VFD sizing

[John MCParticipant @johnmc39344]

I have a 1100w three phase motor I want to run through a VFD. The last time I set up one of these I recall it was important to match the motor to the VFD. Is this still imortant?

I have a 1500w VFD, would this work with the lower rated motor?

Motor 2.5 amps, VFD rated at 7 amps. I also have a 750w VFD, 4 amps, might this be suitable?

John

[John MCParticipant @johnmc39344]

[Thor 🇳🇴Participant @thor]

Hi John,

Your 1500W VFD should handle your motor without problems, just program your VFD with the max current of your motor nameplate. I assume you have a manual for your VFD.

Thor

Edited By Thor 🇳🇴 on 03/02/2023 18:31:41

[not done it yetParticipant @notdoneityet]

If it is a chinese VFD, I would go to the next size up (for the motor) – chinese VFDs may not have sufficient extra power for the motor losses (assuming the motor is not chinese, too!).

I expect the 1500W one would work OK. Any shortcomings might be that of setting a safety parameters such as current limit, but that is unlikely.

Edited By not done it yet on 03/02/2023 18:18:38

[John MCParticipant @johnmc39344]

Thanks for the replies, I'll try the 1.5kw VFD.

John

[SillyOldDufferModerator @sillyoldduffer]

Posted by John MC on 03/02/2023 16:58:57:

I have a 1100w three phase motor I want to run through a VFD. The last time I set up one of these I recall it was important to match the motor to the VFD. Is this still imortant?

I have a 1500w VFD, would this work with the lower rated motor?

Motor 2.5 amps, VFD rated at 7 amps. I also have a 750w VFD, 4 amps, might this be suitable?

John

1500W VFD will be fine with an 1100W motor unless it's misconfigured.

Matching a VFD isn't as simple as comparing watts, though power is the best place to start.

The need for matching is because a VFD isn't just a black box that magically turns 240V single-phase into 3-phase. They provide a multitude of other programmable features and it's how these are configured that constitute 'matching'. There may be hundreds of settings. Examples include:

• setting the base frequency to 50 or 60Hz to suit the motor
• activating speed control and setting the minimum and maximum speeds
• setting the terminals to support safety interlocks and emergency on/off
• limiting maximum output current
• parametrising to optimise VFD output waveform to the inductance of the windings
• parametrising for a soft start: optimising soft-start requires a good understanding of the motor's starting characteristics and the starting load
• parametrising for a slow or fast stop, essentially deciding how quickly the motor acting as a generator will be allowed to dump energy into a big resistor (if fitted)
• Setting for performance or economy
• Stall and slip management
• Torque boost at low speeds
• Vibration control
• and more…

The good news is that modern basic VFDs all seem to come with sensible defaults that will run a motor out-of-the-box.

For amateur use most matching requirements can be ignored because we only buy VFDs as a convenient source of 3-phase and speed control. As long as the motor works we're happy. Our control requirements are super simple compared with, for example, powering the lifts in a tower block. Here the motor on an express lift might be configured to accelerate quickly, whilst all the other lift motors are configured to save energy.

Sensible defaults can't be claimed for older or advanced VFDs. They're for customers who want to optimise by carefully matching the VFD to the motor, and pay for a specialist or send staff on courses. Their manuals are often terse references written for trained installers, without any explanation, let alone an Easy Guide for Newbies! Wiring up is usually trivial compared with understanding how to program the thing from scratch.

I think the trick is to keep it simple by avoiding high-end VFDs unless you understand the manual! And if VFD manuals are a complete mystery, buy a VFD/motor bundle where someone else has set it all up.

Dave

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