Inductance matching drives?

[pdunster]

Hi All,

I would like to seek opinions from other users as to problems with impedance matching on low inductance nmotors.

Most high performance servomotors are built with a very low inductance. Since the magnetic force is proportional to the current flow through the coils, and the current rise time is proportional to the coil inductance, it can easily be seen that the lower the motors inductance the faster the motor can react. The problem here is though that a low inductance is close to a short circuit condition, and if the current rises too fast it can reach an overcurrent situation before the end of the normal PWM cycle.

We often use Maxon motors in our projects, their power to size ratio is really good, but with an inductance only 300uH we usually have to add inductance to the circuit to get good control. We recently modified one of our TEK10 drives and found that by doubling the PWM frequency (hence halving the PWM cycle) we could get good control and eliminated the need for in-circuit inductors.

We have now also modified the firmware on some of our TEK10s to allow for user selectable PWM frequencies to be preset for inductance matching between motors and drives.

I am putting this up to the forum to seek opinions, is this a common problem people have when setting up drives? And should we make this a standard feature of the drives, or just an option?

Any remarks, thoughts or ideas would be appreciated.

Cheers

Pete

[2sm]

I have had problems with this and the TEK10 drive. The peak current limit seems to trip very easily on some motors. It would be good to have some control over this.

[NC Cams]

I seem to recall that we found a similar situation of sorts when we were trying to do high frequency switching of DC motors via electronic speed controls.

My take was this: it takes time for the full surge current to take effect as you saturate the iron core of the motor. Hence, as we increased the frequency of the pulse drive, although the duty cycle remained the same (say 50%), current flow was diminished.

We thought this was cool as we had a limited supply of current (1.2mah batteries) but we failed to consider torque. Although the motors would run at a reduced speed as hoped for, they had a much diminished torque output under throttled conditions.

Apparently, since the current didn't adequately flow to magnetially saturate the armature core, part throttle torque output suffered noticeably.

If you're really worried about over current situations, you might try reducing your instances of ultra high accelerations asked for by the software (as in fast jogs to go from place to place).

It is typically during any high acceleration situation that ultra high currents are generated. We learned that by monitoring peak current and folding it back under extremen current demand conditions, we saved power and stopped cooking motors from them going overcurrent.

Whether you could do that in a servo without encountering follower error remains to be seen. However, anything you do to eliminate the cause of extreme current demands will probably do as much as chopping back the current once you ask/demand it.

[pdunster]

Yes I can see your point, since the current flow through the inductance (motor) is based on time, reducing the time would restrict the total current flow, hence reduce torque.

I think there are two things to consider here, one is the maximum current through the motor in one PWM pulse, the other is the max current over a longer time period with the PWM pulse at a higher ratio. If the PWM ratio is at say 80% the current will go up during the pulse, when the PWM reverses for the 20% it may not reach full current flow in the reverse direction before switching back again, therefore it starts the next pulse with a head start, the current gradually building with each pulse.

Limiting your accelleration will reduce the current build up over a series of pulses by keeping the PWM throttled back, whereas changing either the time period or inductance would change it on a pulse by pulse basis.

I suppose the question comes down to this, does increasing the frequency of the PWM have the same effect as throttling back the PWM over a time period or does it just reduce the current switched on each pulse?

Something to muse over on a long car journey!!!

2sm, if you want to try the variable frequency just send your drive back and I will reprogram it for you, no charge.

[number5]

Well Guys,

What an excellent topic to be greated with on my first day as a member of CNC Zone ( cool site by the way).

I have recently experienced this exact problem with some motors I picked up off e-bay. The motors were Maxon RE-35 motors. They are very powerful motors but extremely small in diameter. I picked them up for $30 each and I got 4 of them. After making a few enquiries, I found out that brand new they a $350 each (not a bad deal I thought).

Anyhow here is my story:

I first purchased 4 "(lizard)" drives and hooked them up. The motion was OK, fairly smooth, but after about 10 mins both the motor and drive were very hot so I shut the system down. I scanned the documentation to make sure I had done nothing wrong in my wiring and after being positive that I was right, I uncoupled the axes from the motor and tried powering them up again. Funny, I got the same result but this time while I was powered up I decided to check everything out with my multimeter. I could not find anything wrong, but then it happened, I let the smoke out of one of the drives. I contacted the manufacturer, who I must say were very helpful and offered me a new replacement drive at no cost, but could not answer why this had happened after I explained the fault (or no fault) finding I did. I recieved the new drive, and GUESS WHAT the same thing .

I then decided to spend a few more dollars and buy the lizards competitors drives "Ru***" . Can you imagine my frustration after another investment of serveral hundred dollars when the exact same scenario occurred. Once again I contacted the manufacturer of these drives and was ignored for about 1.5 weeks but after e-mailing to the point of becomming a spammer I finally got an answer. The total content of the e-mail was the following "Try putting a Resister across the terminals". I e-mailed them back with my motor specs and asked "what Value Resister shout I use?, the reply, well I'm still waiting.

Anyhow, I had been keeping my eye on this new type of drive that had popped its head up called a TEKDRIVE but being the type of person that likes to keep up with the Jones's and not actually be Mr Jones himself I decided to go with the run of the mill stuff that everyone uses (Eg: the above products).

The time had come, however, that I was completely and utterly frustrated beyond belief that I decided to take the plunge and try this new drive. This time though I decided to be smart, so I rang the TEKDRIVE manufacturers first and explained my troubles. LIKE MUSIC TO MY EARS the first thing I was asked after explaining my issues was "What is the inductance value of your motor?", when I said 0.1Mh I almost felt the gasp of air being sucked through the phone. However after a few seconds pause, a reply came back saying, MMMmmm, Ok I will send you a set of drives on a sale or return basis, if they work, send me some money, if not send them back, I think the main problem here, they said, is the ??? frequency (Tecko talk). I was told that they could be set to any frequency via a series of do thises and do thats but the man on the phone said that they would come pre-programmed to suit my motor.

I recieced them two days later and promptly hooked them up (very carefully I must add as I had not parted with any dollars for them) and guess what, well I bet you can guess, THAT"S RIGHT!!!!!!!!! Motors were cool and the TEKDrive was like an icicle. I am still using them now and have been cutting parts like you wouln't believe, these guys saved my life as I was nearly at the point of cardiac arrest.

Sorry for rambling on but I am a convert. I wanted to tell my story because I think that these guys acually know the guts of motor control and not just how to make something that drives a motor.

Don't be afraid to try a new product like I was as you may lose money like I did. Give these guys a call if you want the real deal and get their advice.

THUMBS UP FOR THE TEKDRIVE and the guys who make it.

Regards

Number Five!!!