3-Phase Motor Drive, Induction Generator, Controlling Torque?

[davidsq]

For my application I am performing a study specifically on a motor drive and am in the process of developing the test setup. Right now, the drive is powering a 3-phase, 3HP, 460V induction motor (referred to as the first motor) who's shaft is mechanically coupled to an identical motor (referred to as the second motor). I am powering the motor drive with a 3-Phase, 15kW, 208-480VAC transformer.

My goal is to watch the thermal response of the motor drive's circuitry as the amount of current being delivered to the motor is increased. In order to increase the amount of current being delivered by the drive (maintaining a constant RPM), I would need to increase the amount of torque experienced by the motor. This is where the second motor comes in handy. I am wondering if there is a way to control the second motor in order to vary the amount of torque experienced by the first motor. One colleague suggested a bidirectional motor drive - which I know nothing about and haven't been able to find much information on it.

From what I understand, in order to generate power from the second motor in this setup, the second motor would need to be rotating faster than its synchronous speed (which I believe to be 3450 RPM - the motor's nameplate speed). Therefore I do not believe it would be possible to attach capacitors to the leads and attach a load this way due to the fact that the first motor will not run faster than 3450 RPM. Am I mistaken?

I hope this was clear and I appreciate any and all feedback.

Best,
David

 

[waross]

Drive the second motor with a VFD capable of regeneration. As you lower the drive frequency of the second motor below 3450 RPM it will regenerate and put a mechanical load on the first motor.

Therefore I do not believe it would be possible to attach capacitors to the leads and attach a load this way due to the fact that the first motor will not run faster than 3450 RPM. Am I mistaken?

The capacitors will draw apparent power. The capacitors will draw current and it may look like the they are drawing power, but they will not. Capacitors to supply magnetizing current and resistors to provide a load may be a solution.
The difference between 3600 RPM (synchronous speed) and 3450 is called the slip, or slip frequency. The motor is capable of regenerating but needs to slip to export power.
Your slip is 3600RPM - 3450RPM = 150 RPM
The second motor will happily deliver full power to a system operating at 55 Hz. The VFD can supply such a system.


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

 

[jraef]

You are creating a dynamometer test stand by the way.

An important consideration is the length of time and amount of power you are considering here. The reason is that to put a load on a motor, you have to essentially convert the energy potential of the motor, which is kinetic, into something other than kinetic energy, because your test stand is (I assume) static. So for the most part, that hean either converting it to heat, or converting it back into electricity.

If the duration is relatively short, you can just use a "load bank" of resistors to burn off the enery as heat. If all you are doing is thermally testing the drive power electronics, the motor may not really be necessary at all, just fire the transistors into a resistor bank. That's what VFD mfrs do for heat run testing. If you are also concerned with testing drive performance WRT motor control under varying load conditions, then spin a DC motor and use it as a generator, connected to a load bank. That will cost you far far less than a regenerative AC drive and give you the same outcome. That's what most dyno test stands are.

If the resistor heat is undesirable and/or the test must go on for long periods of tome, use a 4 quadrant DC drive that will convert the regenerated power back into your AC line source. The only reasons to use an AC regen drive and motor is if you will be doing this a lot, day in and day out, and you want to avoid maintenance costs of the DC motor brushes and commutator.

"Will work for (the memory of) salami"

 

[mikekilroy]

do u believe in the KISS principal? are you sufficiently lazy to do this easier yet?

if so, why not just run what you have and do your test? what can't you do? if all you want to do is provide motor 1 a load CURRENT from 0 to drive rating, just do it as you have set up.

ur 150rpm slip motor 2 will provide a full 3hp load to motor 1 if run at 3600+150=3750rpm. MOre at slightly higher speed, less at slightly lower. so what is the problem? motor 2 will simply put this generated power back on the power line feeding it. no extra parts needed. If you power both motors from same power source, what motor 1 uses will be put back by motor 2, less the system efficiency (5%? 10%?)

so to summarize, seems to me from what you want, you simply dial motor 1 to 3450rpm output and your load current will be 0.2amps or so (friction&windage). as you increase motor 1 speed on vfd to 3750rpm you will increase the load along motor 2's rated curve - call it close to linear.

why the mystery? what am I missing?

 

[waross]

Good idea Mike. Lets take it a KISS further.
Belt the motors together with slightly different sized sheaves so that the second motor is over driven. You may be able to find a variable diameter sheave or sheave pair that will allow some adjustment of the speed and load.

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

 

[mikekilroy]

yep. I actually liked the direct 1:1 coupling to be sure the output voltage is at max so no question about output loss measurements.....

 

[davidsq]

Right now motor 2 is NOT connected to a power source and is only being controlled by the coupling to motor 1. I believe a VFD with regenerative capability would be best for the specific tests I need to run. However, when I obtain the VFD would I need to obtain a 3HP (because I will be driving a 3HP motor) or would I need to obtain one that is > 3HP (because the motor I am controlling is acting as an induction generator).

Thank you.

 

[mikekilroy]

if'n u have a vfd on motor 1, why would you want a vfd on motor 2 also? Then u would have to come up with 3hp+ of dynamic braking or more expensive regen to line? why not just plug it into the wall? am I missing something?

 

[LionelHutz]

Mike has the easy answer. Run the VFD up to 60Hz, connect the other motor to the line power and then increase the frequency on the VFD above 60Hz until you get the load you want. You'll have to set the VFD to allow a maximum frequency above 60Hz but that's an easy parameter change.

Just make VERY sure the connection makes the load motor rotate in the same direction as the VFD is turning it. I'd start the load motor first from a stop and observe the rotation before starting it while the VFD is spinning it at full speed. Then, make sure you don't change the order of any connections during your testing.

Assuming both motors have the same 150rpm slip, you'd have to run the VFD to give a synchronous speed of 3900rpm = 65Hz instead of running at a synchronous speed of 3600rpm = 60Hz. At least in theory. In practice it could be less for a number of reasons. We use back to back coupled motors and regen VFD's for loads here and the frequency difference required to get full load never seems to be as high as the theory says it needs to be.