VFD Motor Thermal State

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delectric123

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On the Schneider Altivar 61s I use they can monitor the motor thermal state without any thermal sensor involved. How do they do it? After the motor is stopped the value slowly goes down, so I'm really curious if it can actually monitor it real time, though I can't imagine how.
 
It's not actually monitoring the motor temperature without a sensor, it's just estimating the motor's relative thermal state based on the power it pushes into it. The VFD "knows" roughly how efficiently the motor is operating at all times (because the voltage and current waveforms contain information that can be used to calculate efficiency), it knows how much power the motor consumes, and if you program the motor frame size correctly then the VFD knows approximately how much heat the motor can dissipate. Based on all this information, the VFD estimates the motor's temperature rise. It's not perfectly accurate, but it's more accurate than the old fashioned heater-based overload protection.
 
mpoulton said:
It's not actually monitoring the motor temperature without a sensor, it's just estimating the motor's relative thermal state based on the power it pushes into it. The VFD "knows" roughly how efficiently the motor is operating at all times (because the voltage and current waveforms contain information that can be used to calculate efficiency), it knows how much power the motor consumes, and if you program the motor frame size correctly then the VFD knows approximately how much heat the motor can dissipate. Based on all this information, the VFD estimates the motor's temperature rise. It's not perfectly accurate, but it's more accurate than the old fashioned heater-based overload protection.
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What about when the motor is stopped? Does the drive just calculate the time needed to cool off?
 
They know the current
they know the voltage
so they can calc Z
they know run time from a cold start
the build a map of temp vs time
They adjust for rated hp, current, etc

temp related to Z by
Tt = Tc + (Rh - Rc)/Rc x (Tc + 234.5)
Where:
T(t)=total winding temperature
T(c)=Cold motor (ambient) temperature, C (The motor should be in the ambient environment long enough to reach that temperature.)
R(h)=Hot motor resistance
R(c) =cold motor resistance
234.5 = constant for copper windings
 
delectric123 said:
What about when the motor is stopped? Does the drive just calculate the time needed to cool off?
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Yes it considers run time and stop time. If you cycle drive power though it may reset the data and think the motor is cool when you resume running.

It also will not know if there is dirt accumulation on the motor that may impact it's thermal dissipation properties or if the motor is outdoors and being rained on that it probably can be worked harder without any significant damage.

Add: drive also won't know conditions like added heat from a bad bearing or a missing or broken cooling fan.
 
delectric123 said:
What about when the motor is stopped? Does the drive just calculate the time needed to cool off?
Click to expand...
They will use a standardized motor thermal cooling constant. It's good for probably 80% of the motors out there, maybe 90+% of the Inverter Duty motors out there, and somewhere the manual will tell you to use an inverter duty motor. More expensive Motor Protection Relays will allow you to manually reprogram that cooling constant if you know something more about the motor and installation, ie high or low ambient conditions. But in general, they assume you will not likely be using a VFD on a motor in a high ambient without having de-rated the motor, so their use of a standardized motor will err on the side of caution. The same will be true of a low ambient condition; the thermal model may not allow for restart as soon as it could, but that makes it easier on the motor, not harder.

By the way, most premium drives on the market will do this now, then they will allow you to supplement that thermal modeling with a thermistor input, which in IEC world is required if the VFD driven motor is in a classified area. Also, most now go with a "Retentive a Thermal Model" as well so that you don't reset the thermal modeling simply by cycling power. You have to take an extra step in software to do that, meaning it must be a purposeful act, often referred to as an "Emergency Restart" feature.
 
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