Danfoss vlt AQUA Instruction Manual page 177

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VLT AQUA Drive
Instruction Manual
10.2.4 Derating for Running at Low Speed
When a motor is connected to an adjustable frequency drive, it is necessary to make sure that the cooling of the motor is adequate.
The level of heating depends on the load on the motor as well as the operating speed and time.
Constant torque applications (CT mode)
A problem may occur at low RPM values in constant torque applications. In a constant torque application, a motor may overheat at low speeds due to
less cooling air from the motor integral fan.
Therefore, if the motor is to be run continuously at an RPM value lower than half of the rated value, the motor must be supplied with additional air-cooling
(or a motor designed for this type of operation may be used).
An alternative is to reduce the load level of the motor by choosing a larger motor. However, the design of the adjustable frequency drive limits the motor
size.
Variable (quadratic) torque applications (VT)
In VT applications such as centrifugal pumps and fans, where the torque is proportional to the square of the speed and the power is proportional to the
cube of the speed, there is no need for additional cooling or de-rating of the motor.
In the graphs shown below, the typical VT curve is below the maximum torque with de-rating and maximum torque with forced cooling at all speeds.
Maximum Load for a Standard Motor at 104°F [40°C] driven by an adjustable frequency drive of type VLT FCxxx
Legend: ─ ─ ─ ─Typical torque at VT load ─•─•─•─Max torque with forced cooling ‒‒‒‒‒Max torque
Note 1) Oversyncronous speed operation will result in the available motor torque decreasing inversely proportional to the increase in speed. This
must be considered during the design phase to avoid overloading the motor.
10.2.5 Derating for Installing Long Motor Cables or Cables with Larger Cross-Section
The maximum cable length for this adjustable frequency drive is 984 ft [300 m] for unshielded cable, and 492 ft [150 m] for shielded cable.
The adjustable frequency drive has been designed to work using a motor cable with a rated cross-section. If a cable with a larger cross-section is used,
reduce the output current by 5% for every step the cross-section is increased.
(Increased cable cross-section leads to increased capacity to ground, and thus an increased ground leakage current).
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MG.20.M5.22 - VLT is a registered Danfoss trademark
10 Specifications
10
10-21