Frequency drive

[laketime]

Can you supply a 3 phase 480v motor with 3 phase 208v through a frequency drive and have it still work?

 

[GeorgeB]

Rated torque to about 40% speed, 40% power to 100% speed in most cases. If the motor can be wired for 240V supply, you can get significant improvement ...

 

[Hameedulla-Ekhlas]

Can you supply a 3 phase 480v motor with 3 phase 208v through a frequency drive and have it still work?

For motors there is only 5% voltage drop is allowed and each motor nameplate also shows it.
First of all you are not allowed to do such a work. Suppose, such a work has been done but notice that motor may not work properly and its age comes down.

 

[iwire]

For motors there is only 5% voltage drop is allowed

Where can I find that rule?

 

[stevenj76]

What is the voltage of the DC Bus?

 

[Hameedulla-Ekhlas]

Where can I find that rule?

Voltage regulation standards in North America vary from state to state and utility to utility. The national standard in the U.S.A. is ANSI C84.1. Voltage regulation requirements are defined in two categories:


•Range A is for normal conditions and the required regulation is ? 5% on a 120 volt base at the service entrance (for services above 600 volts, the required regulation is -2.5% to +5%).
•Range B is for short durations or unusual conditions. The allowable range for these conditions is -8.3% to +5.8%. A specific definition of these conditions is not provided.

Other countries have different standards. For example, IEC 61000-2-2 mentions that the normal operational tolerances are ? 10% of the declared voltage. This is the basis of requirements for voltage regulation in EN 50160 for the European Community. EN 50160 requires that voltage regulation be within ? 10% for 95% of the 10 minute samples in a one week period, and that all 10 minute samples be within -15% to +10%, excluding voltage dips.





Effects of low voltage. When you subject a motor to voltages below the nameplate rating, some of the motor's characteristics will change slightly and others will change dramatically. To drive a fixed mechanical load connected to the shaft, a motor must draw a fixed amount of power from the line. The amount of power the motor draws has a rough correlation to the voltage 2current (amps). Thus, when voltage gets low, the current must increase to provide the same amount of power. An increase in current is a danger to the motor only if that current exceeds the motor's nameplate current rating. When amps go above the nameplate rating, heat begins to build up in the motor. Without a timely correction, this heat will damage the motor. The more heat and the longer the exposure to it, the more damage to the motor.

Low voltage can lead to overheating, shortened life, reduced starting ability, and reduced pull-up and pullout torque. The starting torque, pull-up torque, and pullout torque of induction motors all change, based on the applied voltage squared. Thus, a 10% reduction from nameplate voltage (100% to 90%, 230V to 207V) would reduce the starting torque, pull-up torque, and pullout torque by a factor of .92.9. The resulting values would be 81% of the full voltage values. At 80% voltage, the result would be .82.8, or a value of 64% of the full voltage value. What does this translate to in real life? Well, you can now see why it's difficult to start "hard-to-start" loads if the voltage happens to be low. Similarly, the motor's pullout torque would be much lower than it would be under normal voltage conditions

Motors will tolerate a certain change in voltage above the design voltage. However, extremes above the design voltage will cause the amperage to go up with a corresponding increase in heating and a shortening of motor life.



Rules of Thumb for High and Low Voltage

• Small motors tend to be more sensitive to overvoltage and saturation than do large motors.

• Single-phase motors tend to be more sensitive to overvoltage than do 3-phase motors.

• U-frame motors are less sensitive to overvoltage than are T-frames.

• Premium efficiency Super-E motors are less sensitive to overvoltage than are standard efficiency motors.

• Two- and 4-pole motors tend to be less sensitive to high voltage than are 6- and 8-pole designs.

• Overvoltage can drive up amperage and temperature even on lightly loaded motors. Thus, high voltage can shorten motor life even on lightly loaded motors.

• Efficiency drops with either high or low voltage.

• Power factor improves with lower voltage and drops sharply with higher voltage.

• Inrush current goes up with higher voltage.

 

[norcal]

Since when did VFD's boost voltage? I was under the assumption that they were only were operated within a certain percentage of the input voltage, with the exception of some smaller drives (Under 1 HP) that have a 120V input & a 240V 3? output.

 

[charlietuna]

YES----------------- I have a 120 volt(single phase) input to a drive and it runs a 480-three Phase motor.

 

[winnie]

Variable frequency drives generally do not boost their input voltage. Some 'line regen' drives do a slight boost, but for the most part the internal 'DC rail' voltage is set by simply rectifying the input AC voltage.

The output voltage is thus pretty well limited by the DC rail voltage, to the input voltage or less. If you have a 208V input your maximum output voltage will be approximately 208V.

The voltage required by the motor changes with operating frequency. Since we are talking about operation with a variable frequency drive, we have to consider how the voltage required by the motor will change. That '480V' motor needs 480V at 60Hz. At 30Hz that motor will only need about 240V. Additionally, with a VFD you can safely operate a motor at lower 'V/Hz' than nominal as long as your operation can tolerate the reduced output torque capability. All variable frequency drives adjust _both_ output frequency and voltage.

So a normal VFD with 208V input will happily drive a 480V motor with full torque capability up to about 26Hz, and up to about 50Hz with reduced torque capability. (As GeorgeB says in post 2)

-Jon

 

[laketime]

So how do you determine if the VFD will work for the customers needs with this motor?

 

[charlietuna]

You can order a VFD for most any source voltage to support whatever voltage motor you have. As i said i have a 480 volt three phase motor i run off my household kitchen appliance circuit! Thats the way it was ordered.