Variable Power Factor as a Load 2

[tctctraining]

I need to simulate a load with variable power factor for testing purposes. Aside from using cap banks and inductors , I am thinking to use a Synchronous Generator and a voltage regulator to do this. There are couple issues that I appreciate your feedbacks.
1) Is this option feasible?
2) How much variation of power factor do I get.
3) Is reactive power control different from voltage control?
4) Does anybody know of manufacturer of controllers with least expensive parts?

Thanks

 

[dpc]

Well, a synchronous motor would probably make a better load than a generator. You can get a very wide range of power factor, but varying the real kW load will be more difficult.

 

[tctctraining]

Thanks dpc. Do you know any manufacturer who makes small size sync. motors around 1 kw, single phase ,120V. I looked around and only found manufacturer in China!!

 

[jghrist]

I never heard of a single-phase synchronous motor. Is there such an animal?

 

[davidbeach]

The motors for motor driven electric clocks are single phase synchronous motors. That seems to be about as big as they get, and my understanding is that their efficiency is rather poor.

 

[waross]

If you can share with us more details of what you are testing it will help a lot.
The power factor of a motor will vary as the load changes. The VARs remain relatively constant.
You can supply varying Capacitive VARs by connecting a capacitor bank to the output of an auto-transformer. (Variac)
Inductive VARs may be a challenge. I would look for an old, large motor with a poor power factor. For example a 5 kW motor with a rated power factor of 0.9 will draw about 0.97 VARs. If you spin this up to synchronous speed with a smaller motor that is a little over belted the current draw will be almost purely inductive. Again you may vary the vars by varying the applied voltage with an auto-transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

BTW, welcome to Eng-Tips.com

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

I assumed you were talking about three-phase power. Other than clock motors as mentioned, I'm not aware of any single-phase synchronous motors. You could consider a small motor-generator set I suppose. If you run a small three-phase generator from a single-phase motor, you might be a little closer to what you are looking for. But you have not really explained what you really trying to do, so it's hard to know

 

[tctctraining]

Thanks for all replies.
What I trying to do is just to introduce a load with varaible power factor for a device that checks this variation.
One way is to switch on and off cap banks and reactors in parrallel to obtain this, I remember from school that by varying filed voltage I could vary the power factor and therefore I decided to look into this.

I found " Hurst manufacturing " and " Dayton" supplier for these motors.

So a 120V/ 1 HP/ AC motor I thought would do the trick with a controller to change the field voltage!!

I didnt think I needed load, and I wanted to turn this without load.

 

[stevenal]

Without some mechanical load, your real power will only consist of losses. If power = zero, pf also = zero.

 

[tctctraining]

Ok.. This means my thinking is not gonna work.
Back to basics..using capacitors and inductors.

 

[dpc]

You will need some resistance in the load to be able to vary the power factor. The power factor is (roughly speaking) a function of the ratio of real load (kW) to reactive load (kVAR). Without some resistance or other kW load, the power factor will be zero.

 

[tctctraining]

I am still thinking to make that synchronous motor work. Can I use a synch. generator and motor as a load coupled to the shaft with a VFD controlling motor speed?

 

[davidbeach]

You would still need to load the motor.

 

[tem1234]

You can put a resistance (a heater) and utilize the motor only for controling reactive power in parallel with the resistance.

 

[tctctraining]

hmmm..Davidbeach:

How about using a single phase synch. motor and another motor as a load, then by varying speed on Dc motor I will introduce different load and by varying field voltage I can change pf seen through synch. motor.

 

[davidbeach]

You would still need to load the motors to develop the real part of your load. Load the motors as in doing mechanical work, just spinning won't do any good.

 

[waross]

Use a resistive load bank for your kW loading.
Use an auto transformer feeding an induction motor to supply and vary the VARs. You may have to use a second motor to spin the VAR generating motor at low voltages.
The auto transformer will be in parallel with the resistive load bank.
Set he resistive load bank at the kW load you want to test at. Than vary the voltage to the motor to vary the VARs. The power factor will respond to the ratio of kW and KVA.
The VARs developed by the motor will reflect through the auto-transformer and vary your power factor.
Disconnect the start circuit of the motor you will use for the VAR generator and spin it with a second motor.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[tctctraining]

Thank you Waross!
I am not sure about using the second motor.What I understood is for example lowe the voltage to half and then have the second motor coupled with this motor spin it at full speed?
What do I gain with this?

 

[sibeen]

As someone has already mentioned you can use a variac or adjustable autotransformer to power a capacitor to change the leading power factor.

To get a lagging PF I'd use the same variac powering a transformer which has its secondary shorted.

Off course you also need some resistane in the load for a few watts to flow.