Overload setpoint

This is a new one for us. Our steam turbine turning gear motor overload relay is a Schleicher model SBW1007 "Load Monitor". It monitors the phase angle shift between current and voltage. so the set point is in degrees, not amperes. The motor is rated at 480 volts, 3 phase, 60 hertz, 4.7 amps full load.

Their stated operating principle of the device is "With an inductive load, the current trails the voltage by a phase angle (theta). If the load on the motor falls, the phase angle becomes larger, thus cos theta becomes smaller. This change of phase angle is a precise measure of the load change at the motor shaft. The load monitor monitors the phase shift between the voltage and the current under inductive loads in a sinusoidal AC and three phase network." (For the full data sheet visit http://www.smi-online.net/Schleicher/data_sheets.htm and select model SBW1007).

The monitor measures current on one leg and voltage on all three. There's a time delay adjustable up to 30 seconds. The trip setting is in degrees, adjustable between 18 and 90 degrees. We have one motor that wasn't experiencing overload trips that's set at 15 seconds and 35 degrees. The other one had a higher degree setting, was tripping periodically, so I adjusted it to 35 deg. Don't know if that will work until we're back on gear in the fall.

Schleicher may have a better mouse trap, but technicians are accustomed to determining the overload setpoint based on motor name plate data which includes familiar things like full load amperage, service factor, and so on. Nameplates don't mention full load I/E phase angle, let alone the minimum permissible phase angle.

OK, they sometimes give you the full load power factor (but not in this case) and from that we could calculate the full load phase angle if we had it, but even knowing that wouldn't give you the trip set point since you don't know what the phase angle would be at the maximum permissible current. Even our BSEEs are scratching their heads on this one.

Anyone got any suggestions on calculating the trip set point?

Overload it mechanically until you reach the current where you want it to trip. Then, adjust the phase angle setpoint upwards till it trips after the desired delay.

Or, toss the thing and install a normal overload.

I wish we could safely overload it, but that would entail putting some kind of a brake on the turbine shaft while it's still hot and on turning gear, which would raise a lot of eyebrows in operations. I'd guarantee a big no.

Given the situation that nobody on this side of the pond seems to know what to set the thing at is making the idea of replacing it with a normal overload more appealing.

Are you sure that the function the Schneider relay has is used for overload detection? From the description you gave, I would say that the function is used to monitor loss of load on a motor, which in your case would normally translate in the inadvertent disengagement of the turning gear. I don't think that function can be used to detect overload condition, as I wouldn't expect the motor phase angle to shift much from full load to overload condition. On the other hand, if the motor loses its load, then its phase angle would increase abruptly as the dominant load on the motor supply lines would change from a loaded motor running at approx 0.85pf to a motor running off load with a 0.1pf.

I'm convinced it's for overload protection. Here's why:

The Schleicher literature refers to the relay as:

"Overload Detection of Electric Motors without Additional Sensor" (i.e., a CT). Our model SBW1007 operates on the what they call the "Closed circuit Principle", which is given as:

When the supply voltage is applied, the relay switches into its operating position. When the motor is switched on, the adjustable start-up override time at the relay begins. After this time has elapsed, the SBW1007 monitors the pre-selected phase shift cos theta. If the load rises, the phase shift becomes smaller - cos theta becomes greater.

If the preselected phase shift falls below, [sic] the relay switches into its deenergized position after expiration of the fixed response time. If the phase shift exceeds the pre-selected value - back to a normal load - the relay switches into its operating postion after expiration of the fixed release time. The fixed response and release time (1000 ms) prevents the relay from reacting during brief deviations from setpoint value.

Clear as mud, isn't it? The Alstom control logic refers to it as a motor overload, and I know that if I reduce the phase angle setpoint with the turning gear engaged the relay trips and latches. That's when the real weirdness begins.

Upon an overload trip the logic waits 20 seconds then initiates an overload alarm and turns the motor off, disengaging the turning gear. But the logic immediately turns the motor back on causing the gear to reengage. Then it lets the motor run for another 20 seconds, repeats the off-on cycle, runs it for another 20 seconds, and so on. It will do this indefinitely. In fact some operators think it's normal "because it's always done that". (Although unit 2's turning gear doesn't do it. Of course its overload was set at the smaller 35 degrees.)

In addition to phase shift being an unusual way to detect motor overload, it's a pretty strange way to handle a motor overload, don't you think--momentarily stopping then immediately restarting the motor three times a minute? We're trying to find out what Alstom really intended for it to do. Wish us luck.

Greg, based on the figures you cited, the motor size is about 3.5 to 4.0 Hp. Can you provide the Namplate HP, RPM, and Service Factor?

I agree with Steve Myres. There should be no problem to safely overload the motor by say 10-15%! Especially if it has a Service Factor. If you want a simple method contact me off-line! I actually did it to a 19,000 Hp motor, although not to increase load, but to find the cause of vibration! Details are available on request!

BTW, are you sure about the voltage rating? If really 480V, then it's older than me!

Regards, Phil ([email protected])

A motor is considered to be in overload condition when the current it is drawing from the supply is 110% of nominal or above (actually above 100%, but most motor/overload standards accept the 10% overload as part of the continuous capability of the motor). What is the expected change in phase shift for a motor when its load goes from 100% to slightly above 110%. I believe the change in phase shift would be small.

Jojo, as you are probably aware I'm an admitted technocrat! Therefore, by edict, I must disagree with your definition of "overload!"

IEEE Std 100, actually quotes the NEC. The latter's definitive phrase is, "... would cause damage or dangerous overheating!" Overheating occurs when a motor's "design hot-spot temperature" is exceeded. And, hot-spot temperature is a function of ambient temperature!

In conclusion, operation in excess of rated Ampacity is not necessarily a valid indication of "overload!"

Regards, Phil

Yes sure, I agree that if you operate a motor at an ambient temperature of -40C, you can load the motor much more than when you operate it at ambient temperature of +25C. But the point in my argument wasn't whether you operate a motor at its insulation temperature limit or not.

My point was that under the same environmental conditions, what would be the change in phase angle between nominal full load and overload of the motor? Is the change in angle appreciable enough to permit the change to be used to detect overload (instead of using a conventional thermal overload)?

In addition I don't believe that somebody uses systems that cool motors below the ambient air temperature simply to permit it to produce more power without undue internal damage, especially in the case that our friend brought up.

Responding to Jojo's 11-Jul-08 (02:46) comment and questions:

1. Why do you always use unrelated "extremes" to prove a point?

2. The answer to your 1st question is... less than 2 deg, when motor load is icreased from 100% to 110%!

3. I agree with you! The purported gain attributed to using PF angle does not warrant substituting it for the doing it the old-fashioned way... current monitoring!

4. Huh? How is your last question and the poster's problem related?

Regards, Phil

You never find a motor that is exactly 'tuned' to the environment that it is working in. Practically in all motor drives, the motor is running at around 85% to 90% of its nominal temperature rating. The reason is that the users of motor drives cannot change the motor every time the ambient temperature changes. So they have to design the motor to reach max allowable temperature rise at the worst case ambient temperature expected, and for the rest of the time the motor is at lower temperatures than nominal.

More than that there is also the problem that in most cases the thermal overload is located in a more temperature controlled environment than the motor, with the result that there arises a 'mismatch' between the temperature reaction of the overload, and that of the motor (even though in theory both should follow the same temperature curve when the same current passes through them). So the expected protection obtained from the overload is never 100% accurate.

Finally the relationship of my comment to the poster's problem is that in a steam turbine hall, one normally finds the ambient temperature between 20C and 40C. In this range the motor ampacity rating and its allowable temperature rise practically coincide (barring the shift due to the range, as standard motors max insulation tempertaure rise is stated at motor environmental temperature of 25C, with rated ampacity). So when in my post I stated that the motor would be in overload when running at 110% or over, that statement would practically hold, in the poster's problem. Rest assured trying to detect a 2 deg temperature difference between one loading and another will be masked by a number of other environmental/operating factors, other than the one you are trying to test/detect.