Something better than a back EMF diode? - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: February 27, 2016, 05:41:13 pm
content: I'm looking to control 2 30 amp brushed fans. I can imagine that the amount of power coming from the back EMF is going to be considerable. Usually this is dealt with with a reverse polarity diode on the fan. But I'm having a thought what about I take a diode from each side of the fan and connect it to the power rails so that when the fan produces a negative voltage it is effectively connected backwards across the power rail so that it is dumping its power back into the power rail and not just trying to burn it up as it were. I expect a TVS diode would probably be a good idea just to make sure that excessive voltages still do not occur.As I have to fans I could try and do something along the lines of controlling them 180 degrees out of phase so that at 50% duty the back EMF current from one fan would effectively be absorbed by the other fan and that at higher and lower duties something closer to that would happen.Is this a good idea or is it just asking for trouble?

author: SeanB
author_rank: Super Contributor
timestamp: February 27, 2016, 07:16:30 pm
content: You will need 30A rated diodes in any case, and probably will be better off using some really large MOV devices to just clip the hair off the noise. Some ceramic or film capacitors across the motor leads ( around 220n to 1uF) will help with the noise, Just make sure the driver is capable of handling the expected negative voltages as the fan spins down, and also consider that if the fan is turning in ambient airflow it will act as a generator and provide a voltage and current that depends on the airflow direction and speed.

author: Simon
author_rank: Unknown
timestamp: February 27, 2016, 07:25:59 pm
content: I assume by driver you mean my low side switch MOSFETs?Yes the fans will effectively generate voltage if anything from their own inertia as they continue to go round while the power is removed.I found no information about this method of re-feeding back EMF into the power supply. Is there a way of calculating how much power would be stored in the fan?At the expense of making this rather more complicated than perhaps it needs a large bypass capacitor which would probably help store up the back generated power if the duty is less than 50%. For example at 25% duty I would have to wait for 25% of the cycle before the other fan would come on and use up that power. This seems to be how three-phase brushless motor drivers work which helps limit the ripple current in the supply line as the power from one coils back EMF is being used by the other coils that are being turned on.Perhaps at less than 50% duty I should start to increase the frequency of drive so that the offer period on both fan is reduced and there is limited chance of back EMF creating large voltage buildupsNaturally some sort of voltage suppression device will be needed in the event of the worst.

author: SeanB
author_rank: Super Contributor
timestamp: February 27, 2016, 07:47:24 pm
content: So you are using low side switching, which makes things easier drive wise. The body diodes will have to be capable of handling the 30A load, so if not add an external Shottky diode that will do so to save blowing the mosfet up. A diode across the motor is not needed, it will recirculater power happily using the body diode, though you will have to use an 80V rated mosfet ( I assume you are using a 24V supply) and some big MOV or bi direction TVS diodes across the motor terminals, and as you are doing PWM speed control it will be noisy, so use a good supply decoupling capacitor ( they will have to be a good few to get the current rating in aggregate up to 30A so they do not blow up, but at least here ultra low ESR is not really needed, just regular big ass 85c caps) and keep the leads between caps, motor and mosfet short, and then make sure supply and ground traces are terminated at the capacitor leads and not elsewhere. Capacitor across motor will probably be 100-220n mounted near the motor to reduce the noise at the motor so the wires do not radiate. Use a 250V capacitor, preferably potted in a box with the leads leading in and out and with the MOV units there.

author: Simon
author_rank: Unknown
timestamp: February 27, 2016, 07:53:43 pm
content: I'm not sure that would work would it? Surely just putting a diode across the MOSFET in the opposite direction from normal conduction alongside the body diode will effectively put the reversed polarity of the fan motor in series with the supply voltage and drive even more current round and round in circles?What I was hoping to do was during the off cycle use a diode from the positive of the fan to connected to the negative supply and a diode from the negative of the fan to the positive of the supply. In this way when the fan generates back EMF this will be switched back into the power supply which as it happens might be trying to drive another fan that has just turned on or it would charge up a capacitor ready to dump that charge into the next fan.It's actually 12 V so life is a little easier.

author: Zero999
author_rank: Super Contributor
timestamp: February 27, 2016, 08:10:11 pm
content: I think you're talking about recovering the back-EMF and injecting it back into the power supply?To do that, you need to switch both sides of the motor and two diodes are needed to steer the current back into the power supply.

author: Simon
author_rank: Unknown
timestamp: February 27, 2016, 08:20:01 pm
content: I am. Yes I suppose I would need to switch both sides of the fan or one of the diodes would just become an old-fashioned back EMF diode. I suppose that is why most people never bother it's more complicated than it is worth and perhaps has no meaningful gain.

author: Zero999
author_rank: Super Contributor
timestamp: February 27, 2016, 08:50:09 pm
content: Quote from: Simon on February 27, 2016, 08:20:01 pmIt's normally done in a normal H-bridge but as you don't need to reverse the direction, only two transistors and diodes are needed.

author: Simon
author_rank: Unknown
timestamp: February 27, 2016, 08:52:26 pm
content: Quote from: Hero999 on February 27, 2016, 08:50:09 pmI'm not quite sure. If you use a H bridge switching the opposite pair of MOSFETs will just try to actually drive the fan backwards unless you are very clever about the timings or perhaps the body diode in the other 2 MOSFETs would do the job.

author: Zero999
author_rank: Super Contributor
timestamp: February 27, 2016, 09:11:56 pm
content: Yes, the body diodes in the other MOSFETs divert the back-EMF back into the supply.Ignore two of the MOSFETs and their diodes and you have a similar circuit to the one I posted above.

author: Ian.M
author_rank: Super Contributor
timestamp: February 27, 2016, 09:27:34 pm
content: Apart from the inductive 'kick' at turnoff, the back EMF of a brushed DC motor is set by its speed. The motor will accelerate until it reaches a speed where the back EMF balances the supply voltage minus the I*R drop where R is the armature windings resistance. As such it cannot be greater than the supply voltage and no useful amount of energy will be recovered during braking without special measures to boost the back EMF to greater than the DC bus voltage.However shorting a DC motor acts as a fairly strong brake so the simple anti-parallel diode is undesirable if you want to be able to let the motor coast. Short of using an H-bridge, (which with an appropriate control algorithm supports coasting, and both dissipitive and regenerative breaking,) the best option is probably a parallel bidirectional transorb with a breakdown voltage greater than the max supply voltage. It will clip the inductive spike, but not conduct after that decays to the normal back EMF for the current motor speed.

author: station240
author_rank: Unknown
timestamp: February 28, 2016, 01:32:11 am
content: 2nd to last diagram on this page, seems to be what you want.http://www.eternaltruth.net/Science/JOHN%20BEDINI%20MOTOR%20DIAGRAMS%20AND%20LAB%20NOTES.htm

author: Simon
author_rank: Unknown
timestamp: February 28, 2016, 08:43:38 am
content: Quote from: station240 on February 28, 2016, 01:32:11 amApparently there are two types of back emf, the self generating type in the motor and the inductive "kick back". the "generator" type won't be of a high enough voltage to feed back into the supply so I think I'll abandon the idea for now as it's probably more hassle than it is worth and may creat asn amount of expense that is nt appreciated. the picture you refer to is about the same as what Hero posted above.