing machine Fisher-Paykel outrunner.
Max.

 

Never said I have been well exposed to the wonderful world of motors. I can sit here and tell you what I know but I can't tell you what I don't know. How could I?

My reply was just that a reply, to the whole of the thread , not pointing at you. you're not the only one that addressed BLDC motors that aren't my normal thoughts when seeing the term.