Motor Overtemperature

[themroc]

I am running a gear pump using a Siemens micromaster FI Inverter.
The gear pump is connected to a 4kW Motor.
The pump is part of a test rig, at times it is required to run the pump at very low speeds/frequencies.
Under this situations from time to time I encounter a FI cut out due to Overheating of the Motor.
ON the FI Inverter it displays :
Motor Overtemperature I^2 t

Now my Questions:
It is obvious that whenn running with a low speed the ventilator of the motor which is connected to the shaft is not producing a lot of cooling.

I thought to put a fan next to the Motor in order to improve the cooling, would that work? The Motor is thermal class B. Where can I determine that the temperature of the Motor is not to high, I have got some spare thermocouples which I could use, does anyone know where to place them?

Because then by knowing the actual temperature I could disable the error trip of the FI inverter!

 

[electricpete]

You are right that there is less cooling at low speed.

I haven't worked much with this type of equipment. One thing that I was wondering about is the label "I^2 t". It implies a quantity based on current vs time, as such independent of cooling. Therefore I wouldn't expect this particular parameter to trip in this scenario (would have expected a parameter like simulated temperature or calculated temperature). Perhaps the algorithm is a little more complicated than what is suggested by the name ?

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[roydm]

Themroc,
I think electricpete is right on. I'm almost certain that the drive calculates the thermal overload based on Amps and frequency not by measuring the temperature. You may be able to fool it somehow
Regards
Roy

 

[rovineye]

It looks like a calculated temp trip based on time and current.

Check to make sure the correct motor data and duty cycles have been entered as it may be tripping at too low a value relative to your motor. Check the value of that trip as well. It probably trips at something like 100 or 115% of that setpoint max value. Not familiar with this drive but a link below will take you to documentation for your drive.

You could probably change that to an alarm instead of a trip as well, but I would first try to figure out why the computation is incorrect on that trip if you really believe there is no heating issue.

If you don't have devices already embedded in the windings to measure the heat I suggest measuring the winding resistance. That won't pick up hotspots, but will give a general idea of what is going on.

 

[themroc]

Does it in general mean that a hot motor draws more Amps and therefore it would trip at a certain condition?

 

[rovineye]

No, it means that more current equals more heat.

The computation may take into consideration cycling, ambient temperature, service factor of the motor, if the motor is designed for variable speed (compensating for the slower cooling fan at lower speeds), etc.

 

[itsmoked]

Regardless you probably DO need more cooling. If you are running below perhaps 40% speed but still requiring a lot of torque.

You can set up a small blower to blow directly thru the motor or a ducted fan to blow thru the opening entry on the fan housing of a TEFC type motor.

You can use a temp gun to observe the motor's case temperature while running at normal fan cooling high speed for an hour or until the temp gun shows steady state. Aim to stay at or below that steady state with your auxiliary cooling when at low speed for extended periods.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

You typically DO need external cooling for low speed high load operation.

In addition, the MicroMaster is calculating the motor temperature if it does not see a thermistor feedback input. The thermal model is dynamic, so t is making assumptions about the motor cooling properties based upon the operating speed and detected load. If you know what you are doing, you can indeed tweak the motor data, but a better plan is to use a thermistor input to the VFD for real-time motor thermal limit monitoring. Once you tell the VFD that there is a thermistor to look at, the thermal modeling takes that into consideration. To be effective though, the thermistors should be embedded in the motor windings, not on the external case. If your motor was not provided with them, you would need to remove t and take it to a motor shop to have them installed.

 

[ScumPunk]

Hi, All
Not much to add to the advice already given, but here goes:
Which generation of Micromaster is the "FI" drive? I've not heard of that one, only FX and GX, and then on larger versions of the model.
If it's the MM420/440, you can do several things, but it seems a very good idea to make sure that the overtemperature fault/warning is not genuine. If your thermocouple is the flexible wire type, I would push it into the windings via the terminal cover. This will give a fairly good idea of the real winding temperature.
I assume you have performed the motor ID process, and entered the plate data into the drive?
The motor thermal model (which is what is giving the fault here) can be a bit inaccurate, i.e. if the motor is non-Siemens, running at low speeds, etc. So I agree that the best solution is to use a temperature sensor or PTC trip input to the drive, then you can set P601 to 1 or 2 and the drive will then disable the thermal model to an extent.
If you use an external fan to cool the motor, there is a parameter to tell the drive this, so again it will change the model behaviour.
Cheers,
Mort

 

[DickDV]

I've seen enough here to know that the application has the wrong motor if the voltage is 400 or higher and/or the speed turndown is more than 4/1.

First, Insulation Class B is never good enough on 400+ volts running directly off the drive output. Class F is minimum and an MG1 Part31 endorsement is better.

Second, it sounds like a TEFC motor was chosen for an application that requires full torque slower than 1/4 speed. Right out of the textbooks, this should be a TENV or TEBC motor.

Turning off the drive thermal calculations or otherwise tricking it into thinking the motor is something it isn't is hardly the road to reliable long-term performance.

 

[ScottyUK]

Hi Dick,

I didn't realise that there was such a simple link between insulation's thermal rating and the suitability for VFD use at higher supply voltages. Is that because the materials typically used for class B machines inherently have poorer response to high dv/dt than those used for class F or class H, or just that class B machines, especially in small sizes, tend to be older designs which were never intended for VFD use?


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If we learn from our mistakes I'm getting a great education!

 

[edison123]

Scotty

Yes, Dick is right. VFD random winding requires special enamel (it is a dual coating with typical class 180 deg rating and high dv/dt strength) coated wires. Class B and even some class F enamels are unfit for VFD duty.

 

[ozmosis]

Unless I have misunderstood, there is a difference between insulation class and temperature rise when it comes to motors.
True, it is preferred that motors running from a VFD have insulation class F but the temperature rise of the motor could still be Class B.

themroc,
what is the actual fault (or alarm) shown on the Micromaster? If it is a fault and it is the motor overtemperature then you would see F0011:
Vector Variants
Check the following:
1. Load duty cycle must be correct
2. Motor nominal overtemperatures (P0626-P0628) must be correct
3. Motor temperature warning level (P0604) must match
Basic Variants
Check the following:
1. Load duty cycle must be correct
2. Motor thermal time constant (P0611) must be correct
3. Motor I2t warning level must match

if it is a warning of motor overtemp then you would see A0511:
Independently of the kind of temperature determination check:
1. P0604 motor temperature warning threshold
2. P0625 motor ambient temperature

If (P601 = 0 or 1) Check the following:
1. Check if name plate data are correct (if not perform quick commissioning)
2. Accurate equivalent circuit data can be found by performing motor identification (P1910=1).
3. Check if motor weight (P344) is reasonable. Change if neccesary.
4. Via P626, P627, P628 the standard overtemperatures can be changed, if the motor is not a Siemens standard motor.

If (P601 = 2) Check the following:
1. Check if temperature shown in r35 is reasonable.
2. Check if the sensor is a KTY84 (other sensors are not supported)

If you are not getting either of these faults/alarms then let us know what the code is.