50 Hz motors on 60 Hz system

[Isaiah]

We have a large amount of 250kW, 460V induction motors from Italy that show 50 HZ on the name plate (SF of 1 and PF of .89)
They are driven from VFDs, but everything we have is based on 60 HZ… is there an issue with this?


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[zbang]

If the motors are being driven by VDFs, they're probably not seeing 50Hz anyway but whatever the speed requires. The question is whether the VFDs are happy with 50Hz (and they probably are) and whether they know the motor characteristics.

Is this all part of a packaged system or did somebody just decide to buy 50Hz motors?

 

[Isaiah]

If the motors are being driven by VDFs, they're probably not seeing 50Hz anyway but whatever the speed requires. The question is whether the VFDs are happy with 50Hz (and they probably are) and whether they know the motor characteristics.

Is this all part of a packaged system or did somebody just decide to buy 50Hz motors?

Thank you Zbang.
This is part of a vendor packaged system. So I believe the associated VFDs will be fine. Now I have to buy the cable. It’s though finding VFD cable that is non-MC


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[zbang]

Do you specifically need VFD cable? You may not and that opens up lots of possibilities.

 

[Isaiah]

Do you specifically need VFD cable? You may not and that opens up lots of possibilities.

Typically VFD cable needs to be 3/C shielded w/ grounds for harmonics. Do you have any suggestions?


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[JoeStillman]

I assume you mean cables between the drive and motor when you say VFD cables. If it was my design, I'd be using the kind with multiple grounds and overall shield (see Belden). You can use the nameplate amps for VFD cable sizing according to 430.6(C). For the feeders to the drives, see 430.120 &f.

 

[zbang]

Lots of VFDs and motors operate just fine without VFD cable connecting them (e.g using EMT or FMC with stranded THHN inside). There are some 'it depends" in there, but just because there's a VFD involved doesn't automatically mean you need VFD cable. That's the point I'm trying to make. (You'll find at least a handful of discussions about this around the forums.)

Do you need flexible cable at all? We can't tell from here.

 

[JoeStillman]

Lots of VFDs and motors operate just fine without VFD cable connecting them (e.g using EMT or FMC with stranded THHN inside). There are some 'it depends" in there, but just because there's a VFD involved doesn't automatically mean you need VFD cable. That's the point I'm trying to make. (You'll find at least a handful of discussions about this around the forums.)

Do you need flexible cable at all? We can't tell from here.

These motors are pretty big. I'd be concerned about ground current through the frame causing bearing damage. I've seen it happen, even with short cable runs.

 

[petersonra]

VFD cable probably won't prevent bearing current. Belden and other cable suppliers make VFD cables. Your local supply house should be able to get you part numbers and pricing.

 

[JoeStillman]

From www.bearing-news.com

Two Types of Damaging Bearing Currents​

When those spikes of current get to the motor, they don’t go through the windings. Instead, they charge the capacitances inside the motor and then return to the drive. This can cause two kinds of damaging bearing current: High-frequency circulating current in medium-sized motors, 100 hp (75 kW) and up, and rotor ground current, in motors with substandard VFD cable and poor grounding.

Every time the current spikes, there is a risk that those two types of bearing current will flow. And the higher the switching frequency, the more spikes there are.

So if you have a medium-sized motor without an insulated bearing to interrupt circulating current, or if you don’t use a good VFD cable and the motor’s grounding is substandard, then you should use low switching frequencies to minimize the damage due to these currents. Of course, it’s better to use an insulated bearing on medium-sized motors on drives, and always use a good VFD cable, but you can at least reduce the rate at which the damage is done by reducing the switching frequency.

 

[Isaiah]

I assume you mean cables between the drive and motor when you say VFD cables. If it was my design, I'd be using the kind with multiple grounds and overall shield (see Belden). You can use the nameplate amps for VFD cable sizing according to 430.6(C). For the feeders to the drives, see 430.120 &f.

The motor is at 1800 RPM. I’m assuming cable size to the motor should be FLA X 125% - I’m going to be confined to shielded cable due to specifications


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[petersonra]

Here is an example of a vfd cable if you plan to use one.

Belden 29520C 16/3C w/ 3 #18 Grounds 600V XLPE/PVC VFD Cable

Belden 29520C 16/3C w/ 3 #18 Grounds 600V XLPE/PVC VFD Cable . Cut by the foot.

www.wireandcableyourway.com

 

[don_resqcapt19]

These motors are pretty big. I'd be concerned about ground current through the frame causing bearing damage. I've seen it happen, even with short cable runs.

I have installed a number of 500hp and larger motors supplied by VFDs using RMC and XHHW conductors, with no issues. The runs being a few hundred feet long. Also did a couple of 200 hp deep well pumps using non-metallic jacketed MC cable about 1200' long without issues.

 

[zbang]

I'd be concerned about ground current through the frame causing bearing damage.

You can always install shaft grounding devices if this is a problem.

I’m going to be confined to shielded cable due to specifications

That's a different matter.

 

[Isaiah]

I have installed a number of 500hp and larger motors supplied by VFDs using RMC and XHHW conductors, with no issues. The runs being a few hundred feet long. Also did a couple of 200 hp deep well pumps using non-metallic jacketed MC cable about 1200' long without issues.

This is great to know. In our case the conduit is 4”PVC, underground for about 200 feet


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[petersonra]

This is great to know. In our case the conduit is 4”PVC, underground for about 200 feet


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I think with pvc conduit VFD cable is a good idea.

 

[Jraef]

Re: 50Hz 460V motors:

1. Are they really labeled that way on the MOTOR nameplate? I say that because 460V 50Hz is not a standard voltage used in any countries in the world that I am aware of. I think you may be misreading something, or someone mislabeled something, i.e. they created a label for the MACHINE that says this, but that's not what the MOTOR NAMEPLATE actually says.
2. Motors function to produce torque at a specific speed, based on the ratio of V and Hz. So as long as you maintain that ratio, the motor produces the rated torque. Speed then is determined by the frequency and the number of poles in the motor, which (generally) doesn't change, so frequency determines the speed. That's exactly what a VFD does then, changes the frequency to change the speed, then adjusts the voltage to provide the proper V/Hz ratio so that the motor maintains proper torque at that changed speed. So IF the motor nameplate says, for some oddball reason, 460V 50Hz, you just set up the VFD with that as the base motor design and it takes care of the rest for you.
3. The incoming line frequency is totally irrelevant to the VFD, all it does is rectify it to DC. But the VOLTAGE is important, because the VFD cannot increase the voltage above the incoming line. So if the incoming voltage is 460V, the maximum output is 460V, regardless of the output frequency. So if the LINE voltage is 480V 60Hz, you can put out 480V, or 460V or 415V or 400V or 380V at any frequency to match what the MOTOR says it needs.

My rules on cable are:
Metal conduit all the way, FMC at the very end; individual XHHW conductors, triplexed.
PVC conduit; shielded "VFD" cable.
Cable Tray; Shielded tray cable or armored TC.

The shielding is mostly to prevent radiated RFI from the VFD output, think of the variable frequency PWM output signal as if it is a powerful local FM radio transmitter, so the cable becomes the broadcast antenna. If you are in steel conduit, it becomes redundant to the conduit itself, ergo not really necessary. The thing is that outside of North America, steel conduit is not the norm, hence the VFD instruction manuals always saying to use shielded cables.

 

[Isaiah]

Re: 50Hz 460V motors:

1. Are they really labeled that way on the MOTOR nameplate? I say that because 460V 50Hz is not a standard voltage used in any countries in the world that I am aware of. I think you may be misreading something, or someone mislabeled something, i.e. they created a label for the MACHINE that says this, but that's not what the MOTOR NAMEPLATE actually says.
2. Motors function to produce torque at a specific speed, based on the ratio of V and Hz. So as long as you maintain that ratio, the motor produces the rated torque. Speed then is determined by the frequency and the number of poles in the motor, which (generally) doesn't change, so frequency determines the speed. That's exactly what a VFD does then, changes the frequency to change the speed, then adjusts the voltage to provide the proper V/Hz ratio so that the motor maintains proper torque at that changed speed. So IF the motor nameplate says, for some oddball reason, 460V 50Hz, you just set up the VFD with that as the base motor design and it takes care of the rest for you.
3. The incoming line frequency is totally irrelevant to the VFD, all it does is rectify it to DC. But the VOLTAGE is important, because the VFD cannot increase the voltage above the incoming line. So if the incoming voltage is 460V, the maximum output is 460V, regardless of the output frequency. So if the LINE voltage is 480V 60Hz, you can put out 480V, or 460V or 415V or 400V or 380V at any frequency to match what the MOTOR says it needs.

My rules on cable are:
Metal conduit all the way, FMC at the very end; individual XHHW conductors, triplexed.
PVC conduit; shielded "VFD" cable.
Cable Tray; Shielded tray cable or armored TC.

The shielding is mostly to prevent radiated RFI from the VFD output, think of the variable frequency PWM output signal as if it is a powerful local FM radio transmitter, so the cable becomes the broadcast antenna. If you are in steel conduit, it becomes redundant to the conduit itself, ergo not really necessary. The thing is that outside of North America, steel conduit is not the norm, hence the VFD instruction manuals always saying to use shielded cables.

The motor nameplate definitely says 50Hz. And the motor rated voltage is without question 460V since the VFD output is 480V
I am pretty much resigned to using shielded cable since I have PVC conduit and specs call for it


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[retirede]

Re: 50Hz 460V motors:

1. Are they really labeled that way on the MOTOR nameplate? I say that because 460V 50Hz is not a standard voltage used in any countries in the world that I am aware of. I think you may be misreading something, or someone mislabeled something, i.e. they created a label for the MACHINE that says this, but that's not what the MOTOR NAMEPLATE actually says.
2. Motors function to produce torque at a specific speed, based on the ratio of V and Hz. So as long as you maintain that ratio, the motor produces the rated torque. Speed then is determined by the frequency and the number of poles in the motor, which (generally) doesn't change, so frequency determines the speed. That's exactly what a VFD does then, changes the frequency to change the speed, then adjusts the voltage to provide the proper V/Hz ratio so that the motor maintains proper torque at that changed speed. So IF the motor nameplate says, for some oddball reason, 460V 50Hz, you just set up the VFD with that as the base motor design and it takes care of the rest for you.
3. The incoming line frequency is totally irrelevant to the VFD, all it does is rectify it to DC. But the VOLTAGE is important, because the VFD cannot increase the voltage above the incoming line. So if the incoming voltage is 460V, the maximum output is 460V, regardless of the output frequency. So if the LINE voltage is 480V 60Hz, you can put out 480V, or 460V or 415V or 400V or 380V at any frequency to match what the MOTOR says it needs.

My rules on cable are:
Metal conduit all the way, FMC at the very end; individual XHHW conductors, triplexed.
PVC conduit; shielded "VFD" cable.
Cable Tray; Shielded tray cable or armored TC.

The shielding is mostly to prevent radiated RFI from the VFD output, think of the variable frequency PWM output signal as if it is a powerful local FM radio transmitter, so the cable becomes the broadcast antenna. If you are in steel conduit, it becomes redundant to the conduit itself, ergo not really necessary. The thing is that outside of North America, steel conduit is not the norm, hence the VFD instruction manuals always saying to use shielded cables.

Back in the 80s, we had to supply an air compressor with a 460V, 50Hz motor. I can’t remember where it went, but it was an odd duck, for sure. I think we had GE re-nameplate a 440V (maybe 415V?), 50 HZ which dropped the service factor from 1.15 to 1.0.

 

[Isaiah]

Back in the 80s, we had to supply an air compressor with a 460V, 50Hz motor. I can’t remember where it went, but it was an odd duck, for sure. I think we had GE re-nameplate a 440V (maybe 415V?), 50 HZ which dropped the service factor from 1.15 to 1.0.

The service factor on this motor is also 1.0


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