Change Fix speed motor to variable speed

[Electriman]

Greetings folks,

If I want go from fix speed to variable in a couple of years old motor, what do I need to think of?

I have get a VFD, and change my MCC bucket to feed the VFD instead of motor.

Do I need to change a motor?
Do I need to change the cable from TC cable to VFD cable?

How necessary is to change the motor and cable?

Thank you for your replies in advance.

 

[petersonra]

Greetings folks,

If I want go from fix speed to variable in a couple of years old motor, what do I need to think of?

I have get a VFD, and change my MCC bucket to feed the VFD instead of motor.

Do I need to change a motor?
Do I need to change the cable from TC cable to VFD cable?

How necessary is to change the motor and cable?

Thank you for your replies in advance.

You are asking questions that require more information than you have given to answer.

The motor is maybe OK if it is fairly recent vintage. Many newer motors are inverter duty. Check the motor nameplate or catalog cut. If it is not inverter duty putting it on a VFD will lessen its life by some amount that may be very significant. if you only run the thing a few hours a week it may take so long to kill the motor by putting it on a VFD that as a practical matter it just does not matter.

Unless there are some special circumstances, I would not worry about the cabling too much either. It is again a design decision you have to make based on all the factors that apply to this situation.

If the MCC bucket has a MCP instead of a TM breaker you may need to replace the MCP as well. If it is fused or a TM breaker you may just need to remove the contactor and overload.

 

[Electriman]

You are asking questions that require more information than you have given to answer.

The motor is maybe OK if it is fairly recent vintage. Many newer motors are inverter duty. Check the motor nameplate or catalog cut. If it is not inverter duty putting it on a VFD will lessen its life by some amount that may be very significant. if you only run the thing a few hours a week it may take so long to kill the motor by putting it on a VFD that as a practical matter it just does not matter.

Unless there are some special circumstances, I would not worry about the cabling too much either. It is again a design decision you have to make based on all the factors that apply to this situation.

If the MCC bucket has a MCP instead of a TM breaker you may need to replace the MCP as well. If it is fused or a TM breaker you may just need to remove the contactor and overload.

Thanks

 

[kwired]

What is HP rating of the motor?

What is voltage?

Assuming it is three phase.

How long is the conductors between the motor and drive?

Running a dual voltage motor on 208 or 240 with a VFD isn't as hard on motor insulation in a non drive rated motor then if running it on 480 volts. Line reactor on output side of drive also helps with non drive rated motors.

How slow will you run the motor, especially for extended times - motor not designed to be run at low speeds gets less cooling from it's own cooling fan. Some applications may need external cooling if they will run long periods at low RPM and high torque demand, even with a drive rated motor.

 

[Electriman]

What is HP rating of the motor?

What is voltage?

Assuming it is three phase.

How long is the conductors between the motor and drive?

Running a dual voltage motor on 208 or 240 with a VFD isn't as hard on motor insulation in a non drive rated motor then if running it on 480 volts. Line reactor on output side of drive also helps with non drive rated motors.

How slow will you run the motor, especially for extended times - motor not designed to be run at low speeds gets less cooling from it's own cooling fan. Some applications may need external cooling if they will run long periods at low RPM and high torque demand, even with a drive rated motor.

There are various motors with different hp ratings. But they are all 480 V. But I get your points to look more into detail such as speed.

The length of the cable is 100 ft to couple hundreds ft. How does that affect?

 

[kwired]

There are various motors with different hp ratings. But they are all 480 V. But I get your points to look more into detail such as speed.

The length of the cable is 100 ft to couple hundreds ft. How does that affect?

Too long, possibly even for a motor rated for use on VFD's. Most drives will suggest using line reactors on output side for such lengths.

this article can explain what is going on better than I can.

 

[ggunn]

Originally Posted by Electriman

If I want go from fix speed to variable in a couple of years old motor, what do I need to think of?

In a somewhat related story...

A friend of mine had a recording studio with analog tape machines, and he wanted the vary the speed on one of them. He got a 200W solid state bass guitar amp and one of those old audio oscillators with the big clock face on the front, connected the oscillator to the input of the amp, and connected a voltmeter to the speaker output. He set the frequency on the oscillator to about 60Hz, turned on the amp, and turned up the volume until the VM read 120V. He rigged a 1/4" speaker connector and some wire to a 120V outlet and plugged the tape machine into it. He had recorded a 440Hz test tone on the tape, so he plugged the output of the tape playback into his old Conn Strobotuner and twiddled the oscillator frequency until he got the frequency shift he wanted.

It worked like a charm.

 

[Electriman]

Too long, possibly even for a motor rated for use on VFD's. Most drives will suggest using line reactors on output side for such lengths.

this article can explain what is going on better than I can.

My rule of thumb for having a filter or not is 300' long. I am, however, not sure when I use output filter do I still need to pick VFD cables or no?

 

[Dzboyce]

I’m going to install a line filter if there is 50 feet of distance between the vfd and the motor. At 200 or 300 feet I go to a dvdt filter. At 1,000 feet we are going to a sine wave filter. I use TCI and MTE filters.

 

[Electriman]

I’m going to install a line filter if there is 50 feet of distance between the vfd and the motor. At 200 or 300 feet I go to a dvdt filter. At 1,000 feet we are going to a sine wave filter. I use TCI and MTE filters.

Do you use VFD cable or just regular cable?

 

[Dzboyce]

I have never used special VFD cable yet.

 

[kwired]

I’m going to install a line filter if there is 50 feet of distance between the vfd and the motor. At 200 or 300 feet I go to a dvdt filter. At 1,000 feet we are going to a sine wave filter. I use TCI and MTE filters.

Same here, especially for 480 volts applications. Like I said earlier dual volt motors used on low volt rating- don't see as much standing wave damage to insulation when that wave is around half it would be for 480 volts on same insulation.

I have never used special VFD cable yet.

Me either. Shouldn't matter much for standing wave damage either, is more for minimizing RF emissions from your conductors AFAIK.