Question about Diode Sizing for DC Speed controller

So...I suppose the "right" answer will depend not only on the motor but on the load on the motor as well. Assuming an electrical model with a moderate amount of inductance, whatever the inductor peak current is, is what the diode peak current will be, not accounting for high dv/dt and charge being swept out of the diode as it reverses. With an infinite inductance, the current is constant at whatever the motor draws, and the average diode current will just be that current times the percentage of time the diode is conducting. If you're supplying 24V to the motor, the diode conducts zero percent of the time, but if you are driving a heavy load at low speed, so the motor current is 40 amps when the motor voltage is 1 volt, then the diode will be conducting most of the time. Diode dissipation is just the average over one cycle of the instantaneous diode current times the instantaneous diode voltage drop. LTSpice can simulate this sort of stuff quite nicely for you. And you may find that you can save considerable power if you use a half-H-bridge, with a mosfet to the + side, too, though you need a very low on-resistance mosfet to make that work at 40 amps. Oh, and beware of conditions of regenerative braking, where the motor delivers power back to the controller.

Cheers, Tom

"chapmjw" <

** So what does that 40 amp figure mean if you have no motor info ????

Your Q is basically silly, since the answer depends on info YOU say you have not got and must have.

How long is a piece of string ??

......... Phil

Tom,

Thanks for the reply, that's the info I was looking for.

jim

"John Popelish"

** Huh ???

At full PWM width setting, the flyback diode carries no current.

At a low PWM settings, the diode carries nearly the same ( average) current as the motor - but this will be a small value as the effective DC voltage seen by the motor is low.

Worst case dissipation for the diode is about 50 % duty cycle with the motor stalled or near so.

All this assumes the PWM frequency is reasonably high - ie well above 50 Hz.

......... Phil

If the motor is run very near stall, at full current, the diode will carry almost all the motor current. I would use a diode essentially rated for full motor current, and heat sink it for that current. With a 24 volt supply, you can cut the diode losses almost in half by using a Schottky diode, but it will probably cost more than a fast silicon junction diode. Doal diodes are very easy to find, and because they are well coupled, thermally, you can parallel the two sections.

Here is an example that costs less than $6 from Digikey, quantity 1:

Try that again, Phil. If it's locked-rotor, you get lots of current at low voltage. The motor doesn't mind, as long as you keep it cool: zero RPM means it's not turning its own fan, but nothing says it can't be cooled with a fan driven by a different motor.

In the midst of playing in my spare time with a half-bridge to drive up to 20A to a motor at up to 100V (but also 20A at ~10V when driving a heavy load slowly), Cheers, Tom

"Tom Bruhns" "Phil Allison"

** Every word is correct.

** Hardy a precise statement.

Ohms law applies to a stalled DC motor.

........ Phil

Agreed. At near stall, the PWM duty cycle gets pretty low, yet the current gets really high. This is because the motor is producing little back EMF, so a small average voltage can drive a large current.

A motor delivering full rated torque at low speed draws full rated current.

Most drives exceed current limit and won't reach 50% duty cycle at stall.

Yes.

** Replac>> If the motor is run very near stall, at full current, the diode will ** Then your words above are quite wrong.

** Nonsense.

Low speed operation and stopped are not equivalent to " stalled ".

** More non specific language ....... dear, oh dear. ** The rain in Spain stays mainly on the plain ....

( You are not even on the same topic. )

** More complete irrelevances to the actual topic.

" The rain in Spain stays mainly on the plain .... "

The issue I raise is about clear expression and not confusing folk with wrong or ambiguous usage.

Try reading my post again and do NOT disassemble it and post silly replies to its components.

" At full PWM width setting, the flyback diode carries no current. At a low PWM settings, the diode carries nearly the same ( average) current as the motor - but this will be a small value as the effective DC voltage seen by the motor is low. Worst case dissipation for the diode is about 50 % duty cycle with the motor stalled or near so. "

....... Phil

What is really relevant is the relationship between the PWM frequency and the L/R time constant of the motor inductance and circuit resistance. As others have said, all other things being equal, the current is greatest for a stalled motor. But as you pointed out, that current doesn't travel through the flyback diode exact during the periods when the PWM is pulsed off. 50% duty cycle is probably close to the worst-case situation, but in certain situations you might make it even worse with a little different duty cycle.

Robert Scott Ypsilanti, Michigan

In the case of the controller I'm playing with, the output is filtered DC to keep from spraying 100kHz all over the neighborhood. The inductor is 100uH; the input voltage is nominally 160VDC. Current limit is set at 20A. Locked rotor, the motor is nominally 0.4 ohms; it's rated for 90VDC operation at full output. For this case, clearly the diode--or rather the "other" mosfet in this case--sees highest average current when the output voltage is lowest, and the current is at the limit. That would normally be with locked rotor, about

20A*(160-20*0.4)/160, but might be momentarily even worse when reversing the motor. At nominally 80V @ 20A output, the average current in each FET is about 20A*1/2. -- And yes, the application for this motor is such that it is asked to deliver full torque at a wide range of speeds. It's my whole reason for replacing an AC induction motor with the DC motor.

Perhaps Phil is considering a controller with no current limit. I'd just as soon have both my controller and the motor last longer than it would if there were no current limit.

Cheers, Tom

"Tom Bruhns"

** Probably be impossible to get 50% PWM duty cycle into a stalled motor when overcurrent limiting is included.

Anyone know what "begging the question " means ???

......... Phil