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We are going be buying a 15 HP motor, and using a VFD we have, currently not doing anything. While I'm told that typically, you buy a VFD for the Motor (i.e. 15 HP Motor + 15 HP VFD), we couldn't let the Mitsubishi VFD continue to collect dust.

That being said, could a 15 HP power motor be used with a 30 kW VFD (Mitsubishi FR-F740-00620-EC) without harming either unit?

Search on Google and Stack Exchange regarding running a low power on a high power VFD has yielded no results, so here I am asking.

While I can't imagine it being an issue (my thoughts are VFD is just a power supply deal), we figured it wouldn't hurt to be safe and check.

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  • \$\begingroup\$ It depends on your acceleration profile of V/DCR for V/f control. 15Hp will work if you don’t have heavy inertia where Kw may exceed 3kW/hp under full voltage surge load \$\endgroup\$ – Sunnyskyguy EE75 Oct 12 '17 at 21:28
  • \$\begingroup\$ Hmm, I don't have this information as it was only mentioned we are getting a 15 HP motor. No other specs were passed down to me, or what it would be driving (or if it would go over 3 kW/HP). I'll this in mind, and mention it to my supervisor. Thanks! \$\endgroup\$ – Akasaka Oct 16 '17 at 17:39
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Your motor is 15 HP x 750 W/HP = 11.25 kW.

The FR-F740 should handle that OK but you need to set the motor type and power.

enter image description here

Figure 1. Page 153 of the FR user manual.

Start around page 153 of the manual and set up appropriately.

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Actually, only Mitsubishi can answer that properly. As a gross general rule, VFDs work best with motors that are no smaller than 50% of their maximum rating. That's because to function correctly under Sensorless Vector Control and Flux Vector Control, the VFD is required to take very accurate measurements of tiny anomalies in the current waveforms. When the motor is too small, the error range of the current sensors begins to interfere with those measurements. If you are running something like a pump or fan and can use simple V/Hz control, it shouldn't matter at all. I have run a 10HP motor with a 200HP VFD in an emergency, but I had to install an external Overload Relay, because the 200HP VFD would not allow the motor OL current setting to be lower than 90A (the motor FLA was 13A)

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  • \$\begingroup\$ That flew over my head, I had not considered contacting Mitsubishi. My supervisor mentioned that kind of derating, but I wanted to stick with matching the VFD to the motor. However, it is an expensive VFD and we want to use it rather than leaving it to collect dust. \$\endgroup\$ – Akasaka Oct 16 '17 at 17:35
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Some VFDs do not accept very large mismatch between nominal current and lowest possible nominal current of the motor. This issue is regarding the resolution of current measurement range. But let's say that your VFD is able to set nominal motor current such lower.

Another way to increase current and maintain identical conditions is to wire the motor in delta if the motor is designed for 230/400V star/delta. But I guess a 15HP is probably 400/650V star/delta. Anyway, if this possibility exists, then you may adapt the V/f characteristics, so that motor will get 230V output at nominal frequency (50hz) and 400V at \$\sqrt{3}\cdot f_{nom}\$ (87Hz). The output power is increased from 15HP to \$\sqrt{3}\cdot 15HP\$ at 87Hz, as well the output current.

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15 HP = 11 KW. You have a VFD which can drive motors upto 30 KW, with atleast 10% overload capacity. You can configure the motor data, according to the 15 HP motor, Most brands of VFDs accept lesser rated motor data. Be sure to set the current limits (Stall current, Limiting current etc) There wont be any issues if you enter motor parameters strictly according to the Motor rating plate unless the Mitsubishi vfd accepts the 11 KW motor parameters.

Also remember to carry out reforming of DC link capacitors , if the VFD is kept unpowered for more than 2 years.

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