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Most VFD manufacturers today make their products with two ratings; a Variable Torque rating, and a Constant Torque rating.
The Variable Torque rated product is designed to operate at 100% of its output current rating continuously, and 110% for one-minute.
The Constant Torque rated product will operate continuously at 100% of its output current rating and 150% for one-minute, and some will have a 200% overload rating for a few seconds.
(This is predicated on the understanding that rated ambient temperature and specified altitude are not exceeded... otherwise derating must be applied accordingly)
HEAT, being the enemy of solid state devices, is the limiter of overload rating at the design phase of product development. When you operate the VFD in the overload region, you are raising the junction temperature of the solid state power devices. This heat is conducted into the heatsink structure, which will cause the temperature of the heatsink to increase. If additional and adequate cooling is not applied, you very quickly get to a destructive thermal situation with device failure imminent.
In the design phase, the engineers include protections to prevent the VFD from getting to that point. Current and Heatsink Temperature are monitored, and when the design limits are exceeded, the VFD will shut down. Restart will then only be possible when conditions (current and temperature) have returned to normal.
Protection of the motor was once the responsibility of the user; inclusion of an inverse time thermal overload device in the circuitry supplying power to the motor; the use of embedded thermal measuring devices in the motor windings (RTDs for instance); or magnetic trip devices that sense the rate of rise in motor current; are but a few of the protections afforded to motors.
Today, and for at least the last 15 or so years, VFDs have included the inverse-time function which synthesizes the temperature in the motor based upon Current over time and some even include speed as a parameter in the calculation.
(Check the operating manual for your particular VFD to see what motor protection it provides)
That addresses the hardware. Now lets look at the application.
99% of applications operate within the design limits of the equipment. Your application falls into the 1% category.
Here are some things to consider:
Increase the rating of the VFD so that it can handle the overload situation without activating any of the protective functions.
For example ... if its a 100 HP application .... and has to operate at 150% for a period greater than 1-minute, then provide a 150 HP VFD. You could also up the HP of the motor to accommodate operation in the overload range for a prolonged period of time. Sometimes the motor manufacturer will offer a 100 HP motor in the next larger frame size to accommodate the increased heating. But you would still need to upsize the VFD to accommodate operating in the overload region.
In situations such as this, it is ALWAYS best to consult with the motor manufacturer, the VFD manufacturer, and the machine (driven equipment) manufacturer. Let them tell you what is needed. That way if it doesn't work, they're on the hook .... and you're not.
H-t-H ...
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