motor starting with smaller generator using start delays 1

[thermins1]

Awhile back I posed this question. Scotty uk replied, but I never could pull up his comments, so I'll try again, and this time I'll leave my e-mail address for replies. I have an insulation machine which uses two 13 amp industrial blower motors and one 15 amp 1 hp drive motor. Because of the spike on startup, I have to use a 17 kw generator which is large and heavy to be hauling around to our jobs. If I were to use time-delays of say a half second on each motor, would that be sufficient to be able to down-size my generator to a more manageable 8 kw? Also, how can I measure the duration of the spike so I get the correct delays, and where would I get these delays? I'm figuring 13+13+15 amps X 120 volts = 4,920 watts being what is actually necessary to run these motors. Hope I'm barking up the right tree. Any info would be extremely useful! Thanks a lot! Larry

 

[MikeHalloran]

I built a show display that used a 1HP high pressure blower, and kept tripping the 15A breakers provided at the shows.

With a current transducer and a graphing voltmeter, I found that the starting current peaked at 48A, and stayed above 40A for about five seconds.

By dumb luck, I had bought it with a 115/230 motor, so there was an option, because it was running with paired windings in parallel. I split out one winding, added a 50A solid state relay and a six second time delay, and started it without half of one winding. So the blower would start spinning up with a lot less violence than before, and would then audibly 'shift gears' and run up to full speed. That limited the peak current to 38A, which most show breakers will tolerate.

If I had to do it again, I'd buy a 230/3 motor and use a solid state speed control to soft start it. That might be an option for you, since you can now get 115/115 controls.





Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

Time delays, maybe 5 or 10 seconds. Leave enough time for the generator to stabilize after each motor starts.
My rule of thumb is the sum of the motor currents plus 200% of the last motor to start. That comes out to almost exactly 8KW here. 13A+13A+15A=41A, 41A+(2x13A)=67A, 67Ax120V/1000=8.04KW.
Based on one of the 13 amp motors being the last to start.
The current ratings are close enough that you would probably be able to start the motors in any order.
There are two issues that limit the ability of a generator to start a motor or group of motors.
One issue is the voltage drop and the ability of the Automatic Voltage Regulator (AVR) to hold the voltage up.
This varies widely on small generators and not all small generators use an AVR. Without an AVR the voltage drop may be much more.
The other issue is speed and frequency drop when starting a heavy load.
If the motor is oversized for the rating the speed and frequency will hold up much better and a larger motor may be started.
The short answer, My recommendation for staggered starts would be an 8KW set and about 5 second delays.
respectfully

 

[ScottyUK]

Larry,

What identity did you have when you posted the question? Your current one is brand new. I had a look through my old posts and drew a blank.

waross' rule of thumb is in the right ballpark, but I would be looking for 10kVA rather than 8kW simply because that is more likely to be what is on the nameplate. 8kW describes the engine, 10kVA describes the generator, and '10' sounds better than '8' to the marketing guys so guess which one they publish! Bill makes some other good comments, especially regarding voltage stability during starting. The blowers will probably be OK to start because they will shed load as voltage and frequency drop and then pick up speed as the set recovers. The other drive might struggle more if the voltage regulation is poor, possibly leading to a stall if voltage drops too far.

See if you can get a demo set and try it before you commit money to a purchase.


----------------------------------

Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[thermins1]

Thanks to all for the replies. I'm a little concerned about using 5 second delays, as this would create a total of 15 seconds before we start blowing insulation. Because we start and stop this machine often during the course of a day, (maybe a hundred times or more) this would add considerably to the amount of time we are on the job, and, depending on which motor starts, 1st, 2nd, or 3rd, we run the risk of clogging the blowing hose for lack of blowing power while waiting for the next blower to come up to full power. Seems to me these motors spike out at approx a 1/2 second when starting. (I had hoped the spike itself might be 1/10th second). Do I need an oscilloscope to see the actual spike's duration before it stabilizes to full-run and current draw, or would a voltmeter or ammeter get me pretty close to the timing? What about a freq drive, (which I know Nothing about) or a capacitor to dump "extra" voltage onto a starting motor? A perplexing problem to me, I assure you. Thanks again. Larry

 

[itsmoked]

It's hard for us to say much more not standing in front of the machine. If you have special considerations like insulation clogging and high start/stop frequency then you may be stuck with Big Bertha the generator.

If you feel the motors are at full speed in a much shorter time you are welcome to reduce the inter-motor delays. The delays need only be short enough for the various motors to reach 95% or so percent of their running speed.

You may be able to recognize the best sequence for starting the motors too. You can test all this using whatever generator you have and just listening to it. As you put delays into it does it sound less hammered that with a mass startup?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com