Electrical service, 240V; Motor nameplate, 230V

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Jon456

Jon456

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Colorado
There is 3-phase utility service to a remote pump house, used to pump rain and groundwater off a large commercial property. One pump is very old and there is no longer any nameplate on the motor. The second pump is fairly new. Both the old and the new pumps are on (seperate) soft-start controllers. The nameplate on the new pump motor lists the following:

HP: 40
SF: 1.15
Hz: 60
Volts: 230/460
Amps: 100.0/50.0
SF Amps: 117.0/58.0
PF: 84.3

According to POCO, the service is 240V 3-phase delta with 208 stinger leg.

Yesterday, I read a post (which I can't seem to find now) in this forum that stated the service must match the nameplate data. So now what?
 
Your 230 nameplate motor will be fine on the 240 volt system.
The 208 hi-leg stinger has no effect on your 3 pahse loads.

(sorry Bob, stepped on you)
 
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Thanks for the quick replies and reference information. POCO reps have been out at the pump house on numerous occassions: once when the pole transformer failed during a winter storm and had to be replaced; and a number of times to try to diagnose phase imbalance problems that continue to plague the pumps. I would have thought someone would have said something by now if the installation was wrong. But that post I read yesterday got me worried.
 
Some of the far more learned than I can explain what I am referencing, but over the years I have encountered several pump situations which have 2 pot
240/120 delta 3 phase supplies which for reasons beyond my comprehension reek havoc with pumps, especial submersible pumps.
We had a water-crest farm in our area that had 20 or 30 pumps that were constantly going out. The brainiacs from TVA were called in and concluded that due to the nature of the loads, only a "true" 3 phase system with 3 phase primary and 3 pots would alleviate the problem. At much expense POCO changed the system and the problems disappeared.
The engineers had a great explaination which was way over my head,
 
Jon456 said:
Thanks for the quick replies and reference information. POCO reps have been out at the pump house on numerous occassions: once when the pole transformer failed during a winter storm and had to be replaced; and a number of times to try to diagnose phase imbalance problems that continue to plague the pumps. I would have thought someone would have said something by now if the installation was wrong. But that post I read yesterday got me worried.
Click to expand...

Don't know who the POCO reps were, but they may very well have been lineman. Most lineman fail to realize that an open delta is only good for 87% of transformer bank rating. Normally the larger pot is used for lighting loads (120/240) So if you had a 10 kVa and a 15kVa XF, the kva rating for the bank would be about 22 kVa. Once you start loading these banks down, as many engineers like to do to control line loss, you start to have voltage imbalances that cause the motors to heat up excessively, thus shortening their life. Another thing that adds to the voltage imbalance is improperly matched impedances in transformer banks. this just adds to the voltage imbalances. Aren't motors rated for about 1% voltage imbalance anyway? A 2 pot bank is generally saved for light 3 phase loads along with single phase loads. A lot of POCO's will try to stretch out these banks with v phase feeding them. Your motors are cheaper than the POCO's third phase and the additional XF, especially if the nearest third phase is 3 or 4 miles down the road. When we have a high leg pot go out on a large load, I like to upsize the XF based on the demand reading on the meter. A larger XF is also cheaper than a third phase if the load isn't too large.
 
I'd be surprised if you encountered anything different! It's a rule that appliances have different 'nominal' ratings than the electrical service.

Let's look at a typical clothes dryer as an example.

The household supply is described as 120/240. These are the PoCo nominal ratings.

The NEMA norm is for the motor rating - the nameplate marking- to be 230v. Relax, though; NEMA also expects that motor to operate without harm at any voltage within 10% of the nameplate voltage.

Oddly enough, the plug for the dryer has a nominal rating of 250v., following a different NEMA standard. Go figure.
 
The voltage differences you are observing are actually industry standards. There are two standard ANSI voltage potentials, the "Distribution Voltage", meaning the voltage that the power utility distributes to customers, and the "Utilization Voltage", meaning the design voltage used by end-point equipment manufacturers for things like motors, lighting, power supplies etc. The Utilization Voltage levels are always lower than the Distribution Voltages because they have to assume a certain amount of voltage drop from the main transformer to the end point load. For for a 240V Distribution system, the Utilization voltage is 230V, same issue for 480/460V, even 120/115V.

208V is kind of an odd duck, most 230V equipment will work fine on 208V, but has no tolerance for added VD because 230-10% is 207V, so it's only 0.05% of 208. Because of this, some motor mfrs have a tripple ratred motor, 208-230/460. What they do behind the scenes is design the windings as 220/440V but with added tolerance, so it's 220 +-15% instead of 230+-10%. That way the 220-15% is the same as 208-10%, and the 440+ 15% is the same as 460+10%. Otherwise, the official Utilization Voltage for a 208V system is 200V.
 
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