TUTORIAL

TITLE: BASIC CONCEPTS

Department of Industrial Electric Power Conversion Basic Concepts

Page 2 of 2

Power

1. A calculator with an internal 3-V battery draws 0.4 mW when fully functional.

(a) What is the current demand from the supply? Ans: 133µA

(b) If the calculator is rated to operate 500 h on the same battery, what is the

ampere-hour rating of the battery? Ans: 66.5mAh

2. A small, portable black-and-white television draws 0.455A at 9V.

(a) What is the power rating of the television? Ans: 4.095W

(b) What is the internal resistance of the television? Ans: 19.78Ω

(c) What is the energy converted in 6 h of typical battery life? Ans: 88452J

3. If a home is supplied with a 120-V, 100-A service, find

(a) the maximum power capability. Ans: 12kW

(b) Can the homeowner safely operate the following loads at the same time?

I. 5-hp motor

II. 3000-W clothes dryer

III. 2400-W electric range

IV. 1000-W steam iron Ans: 10.13kW < 12kW (Yes)

Efficiency

4. The motor of a power saw is rated 68.5% efficient. If 1.8 hp are required to cut a

particular piece of lumber, what is the current drawn from a 120-V supply? Ans: 16.34A

5. A motor is rated to deliver 2 hp.

(a) If it runs on 110 V and is 90% efficient, how many watts does it draw from the

power line? Ans: 1657.78W

(b) What is the input current? Ans: 15.07A

(c) What is the input current if the motor is only 70% efficient? Ans: 19.38A

6. A 2-hp motor drives a sanding belt. If the efficiency of the motor is 87% and that of the

sanding belt 75% due to slippage, what is the overall efficiency of the system? Ans:

65.25%

Energy

7. In 10 h an electrical system converts 500 kWh of electrical energy into heat. What is the

power level of the system? Ans: 50kW

(a) If the applied voltage is 208 V, what is the current drawn from the supply?

Ans: 240.38A

(b) If the efficiency of the system is 82%, how much energy is lost or stored in

10h? Ans: 90kWh

8. What is the total cost of using the following at €0.09 per kilowatthour? Ans: €2.19

An 860-W air conditioner for 24 h, a 4800-W clothes dryer for 30 min, a 400-W washing

machine for 1 h and a 1200-W dishwasher for 45 min.