Electrical/Motor Question

Hello,

I'm trying to get a blower motor to run in the field. I had an electrician provide 100A single phase 230v service. This service was specified by others.

I have two motors, a PLC and two starters, HOA switches, etc.

My PLC and all associated I/O are working as expected. My lower amp-draw motor works perfectly.

My problem, one of the two motors (higher current draw) trips it's breaker about 10 seconds after running.

The motor specs:
single phase
230v
21 A (28 FLA)
5.5 HP

It's driving motor starter:
50A Max
32-42A Thermal overload

It's driving Breaker:
32A D-Curve, 2-pole

First, I'm no electrician. I am wondering if someone could share a "rule of thumb" regarding sizing breakers/starters/overloads for motors. I would say I was a little careless here.

Second, Can I bypass the 32A breaker and use the starter with thermal overload for a few days until a new breaker arrives?

Third, if I really should use a breaker always, can I substitute the 2-pole for a 3-pole, and only use L1 and L2?

Thanks All!!!!!!

1) 'if I really should use a breaker always...'. Absolutely.

2) What do you mean by 'can I substitute the 2-pole for a 3-pole, and only use L1 and L2'? Isn't this a single phase 2 pole system?

Bypassing a breaker and going to a 2 pole can't be recommended here. You are making a broad assumption that everything else is okay, and only a sizing issue exists. If you are wrong, you burn up the motor and maybe burn the place down.
I don't see a sizing problem, other than the overload is too big.
If you're no electrician, then find one! This is what they do for a living.

Hi keithkyll,

Yes, I would like to have an electrician out here, but that's not an option right now (long story)

I have a single phase system. I am using two hot wires to the motor, therefore, I am using a 2-pole breaker.

A local electrical distributor has a 3-pole breaker in-stock and told me that I could substitute it (buy it from them) for my 2-pole. I am using a DIN rail mountable breaker. I am also trying to learn to size these in the future so I know what to ask for.

I'm trying to be careful by asking questions. I do not want any safety issues, of course.

I am trying to get my electrician back today but it looks impossible.

Thanks,

Tim

I would first look at the overload trip setting of the breaker. The overload trip setting is adjustable. The breaker should be sized for 125% of namepalte current.

There is nothing prohibitive about leaving one pole in a breaker unused - although the breaker contacts are connected, each pole trips individually.
So you can place a three pole breaker on a single pole application and use only two of the three poles in it. I would not personally make that a permanent installation however.

Now to the root of the problem. Have you placed an ampmeter on the motor to see how much current it is drawing? You need to first know how much current the motor is drawing but here are a few possibilites,

1. a problem with the breaker, some breakers, sometimes even new ones, are weak.
2. the breaker is undersized
3. the motor is undersized for the load.
4. Problem with the motor

edit:

I am using a DIN rail mountable breaker

Click to expand...

Be careful with those din rail mounted breakers, most of them are only rated as supplemental protectors and are not suitable for motor branch protection.

Your about to smoke something here. If you have a 32A breaker driving a motor rated at 28A and the breaker is tripping in 10 seconds then your motor is severely overloaded.

Your 32A breaker is a little small but not that much (125% higher then motor FLA is the rule). But your overload is too big.

Call a reputable electrician to come have a look at your setup.

28 * 1.25= 35 AMP Breaker overloads should already have the 125% added in so get one rated for 28 to 30 amps.

TimD said:

My problem, one of the two motors (higher current draw) trips it's breaker about 10 seconds after running.

The motor specs:
single phase
230v
21 A (28 FLA)
5.5 HP

It's driving motor starter:
50A Max
32-42A Thermal overload

It's driving Breaker:
32A D-Curve, 2-pole

First, I'm no electrician. I am wondering if someone could share a "rule of thumb" regarding sizing breakers/starters/overloads for motors. I would say I was a little careless here.

Second, Can I bypass the 32A breaker and use the starter with thermal overload for a few days until a new breaker arrives?

Third, if I really should use a breaker always, can I substitute the 2-pole for a 3-pole, and only use L1 and L2?

Thanks All!!!!!!

Click to expand...

Tim, to answer your questions, get an UGLY's book. These have some very good "rules of thumb" but you need to use actual nameplate data for designing equipment.

Second, you do need an electrician out there to help you out and keep you safe. If you don't know what you are doing, then don't touch anything. It's not worth your (or anyone else's) life.

Your motor data is WRONG somewhere. A 5 HP motor on 230VAC pulls about 15 amps. A 7.5 HP (which is what I bet you have) pulls 22 amps at 230 VAC. When using a circuit breaker, the 7.5HP should have a 45A breaker on it. (If it was a 5 HP it should have a 30A). The starter size needs to be a size 1 (which it looks like you have) and the wire size needs to be #10 AWG or larger with a ground.

In addition, you need to understand how a breaker works. If you substitute a 3 phase for a 2 phase (2 pole) breaker, you will not do anything to help you out. If there is excessive current on only 1 leg of a breaker, it will trip.

Finally, there is a lot more to designing a system than putting in breakers and overloads. You must know the available fault current at the panel and all the components in the panel must be rated to handle that current. For example, if you have 40,000A availble, you can not use a 22,000A breaker or starter. You also need the breaker curves to co-ordinate your system. Just because you have a 100A breaker feeding a 45A breaker, under a fault condition, the 100A breaker could trip first causing un-necessary equipment outage. Lastly, you need to be able to reduce the arc-flash incident energy if the system has an unacceptably high amount of energy available. At a minimum, you need to do the calculations to determing the amount of incident energy and label the panel for the appropriate level.

I would suggest in addition to hiring an electrician, you hire an engineer to go over your design and make sure you didn't have anything wrong. There is a lot to doing this right and I think you should take the time and do it right. If this is your design and something is wrong and it gets someone hurt, you are responsible.

Hi all,

OK, first, a BIG thanks for all the replies.

Second, I got in touch with the master electrician at the local sshop we were using, and he is coming out to check, whew.

Third, I always use these DIN breakers. Granted, never on motors this large... I hope this is not an issue, or else I need to rethink things, perhaps use fuses?

Finally, I want to add that my 100A circuit is supplied by a fused disconnect, as well as a 100A breaker at the main panel.

Hi all,

OK, first, a BIG thanks for all the replies.

Second, I got in touch with the master electrician at the local sshop we were using, and he is coming out to check, whew.

Third, I always use these DIN breakers. Granted, never on motors this large... I hope this is not an issue, or else I need to rethink things, perhaps use fuses?

Finally, I want to add that my 100A circuit is supplied by a fused disconnect, as well as a 100A breaker at the main panel.

Thanks!!!

The motor overload relay provides the running overcurrent protection for the motor. These need to be adjusted to match the nameplate amps of the motor (although there are some rare cases when adjustment multipliers need to be considered). Do not oversize this protection.

The breaker provides short circuit protection to the motor therefore it is normally sized at least at 175% of the motor FLA (the NEC allows even higher settings).

Because your breaker waits 10sec before it trips, it appears your motor is not accelerating the load very fast and your breaker is operating in its short time range. There are no standard trip curves for breakers. You need to measure your amps at 10sec (just before the breaker trips) and compare them to the trip curve of your specific/intended breaker.

TimD said:

Third, I always use these DIN breakers. Granted, never on motors this large... I hope this is not an issue, or else I need to rethink things, perhaps use fuses?

Click to expand...

The DIN breakers MUST have a UL489 rating if your are going to use them for motor branch circuit protection, no matter what the motor size is - most DIN rail breakers do not have this rating, but a few do.

Is the blower installed with the ductwork or is it running free air?

Could this be the issue?

http://www.plctalk.net/qanda/showthread.php?t=33130&highlight=fan+overload

Andy

Ok,

The electrician showed up and we talked about the overcurrent issue.

He suggested using 50 Amp time delay fuses (2 poles) instead. He said there is nothing wrong with breakers, as long as they are sized correctly, but I would have better luck getting a fuse block and fuses today.


Thanks for all the advice. I'll avoid this next time by doing more homework and gettting the electricians to wire the motors!

Many Thanks,

Tim

AndyT,

The blower has all piping installed. The one that works, does not...

Tim