VFDs & 3 Phase Motors

[Bob La Londe]

VFDs & 3 Phase Motors

All-righty then. This may sound stupid, but when you run a 3 phase motor off of a VFD what is your speed range relative to the original speed of the motor?

Approximately of course. Mileage will vary I am sure.

[keebler303]

Theoretically you can go 0 speed to somewhere around 5 times the rated speed.

Realistically, if you go too slow, your motor will not have adequate cooling and will overheat. If you go too fast, your bearings will fail early and you will have increased vibration.

A general purpose motor is generally good for constant torque down to 1/4 of rated speed. You can go slower but the motor will get hotter and will not be putting out nearly as much power. You can go up to the maximum speed of the motor with constant horsepower (decreasing torque with increasing speed).

If you need a wide range, a blower cooled vector duty motor and closed loop vctor drive will give you full torque from zero to rated speed and full horsepower to around 2x rated speed.

Matt

[Al_The_Man]

RPM is dependent on applied frequency.
Most 3ph induction motors are 2pole or 4 pole.
The base speed of a 2 pole motor operating on 60Hz will result in something under 3600rpm, a 4 pole just under 1800rpm at no load.
On 50Hz this will be <3000 & 1500 respectively.
If running a 4 pole non-vector rated motor on a VFD, you may not want to go much above 120Hz (x2 base rpm).
With a 2 pole non-vector motor, it is most likely already running near the maximum rpm it is designed for, so use caution in going over 60Hz with a VFD.
The minimum rpm is dependent on whether the VFD has Vector angle detection or better still pulse or encoder feedback.
Vector rated motors, check with the manuf.
Some high quality Vector VFD's will go down to full torque at 1 rpm.
Here endith the lesson
Al.


.