Scenario - install blower package with a 20hp 480v 3Ph Blower motor and a 1/3hp 480v 3ph cooling fan motor. Is it OK to feed the 1/3hp motor tapping off the main motor with a set of inline fuses to protect the smaller motor. The smaller motor will start when the bigger motor is started. Both are in a cabinet that is accessible with a cabinet key. I say this is not good practice but I am not sure of the code requirements. Help?
Tapping off a large motor to feed a smaller motor using only one starter
- Thread starter MontanaIR
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It's "legality" may rest in some details not provided.
If it is a listed control cabinet, prewired in that manner then the NRTL listing covers the design.
If it's a listed control cabinet and you are adding the second set of fuses, you would be altering a listed piece of equipment.
If it's a field design, if your disconnect means meets 240.40/430.112, and your fuses are sized to provide 430.32 Overload protection and 430.52 short circuit protection, the only "violation" I can think of would be an increase in HP over the starter rating.
I have seen similar arrangements in listed control cabinets, but, for the most part the ones I have seen provided a separate overload device and tapped off the contactor ahead of the 12sdt motor's OL device.
If it is a listed control cabinet, prewired in that manner then the NRTL listing covers the design.
If it's a listed control cabinet and you are adding the second set of fuses, you would be altering a listed piece of equipment.
If it's a field design, if your disconnect means meets 240.40/430.112, and your fuses are sized to provide 430.32 Overload protection and 430.52 short circuit protection, the only "violation" I can think of would be an increase in HP over the starter rating.
I have seen similar arrangements in listed control cabinets, but, for the most part the ones I have seen provided a separate overload device and tapped off the contactor ahead of the 12sdt motor's OL device.
petersonra
Senior Member
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- Northern illinois
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- engineer
I think the general concept is OK. The devil is sometimes in the details though.
I would want an extra set of overloads on just the fan so that if the fan quits the main motor is also shut off, but I do not think this is actually required.
I would want an extra set of overloads on just the fan so that if the fan quits the main motor is also shut off, but I do not think this is actually required.
Tapping off a large motor to feed a smaller motor using only one starter reply
Tapping off a large motor to feed a smaller motor using only one starter reply
Thanks Augie47 and Petersonra. Both answers helped. I forgot to mention the cabinet is just a sound deadening enclosure as the blower is a rotary lobe type that is extremely noisy. The cabinet itself is not designed to be an electrical enclosure. It is a metal case with sound deadening foam lining it. I am not comfortable running the small motor off of the larger motor T-leads even with fused protection. Thanks again. Any other information or ideas are welcome.
Tapping off a large motor to feed a smaller motor using only one starter reply
Thanks Augie47 and Petersonra. Both answers helped. I forgot to mention the cabinet is just a sound deadening enclosure as the blower is a rotary lobe type that is extremely noisy. The cabinet itself is not designed to be an electrical enclosure. It is a metal case with sound deadening foam lining it. I am not comfortable running the small motor off of the larger motor T-leads even with fused protection. Thanks again. Any other information or ideas are welcome.
From what I understand that you are doing you have to target the overload protection of both motors by using a dedicated OLR sized for each motor. The contactor should be sized to handle both motors. I would connect each OLR to the load side of the contactor feeding the individual motors. The wire the N/C contact in series with on another and in series with the coil of the contactor. The only other concern is the SC protection which is usually provided with the disconnect on the line side of the contactor using either fuses or an MCP. To provide adaquate protection for the small motor may be a bit of a problem when you consider nuisance tripping due to the larger motor is you try to down size for the smaler motor.
As such you may consider supplimentary protection for the smaller motor on the load side of the contactor which feeds the smaller motor to cover the SC protection as you had suggested. You may also use fuses if the motor MFR specifies a give fuse for OL protection. However, depending upon what function the smaller motor is doing my concern would be if the 1/3hp motor takes out a fuse there is no may to shut down the larger motor. WIth 2 N/C OLRs contacts wired in series if either trips the contactor wil drop out.
kwired
Electron manager
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- NE Nebraska
Motor overload on the fan is required, but does not have to interrupt anything but the fan. The main motor should have its own overload.
Shutting down the main motor if the fan should quit is a design issue. Most of the time I would likely do this with an aux contact on the fan controller wired in series with the controls to the main motor. For a multimotor machine it is sometimes common for all motor overloads, or associated groups of motors to all have their overloads in series with a portion of the controls instead of directly controlling its associated motor controller.
Interesting. As I understand it you are proposing 1 contactor to control both motors and an dedicated OLR for each motor.
Should the OLR for the smaller motor trip what is used to take that motor off line? In order to do so would 2 separate starters be required?
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petersonra
Senior Member
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- Northern illinois
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- engineer
This seems like a lot of effort to save a small amount of money.
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
You can feed the 1/3HP motor from the same circuit without issue, but you must provide two circuit elements that would be SPECIFIC to the 1/3HP motor, because these circuit elements for the 20HP motor are way to big for the 1/3HP motor:
1) Overload protection
2) Branch short circuit protection.
You can accomplish this with one of those new little IEC style Manual Motor Starters, they are a combination of a disconnect device, a motor circuit protector and an overload relay all in one so they provide everything you need except the on-off capability (other than manual). Since the intent is that the 1/3HP motor comes on at the same time as the 20HP motor when you close the bigger contactor, you do not necessarily need a separate contactor for the 1/3HP motor.
If you are using an Across-The-Line (FVNR) NEMA size 2 starter for the 20HP motor, you can feed the 1/3HP motor from the T leads, because a NEMA size 2 starter is good for 25HP. But you have to understand that by tapping on the LOAD side of the OL relay on that starter, you are ADDING the motor amps of that 1/3HP motor to the amps that the overload relay of the 20HP motor is seeing. So in essence, the OL relay for the 20HP motor would trip earlier if the 1/3HP motor is running. But there are no allowances for this in selecting and sizing the OL heaters or settings, the settings for the 20HP motor must still be based on the 20HP motor or you ruhn the risk of damaging it. So it's just something you will have to live with but on a blower, I wouldn't lose any sleep over it.
If you are using an IEC contactor rated for 20HP exactly, there is very little overhead in the sizing of those devices so unless it too is rated for a slightly higher amount, I would not connect it that way. The contactor will need to be able to handle the Locked Rotor Current of both motors when they start up, and IEC contactor sizing is not very forgiving. In that case, I would tap in to the LINE side of the 20HP starter and have a completely separate full starter for the 1/3HP motor.
1) Overload protection
2) Branch short circuit protection.
You can accomplish this with one of those new little IEC style Manual Motor Starters, they are a combination of a disconnect device, a motor circuit protector and an overload relay all in one so they provide everything you need except the on-off capability (other than manual). Since the intent is that the 1/3HP motor comes on at the same time as the 20HP motor when you close the bigger contactor, you do not necessarily need a separate contactor for the 1/3HP motor.
If you are using an Across-The-Line (FVNR) NEMA size 2 starter for the 20HP motor, you can feed the 1/3HP motor from the T leads, because a NEMA size 2 starter is good for 25HP. But you have to understand that by tapping on the LOAD side of the OL relay on that starter, you are ADDING the motor amps of that 1/3HP motor to the amps that the overload relay of the 20HP motor is seeing. So in essence, the OL relay for the 20HP motor would trip earlier if the 1/3HP motor is running. But there are no allowances for this in selecting and sizing the OL heaters or settings, the settings for the 20HP motor must still be based on the 20HP motor or you ruhn the risk of damaging it. So it's just something you will have to live with but on a blower, I wouldn't lose any sleep over it.
If you are using an IEC contactor rated for 20HP exactly, there is very little overhead in the sizing of those devices so unless it too is rated for a slightly higher amount, I would not connect it that way. The contactor will need to be able to handle the Locked Rotor Current of both motors when they start up, and IEC contactor sizing is not very forgiving. In that case, I would tap in to the LINE side of the 20HP starter and have a completely separate full starter for the 1/3HP motor.
kwired
Electron manager
- Location
- NE Nebraska
Sorry if you misunderstood, I was not necessarily proposing 1 contactor, though I suppose it could be possible. Like Jraef said two things are necessary for each motor, branch circuit ground fault/short circuit protection, and overload protection. If both are provided you are good to go even if there is only one contactor involved. In the example you just gave if the OLR for the smaller motor would trip then the OLR would need to be in series with the contactor coil and take both motors off line when it trips. Still NEC compliant, as long as the smaller motor also has short circuit and ground fault protection somehow.
It is of my opinion that this would be the way to go. The question would be if you wanted the larger motor to continue running if the smaller motor's OLR trips or the other way around. But the fact remains that should the GF/short protective device trips you would have to add an aux smitch to that device should it be desirable for the contactor open also if you find it necessary to shut down the larger motor atathe same time.
Bottom line is that it depends upon how you want the 2 motors to be controlled together, at the same time or separately.
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