PWM of Brushed PM Motor Outside(2X) of Nominal Voltage Rating
Miner_with_a_PIC
Posts: 123
Are there any reliability(burning out of brushes etc.) concerns [or design issues] if a brushed permanent magnet motor is PWMed outside of its rated voltage (2X in this design)? The speed needs to be controlled within a range where the effective supply to the motor is between 1.5 and 2.5V as desired by the software. As the battery is depleted there will be a feedback loop to determine the current battery voltage and alter the PWM accordingly. Battery life is important so quiescent current must be minimal ( < 50uA, lower the better) . Using a simple transistor to ground would be cheap and easy to implement but may have negative effects which are to be avoided in this consumer product design. Any inputs on efficiency, reliability or pitfalls (noise etc.) to be wary of would be very much appreciated.
Hardware Specifics:
Motor >> 1.5 to 3V toy motor with running current in the range of 150 to 250 mA unloaded. Long mechanical time constant (motor stops over ~0.5-1 second).
Battery >> 4 AA with output ranging from 6.4V (fresh batteries) with depletion cutoff of 3.6V (Propeller 3.3V + Vdropout).
Hardware Specifics:
Motor >> 1.5 to 3V toy motor with running current in the range of 150 to 250 mA unloaded. Long mechanical time constant (motor stops over ~0.5-1 second).
Battery >> 4 AA with output ranging from 6.4V (fresh batteries) with depletion cutoff of 3.6V (Propeller 3.3V + Vdropout).
Comments
The general rule of thumb is to not go over the nominal voltage rating of permanent magnet DC motor.
Well maybe 25% over is OK.
The main problem is not excessive current as the current is basically independent of voltage.
The problem is excessive motor rpm which can damage windings through centrifugal force.
Another problem is excessive brush arching.
I would recommend dropping the voltage with an NPN emitter follower circuit using an MJD31C1G.
Use a small MOSFET, STD50NH02L-1, as the PWM control driven by the Prop Pin.
Duane J
The motor 'sees' the average voltage, and it sounds like you are PWM regulating the Motor to lower the voltage, so that should be ok.
If battery life matters, you want to avoid over-drive of the motor anyway, and you would want to tolerate large Vcc dips from inrush (starting) current. ie you do not want an aged/low battery + motor start to reset your controller.
For best efficiency, you need a Synchronous Rectifier on the Motor, and that usually means low threshold NFET/PFET.
If you need reverse battery protection, another FET, diode connected saves power.
-Phil
If the voltage is in excess of the motor's rating arching may damage the brushes. Especially with miniature types.
Duane J
Using 9V and PWM would have a failure mode where the PWM output gets shorted/stuck high and causes the motor to probably burn out. Might be an issue to check out.
Yes and no. The breakdown of air is ~3 V/µm, and the arc effect is actually driven by the current (which does average) and the inductive flyback voltage, generated as the brush wiping contact releases the current.
That voltage, far exceeds the difference between 1.5 and 3.3V.
I have been feverishly following your design with substitute parts and find it to be a great solution and something I hadn't thought of. I did however make a change, partially due to parts on hand as well as cost considerations . The ground transistor was completely removed and instead the PWM signal was routed into the base of a Darlington transistor pair (via a current limiting resistor) made from 2 NPNs with common collectors connected to the 6V. I do suffer two Vbe drops but the design can tolerate this (results in ~ 2.05V @100 % duty cycle). Thank you for sharing, this saved some parts and seems like it would be robust.
Phil,
You are absolutely right and please forgive my lack of full description of the intended purpose, its really difficult to cover all the bases in these posts. The motor will run a maximum of 10 seconds per event and no more than 6 events per day, which amounts to a measly 5 or so mAh per day for a 250 mA load. For alkaline AA batteries this is peanuts, however idle quiescent current during the remainder of the time is a major concern. I do like an energy conserving designs but as Mark_T mentioned there is always the chance that thing either get electrically shorted or software goes haywire resulting the motor and attached going fast forwarding. I did test PWMing 6V directly and indeed if the frequency is high enough (50KHz ish) then current savings are definitely realized. Thanks for your advice.
Mark_T,
Software going haywire more than something shorting (on a finished PCB) is a fear that I have. Unfortunately motors are attached to assemblies and its more than brushes that will bare the consequences.
jmg,
Its unclear what will happen to the brushes, there appears to be two conflicting opinions which adds to my concern. I suspect the reality depends on the motor speed, condition of the brushes, etc etc etc. The good news here is that the motor speed will be unchanged so the motor will run mechanically as it normally would, the coils don't care, so its all down to arcing due to increased voltage potentials as well as potentially larger current flows during the on portion than the motor might normally see...The only way to know really is to do life testing which shouldn't be too time consuming as I can simulate nearly 4 years of expected usage in a 24 hour period. I only have one geared motor assembly and would hate to kill that in a life test session. Something to consider is than motors naturally have a reverse [edit: Forward, same polarity as Vin] EMF due to there rotation so under normal conditions (non PWM) the input voltage vs the at speed armature coil's induced voltage should be small, when a 6V source is introduced in the PWM case the EMF difference is relatively large. The only time a motor normally sees large EMF differences is during start-up, something to think about.
Some are surprisingly cheap, like this one (2500 : $0.34452)
http://www.richtek.com/product_detail.jsp?s=768
Works down to 3V, and a small inductor will drive with DC, and remove any RFI and brush issues.
You can also soft-start the motor, if life-cycles matter, and it is small and easy to enable and control.
I had thought of this, but the added cost of an inductor, filter caps and the schottky diode were undesirable. Also I need to vary the voltage in software which could only be done by PWMing the enable pin which is unconventional usage for these parts (me thinks). I agree though that this is a clean design and probably the way I would perform this around the lab where reliability and RFI are concerns take a front seat to cost/complexity.
No, you could use the Feedback pin, and drive a simple DAC (RC) into that.
Use the ENable pin, for deep sleep.
That's an interesting approach but I'd like to keep the buck converter's ability to regulate if possible. So I could either raise the ground on the feedback divider using the RC output or mix the RC output into the divider at the feedback pin via a resistor (higher impedance than used in the RC network). I think the latter method would be most stable, was this the connection method you were thinking of?
Selecting the right drive resistor size from the RC to the regulator's feedback divider could help eliminate safety concerns of over driving the motor and the output would be DC with the customary buck switching ripple. This is the cake and you can eat it too solution...of course the cake isn't without a little added cost.
I then can't see why you need to use 4 AA batteries to get 6V or so.
Why not use only 2 AA batteries to get 3V or so. You would get still get 2.05V till the batteries are practically dead.
The second circuit drives the MOSFET in direct digital PWM mode.
The third circuit drives the MOSFET in indirect analog PWM mode.
You choose the R and C to suit your PWM signal.
Digital PWM is the most efficient.
Analog PWM is the kindest to the motor. Practically no RF. Although it will dissipate some power but would not need a heat sink.
Please consider the STD50NH02L-1 MOSFET as it would have only 5mV Drain to Emitter voltage drop at 250mA so it can operate the motor full speed,(2.05V), all the way down to a dead 3V battery. Its really a very small size package and only $0.40 in unit quantities.
You don't need 6V, only 3V and a good transistor.
Duane J
Your original post to this forum has seeded all sorts of experiments on my breadboard this evening and opened up many possibilities outside the scope of the current project. Many thanks for all the advice, its very sound and helpful.
The reason for using 4 AA batteries is that I'd like to get as much juice out of those batteries as is possible with most of the head room afforded for the propeller chip's 3.3V, so I can get away with depleting the batteries down to a termination voltage of 0.9V. It might be higher given the motor induced voltage droop as the internal resistance of the batteries increases. At this point I am pretty much sold on the buck idea which has also evolved to a PWM driven buck with a transistor to ground rather than the positive rail (that is a home brew buck converter, not a dedicated PMIC). It works beautifully, very little ripple and very controllable. I will determine the battery voltage in software and use this to set the PWM duty cycle accordingly. The straight PWM method caused unpredictable motor speed as the voltage didn't seem to be linear with duty cycle, the buck converter is much more predictable.
If I run into software control issues and/or the cost gets prohibitive I will definitely fall back on the emitter follower design with very little reservation. Its simple and very cheap to implement.