VFD used instead of multi speed winding

[bpk]

I don't think this would create any issue but wanted to check with the experts first. I have two motors driving centrifugal fans on an AHU unit, supply and return air. They are both dual speed windings, using 2 speed magnetic starters. If I was to remove the magnetic starters do you see any issues with connecting these motors to a VFD using just the high speed windings? The motors are identical with continuous duty ratings, and 1.15 SF.
The only reason Im not just leaving as is, is due to the interlock tabs being broken off of each existing starter - I would think being the motors are only 3hp each, a single VFD would be more economical than replacing both dual speed starters- or trying to find parts.
Thanks in advance

 

[EC Dan]

You need to verify that the motors are inverter-duty rated.

 

[Jraef]

You need to verify that the motors are inverter-duty rated.

Other than this issue it would be fine. If this is 240V or 208V even this is not much of an issue.

Also... assuming 3 phase. While there are VFDs for single phase motors not all single phase motors can be run from a VFD.

 

[ptonsparky]

I'm curious as to why they have interlocking tabs. Is it one or the other, not both?

You would still need to have independent overloads for each motor plus do all the control to ensure things work as they should. Not impossible by any means, but once you change it, you've bought it.

 

[bpk]

I'm curious as to why they have interlocking tabs. Is it one or the other, not both?

You would still need to have independent overloads for each motor plus do all the control to ensure things work as they should. Not impossible by any means, but once you change it, you've bought it.

Each motor is totally independent, but controlled exactly the same. The starters have the interlocking tabs just the same as a forward/reversing starter to ensure both sets of windings cant be energized at the same time.

 

[Besoeker3]

Could the low speed higher torque be an issue?

 

[tom baker]

The vids may need to have a minimum speed set, as the motors may need a certain cfm for cooling, if the motors are in the ductwork

 

[Jraef]

Could the low speed higher torque be an issue?

If he were using a gear or belt reduction, it could be. But if he were using a 2 speed motor, the low speed would be less (or worst case, the same) torque as the high speed.

 

[Jraef]

You would still need to have independent overloads for each motor plus do all the control to ensure things work as they should. Not impossible by any means, but once you change it, you've bought it.

Good catch, I missed the "single VFD" aspect, so yes, there would need to be separate OL relays for each motor down stream of the VFD, wired so that if either motor overloads, it would shut down the VFD. An alternative would be to use the little IEC style Manual Motor Protectors as the OL device, because they will trip and open only the individual overloaded motor.

 

[Besoeker3]

If he were using a gear or belt reduction, it could be. But if he were using a 2 speed motor, the low speed would be less (or worst case, the same) torque as the high speed.

I don't quite follow that. Why would the low speed be less or the same torque?

 

[Jraef]

I don't quite follow that. Why would the low speed be less or the same torque?

There are 3 different types of 2 speed motors; Variable Torque, Constant Torque and Constant HP. In the VT version, HP is 1/2 or 1/4 at Low speed vs High speed. In the Constant Torque type, the torque remains the same between the two speeds. So in either of those versions, the torque is less than or equal to the High speed value. Fans are Variable Torque loads, meaning the load itself needs less torque at the lower speed, so the simplest form of 2 speed motor for a fan is the VT version. MAYBE there is an offhand chance that they might have used a CT motor(because that's all they had).

Only the Constant HP type would have more torque at the lower speed, and they are rare because they are larger and the most expensive, plus they are totally unnecessary for a fan. They are (were) used on machine tools but have been all but supplanted by VFDs now by just buying a motor with the highest required torque and using the VFD to change speed.

 

[Besoeker3]

There are 3 different types of 2 speed motors; Variable Torque, Constant Torque and Constant HP. In the VT version, HP is 1/2 or 1/4 at Low speed vs High speed. In the Constant Torque type, the torque remains the same between the two speeds. So in either of those versions, the torque is less than or equal to the High speed value. Fans are Variable Torque loads, meaning the load itself needs less torque at the lower speed, so the simplest form of 2 speed motor for a fan is the VT version. MAYBE there is an offhand chance that they might have used a CT motor(because that's all they had).

Only the Constant HP type would have more torque at the lower speed, and they are rare because they are larger and the most expensive, plus they are totally unnecessary for a fan. They are (were) used on machine tools but have been all but supplanted by VFDs now by just buying a motor with the highest required torque and using the VFD to change speed.

Let me try it again. The motors are centrifugal fans from the OP. Thus the lower speed motor would need less torque. But it could have more available torque.

 

[petersonra]

I don't deal with 2 speed motors much but I seem to recall you can replace a 2 speed starter with a reversing starter and an extra contactor.

 

[ptonsparky]

Might be easier to find IEC equipment and build the two speed starters than find a VFD.

 

[garbo]

I don't think this would create any issue but wanted to check with the experts first. I have two motors driving centrifugal fans on an AHU unit, supply and return air. They are both dual speed windings, using 2 speed magnetic starters. If I was to remove the magnetic starters do you see any issues with connecting these motors to a VFD using just the high speed windings? The motors are identical with continuous duty ratings, and 1.15 SF.
The only reason Im not just leaving as is, is due to the interlock tabs being broken off of each existing starter - I would think being the motors are only 3hp each, a single VFD would be more economical than replacing both dual speed starters- or trying to find parts.
Thanks in advance

Have replaced 2 speed motors with a VFD with no problems. Never had a problem with a lot if old ( over 20 years ) motors that were not marked drive rated. Had issues with inspectors & an insurance company where a company had several 7.5 to 20 HP two speed cooling tower fan motors that did not have disconnect switches on the roof. They tried cutting corners by having a stop button on roof. Replaced two speed starters with drives and company maintenance guy was extra happy because v belts lasted at least twice as long with a 20 to 30 second ramp up time that stopped belts from slipping.

 

[garbo]

You need to verify that the motors are inverter-duty rated.

Have been hearing that forever. At the large hospital that I retired from had drives installed on just about every motor 5 HP & higher. Original motors date back to 1962 when building was built.Had to have over a hundred 30 plus year old motors that were updated to run on VFD'S. with very few burning up. Majority ran 24/7 with no problems. When an old motor did finally burn out they did replace it with a inverted rated energy saver motor.

 

[garbo]

I don't think this would create any issue but wanted to check with the experts first. I have two motors driving centrifugal fans on an AHU unit, supply and return air. They are both dual speed windings, using 2 speed magnetic starters. If I was to remove the magnetic starters do you see any issues with connecting these motors to a VFD using just the high speed windings? The motors are identical with continuous duty ratings, and 1.15 SF.
The only reason Im not just leaving as is, is due to the interlock tabs being broken off of each existing starter - I would think being the motors are only 3hp each, a single VFD would be more economical than replacing both dual speed starters- or trying to find parts.
Thanks in advance

I have installed VFD'S on two speed motors. Just always connected it to the high speed winding. Does any company still make two speeds motors. Back in the 1980's we had problems on several two speed motors on cooling towers. They never installed expensive 6 pole safety switches on these roof top units. Luckily the two speed starters were located indoors close by so we ran conduit from the circuit breakers that supplied power to two speed starters up to roof then feed contactors from the roof disconnects.

 

[GeorgeB]

Have been hearing that forever. At the large hospital that I retired from had drives installed on just about every motor 5 HP & higher. Original motors date back to 1962 when building was built.Had to have over a hundred 30 plus year old motors that were updated to run on VFD'S. with very few burning up. Majority ran 24/7 with no problems. When an old motor did finally burn out they did replace it with a inverted rated energy saver motor.

With 200V or 230V motors (on 208 or 240V systems) I've not heard of many problems. With 460 motors on 480 systems, I understand there are more issues. Which were you using?

 

[winnie]

Let me try it again. The motors are centrifugal fans from the OP. Thus the lower speed motor would need less torque. But it could have more available torque.

Quite correct, it is possible that the two speed motor is designed to have more available torque at the low speed.

But as Jraef notes such a motor is more expensive than a motor that had the same or lower torque available on the low speed winding. Since the load requires less torque at lower speed it is very unlikely that such a motor would have been used.

Jon

 

[garbo]

With 200V or 230V motors (on 208 or 240V systems) I've not heard of many problems. With 460 motors on 480 systems, I understand there are more issues. Which were you using?

All 480 volts. They had two new 1750 HP chillers going in line when I retired. Control panel for each motor was at least 18' long & feed by 13,200 volts to a step down transformers. Drives ran on 4,160 volts. Back in 1980 we were told that they would never have the electronices able to handle 480 volts. I never had more issues with drives on 480 volts. Worked, troubleshoot ,& repaired 480 volt drives up to 250 HP. Tell you one thing when something shorts out on 480 you get a lot more noise. Replaced the what Danfoss called the 6 pack ( IGBT module ) on a 480 volt 125 HP drive. For some reason even though the motor rang & meggered great I wanted to call in the dive tech but boss said hurry up and get it back on line. I placed drive on hand and ramped it up slowly. Before it got to around 20% of full speed got a great big boom that echoed off concrete walls then a lot if smoke. Tech came in and asked me if I replaced the little cheap control board and told him no because I did not know it had one. When the cheap board goes bad it destroys the IGBT'S. Only had thing with 480 volt 3hase drives the DC Buss can be as high as 675 volts DC and some drives take over 20 minutes for large capacitor bank to discharge so nobody waits. Shut off 480 volt power and imediatly start troubleshooting. Got hit several times due to rushing. Besides a say 480 volt drive rated for 100 HP probably a thousand dollars cheaper then a 240 volt drive for same motor horse power. Never got involved with prices. Would send drive salesmen a picture of existing drive or motor nameplate and tell him to send a quote. They have came down a lot in price & size. The first 100 HP drive that I installed around 1984 had a cabinet 3' wide and 6' high now maybe a foot wide and 2.5 ' with a built in disconnect switch.