Question about VFD

chknifemaker

chknifemaker

Joined
Jun 30, 2013
Messages
207
If I had a 3 phase motor that ran at 1800 rpm will a VFD make it run a little faster or does a VFD only make a motor go slower?
Thanks
 
The KB VFD's can be set to run the motor at normal or 2x speed. I'm not sure about the other ones.
 
You set it and leave it. You don't switch it back and forth.
 
Though it really doesn't matter that much, my understanding is that the motor will have the most available torque at its "native" RPM, so though you are doubling the speed, you do have a little less torque as you increase rpm above 1800. On my setup, I used pulley size to get the belt speed I wanted at 1800rpm and then used the VFD to slow the motor. This gives me the most torque when hogging at high RPM, though I do lose torque with low RPM. In fact, if I go too slow for too long (with the belt really crawling), I end up getting an error on my VFD and have to reset.

--nathan
 
Almost all VFDs can run faster than the nominal mains frequency.

Simplifying things a bit;

The voltage varies with the frequency from Zero to the motor base frequency and the motor runs at constant available torque from minimum to the base (rating plate) frequency. The Torque available is directly related to the current (Amps). Power is Volts times Amps.

Once it reaches the design frequency/voltage, it stops increasing the voltage (because it is already at the maximum the motor can safely take) and only increases the frequency. The motor speed continues to increase, but the torque available reduces as the speed goes up, giving constant available power. (power is torque times RPM, so torque is power/RPM)

The simplest V/Hz VFDs have a linear relationship between Voltage and Frequency and will generally run a 4-pole motor (rated for about 1500/1800 RPM on 50/60 Hz mains) from 10 Hz to 120 Hz (about 300 to 3600 RPM). Below about 10 Hz, they often start to feel "coggy".

Sensorless Vector drives have some fancy electronics that calculate the power factor of the motor on the fly and tweak the output Voltage to maintain the rating plate power factor. These are able to run smoothly down to very low frequencies.

It is worth mentioning that the torque reduction above rated speed is not necessarily a bad thing. If you have a 2 HP 1800 RPM motor running to 3600 RPM on a VFD and a 2 HP 3600 RPM motor running to 3600 RPM on a VFD, the torque at 3600 RPM will be the same on both motors. Below 3600 RPM, there will be more torque available from the 1800 RPM motor at all speeds.

Electrically, it is also possible to drive a 3600 RPM motor above its design speed (120 Hz would give 7200 RPM). However, most motors are only designed to run to 3600 RPM (it's down to manufacturing economics; only the static windings need to change to get an 1800 RPM and a 3600 RPM variant in the same casing. The bearings, etc will be the same for both) and doing this takes you into unknown territory mechanically. I would not recommend it.

Once programmed to run from 0 or 10 Hz to 120 Hz, the control potentiometer provides adjustment within this range.

On most VFDs I have used, the default maximum frequency is the mains frequency. It's just a safety thing to stop anyone overspeeding a machine when they first fire it up. To go faster, you just have to change the setting.
 
With doubling of the motor speed with the 2x option, you will have half the torque at the 2x speed and full torque at the motor plate speed. This info is from the kb ac drive manual.
 
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