Motor question for the elecitronically inclined

[Ray Cover]

OK here is the deal.

I am putting together a lathe for my bench and I have an electrical question for those of you who have such knowledge. I thought it would be handy for shaping punches, refinishing screws, etc.

It is my understanding that to find the amps that a motor will pull you divide the watts by the voltage.

So I ordered a 24v. 350 watt motor which should pull 14.5 amps max.
a 24v 15 amp power supply
and a 15 amp 24v motor speed controller.

The motor arrives today and it is marked
24v dc
350 watts
Rated Current 19.2 amps

what is going on here. If the motor pulls 19.2 amps, shouldn't it be a 460 watt motor instead of a 350 watt motor?

Will this still work with the 15 amp powersupply and controller without overheating them?

Thanks,

Ray

 

[msar24]

Motors can be rated at the maximum rather than the expected operating parameters. This higher amp rating may be the continuous power rating which is basically the amperage you can operate without overheating and causing damage to the motor. You should be able to contact (or look on the internet) the supplier or manufacturer to find out.

 

[coincutter]

Might be a rating for the start up draw which is generally higher than the run time rating. I threw a 13 amp compressor on a dedicated 15 amp circut and it blew the whole panel. Cold start was somthing like 20 amps so back it went.

call rod cameron or email him
he is on top of that stuff - more lathes in his shop than you can throw a stick at.

 

[SamW]

Ray, under load the motor will draw more current. Too much load will draw more than the rated current for the motor and it will overheat. I fly electric motor powered model aircraft and todays powerhouses can draw 100 or more amps and have 2 or 3 thousand watts of power, or more. Those I use are more in the 10 to 20 amp range at 11.1 volts. The motors are rated at so many rpm per volt and will draw more current under load to maintain that rpm. It sure is different than engines!!! S

 

[Ray Cover]

This little lathe is not going to stress this motor. This motor is out of an electric scooter capable of pulling my fat hillbilly butt up the hill so overload on the motor is not my concern.

What I am concerned about is the motor pulling too much current and burning out either the PWM speed controller or the power supply.

Ray

 

[TallGary]

Ray,

There are a few ways to work this problem with what you have at hand.

If the power supply or the PWM speed control has overload protection -- you are unlikely to damage either of them. If they are inexpensive, you may not have that option available.

Some speed controls can be fitted with additional heat sinks that increase the power handling capability of the control.

Along that same line, putting fans on the supply and control can help save each of them.

You can usually parallel to identical DC power supplies to (nearly) double the DC supply capacity but that won't help the speed controller.

Another way is to put a DC ammeter in the circuit between the power supply and the speed control and watch the current draw as you increase the speed on every start-up to protect the supply and speed control. The nice thing about DC motors is that they can be started with very little current at low voltages.

Generally speaking, with DC motors, if you decrease the input voltage you will reduce the speed output and the current input to the motor. If the PWM will stand it and the power supply is adjustable, decreasing the power supply output voltage to 18V should protect the power supply and the PWM. The down side of this is that dropping the voltage to 18V derates the entire assembly to about 270 Watts.

Hauling someone's fat butt up a hill is as much a matter of gearing as power input, but I catch your drift....

Gary

 

[rod]

Hello Ray!

I am enjoying your DVD, thanks for putting the work into this!!

I take it you have a DC low voltage motor because you want variable speed, or some other reason?

In Direct Current DC Power is simply Volts x amps P=VI

Other motors, such as AC induction motors can sometimes take about ten times their rated steady state power at start up, when the acceleration kicks them from stop to full speed., but that high amperage lasts only a fraction of a second and has no time to heat anything up unduly.

After that the motor only draws what it needs to overcome the braking force of your tool trying to slow it down. For example a half horse motor draws next to nothing when all you are doing is working on a tiny piece.

Gary has a good handle on your DC set-up, so I would follow his posting.

best

Rod

 

[Ray Cover]

I went with a DC system and a PWM motor control for three main reasons.

1. was to have a variable speed lathe at the turn of a potentiometer dial rather than having to change belt pulleys and have a wider speed range to boot.

2. With the PWM I have close to full torque even at the low speeds

3. DC motors in this size range tend to pull more torque than ac motors without getting into very expensive electronics.

I can buy these little scooter motors for about $25 shipping included. would I be better off just spending the $25 and buying a 250w 24v motor that has about a 14 amp max draw. I have found one for $11 + $10 shipping. I can always ebay this one off or save it for a future project.

electronics and motor I have about $75 in this drive system. for the potential performance that seems very reasonable to me.

This little antique model makers lathe is very nice. Low serial number of 66 and has never been used. It is kind of half toy and half tool for me. There are several times like refinishing screw heads or making a quick delrin mandrel to hold an odd pen part that it would be handy to have next to the bench. I don't need to take the kind of cut on this that I can on the big Sheldon tool room engine lathe I have in the garage but I would like it to have enough umph! to make small parts on.

Ray

 

[Bill Tokyo]

Hope you have that lathe bolted down

350 watts is just about 1/2 horsepower. The conversion is that 756 Watts equal a horsepower.
Basically you need about 1/3 horsepower for most smaller lathes, and lots of the Chinese 6 inch and 8 inch
swing ones come with about 1/8 horsepower and people don't find them under powerful.

Both the Taig and the Sherline have 1/3 horse motors pushing them. I also have to say that for many things they are a bit overpowered. I find most grinders and buffers are in the 1/8 to 1/4 HP range.

Bill

(who just bought a 120 Watt universal motor for his lathe. I'm looking forward to delivery tomorrow)

 

[Ray Cover]

I want to thank everyone for their input on this.

I have done some checking with Paul Hamler. I know Paul worked a lot with DC systems on a lot of his miniature shop equipment so I had to ask him.

Here is the story. The 19.2 amps is the max draw before the motor burns out if stalled. The running amperage should be in the range of the 14 - 15 amp at top speed under load. at normal running it will much less than that. No more load than this motor will have running this little watch lathe it should never even approach the max limit of the power supply or the controller and overheat them.

This affords me some extra protection. I have found a couple 24V computer fans with such a low amperage draw that I can run four off the power supply and only draw a half amp. So I have the protection of the fans constantly cooling the electronics to keep them cool as a precautionary measure.

Hopefully in a few weeks I will have this little baby up and running.

Now that the electronics are figured out I need to find a local welding shop that can weld up a tiny chip pan/ splash guard for me.

 

[FL-Flinter]

Ray,

Paul is very much correct, the 19.2 rating is what's known as "LR" or "Locked Rotor" meaning that if you jam the shaft solid, that's the maximum current it'll draw until it goes up into a ball of fire.

"FL" or "FLC" (Full Load Current) is a rating of how many amps or watts is consumed with the motor operating at full loading IE: a 7.5 Hp motor running with a 7.5 Hp loading. Generally, every fractional Hp motor you come across will only have a "LR" rating on it unless you get into the high dollar specialty motors.

I'm not familiar with the PWM controller but it sounds like either a pulsing or current limiting unit, these are the best type of controls to use since both types apply full voltage to the motor at all times. Operating a motor at reduced voltages causes it to draw more amperage, the motor doesn't know any different and it will take whatever voltage you give it and try to run at full speed and power, the only way it can do this is to draw more current - something has to give. A pulse controller will apply short pulses of full voltage and amperage to the motor and the speed is limited by the duration and frequency of the pusles. A current limiting control does exactly that, it limits the amount of current being sent to the motor. With either of these controller, the motor will produce near full torque output at lower speed but will not overheat.

AC motors can also be speed controlled by voltage but again this will burn them up in no time. AC motor controllers vary the frequency to obtain speed control, as with the pulse or current controllers for a DC motor, VFD's (variable freuency drives) allow reduced RPM operation without causing the motor to overheat as the good VFD's will also limit the current supply in relation to the motor size being used. If the motor is in an overload condition, the VFD should trip out to protect the motor. VFD's are simply an advancement of the soft-start technology that is becoming far more prevelent in smaller motors.