Repeated failures of a motor drive

  • Thread starter "Þórhallur" Ragnarsson
  • Start date
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Thread Starter

"Þórhallur" Ragnarsson

The HVAC system in our 4 year old vocational school building is supposed to be capable of maximum 15000m3/h to ventilate an area of 2100m2 (approx. 6500m3) occupied by 140-160 students (age 15 up) and teachers from 8:00 to 16:00 and maybe some 10-20 students/staff from 16:00 to 21:00

The system has an air intake with a centifugal blower driven by V-belts from a WEG type 132M-4 motor rated 400/690V, 50Hz, 7.5kW, 14.2/8.23A the blower is running at approx 70% of motor-rpm The motor is connected to a SEW Eurodrive type MC07A055-5A3-4-00 (rated 5.5kW)

(the exhaust blower is similar setup, a bit smaller and has run without trouble)

The maximum motor rpm is set at 1250. the minimum motor rpm is set at 900 Ramp up time is 20s and ramp down is 45s

Now at the time of writing the motor runs at 1210rpm with phase current 8.3A according to the drive display.

The rpm of the system is regulated by a PI temperature regulator. Minimum speed is supposed to be adequate for air supply at all times (I really wonder if that is correct, but that is another story)

The HVAC also slows down to minimum speed at 21:00 and speeds up again at 7:30

The drive has stopped several times with error F-07 = DC overvoltage. I do not know for sure when that happens, just that it on or after 21:00 and before 8:00 The manual suggests that error F-07 indicates bad brake resistor or connections.

The drive does not have a brake resistor installed.

The drive has also failed totally (output transistors) 6 times in the past 4 years.

When the drive failed first time it was replaced by the next larger version (rated 7.5kW) That setup failed also several times and now it has been replaced with the "size that was designed " i. e. 5.5kW type.

The motor has also been replaced once because of "suspected" insulation failure.

Can anyone see a problem(s) with this setup and/or suggest a solution? Is the drive too small or wrong type? Is this type of drive less reliable than some other type? Is the motor unsuitable for the drive? Is a brake resistor needed? Any other remarks?

These repeated failures are getting very tiresome for staff and students, repairs now take ages (last time 4 weeks) while the relevant parties battle over who shall pay for the repair.

I have suggested to the HVAC designer that he might try to add a brake resistor, to absorb any possible overvoltage from regenerative braking. He refused and said "There should be no regeneration, the ramp down time is more than adequate, we do not do such experiments here".

Best regards.

Thor

Þórhallur Ragnarsson Electronics technician/teacher
 
S
Yes, DC bus overvoltage is caused only by regeneration as far as I am aware. Large fans have heavy wheels where inertia can become the
governing factor in hp selection, so the supply fan may be more prone than the exhaust fan to this issue (higher pressure fans would also tend to have heavier wheels per CFM.

45 seconds does seem like an adequate deceleration time, but a brake resistor is fairly cheap to try as a solution to such a persistent problem, so why not give it a try?

Your ventilation rate, a MAX of 0.38 CFM/ft2 seems quite low. I would be more inclined to provide at least 1.0 CFM/ft2 as the max, but a change of that magnitude will be hard to achieve at low cost.

--
Steve Myres, PE
Automation Solutions
(480) 232-6862 Mobile
(480) 813-1145 Office
 
W

William Sturm

You would only need a braking resistor if you are trying to slow a large inertia load faster than it would naturally coast down. If you disable the drive, does it take longer than 45 seconds to slow from the higher speed to the lower speed? If so, a braking resistor may help. You might also try to monitor the DC bus voltage while the fan is slowing. Most drives have a high voltage limit, after that they will shut down.

A more likely problem is the length of cable from the drive to the motor. If it is longer than 50 to 75 feet, then you should have a three phase line reactor in series with the motor. The line reactor is probably a good idea anyways, it could make your motor last longer. The reasoning is that the sharp edges of the generated AC waveform create high freqeuncy voltage spikes. These can cause standing waves in your motor cables, which can cause abnormally high voltages in your cables and motor windings. Eventually, these spike will burn through the motor insulation and cause momentary arcs (short circuits) in you motor and/or cables.

Hope this helps,
Bill
 
P

Phil Corso, PE

Responding to P. Ragnarsson's Oct 6, 12:53am query... answers to the
following questions would be very helpful:

Q1) What are ambient and electrical equipment temperatures for drive
panel location?

Q2) What are ambient and electrical equipment temperatures for motor
location?

Q3) What are specific nameplate rated values for motor, i.e., speed,
enclosure type, and temperature-rise?

Q4) What has motor supplier said about lower speed operation?

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])
 
C

Curt Wuollet

Sounds like a classic case of the overrunning load. The over voltage is caused by the motor generating voltage as you are trying to slow it down. Many drives freak when trying to start a blower that is already spinning. And with a
inlet/outlet blower system, if one blower is running the other likes to windmill in the air flow. Braking would probably help. You need someplace to dump enough energy to let the drive stay in control.

Regards
cww
 
T

Thorhallur Ragnarsson

Responding to Phil Corso´s questions:
Thank you for the reply, here are some incomplete answers to your questions:
> Responding to P. Ragnarsson's Oct 6, 12:53am query... answers to the following questions would be very helpful:
>
> Q1) What are ambient and electrical equipment temperatures for drive
> panel location?

A1) The drive has internal cooling fan, ambient air temp going into the drive is 44°C, (111°F) the air going out is 60°C (140°F), I did not have suitable probe to measure heatsink temp but it´s very hot.

> Q2) What are ambient and electrical equipment temperatures for motor location?
A2) ambient motor temp 10° (50°F) to max 25°C (77°F) depending on weather. At this time ambient temp. is 18°C (64°F) and the motor itself is cool to the touch, surely less than my body temperature.

> Q3) What are specific nameplate rated values for motor, i.e., speed, enclosure type, and temperature-rise?

A3) WEG type 132M-4 motor rated 400/690V, 50Hz, 7.5kW, 1470rpm, IP55, fan on one end, temp rise not specified as far as I can see. (I think this is one of their "green" hi eff. types)

> Q4) What has motor supplier said about lower speed operation?
A4) I can only quote manufactureers (WEG)website "...the company has officially announced that our standard off-the-shelf product is able to withstand 1,500Vpeak and 5,200V/µs at 5kHz. This covers more than 95% of the applications with PWM drives today." I understand this applies to the motor we have here.

I suspect A1) is our main concern, although the drive manufacturer (SEW-Eurodrive) does specify max ambient temp. to be 50°C (122°F) for up to 100% nominal current and up to 60°C with derating.

Best regards
Thor
 
P

Phil Corso, PE

Responding to Thor's Oct 8, 9:29pm answers... analysis points to the drive:

Drive.
Assuming the drive enclosure surface area is 2,500 cmq (cm^2), then the
16°C temp-rise over the 44°C ambient yields an internal heat-load of
60 to 80 Watts. Failure-rate, 6 times in 4 years, is excessive. But,
their exact causes (e.g., cpu, power components) are unclear!

Motor.
Assuming the motor is equivalent to a NEMA TEFC enclosure, then the
rated full-load temperature-rise is 40°C over a 40°C ambient. For
current operation, your "cool to the touch" observation means that the
motor's carcass temp is less than 40°C, say 35°C. Thus the temp-rise
above ambient is 17°C. This indicates that the load factor is
(17/40)^2 or about 20% of rated. There has been one failure, but its
cause is not clear.

Conclusions/Recommendations.
1) The motor temp-rise is certainly tolerable for the assumption
stated.
2) The drive's internal heat-load does not appear excessive. However,
it may be when compared to the motor's relatively low output... about 3%
for a 20% load-factor!
3) The drive's inlet cooling-air temperature is too high!
4) Relying on the most revered electronic engineering tenet, "heat is
the number one enemy of electronics" begs the question, "Can't you
provide the drive enclosure with lower temp cooling air.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])
 
Correction to my earlier Oct 9, 2:19pm comments on the subject... boy, is my face red!

Load factor is proportional to the ratio of temp-rise to rated tempe-rise, and not the square of the ratio. Therefore, make the following changes:

a) Penultimate sentence in paragraph titled 'Motor', "This indicates that the load-factor is (17/40) or about 43% of rated."

b) Sentence 2) in paragraph titled 'Conclusion/Recommendation', "... about 1.1% for the 43% load-factor case!"

For determination of load-factor, see thread #1026164342 "Motor Load Factors", dated Dec 31, 2002!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])
 
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