Lathe DC motor power supply

[Hero999]

No, I'm talking about putting the dimmer ahead of the transformer which will work - it's been done for years with halogen lamps and should work just as well for a motor.

A capacitive filter on the DC side of a 90V transformer is a bad idea because it will increase the RMS voltage to the motor to above 90V.

I think using an isolation transformer is a good idea anyway for safety reasons.

Phase control can be done to reduce the RMS voltage to 90V but it isn't as simple as setting the output to 75% of the full setting. You need to look at the shape of the expected waveform and calculate the required delay for an RMS voltage of 90V.

Another idea is to use a 90V transformer and rectifier with a traditional PWM MOSFET speed control on the output; this will give higher torque at lower speeds but it's not as easy as simply connecting a lamp dimmer before the transformer.

 

[LatheMan]

I have **broken link removed**, and **broken link removed** already.

 

[LatheMan]

A capacitive filter on the DC side of a 90V transformer is a bad idea because it will increase the RMS voltage to the motor to above 90V.

...

Phase control can be done to reduce the RMS voltage to 90V but it isn't as simple as setting the output to 75% of the full setting. You need to look at the shape of the expected waveform and calculate the required delay for an RMS voltage of 90V.

That is ONLY true for certain transformers. If my transformer put out 5v, no capacitor in the world could make the RMS 90 volts.

Another idea is to use a 90V transformer and rectifier with a traditional PWM MOSFET speed control on the output; this will give higher torque at lower speeds but it's not as easy as simply connecting a lamp dimmer before the transformer.

Read my earlier posts and you will see, that that is EXACTLY what I am doing.

I just want to power the CONTROLER, not the motor.

 

[LatheMan]

I will try to be more concise in my answers, as I get the feeling that people are not reading my posts before replying.

>Shortbus - Thanks for those links. And your right, the fan wouldn't work well in reversal.. I may just add a muffin fan to it.

>tcmtech - The Pulse Width Modulation controller that I have takes DC inputs. That's why I am trying to get 90VDC from AC. (my last AC motor burnt its start up clutch. poor quality chinese contacts.)

>BrownOut - Thanks for reading my previous posts. I thought so too. To make smooth DC to power my controller, some kind of transformer is going to be needed.

 

[Hero999]

That is ONLY true for certain transformers. If my transformer put out 5v, no capacitor in the world could make the RMS 90 volts.

I think you've missed my point.

If you connect a 90V transformer to a rectifier and measured the RMS output voltage, it would be 90V, minus any rectifier losses.

If you connected a large electrolytic capacitor to the output of the rectifier, the RMS output voltage will be closer to the peak voltage which in this case is 90√2 = 127V minus the rectifier loses, which will burn the motor out and is as worse than connecting it to the mains via a rectifier with no transformer.


You don't need a clean DC power supply to the motor, the motor will run quite well from a 120Hz fully rectified sinewave. It is nice to have a clean supply because it will make the motion smoother but I don't think that's important for a lathe which will have a heavy chuck which will act as a flywheel and smooth the motion.

The question is, what's the voltage rating of the controller?

If you've got the 190V version, then you can use the unfiltered 90V transformer and rectifier.

If you've got the 90V version, then you'll need to use a lower voltage transformer and stick a huge filter capacitor on the output.

60VAC is probably a bit on the low side because you have to account for voltage drop and ripple but it'll probably be fine. I'd recommend 70V but it'll probably be too much for the controller which will be rated to 100V peak.

Read my earlier posts and you will see, that that is EXACTLY what I am doing.

I just want to power the CONTROLER, not the motor.

That's easy, I'd recommend using another small 5V power supply.

If you're using a toroidal transformer, you could simply add another lower voltage winding, of thin magnet wire and cover it with insulation tape. This could be used to drive a bridge rectifier connected to a capacitor and 5V linear regulator, such as the LM7805. This will save the cost/space of another transformer but you'll have to figure out the volts/turns and go to the trouble of winding another coil.

If you're using a traditional laminated transformer, it probably won't be easy to add another winding so you could use a small 9V 3VA transformer to drive the rectifier, capacitor and linear regulator.

 

[tcmtech]

If your motor is a powered field and not a permanent magnet field you will have all sorts of problems trying to run it off of your PWM system.
For one it wont reverse regardless of what polarity you give it if the armature and fields are wired in parallel. One has to be reversed from the other to change direction.

Also the feeding the armature and the fields from the same power source will not give you any real speed control. you will get more of a variable torque effect instead. To use that motor you have posted with your PWM controller the fields will need their own independent power source.

At this point what I see is an unnecessarily complicated and likely somewhat costly way of doing something that should be simple, easy, and fairly cheap as well.
For what a step down transformer that can carry that motor will cost plus the additional other parts you could have bought a good Variac, bridge rectifier, big capacitor, and a simple 90 volt field source power supply and had a basic but reliable system.

A good surplus or NOS 1+ KVA variac is only around $50 and often times less. Add in the rectifier and capacitor for $10 more and a 90 volt field power supply for about $20 more and your good to go for around $100 including shipping.

Heavy duty Variac. **broken link removed**
Bridge rectifier. **broken link removed**
Big capacitor. **broken link removed**
90 volt field power supply. **broken link removed**

the prices shown are common at any time for any similar devices as well.