motor speed and VFDs

[cvirgil]

This one is an industry "rule of thumb" i hear often.

Can someone explain the reasoning for motors overhetaing when motor speed is reduced to 33% under VFD control.

 

[VicP]

The motor cooling fan is attached to the motor shaft. If the motor is running slowly, the fan does not move much air and the motor overheats. If the motor must run for long periods at slow speed, an external fan is required.

 

[jghrist]

See Motor shaft voltages and bearing currents and their reduction in multilevel medium-voltage PWM voltage-source-inverter drive applications, Fei Wang, GE Ind. Syst., Salem, VA; IEEE Transactions on Industry Applications, Sep/Oct 2000, Volume: 36, Issue: 5

Abstract:
This paper presents test results on problems and solutions of motor shaft voltages and bearing currents in medium-voltage pulsewidth modulated (PWM) drive systems. Tests show that multilevel medium-voltage PWM voltage-source inverter drives can cause motor bearing currents, similar to a low-voltage PWM drives, even with one motor bearing insulated. Common-mode voltages generated as a result of PWM switching are observed on motor windings and capacitively coupled to the motor shaft, leading to bearing currents. Potential solutions, including altering common-mode circuitry, changing the grounding scheme, providing common-mode filtering, and grounding the motor shaft, are investigated. Test results on their effectiveness are presented

Also see:
Modeling of motor bearing currents in PWM inverter drives , Shaotang Chen; Lipo, T.A.; Fitzgerald, D.;
Industry Applications Conference, 1995. Thirtieth IAS Annual Meeting, IAS '95., Conference Record of the 1995 IEEE , Volume: 1 , 8-12 Oct. 1995

Effect of PWM inverters on AC motor bearing currents and shaft voltages, Erdman, J.; Kerkman, R.J.; Schlegel, D.; Skibinski, G.; Applied Power Electronics Conference and Exposition, 1995. APEC '95. Conference Proceedings 1995.,Tenth Annual Issue: 0 , 5-9 March 1995, Page(s): 24 -33 vol.1

Source of induction motor bearing currents caused by PWM inverters, Chen, S.; Lipo, T.A.; Fitzgerald, D.; Energy Conversion, IEEE Transactions on , Volume: 11 Issue: 1 , March 1996, Page(s): 25 -32

Bearing currents and their relationship to PWM drives, Busse, D.; Erdman, J.; Kerkman, R.J.; Schlegel, D.; Skibinski, G.; Power Electronics, IEEE Transactions on , Volume: 12 Issue: 2 , March 1997, Page(s): 243 -252

Minimizing electric bearing currents in ASD systems, Link, P.J.; Industry Applications Magazine, IEEE , Volume: 5 Issue: 4 , July-Aug. 1999, Page(s): 55 -66