AutomationDirect.com offers so many motor choices
how do you find the right one for your VFD application? Inverter rated, inverter duty,
high performance inverter duty, what do all these things mean?
All these terms mean the motor can be used with a VFD. Inverter duty or inverter rated
means the same thing and are simply inexpensive motors that are designed with VFDs in mind. High
performance inverter duty motors are more costly but aren’t as limited as inverter rated motors.
All three-phase motors sold by AutomationDirect.com can be used with an inverter or VFD.
To determine what’s best for you let’s look at your application.
First, if you’re not sure what size motor you need, please see the video linked above. There’s also a link to this
video in the description if you want to view it later. What you need to determine after size and
speed is if your application is constant torque or variable torque. A constant torque application
means the torque demand stays unchanged at lower speeds such as with conveyors, extruders, etc.
A variable torque load means as the speed slows the torque decreases and as speed increases the
torque increases. Good examples of these are centrifugal pumps, fans, etc.
Since these run at slower speeds, less air or water is moved so there is less load on the motor.
Now that we understand that let's go find a motor. Let's assume we need 1hp at 1800rpm.
These are inexpensive rolled steel inverter rated IronHorse motors.
We can see that the minimum speed with constant torque is 440rpm and with variable
torque it is 176rpm. Another way to say that is the CT speed is 4:1 and the VT speed is 10:1.
Some inverter rated motors may go as high as 10:1 constant torque and 20:1 variable torque.
So, let's compare that with a high performance inverter duty motors.
Let's look at these Marathon Black Max motors. We can see these have a 1000:1 CT speed range,
meaning an 1800rpm motor with a full load speed of 1750 can be run as low at 1.75rpm.
These high performance motors are optimized to withstand the heat generated at low speeds for
constant torque applications. Variable torque on these is usually 10:1 or 20:1 which is similar
to a lower cost inverter-rated motor. Utilizing these motors for a variable torque application
that often runs at low speeds will provide a benefit in lower heat and longer lifespan,
but this will come at a cost premium compared to an inverter rated motor.
So picking the right motor depends on the torque type and how slow the motor needs to run
for your application. A shortcut to all this would be that high performance inverter duty motors are rated
for constant torque of 20:1 or greater. Inverter rated motors have a lower constant torque ratings as
we saw earlier and may generate more heat at lower speeds. AutomationDirect is providing the products and
support you need to make us your common sense way to buy industrial controls.
If you need more help picking a motor please see our tech support options here.
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