OT:Motor running costs

dfgreap2234 · Jan 7, 2006

Hi all,

Can anyone help me out with a little homework question?

I have three motors (5.5kW) all fed from the same supply, when monitored these are the results produced:

33.3 Amps/phase
7.8 kVA/phase
5.8 kW/phase
5.2 kVAr/phase
0.74 lagging power factor
electricity unit cost 7.5p

I need to know how much each motor costs to run per hour.

As far as I can tell most of the information is un-necessary, and the kVA figure is the only one I am interested in?

So would the running cost per motor per hour be:

7.8 /3 (for number of motors) * 7.5p

or would I need to multiply this by three because there are 3 phases to take into account.

Please let me know if I am way off the mark

PLucas · Jan 7, 2006

dfgreap2234 said:
and the kVA figure is the only one I am interested in?

I assume that your electricity cost is 7.5p per kW, that is what you need to work out.

I will give you a clue, kW is the adjacent side, kVA is the hypoteneuse, power factor is the cosine of the angle.

I have just notice that you quoted a kW reading per phase

5.8 kW/phase

So, the answer might just be 5.8 * 1.73 * 7.5

5.8 = Kw
1.73 = root 3 (for 3 phase)
7.5 = price in pence per Kw/H

You don't state whether these readings were for all 5 motors or just one.

Anyway, hopefully what I have written will give you some help

Paul

dfgreap2234 · Jan 7, 2006

Paul,

Thanks for replying.

The readings are taken from the incoming supply to all of the motors, ie. they cover all three of them.

I thought that with a power factor of less than one. You would be billed for more power usage than you actual need therefore the motors take the kW figure but the supply usage is the kVA figure.

The unit cost is kW/h.

I am sorry I don't really understand alot about the effects of power factor to the supply (yet).

dfgreap2234 · Jan 8, 2006

Or can anyone give me a good internet resource on this topic??

Lancie1 · Jan 8, 2006

dfgreap2234,

To answer the question, you need to know exactly what "electricity unit cost 7.5p" means. Many power companies (here in the US) charge for kW/Hour for residential customers, but for industrial customers (with many small motors) they may bill for kW AND power factor. For example, If the power factor drops below 0.65 for any 15-minute period, then a penalty or surcharge will apply. On the other hand, some US power companies meter the KVA and KW, and have sliding scales over a range for each, which is another way to charge for the power factor.

Perhaps in Wales the rate structure is a known quantity that never varies?

Bottom line: You have to know the local power billing structure in order to determine the actual cost.

darrenj · Jan 8, 2006

Lancie1 said:
dfgreap2234,

To answer the question, you need to know exactly what "electricity unit cost 7.5p" means. Many power companies (here in the US) charge for kW/Hour for residential customers, but for industrial customers (with many small motors) they may bill for kW AND power factor. For example, If the power factor drops below 0.65 for any 15-minute period, then a penalty or surcharge will apply. On the other hand, some US power companies meter the KVA and KW, and have sliding scales over a range for each, which is another way to charge for the power factor.

Perhaps in Wales the rate structure is a known quantity that never varies?

Bottom line: You have to know the local power billing structure in order to determine the actual cost.

Wow .65!! in ontario its anything below .90 for 15 mins..

Lancie1 · Jan 9, 2006

Darren,

Here the rate and power factor charge is dependent on the type of power being purchased. For a local city (Florence, Alabama), the commercial rate has the 0.65 power factor penalty level. For industrial rates, the power factor penalty kicks in at a much higher power factor. Each non-residential customer can negotiate the type of power they wish to purchase. The more firm (non-shedable) the power, the more it costs.

OT:Motor running costs

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