We recently changed out a 200 HP 480V 1800 RPM three phase motor driving a centrifugal fan and are having a problem with heaters. The motor is taking about 15 to 20 second to get up to speed. In that time the heater trip the motor off line. Note: The first motor appears to have had a bearing failure not electrical.

Any ideas on what would cause a new motor to ramp up so slowly?

Most important check the motor is wired correctly. If the run configuration is supposed to be delta and you connect in wye, then you have reduced torque.

An easy check is currents - make sure they are balanced.

Check the voltage... low or unbalanced voltage can cause a problem (unbalanced voltage would show up as unbalanced current).

Less likely, it might be something with the replacement motor... could be the wrong rating? (wrong horsepower, voltage, supply frequency) Could have an internal problem if recently rewound. Severe rotor bar problem can cause torque reduction.

Also you might double-check the fan/system lineup. From a load torque standpoint, centrifugal fan starts best with dampers throttled toward closed, axial flow fan starts best with dampers open (there may be other factors creating restrictions on how you position the dampers)

Thanks for the quick reply.

No dampers to unload the fan. Motor is brand new.
We're checking the voltage voltage at the moment.

Basically the motor just want get up to running speed before the heater take it of line.

This message has been edited. Last edited by: Waylon M,

One longshot to consider (after you look at the other things). A dramatic change in air temperature might affect the load torque required which might cause a problem if you had marginal accelerating torque to begin with. I'll bet you're getting the same cold snap that we're getting here in TX. It's kind of a longshot (especially if you have started it ok in similarly cold weather before), but you never know.

We took the belts off and ran the motor 20 minutes.

460V
US Motor 447T Frame (standard off the shelf motor)
1800 RPM turning 1792 w/slip
Across the line starter.

Note: voltage drops to 422 with load while trying to start.


Amp draw no load is 56, 57, & 58 amps.
LR amp about 680-700 then drops to 200 to 220 amps before the heaters take it off line.
Detla Wiring configuration.

So if I understand correctly the motor ran fine when uncoupled (~ 60A and balanced) but trips in 10 or 20 seconds when starting loaded.

How many horsepower? I'm guessing 100 or 150?
200 - 220 amps seems pretty high relative to LRC and may indicate that the motor never made it up to speed (load torque too high or motor torque too low).

Did they perhaps get an incorrect sized sheave (too large) onto the replacement motor? That could certainly reduce your torque.

Voltage dropping down to 422 is not that bad, depending on where you're measuring it (it's certainly ok if you are measuring close to the motor). Motors should certainly be able to start at a terminal voltage of 90% nameplate, and many are specified to start at 80%.

Waylon

Attached is the nameplate from a similar 447T-Frame, 200 HP motor in MotorMaster.

The idle amp draw from motors of this size and type range from 59 to 65Amps with LRA of 1490 to 1600 Amps and full load current draw from 225 to 235 Amps.

If the LR amps you read were in the 680 to 700Amp range, there may be something wrong with the connections (or the amp meter you used did not catch the LRA). Can you post the connection that you have for the motor?

Does the motor make noise during startup?

Howard

nameplate.ppt (206 Kb, 13 downloads)

Waylon,
If I may throw another question at you: when running no load, did motor come up to speed right away? As the other posts suggest, rotor bar problems will cause the motor to have less torque, speed drop, and longer time to "get over the hump" A motor with severe rotor bar issues tend to emit a very noticeable "growl" when energized and take "forever" to reach rated rpm.
Thanks, Jeff

Motor runs up quickly with no load. Makes no sound and appears to run just fine. Unfortunately I have no name plate information off the old motor. Salt generally does a good job at removing such information and the old motor predates our detail record keeping. I do know the old motor was a WEG of like kind in speed and horsepower. We guessing that US motor may have less starting torque than the motor it is replacing. We’ve made no changes in pulley dimensions or anything else mechanical. Our local rep has suggested upgrading the heaters. This is a continuous application which isn’t subject to frequent stop start cycling.

Thanks for your comments. Until the heaters arrive here we're still looking for other possibilities.

Greetings Waylon,
I don't recall seeing the reason the previous motor was changed out. Why was that? Did it ever trip the overloads?

We've had cases recently where the overload unit was the problem rather than the motor. The units I mention were not the older heater type overloads. The problem with our overloads was false tripping of the integral phase imbalance function of these overload units.

Also, how are you gauging when the motor comes up to speed?

I would think 680 to 700 amps sounds like normal starting current for this size of motor on a variable torque application like an AHU fan.

One last thought, you might check belt over-tightness and make sure you're not putting undue stress on the bearings.

J-

Just upgraded the heaters and the motor appears to be running up to speeds without tripping out.