ideas for economical testing of AC and DC motors?

[eeinpa]

I've been asked (one day ago!) to think about ways to cost-effectively give a small motor shop the ability to test run a variety of AC and DC motors, probably 10-100hp. Because of cost limitations, I'm going to say these would be "no load" tests only. It would be ideal if we could find some used or surplus commercial equipment and adapt it to this use, as I can't imagine many companies are building test gear like this, and it thus must be quite expensive.

Motors typically include:
230 and 460V ac motors
180 and 230V shunt field DC motors
"special" 195V 120 Hz motors

For 230/460 AC motors, it would seem off the shelf VFDs are an obvious solution. I presume the built-in protective features of these drives would give reasonable protection in case of a fault. Comments?

For the 195V 120Hz motors, it would seem a VFD set for 200Vac input (via a multitap transformer?) is a great solution. Again, any thoughts?

For DC motors, they already have a very beefy rectifier that could handle almost any conceivable DC situation. The problem then becomes how to generate a variable 3 phase input voltage to the rectifier, with some form of current limiting. I have a feeling that most VFDs are NOT going to like driving a rectifier. And do any of them actually produce a variable-voltage waveform? What are my other options? Chopper drive? M-G set? Any leads on surplus equipment?

Finally, does anyone have experience using VFDs and motors as test loads for motor testing? I found an earlier post that hit on this, but the crucial parts of it were cut off! I'm not sure we do our biggest motors this way, but perhaps the smaller ones...

Ideas gratefully welcomed!

 

[waross]

It would be a great help if you describe the electricval service to the shop. How much spare capacity is available for motor testing? Voltage? Three phase?
respectfully

 

[eeinpa]

There is an 800 amp 480V 3 phase supply to the shop which was installed with the intention of adding motor test equipment. I think the deal for purchase of the original equipment fell through, thus nothing has been done.

Due to the relatively low volume through the shop, it would be very nice if modern electronics (e.g. VFDs or chopper drives) could be used to cost effectively cover a range of voltages and/or horsepowers. I'm a controls person, so I can handle that end of things, I just need to know more about how to get the voltages and currents required for driving this equipment.

Thanks!

 

[eeinpa]

While browsing the web I came across the Polyspede DC drives such as the HP and PRD series. It looks like these can drive motors up to 100 hp for about $2500, which doesn't seem too bad. At brief inspection it seems they can support multiple field voltages as well. Anyone have experience with these or something similar from another vendor, used to drive a variety of motors (rather than one fixed motor load)?

Thanks

 

[djs]

Hello;
You can probably make do with just one VFD. Most are programable in terms of the V/F ratio, and most 60Hz VFDs will go to 120 Hz and more.
Most modern VFDs are now using MOSFET transistors for the outputs. I do not think connecting a three phase rectifer bridge to the out will effect this type of VFD. The output is a PMW wave. This should result in a variable DC voltage from the rectifier. Will have to have a load connected to it to see the varying voltage.
The newer vector drives however will not work with a rectifier unless you can turn off the vector control.

 

[eeinpa]

It would be really slick if I could pick up a used/surplus 50hp VFD, use it to test AC motors and plug it into our existing rectifier to test various DC motors. I will try contacting a few applications engineers at drive manufacturers and see what they think of driving the rectifier... I might also try putting one of my little $125 VFDs into a rectifier and a small DC motor and consider it a cheap experiment. BANG!

I thought most drives were now using IGBTs, not MOSFETs. Not sure the technology makes a lot of difference as long as (as you have noted) the drive is not trying to do vector control. I presume it does vector by sensing back EMF of the rotating element? Obviously that won't work with a rectifier. I've tried to quickly find an explanation of how sensorless vector control works, with no luck

Incidentally, I was in a shop today which powers field and armature during no-load tests with large 3 phase variable autotransformers. Checking some prices on the web, the pair would probably cost nearly $10k new. I think I'd prefer a few SCRs or IGBTs...

 

[LionelHutz]

I don't believe you will be able to successfully feed the output PWM signal of a VFD into a rectifier and get DC.

If you have 208VAC or 230VAC 3-phase then you can easily configure the right soft-starter as a voltage controller or current controller via an analog signal. Feed the AC to the rectifier to get your DC.

With some programming you can also use the soft-starter to test AC motors. The only thing you can't test is varying frequencies.

 

[eeinpa]

At some point talking about this, someone pointed out that a VFD into the rectifier does nothing about field supply. Perhaps I could do it for series motors, but probably not practical for shunt field motors.

The softstart is an idea I will have to look into. The only ones I have encountered are small (few hp) and don't seem to have any control inputs. Again, I will have to look at how to generate a field supply of at least 10 amps. Perhaps a small soft start for field control and a larger one for armature...

 

[eeinpa]

Could anyone with experience with a variety of soft starters provides recommendations on which ones I might look at as possibilities for driving a rectifier for powering series field DC motors? I would need the ability to control voltage and current, either via control voltages (less desirable) or a serial link.