help with small motor load and fusing for it

[bob1112]

Here is the short story. bowl tumbler lost a lower bearing on the shaft that goes through the center of the bowl and holds the jiggle weights. I forgot to fuse my motor correctly so when the bearing failed, it took the motor with it. I replaced the bearings and got a used 1.2HP open drip motor (previous was a 1.0HP TEFC). I upsized the motor slightly because it seemed to need a tad more. Both motors are single phase cap start motors.

When the tumbler starts with the 1.2HP motor, it starts very slow and takes about 1-2min to get up to speed. With no load it zings right up. I am used to seeing about 10X the running amperage for inrush at start but the new motor only pulls about 40A on start. It then calms down to 10-12A while getting up to speed. It settles 10min later to around 7A. These motors are connected via 240 volts. My numbers look like the motor is about 50% efficient! I know the bearings are right and not in a bind. Purely a motor issue here.

I am wondering if changing the capacitor on the motor will help me get up to speed or if this is strictly a motor issue? I am sure the cap is good and working but "maybe" it could be weak? I am also wondering why the motor does not pull more power since it takes so long to get to speed. Should it not be pulling big amps to get there?

Because of the previous motor BBQ, I need to fuse this thing up. What do I need? I want to fused pretty close to load amperage so it will blow if a bearing goes again. Is there a book of knowledge on fuses and there time delays and such? Are motor fuses like breakers in that they need to be sized 20% above full load? ie, do I want an 8A fuse for a 7A load or a 10-12A?

With all these problems with stupid single phase motors, I am thinking of adding a 3ph motor and VFD so I can change the frequency of the machine. I think my bearings are all good for another 200-300 rpm. Would be nice to have the protection of a VFD too.

[Al_The_Man]

Changing the capacitor will not help, I think you have either a motor size issue or a design issue, a motor that is not up to speed or even close appears as an extremely high load across the supply, and probabally will not last long.
Al.

[NC Cams]

The motor, from what I've leanred by playing with them, is doing a wye/delta switch at start up. THey start in wye to reduce current draw ans then when the speed is high enought, a speed sensitive switch chages the input connectiosn to delta and "high" speed operation.

AFIK, the cap merely serves as a "booster" so that it stores up energy for an instantaneous "jolt/boost" so that inrush current is minimized/controlled. Sort of like an electronic flywheel.

Although calculations may suggest that a smaller HP motor will do the task, a common fallacy is to design to the numbers and fail to consider/provide for the MASSIVE amounts of torque the may be needed for start up or other peak load conditions. IN short, althugh 5 hp may suffice for oepration, you may need the startup torque of 15-25 hp to get the ball rolling.

Might want to give the engineers at various motor suppliers a call. I'm sure they have an applications department that has fixed you' problem before. Don't be surprised if they pull a " you have this but you really need THAT" sort of deal on you.

[bob1112]

Well, the original motor was a 1HP motor and performed flawlessly until it nuked. I noticed it would slow a bit when the bearing was going out. I figured 1.2HP would allow me to throw a little more weight for counters and get more amplitude. The motor actually sucks and not sure why.


Regarding wye/delta switching. Generally there has be special switch gear in th motor for that. The change of wye or delta is how most motors are switching from high or low voltage as far as I remember.

I am just dumb founded as to why the motor does not pull more power. ON the 1HP motor, when it would bog down from a bearing problem, the amps would climb right out until it either burned up or tripped the breaker. Not the case with this one. Seems to just be happy with slow performance.

[Geof]

Do a bit of Googling about motor designs. Some are intended for starting under load others are intended to spin up unloaded. It is possible your 1hp was specifically designed to start with a load but the larger motor is not.

Another thing to consider when you have a high inertia load that does not take much to keep it running once it is at speed is either a fluid coupling or a centrifugal clutch. These allow the motor to come up to about 75 - 80% speed unloaded, at which point it is developing decent torque and not drawing excessive current and can handle the drive being taken up from there on. I retrofitted a big bandsaw one time, with the fluid coupling it came up to speed faster than when it was direct drive, and the motor did not get as hot.

[Al_The_Man]

Star/Delta start applies to 3 phase motors, you mentioned this is a 1 phase?
IIRC the code states that all heavy load starting motors, the protection can be increased but should not exceed 225% of the FLA.
Al.

[bob1112]

The fluid coupler is out due to space issues. Also, I can probably find another motor for what that coupler will cost me. I cannot remember how to determine if a motor is able to start under load. Idea? I think my first mistake was settling for an open drip motor instead of TEFC but this motor in a 1ph is HARD to find. a dime a dozen in the 3ph variety. I am looking at VFDs now. If I have to change this motor, I am strongly considering just putting a VFD on it. This way, when I move it to the next shop with 3ph power, it will be a no brainer. I planed to get a 2HP VFD to handle the 1HP load at 66% of rated capacity on the VFD. Just have to make sure the VFD will not fault out with a leg missing. Out Yaskawa drives never had an issue with this.

[TOTALLYRC]

Hitachi makes some nice drives that deliver rated power on three or single phase.
Mike