Three Phase Dual Voltage Motor

Status
Not open for further replies.

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
080417-11411 EST USA

eeeee:

If a 240 motor is connected across 480 we know for certain that the magnitizing current will be very high. Run the experiment I previously described with an unloaded transformer and you will see the rapid rise in current as voltage increases. This produces lots of heat. That is why excessive input above rating on a continuous basis burns up these devices.

.
 

mull982

Senior Member
gar said:
080417-0620 EST USA

All inductors (coils in transformers, coils in motors, and plain inductors) with a magnetic material core (steel, iron, etc.) exhibit core magnetic saturation as the magnetic flux density increases. The maximum flux is proportional to voltage and inversely to frequency.
.

This is a great thread and I'd love to hear more on the voltage vs current relationship.

The one question I had based on the above statement, was that I always thought flux was a result of a change in current di/dt? Above you stated that flux was related to a change in voltage?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
The volt-time integral defines the flux level and the current will be whatever it has to be.

It is based on e = N df/dt . Where e is the induced voltage, N is a constant including number of turns, and df/dt is the rate of change of flux. There has to be an equality of the source voltage and the generated voltage from the coil. The source voltage in this case is the forcing function. So the current will be what it has to be to produce the flux necessary to induce the voltage to balance the applied voltage.

If we consider a series circuit of a battery, switch, resistor and inductor. Close the switch and a current will gradually increase and store energy in the magnetic field.

At any time open the switch and the inductor will try to maintain the same current flow in the same direction, polarity reverses across the inductor to achieve this continuity, and the voltage will build up to whatever level necessary to maintain the current. In practice this means an arc will occur across the switch contact. Again the same equation from above applies. Here current is the forcing function and the voltage will be whatever is necessary.

.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Flux is the result of _current_, not rate change of current. When you have ferromagnetic material in the magnetic circuit, the amount of flux produced by a given current will be increased dramatically. DC current will produce a DC magnetic flux.

_Changing_ flux will induce voltage in coils enclosing the flux path. So if you have AC current flowing in a coil, it will create AC magnetic flux. The AC magnetic flux linking the coil will induce a _voltage_ in that coil.

When you apply an AC voltage source to a coil, sufficient current will flow to create enough changing flux that the induced voltage balances the applied voltage (less any voltage drop due to resistance). The higher the applied voltage, the greater the required rate change of flux, as well as the greater the required maximum flux, and the greater the required maximum current flow.

Note the cool trick: the voltage induced is proportional to the _rate change_ of magnetic flux. But the _total flux_ at the peak of a flux cycle depends both on the rate change and the duration of the cycle. The higher the frequency, the greater the voltage for a given peak flux.

Because ferromagnetic material increases the net flux produced by a given current, it will also increase the rate change of flux produced by a given change in current. Ferromagnetic material greatly decreases the amount of current which must flow for a coil to balance the applied voltage. Once the material saturates, it can no longer provide this benefit; any increase in voltage requires a much larger increase in current to get the necessary flux.

-Jon
 

crossman

Senior Member
Location
Southeast Texas
winnie said:
When you have ferromagnetic material in the magnetic circuit, the amount of flux produced by a given current will be increased dramatically.

Jon, did this end up what you meant to say? I am of the opinion that X amount of current in a coil produces Y amount of flux around the coil regardless of what other material is in the vicinity. What the iron core does is increase the flux density in the vicinity of the coil - not the total amount of flux produced. Without the iron core, much of the flux "escapes" into space around the coil.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
crossman said:
Jon, did this end up what you meant to say? I am of the opinion that X amount of current in a coil produces Y amount of flux ....

I'm pretty sure that the magnetic core actually increases the amount of flux.

Consider that the voltage developed by a coil depends upon the rate change of flux coupled to the coil. The more flux developed by a given amount of current flow, the greater the self inductance. All the flux created by the coil is coupled to the coil. So the fact that the inductance of a coil increases when a magnetic core is added means that for the same current flow there is more flux developed.

-Jon
 

rattus

Senior Member
Winnie is right of course:

Winnie is right of course:

winnie said:
I'm pretty sure that the magnetic core actually increases the amount of flux.

Consider that the voltage developed by a coil depends upon the rate change of flux coupled to the coil. The more flux developed by a given amount of current flow, the greater the self inductance. All the flux created by the coil is coupled to the coil. So the fact that the inductance of a coil increases when a magnetic core is added means that for the same current flow there is more flux developed.

-Jon

With a coil wound on an iron toroid for example, flux is proportional to the permeability of the iron which is dramatically higher than that of air. Another way to look at it is that the iron core dramatically reduces the "reluctance" of the magnetic circuit. This allows more flux to "flow" so to speak.
 

mull982

Senior Member
Ok, so now that we know the properties of flux and how it relates to voltage, how does this apply to crossman's question?

crossman said:
Okay for the rest of you....

Given: a 480 volt 3-phase 10 HP induction motor is connected correctly for 480 volts. The motor is connected to a large fan and the motor is drawing 9 amps with a supply voltage of 480.

If I change the voltage to 470, does the current go up or down?

If I change the voltage to 450, does the current go up or down?

If I change the voltage to 490, does the current go up or down?

If I change the voltage to 510, does the current go up or down?
 

rattus

Senior Member
mull982 said:
Ok, so now that we know the properties of flux and how it relates to voltage, how does this apply to crossman's question?

I would expect the no-load current to increase as the voltage increases.

For a constant mechanical load, I would expect the load current to decrease as the voltage increases.

For a fan or pump, I would expect the load current to increase as the voltage increases.

But, doubling the voltage causes the permeability of the iron to drop drastically (saturation) and the current goes up drastically resulting in overheating.
 

crossman

Senior Member
Location
Southeast Texas
For rattus and winnie:

rattus said:
Another way to look at it is that the iron core dramatically reduces the "reluctance" of the magnetic circuit. This allows more flux to "flow" so to speak.

I would argue that the reluctance affects the path of the flux, not the amount of flux.

I need to do some research on this. I am still thinking that 50 amps of current produces a certain amount of flux regardless of other objects in the vicinity. Now, these flux lines will attempt to follow the path of least reluctance.

Interesting thoughts and I want to know more.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
crossman said:
Okay for the rest of you....

Given: a 480 volt 3-phase 10 HP induction motor is connected correctly for 480 volts. The motor is connected to a large fan and the motor is drawing 9 amps with a supply voltage of 480.

If I change the voltage to 470, ...450 ... 490...510

Okay, I am guessing that the actual load on the motor is about 7 hp, based on the current flow number.

I am guessing that this is a 4 pole motor, with a nominal speed of 1725 RPM at full load and 1800 RPM synchronous speed.

At nominal voltage and the given load, I expect an actual speed of 1740-1750 RPM.

With a 10% drop in supply voltage, I expect no more than a 0.5% drop in motor speed; similarly a 10% increase in supply voltage will increase load speed by less than 0.5%.

Fan load power varies as the cube of speed, so a 0.5% drop in speed works out to about a 1.5% drop in power. So when the voltage drops 10% (to 432V) the power will drop 1.5%, and the _real_ current will have to rise by about 9.5% to make up the input power. At the same time, the magnetizing current will drop by 10%. I expect the real current to dominate, and the actual current consumption to increase...but if the motor has poor power factor, then it is quite possible that the total current would be the same.

Doing a similar analysis on a 10% increase in supply voltage, I expect current to drop, unless the motor has poor power factor and is close to saturation, in which case the increased magnetizing current might mean greater net current flow. The _real_ current flow (delivering power to the load) will drop.

-Jon
 

rattus

Senior Member
crossman said:
I have always wondered what type of mechanical load would be a "constant horsepower" load?

How about a crane driven by a synchronous motor--not that you would ever see that. But a crane driven by an induction motor would be close.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
crossman said:
For rattus and winnie:
I need to do some research on this. I am still thinking that 50 amps of current produces a certain amount of flux regardless of other objects in the vicinity. Now, these flux lines will attempt to follow the path of least reluctance.

Interesting thoughts and I want to know more.

So a given current flow in a wire will produce a _fixed_ amount of 'MMF'.

This MMF will produce flux, the amount of flux proportional to the amount of MMF and inversely proportional to the reluctance of the path.

As an analogy, think voltage and resistance. A given charge displacement will produce a given _fixed_ voltage. Doesn't matter what is around. But the voltage will cause current flow, and that current flow does depend upon the surrounding materials. (The analogy falls apart in that once the current does start flowing, the charge displacement will change, but this should give the idea.)

-Jon
 

crossman

Senior Member
Location
Southeast Texas
winnie said:
Doing a similar analysis on a 10% increase in supply voltage, I expect current to drop, unless the motor has poor power factor and is close to saturation, in which case the increased magnetizing current might mean greater net current flow. The _real_ current flow (delivering power to the load) will drop.

Jon, thank you for the great analysis. I see what you are saying.
 

crossman

Senior Member
Location
Southeast Texas
winnie said:
So a given current flow in a wire will produce a _fixed_ amount of 'MMF'. This MMF will produce flux, the amount of flux proportional to the amount of MMF and inversely proportional to the reluctance of the path.

Okay, I am changing my thoughts on this one too. Thank you Jon and Rattus for the exchange of info. But, I still need to do some pondering on this one just to get it snuggly confirmed in my brain.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
080418-1114 EST USA

Results from an experiment on a single phase capacitor run fan in free space. It is a shop fan on a post. Must have a fairly high rotor resistance because there is a substantial speed variation with voltage. Source was a 7.5 A Powerstat. Motor tag Magnetek 115 V 60 Hz 3.4 A. Measurement with Fluke 87 III True RMS

80 V ..... 1.61 A
90 V ...,. 1.77 A
100 V ... 1.90 A
110 V ... 1.90 A
120 V ... 1.83 A
130 V ... 1.70 A
140 V ... 1.60 A

This is as high as I can go easily. But at some point magnetizing current is goint to start to rise rapidy.

Would also be interesting to monitor power input.

.
 

arnolds

Member
Three Phase Dual Voltage Motor

First of all the windings in a 3- phase 230/460V dual voltage motor can handle 480 volts. Second we know that there are 9 leads available to connect to but actually the motor has 12 leads with 6- separate coils that get wired together either series or parallel to create the 3- individual windings or phases and each winding has 2- coils. Lets say the motor is a 3-phase 1-1/2HP dual voltage motor 230V-6 Amps/460V-3 Amps and you want to feed it with 480V. High Voltage Connection. You would wire the coils in series. 2- coils per winding, in series each coil would have 230V and 3 amps. Since they are wired in series the total voltage would be 480V and the total amps would still be 3 amps. Now if you feed the motor with 230V. Low voltage connections you would wire the coils in parallel to create 3- individual windings. You still have 2- coils per winding and since they are wired in parallel each coil would get 230V. And the total voltage would be 230V. but in parallel each coil would only get 3 amps half the amperage but total amps would be 6. Now if you connect 480V to a low voltage connection (Parallel) the voltage on each coil is 480V and since they are wired in parallel the total voltage is still 480V. Now in parallel each coil would only get half the amperage but the total amperage remains the same. Here is where I am alittle fuzzy I spoke to a motor manufacture and the tech told me that the motor would not get damaged and that incorrectly wiring the motor to a higher source voltage would have very little effect on the motor. But I am thinking that higher source voltage you would have more resistance that could create excessive heat.
 
Last edited:

rattus

Senior Member
Arnolds, If the motor is wired correctly for high and low voltages, it will draw half as much current at 480V as it does at 240V.

If we apply 480V to a motor wired for 240V, the motor will fry in short order.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
080420-2045 EST USA

To add to my previous data on the single phase fan motor.

Previous data:

80 V ..... 1.61 A
90 V ...,. 1.77 A
100 V ... 1.90 A
110 V ... 1.90 A
120 V ... 1.83 A
130 V ... 1.70 A
140 V ... 1.60 A

slight discontinuitity because of different motor temperature and maybe some other factors, like air temperature and its effect on the fan.

new data:

120 V ... 1.88 A
130 V ... 1.78 A
140 V ... 1.69 A
150 V ... 1.63 A
160 V ... 1.62 A
170 V ... 1.64 A
180 V ... 1.70 A
190 V ... 1.80 A
200 V ... 1.95 A
210 V ... 2.15 A
220 V ... 2.39 A
230 V ... 2.66 A
240 V ... 2.92 A

I was very surprised that core saturation did not show up at a lower voltage. This does show a low voltage max at 100 V, a min at 160 V, and a higher value at 240 V, From here on I expect the current to do nothing except go up until quick burn out. Even throught the current does not show a major problem with substantial over voltage I would not be inclined to operate above 130 V without substantial testing and consulation with the manufacturer. Above 120 V there was only minor speed change.


On a Baldor grinder single phase motor unloaded and rated 115 V 3.1 A 3600 RPM I got the following results:

120 V ... 2.35 A
130 V ... 3.06 A
140 V ... 3.7 A
150 V ... 4.8 A
160 V ... 5.7 A

This goes into saturation much more quickly.

In all the above tests the power factor was low.

.
 
Status
Not open for further replies.
Top