DC Motor ratings

[Chazz]

Good day people, my lathe has a 0.75HP, 115VAC motor, I plan to upgrade to a 1.0HP DC Motor with PWM for variable speed. My motiviation is the fact that I got a free 24VDC power supply I think I should be able to use.  It is a Phoenix Connect 'Quint' 115VAC, 15Amp, single phase input to 2 x 24VDC x 40Amp output.  My question is this, in my research (if my VERY basic electronics knowledge is right) I can get a 1.0HP DC motor in 3 different ratings that should? work with my power supply, but I don't know what the difference is, ie which would be better\worst? I can get 12VDC x 80Amps, (with a step down transformer) or 24VDC x 40 Amps, and 48VDC x 20Amps?

Is there any torque difference, especially at lower RPM's?  Or would the only difference show up in the power bill?  

Thanks for any insight,
Chazz

[John Rudd]

I cant see the 12v motor lasting very long on a 24v supply.....

hmmm, I smell burning 


Torque generally isnt affected that much when a motor is run on PWM, but there will some a reduction, as torque is a function of speed.
 
I'd put money on the 48vc motor being the best fit...

[Chazz]

Oops, sorry about that, I edited the post, thanks.  After further thinking, if I use the 1 output for the motor, would 24VDC x 40Amps be an adequite supply for say, electroplating?


Regards,
Chazz

[kwackers]

Torque generally isnt affected that much when a motor is run on PWM, but there will some a reduction, as torque is a function of speed.
 
I'd put money on the 48vc motor being the best fit...

Is that true? I thought power was torque with respect to speed, or do you mean as a generalisation for electric motors?

The only thing I can think of is a higher voltage motor will have greater impedance (and possibly DC resistance losses) due to more turns of thinner wire. Driving it off PWM might take longer for the current (and thus flux) to build up?
I think there are many pros and cons for each motor, quite possibly a lot will balance out. The availability of a suitable controller would probably be the deciding factor for me.

[John Rudd]

No, It was meant in general terms....

I take your point about the flux issue, granted a higher voltage motor may take longer, without knowing the type of dc motor ( with a PM generated field?) it is a bit difficult to make a choice...

Controller wise, are we talking commercially available units or a diy build ?

[Chazz]

This is the controller Ii am contemplating.

http://www.minarikdrives.com/catalog/page21.pdf

Cheers,
Chazz

[John Rudd]

Those controller are for motors with a PM field....

They do not have a supply for a motor with field windings...

[Lew_Merrick_PE]

This is the controller Ii am contemplating.

http://www.minarikdrives.com/catalog/page21.pdf

Chazz,  While I have to admit that it has been more than 25 years, I have had nothing but bad experiences with Minarik.  I swore then (and have stuck to it) to have nothing to do with them ever again.  There was one project where I paid them a premium for Baldor motors and what they delivered were Chinese knock-off's (that were "problematic" until Baldor stepped in to fix the problem).  At another point, I had stopped to get gas and a Minarik catalog was on my dash.  A complete stranger walked up to me and asked if I wanted to join in a lawsuit against them.  I have been sorry that I did not take him seriously.

I find Futurlec (http://www.futurlec.com/index.shtml) and Automation Direct (http://www.automationdirect.com/) to be the places I purchase such components from when I need them.

[John Hill]

Hmmm,  I think you would be OK using a 24V motor on 48V provided the PWM is never more than 50%.

[John Rudd]

Hmmm,  I think you would be OK using a 24V motor on 48V provided the PWM is never more than 50%.

Nononononnonooooo...

PWM works by modulating the width of the pulse rather than the amplitude (AM amplitude modulation...Long Wave Medium Wave radio? FM )

With PWM the amplitude is always the same as the supply voltage ( give or take ) so the windings still see 12 or 24v or whatever...

[John Hill]

PWM works by modulating the width of the pulse rather than the amplitude........................

Indeed that is true  but the limitation on the motor is not voltage but heat.... 

Higher voltage pulses will give more torque than lower voltage continuous voltage (thats why we use PWM rather than voltage level adjustment to vary motor speed).  Double the voltage and half the time gives the same watt hours per hour.

[DMIOM]

At the lowest level, its the same PWM that is used by good stepper drivers. I use a PSU of app. 50v smoothed DC into Gecko drives, thence into steppers whose nominal DC rating is, from memory, about 5v ! - the trick is that the drive is set up to limit the current through the motor coils.

True, un-managed PWM could be dangerous - but a higher voltage + careful PWM can get a far better response from a DC motor.

Dave

[picclock]

@ John Hill

>> Double the voltage and half the time gives the same watt hours per hour.

Not True !!!

Power is related to the square of the current. So double the voltage gives 4X the power.

Using Power=Amps X Volts or Power = Amps X Amps X Resistance. (P=IV, P=I squared R)

Example : A 1 Ohm load with a 1Volt supply will dissipate 1 watt of power. A 1 Ohm load with a 2V supply will dissipate 4 watts of power. 

So a 50% pulse width at twice the voltage will double the power in a resistive load.

However, in a motor, things are more complex. When supplied with DC, a motor will generate a voltage almost equal and opposite to the voltage supplied. The almost bit is where the motor losses (winding resistance etc) get their power from.

When a motor is pulse width modulated the peak current is determined by the pulse frequency, motor inductance, motor resistance, and the back emf (motor generated voltage).

Its quite possible that at double the voltage some significant problems could occur. Likewise, just connecting the motor across the supply, at turn on when its at rest,  the inrush current flowing will probably exceed the 20 Amps your PSU can supply (Assuming the winding resistance is <1 Ohm). 

IMHO Use a PWM controller with a frequency high enough to limit the current in the motor windings. (by means of the motor inductnce)

Alternately (and more simply ) get a controller with a built in current limit. 

Hope this helps a bit.

picclock

PS 1HP is about 32 Amps+ at 24V.

[John Hill]

@ John Hill

>> Double the voltage and half the time gives the same watt hours per hour.

Not True !!!

I am a dolt!

However, I still have a soft spot for my principal assertion that heat is the limitation on the motor. not voltage per se.