Inductive load

[RUSO]

What's an inductive load? Why should I care? What about impedance load?? Got a job to spec a variable transformer, but I"m all of a Mechanical engineer. I'm dealing with 115V to be transformed up(135V) and down (24v). Thanks..

 

[nbucska]

Did you select transformer? Where can I find its data ?
What does the load consists of ?
<nbucska@pcperipherals.com>

 

[RUSO]

I have not selected the transformer yet. Current draws can be from .1 amps all the way to 8 amps. It will be ussed to run shaded pole, universal, and split capactor motors, depending on what voltage they are wound for.

 

[Lewish]

OK, try it again. What are you using the 135 volts for? If it is to run the motor, don't! Just put the motor across the 115.
Now, what are you using the 24 volts for?

 

[RUSO]

I use this voltage range all the time on our motors, within limits of course, and quite safely. I have driven a 115 motor to 135 safely in our lab. The variable transformer will be used to develope test beds for our customers applications. We make 24 volt AC motors upto 115, but it's helpful to have some extra voltage. This way I make the motor stronger in case it doesn't move the load. Once I find out the operating parameters at the higher voltage, I rewind the motors to safer voltages.

 

[melone]

Try doing a search on this site for inductive fiyback or spikes. I know that I have personally asnwered that question at least 3 times.

 

[PUMPDESIGNER]

I do not think anyone answered your question.
One way of looking at inductive versus resistive loads is ...
On an inductive circuit current precedes voltage,
On resistive loads voltage precedes current.

That definition should warn you that an inductive load has the potential to strain an electrical system at the moment of energizing the circuit due to the voltage lag and high inrush current, whereas resistive loads do not produce that strain.

This high inrush feature of inductive loads therefore requires a different set of calculations and equipment to account for the starting surges, which is why the NEC has all those special rules for motors.

Richard Neff
pump@shadow.com Richard Neff
Irrigation Craft

 

[melone]

PUMPDESIGNER, do a search on this website for inductive loads, inductive spikes, etc. and you will see why I made the suggestion. I can confidently say, that I have explained how, and why inductive spike occur on at least 3 seperate occasions.

 

[CarlPugh]

Variable transformers are normally rated for current. In your applicatton whether the load is resistive or inductive probably doesn't make any difference.
Most variable transformers are not isolated. In this application a 115/115 volt isolation transformer before the variable transformer is probably preferable.
Most variable transformer have a connection so that the output voltage can be higher than the input voltage. 115 volt input and 0 to 135 volt output is probably typical.

A WORD OF WARNING: Place a fuse or circuit breaker in the variable transformer output. Do not depend on a fuse or circuit break in the variable transformer input to protect the variable transformer or the load.

 

[Bung]

Pumpdesigner is not quite right with his definitions. For an inductor, the current lags the voltage, so on switch on there is full volts, no current.

For a resistor, current is in phase with voltage.

For a capacitor, current leads voltage, ie at swith on voltage collapses and current is high.

Bung
Life is non-linear...

 

[gjones33]

Hi all,

Please be careful with variable transformers and do not place isolation transformers or other inductive loads in the primary circuit. It is often forgotten that a variable transformer can also act as an auto transformer with a ratio of turns below the tap to the turns above the tap and under certain conditions this can produce several thousand volts on an open end or in other inductive circuits connected to the high end.

Hi Bung, the reason there is a high inrush current to an inductive load like a motor is because normal full load current (FLC) occurs when the back emf is established i.e when the motor has attained full speed. At zero speed the impedance is very low and in extreme cases the current can be some ten times greater than FLC.

Cheers,
G

 

[CarlPugh]

I have used isolation transformers before the variable transformer and after the variable transformer with no problem.
Using an isolation transformer after the variable transformer adds a fixed inductance, so that at low voltages, the regulation is worse, and short circuit current is less. It also allows the use of a series/parallel connection on the isolation transformer secondary allowing a greater range of voltages and currents.
Using the isolation transformer before the variable transformer makes the regulation better and short circuit currents higher at low voltages.
Whichever approach is used, don't forget the fuse or circuit breaker in the variable transformer output.

 

[Bung]

gjones, an inductor and an induction motor ain't quite the same thing. Lock the rotor of the induction machine, and then it is an inductor. That it draws a very high current (locked rotor current) isn't in dispute - all I'm saying is that at time t=0 the voltage is established first in order to &quot;force&quot; current into the inductor to allow energy storage in the magnetic field.
Bung
Life is non-linear...