Hegner variable speed lathe running at half speed

[RogerS]

First let me preface this by saying that the guys at Hegner have been very helpful and my next course of action is to send the variable speed unit back to them for checking.

But before I do...am I missing something obvious?

It all started when I thought I'd check to see how close to the printed RPM on the speed control unit the actual RPM was. I've got a laser tachometer and used that. I was surprised to see that the actual RPM was about half that on the printed label. There are two pulley ratios and I confirmed I was looking at the right ratio. The belt isn't slipping. To check the tachometer I measured the speed of my DW625 router...20,000 max rpm and I measured 19500rpm so I reckon the tacho is OK.

The motor has three connections from the variable speed unit. The motor itself has three windings. I have measured the resistance of the three windings and they all appear similar. When I measure the voltage between any two connections then they are all similar. They all vary in amplitude as the speed control is increased.

Any advice very welcome.

 

[CHJ]

When I measure the voltage between any two connections then they are all similar. They all vary in amplitude as the speed control is increased.

Any advice very welcome.

Surely if it is 3PH the Amplitude (Voltage) should be reasonably constant but the frequency should change, and possibly PWM (pulse width modulated) to maintain power/speed under load.

What are you measuring the voltage with? you may not be seeing true mean voltage.

Can you measure the frequency of the invertor output, this will give you a direct ratio to motor speed, what speed is the motor turning at when supplied with 50 HZ?

 

[RogerS]

Surely if it is 3PH the Amplitude (Voltage) should be reasonably constant but the frequency should change, and possibly PWM (pulse width modulated) to maintain power/speed under load.

Yes..that's what I thought (with my shaky knowledge of 3ph motors)

What are you measuring the voltage with? you may not be seeing true mean voltage.

digital multimeter ..very high input impedance.

Can you measure the frequency of the invertor output, this will give you a direct ratio to motor speed, what speed is the motor turning at when supplied with 50 HZ?

No..I don't have a frequency meter..but I guess it's getting to the point that there may be something wrong with the variable speed control...

 

[Scrit]

From my experience of uising 3-phase induction motors dropping 20% in voltage doesn't make a heck of a lot of difference, so as long as you're getting 350 to 415 volts phase to phase (415 volts phase to phase being the equivalent of a 240 volt phase to neutral) then it should be OK. I'm assuming that the motor is a 380/415 volt motor rather than a dual/multi-voltage type. Some three phase motors are wired to run on all sorts of odd voltages. If you're seeing those sorts of voltages then the frequency of the poser output has to be suspect I'm afraid

As an aside, is the motor a "two-pole" (i.e. a one-speed) type or a "four-pole/two-pole" type? It might be useful to catch site of the motor rating plate to answer this question

Scrit

 

[CHJ]

What are you measuring the voltage with? you may not be seeing true mean voltage.

digital multimeter ..very high input impedance.

Maybe but it is probably making assumption that it is looking at a standard sine wave, If PWM output you will have a square wave of varying duration and it MAY be giving erroneous reading, dependant on the algorithm used.

Edit: as Scrit says, voltage should make very little difference to speed under no load conditions, may even run on half voltage.

Do you have access to an oscilloscope? both frequency and waveform/voltage peak would be measurable.

 

[CHJ]

Roger, does your Speed unit use one of these inverters? mine uses the Jaguar Cub pattern.

Some useful disc patterns for checking spindle speeds if you do not have tachometer.

 

[RogerS]

Many thanks for the help, guys. Much appreciated.

Chas - I do have a 'scope (somewhere ! It's not been used for years and so probably may not even fire up!!).

I decided to pack up the unit and send it to Hegner, as even if I had a 'scope' and that confirmed that something was wrong with the unit, I'd still be back at square one as it would take me an age to investigate/diagnose/mend the unit even assuming that I could find a circuit diagram and stir up the particular grey cells that specialise in electronics and have lain dormant for about 30 years. It's a German unit, by the way, IIRC.

So plan a. Let Hegner investigate the problem. Ideally it will be something simple like an incorrect link (a Freitag job :wink: ). Worst case will be new inverter in which case I'll keep the old one and investigate it further at my leisure. Following the simple rule of 'black, broken or smoking'. :lol:

 

[RogerS]

Excellent service from Hegner. Sent to them Saturday and just received it back (Tues pm).

And the problem?

Somehow it had dropped its' programming and so a quick reprogram by Hegner and we're away.

I have an initial one-day turning course in a couple of weeks time and so glad that my lathe will be up and running for me to practice on when I return.

 

[CHJ]

Excellent service from Hegner. Sent to them Saturday and just received it back (Tues pm).

And the problem?

Somehow it had dropped its' programming and so a quick reprogram by Hegner and we're away.

I have an initial one-day turning course in a couple of weeks time and so glad that my lathe will be up and running for me to practice on when I return.

Glad to hear it's sorted.

Roger have you got the program parameters/key sequences etc. from Hegner, if not I would ask them for the Invertor programming details and default settings for your motor. Usually no more difficult than setting up a DAB radio or Video. :lol: :twisted: On mine one parameter is all that is needed to halve the output frequency.

Does the unit by any chance use a lithium battery to backup the memory?
Could you have lost setting by not having power on it for some time?

We had machines go down because company policy was to kill all power to a facility when not in use, several years down the line when the company who did the programming went out of business and we found that they had volatile memory installed we could not get them back up. And procurement in their bid for lowest tender never bought the software backup.

Best of Luck on your course.