What does Usable at 208 mean? Can a 230 motor be used at 206v? 4

[TimWPH]

Hey guys,

I have (5) new sidewall propeller exhaust fans with 3HP 115/230 volt, 60 Hz, single phase 1725 Rpm fan motors with a 1.0 service factor. At the bottom of the tag it also says usable at 208 volts 15.5 amps. The system they are operating on is 208 volt 60 Hz.

I am having trouble with several of theese motors never coming up to speed and kicking the breaker and the more we use the fans, more fans start to have the same problem. It started out with (2) doing this and now it is (3). They do not do this every time but after they have cycled on & off a few times they start to kick the breaker and continue to do this every time until cut off and left off for a while.

When they come on & run they will pull about 77 amps for 3 seconds and then come up to speed and pull about 16 amps. When it doesn't come up to speed it will run and pull about 77 amps until the 30 amp breaker supplying it trips. We have checked start & run capacitors and they all check good.

The panel they are being fed from is supplied power by a 460 three phase primary to 208v single phase secondary transformer. Each fan is supplied by a seperate circuit form a breaker panel. All the wiring has been checked and is of suffiecent size according to the NEC and was increased by one wire size every 100 feet as specified by the electrical engineer on this job. We have checked the fans and belts and everything seems to be turning freely.

With the fan not running its circuit is about 213 volts at the fan. With it running at speed it is about 213 volts at the fan. When it comes on and does not come up to speed it reads about 184 volts while it is pulling the 77 amps but the breaker panel the circuit is coming out of does not drop voltage.

I think the motor should be either a true 208 volt or 200 volt motor but the supplier assures me there must be a power problem because the motor says "Usable at 208 volts".

What exacty does usable at 208 mean? Does it mean it can be used at a voltage 10% below 208 and work fine or does it mean it it is O.K. down to 208 volts and below that it is not reccomended?

Any help would be appreciated.

 

[waross]

Hi, jraef; Re your post on capacitors. Possibly the cheapest fix may be to increase the size of the starting capacitor by 50% to 100%. I had an instance where some gate closers were undersized. (pull chain type for rolling gates) The customer bought 1/2 HP but needed at least 3/4 HP for some very heavy gates. The motors would not start to turn at all. Before ordering a pair of 3/4 HP motors, I doubled up the starting capacitors. That was over 10 years ago. I was over to visit the client last week and the gates are still working well.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TimWPH]

Waross,

I will try larger start capacitors. Once the motor gets to full speed the voltage rises back to 213 volts.

Thanks to everyone for their time.

 

[waross]

I should mention that my installation was in Honduras and the motors were sourced from the US. no thought of returning the motors, and no loss if they failed in service with the larger capacitors. Fortunately the addition of capacitors was successful. I would expect the fans to start once a day, one of these gates would start a minimum of 8 times a day, 6 days a week.
Let us know how extra capacitors work for you.
Tanks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Compositepro]

Is there an optimum capacitance for motor starting? It seems an infinitely large capacitor would be equivalent to a wire, so why use a capacitor at all? I've wondered about this before.

 

[waross]

A wire will not shift the phase relationship between the voltage and current. A capacitor will.

Bill
--------------------
"Why not the best?"
Jimmy Carter