A DIYer’s Observation, Induction Motors

Rich Barber writes about a curious situation he describes below. I figure this is a learning situation for many of us, and there might be some fun applications as well. I tag on a very expensive lesson I learned, and it was a rarity, as I learned it at a friend’s expense VS my own. 

Subject: Observation

 Message: Hello George, Interesting experience last week. 5KW Onan repeatedly tripped 15 A breaker when trying to start 1.5 HP drill press. So…I turned on my 8″ bench grinder which takes several seconds to reach full speed. With the grinder running the drill press stated without a problem. I am thinking must be either a capacitance issue or grinder inertia is helping generator.


Now of course there’s a lot of questions to ask here, some that come to mind is the spindle speed of the drill press, is it set up to run at 6000 RPMs, did this cause current to remain high for a longer period at start up, is this an older drill press with considerable mass attached to the spindle, is there something wrong with the start winding circuit.. on and on..

But at the end of the questions, Rich notes, the drill press starts when the grinder is on and idling.

I shared this email with Bill Rogers, his input helped me focus on the more probable explanation.  We don’t know why a five KW Onan didn’t start the drill press, but we can guess why the grinder aided and prevented the circuit breaker from blowing durign the start up. Bill, like most Engineers would collect more data before forming any conclusions. 

Here’s the theory:

The Onan generator  is stable in RPM, same with the induction motor grinder, Rich then adds the drill press which is a good sized load, and we can’t assume the 5kw generator is wired for 120VAC only, we don’t know if this unit is using the entire stator winding or one side, thus, we have less idea as to how much voltage droop we might see during this start, exactly how well the governor is functioning is yet another unknown.

We do know that the grinder is running at some slip speed WHEN the generator and grinder are stable and no other loads are popping in our out, wiht this stable situation we dump this larger motor across the line, and alternator sees what looks close to a short for an instant or longer with the rotor at rest.  The generator lost RPM, and the idling grinder now finds itself running well above slip speed, in fact it’s now become a generator and is converting the energy stored in the rotating mass into electrical energy and back into the line where the Drill Press is able to make use of it. With E held higher, I stays low enough to keep the breaker from tripping and  and the drill press motor builds enough RPM to produce the CEMF necessary to shut off that big current draw seen durign start up.  We might place a good ‘clamp on’ amp meter to the line feeding the grinder and actually measure it’s contribution to starting the drill press.

I have a seven horse power 37xx RPM single phase motor, I also have a 50 pound flywheel I’ve been wanting to fit to this motor and use it as an accumulator of sort to play with. The desire comes from a bonafide observation I made in a glass blowing shop 10 or more years ago in Cindy Miller’s, ‘Hot Shop’ south  of Seattle near Lake sawyer. she’s a glass blower.

Cindy worked with a sales type who eventually sold her an 8KW natural gas generac auto start generator. The high priority load was the furnace blower, and a few overhead fluorescent fixtures. It was assumed (by the generator salesmen) the 8KW Generac was plenty enough to start this motor.

Here’s something to remember whether you’re an electrical engineer or hobbyist.  The person who designed this masterpiece of a blower used a high RPM motor, they also used poured cast iron to create a very nice looking blower with considerable mass. Of course it was likely designed by a glass blower and on commercial power with less distribution losses, all was well.  To listen to this motor start up was memorable, a slow spool up, sounded a little like a cream separator winding up. 

Hot shops are an interesting environment, once the furnace is up to temp, the blower may run a year or more. With that understanding this glass shop was losing blower motors.. They’d shut off the furnace, and the motor would not restart! After a lot of screwing around it was determined that the 8KW Generac could start this specialized blower motor once, AND since the start was so marginal, it would burnout the start winding with the start. the problem is the unit might run happily for a year and it took some time to realize that the all too small Generac generator OR the poorly designed blower was at fault.

And older man with some experience in perspective knows how the argument goes.. the salesmen knows it was the designer of the blower at fault.  The person designing the blower might expect any backup generator would be just as capable as hoover dam in starting his appliance.

I share this experience because I have more than once ignored what is on that motor shaft, and just looked at the electric motor manufacturer’s literature. If the glass shop would have had someone like Bill Rogers on site to watch that slow spool up, there would have been concern from the beginning as to how long that start winding was in the circuit and just how toasty it was getting. The majority of us look for these things more closely AFTER we have opened our wallets and bought an expensive replacement motor.          

OK, food for thought, there’s soft starters out there, would they help, and under what conditions? Some of use might think a voltage regulator or the lack of same was at fault, but I thing the Generator ran out of grunt, that little extra stored energy in the grinder likely powered the drill press through the start.

All the best, and than you Rich for sharing your observation!


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3 Responses to A DIYer’s Observation, Induction Motors

  1. Greg says:

    hmmm? good one.
    Now I want to test this.

    Maybe if I get my grinder going and on the same leg check the Pf with the grinder on and off.

  2. _Jim says:

    I’m with George; the grinder motor ‘assisted’ in the starting when the RPMs on the generator dropped and the grinder was now *above* generator speed, the grinder was able to provide current (and energy) back onto the line and provide (it looks like) enough current such that the breaker did not pop.

    The breaker likely still saw a large surge in current, perhaps above rating *but* below the period of time necessary to trip it … any breaker (or fuse) is rated for a percent overload condition with an associated time period … it amounts to a ‘time constant’ that is innate to any particular fuse or breaker. The grinder provided enough surge ‘reserve’ current (and energy) to get the drill press up-to-speed and then draw normal ‘run’ current (as opposed to a high starting currents) through the breaker.


  3. Don Boyd says:

    A split phase motor is a low priced version of the capacitor start motor. The start capacitor is missing and the start winding uses smaller wire to increase resistance in the winding to limit starting current. The starting torque of the split phase motor is low, much lower than the capacitor start motor. If the starting current is measured with a clamp on ammeter, the needle will most likely peg when voltage is applied to the motor. On the other hand, the capacitor start motor starting current will be much less and the starting torque will be much higher, and startup will be much faster and lighten the load on the power source, in this case the standby generator. If you have the misfortune of having a motor without the starting capacitor, you can cut one of the leads to the centrifugal start switch, hook a pair of wires to ends of the wire just split, and route them to the outside of the motor, where they can be hooked to an electrolytic ac start capacitor of similar value to one on an equivalent sized capacitor motor. The capacitor adds capacitive reactance to the starting coils which limits starting current and provides additional phase shift to the current which increases starting torque and lowers the total starting current and possibly keeps from over
    loading the standby generator.

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