Measuring Instruments--part 3: Ammeters

AC Ammeters

Shunts can be used with AC ammeters to increase their range but cannot be used to decrease their range. Most AC ammeters use a current trans former instead of shunts to change scale values. The primary of the transformer is connected in series with the load, and the ammeter is connected to the secondary of the transformer. Notice that the range of the meter is changed by selecting different taps on the secondary of the current transformer. The different taps on the transformer provide different turns ratios between the primary and secondary of the transformer. The turns ratio is the ratio of the number of turns of wire in the primary as compared to the number of turns of wire in the secondary.

+++++25 A current transformer is used to change the range of an AC ammeter. AC ammeter Alternator Current transformer Load

Calculating the Turns Ratio

In this example, it’s assumed that an AC meter movement requires a current flow of 100 milliamperes to deflect the meter full scale. It’s also assumed that the primary of the current transformer contains five turns of wire. A transformer will be designed to provide full-scale current readings of 1 ampere, = amperes, and 10 amperes. To find the number of turns required in the secondary winding, the following formula can be used:

Np ___ Ns =

Is __ Ip

where

Np = number of turns of wire in the primary

Ns = number of turns of wire in the secondary

Ip = current of the primary

Is = current of the secondary

The number of turns of wire in the secondary to produce a full-scale current reading of 1 ampere can be calculated as follows:

= ___ Ns

= 0.1 A

_____ 1 A

Cross-multiplication is used to solve the problem. Cross-multiplication is accomplished by multiplying the bottom half of the equation on one side of the equal sign by the top half of the equation on the other side of the equal sign.

0.1A N_s = 5A -turns

N_s = 50 turns

The transformer secondary must contain 50 turns of wire if the ammeter is to indicate a full-scale reading when 1 ampere of current flows through the primary winding.

The number of secondary turns can be found for the other values of primary current in the same way:

Current Transformers (CTs)

When a large amount of AC must be measured, a different type of current transformer is connected in the power line. These transformers have ratios that start at 200:5 and can have ratios of several thousand to five. These current transformers, generally referred to in industry as CTs, have a standard secondary current rating of = ampere AC. They are designed to be operated with a 5-ampere AC ammeter connected directly to their secondary winding, which produces a short circuit. CTs are designed to operate with the secondary winding shorted.

A current transformer is basically a toroid transformer. A toroid transformer is constructed with a hollow core similar to a doughnut. When current transformers are used, the main power line is inserted through the opening in the transformer. The power line acts as the primary of the transformer and is considered to be one turn.

+++++26 A toroid current transformer.

+++++27 Toroid transformer used to change the scale factor of an AC ammeter. Alternator; Current transformer; Power line acts as a primary winding of one turn.

+++++28 The primary conductor loops through the CT to produce a second turn, which changes the ratio. Alternator Load

The turns ratio of the transformer can be changed by looping the power wire through the opening in the transformer to produce a primary winding of more than one turn. For example, assume a current transformer has a ratio of 600:5. If the primary power wire is inserted through the opening, it will require a current of 600 amperes to deflect the meter full scale. If the primary power conductor is looped around and inserted through the window a second time, the primary now contains two turns of wire instead of one. It now requires 300 amperes of current flow in the primary to deflect the meter full scale. If the primary conductor is looped through the opening a third time, it will require only 200 amperes of current flow to deflect the meter full scale.

Clamp-On Ammeters

Many electricians use the clamp-on ammeter. The jaw of this type of meter is clamped around one of the conductors supplying power to the load. The meter is clamped around only one of the lines. If the meter is clamped around more than one line, the magnetic fields of the wires cancel each other and the meter indicates zero.

+++++29 (A) Analog type clamp-on ammeter with vertical scale. (B) Analog type clamp-on ammeter with flat scale. (C) Clamp-on ammeter with digital scale.

+++++30 The clamp-on ammeter connects around only one conductor.

The clamp-on meter also uses a current transformer to operate. The jaw of the meter is part of the core material of the transformer. When the meter is connected around the current-carrying wire, the changing magnetic field produced by the AC induces a voltage into the current transformer. The strength and frequency of the magnetic field determine the amount of voltage induced in the current transformer. Because 60 hertz is a standard frequency throughout the country, the amount of induced voltage is proportional to the strength of the magnetic field.

The clamp-on type ammeter can be given different range settings by changing the turns ratio of the secondary of the transformer just as is done on the in-line ammeter. The primary of the transformer is the conductor around which the movable jaw is connected. If the ammeter is connected around one wire, the primary has one turn of wire compared with the turns of the secondary. The turns ratio can be changed in the same manner that the ratio of the CT is changed. If two turns of wire are wrapped around the jaw of the ammeter, the primary winding now contains two turns in stead of one, and the turns ratio of the transformer is changed. The ammeter will now indicate double the amount of current in the circuit. The reading on the scale of the meter would have to be divided by 2 to get the correct reading. The ability to change the turns ratio of a clamp-on ammeter can be useful for measuring low currents. Changing the turns ratio is not limited to wrapping two turns of wire around the jaw of the ammeter. Any number of turns can be wrapped around the jaw of the ammeter, and the reading will be divided by that number.

+++++ Looping the conductor around the jaw of the ammeter changes the ratio.

+++++DC-AC clamp-on ammeter.

+++++Basic Hall generator. Current Generator Semiconductor, Zero-center voltmeter

+++++ The presence of a magnetic field causes the Hall generator to produce a voltage.

Current Generator: The current path is changed.

A voltage is produced in the generator. Magnet

+++++ If the magnetic field polarity changes, the polarity of the voltage changes.

Current Generator Direction of current changes because of change of magnetic polarity.

Polarity of voltage changes. Magnet.

DC-AC Clamp-On Ammeters

Most clamp-on ammeters that have the ability to measure both DC and AC don’t operate on the principle of the current transformer. Current transformers depend on induction, which means that the current in the line must change direction periodically to provide a change of magnetic field polarity. It’s the continuous change of field strength and direction that permits the current trans former to operate. The current in a DC circuit is unidirectional and does not change polarity, which would not permit the current transformer to operate.

DC-AC clamp-on ammeters use the Hall effect as the basic principle of operation. The Hall effect was discovered by Edward H. Hall at Johns Hopkins University in 1879. Hall originally used a piece of pure gold to produce the Hall effect, but today a semiconductor material is used because it has better operating characteristics and is less expensive. The device is often referred to as a Hall generator. ==== the operating principle of the Hall generator. A constant-current generator is used to supply a continuous current to the semiconductor chip. The leads of a zero-center voltmeter are connected across the opposite sides of the chip. As long as the current flows through the center of the semiconductor chip, no potential difference or volt age develops across the chip.

If a magnetic field comes near the chip (), the electron path is distorted and the current no longer flows through the center of the chip. A voltage across the sides of the chip is produced. The voltage is proportional to the amount of current flow and the amount of current distortion. Because the current remains constant and the amount of distortion is proportional to the strength of the magnetic field, the voltage produced across the chip is proportional to the strength of the magnetic field.

If the polarity of the magnetic field were reversed, the current path would be distorted in the opposite direction, producing a volt age of the opposite polarity. Notice that the Hall generator produces a voltage in the presence of a magnetic field. It makes no difference whether the field is moving or stationary. The Hall effect can therefore be used to measure DC or AC.

+++++ Basic series ohmmeter. R2 Meter movement.

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