AC and DC Motors

Question 1

How does a motor create rotational force?

Answer 1

By circulating current through a magnetic field.

Question 2

What is the magnetic field in a DC motor called?

Answer 2

Magnetic flux or just flux.

Question 3

How is the magnetic field generated in a DC motor?

Answer 3

It is generated by a separate magnetic circuit called the motor field.

Question 4

What is the magnetic flux in a DC motor dependant on?

Answer 4

The core material, the number of windings and the amps circulated through these windings.

Question 5

How many fields does an AC induction motor have?

Answer 5

An AC induction motor has only one circuit for both flux- and torque-produc- ing current. The vector summation of flux current and torque current repre- sents total current.

Question 6

How does a DC motor run?

Answer 6

A DC motor runs when DC voltage is supplied to the armature. As it spins, it also generates voltage inside itself like a generator. This internal voltage is called the counter electromotive force or CEMF. CEMF is dependent on the field current (flux) and the motor speed.

Question 7

What determines the speed of an AC motor?

Answer 7

The speed of an AC motor depends on its construction. The higher the input frequency, the faster it spins. This is because the input frequency determines the speed of the rotating field inside the motor that is generated by the stator windings. The speed is determined as:
synchronous speed = (120 × AC frequency) / number of poles

Question 8

What happens when there is a mechanical load on an AC motor?

Answer 8

When an AC motor has a mechanical load, it is no longer synchronized with the input frequency. The speed difference between no-load speed and full- load speed is called slip. It is expressed as a percentage.

Slip = (synchronous speed - full-load speed × 100) / synchronous speed

Question 9

Define Motor Field

Answer 9

The group of stationary coils of a DC motor’s magnetic circuit that provides the magnetic flux and torque flux to allow torque (and motion) to develop when current is passed through the DC motor’s armature.

Question 10

Define Magnetic Flux

Answer 10

Magnetic Flux · The stator is the group of stationary coils of an AC induction motor’s magnetic circuit that provides the magnetic flux and torque flux to allow torque (and motion) to develop at the rotor.

Question 11

Define Counter Electromotive Force (CEMF)

Answer 11

The internal voltage of a DC motor generated as it spins, like a generator.

Question 12

Define Slip

Answer 12

The ratio of speed difference between no load speed and full load speed to no load speed in an AC induction motor.

Question 13

Define Armature

Answer 13

The rotating part of a DC motor.

The armature is made up of the armature coils and commutator mounted on the shaft. Carbon brushes deliver the power to the armature.

Question 14

Define Rotor

Answer 14

The rotating part of the AC induction motor.

Question 15

What are Poles?

Answer 15

Poles are coils symmetrically spaced inside the motor (stator windings).

Question 16

Define Load

Answer 16

Load is the percentage of a drive/motor system’s rated current or amperage capacity in use at any given time.

Full load means 100 percent of rated capacity.

During overload, the capacity of the drive/motor system is exceeded.

Many drive/motor systems can operate at more than 100 percent load for a short period of time.

Motors are protected from prolonged overload conditions by thermal relays or by built-in features of the drives.

Typical overloads include acceleration power or impact process loads.

The percentage of current or amperage a motor/drive is using while running. Full load means 100% of rated capacity.

Question 17

Define Efficiency

Answer 17

Efficiency refers to the ratio of electrical input power to mechanical output power.

Question 18

Define Duty Cycle

Answer 18

In a typical operation, short periods of overloads are usually followed by extended periods of lighter loads. The ability of a drive/motor system to accommodate these variations is called its duty cycle.

The duty cycle pro- vides a more realistic measurement of a drive/motor system’s capability to handle overloads during routine use than a measurement focusing only on its overload capacity.

Question 19

Define Motor Terminal Volts

Answer 19

The volts directly applied to the motor terminals.

Question 20

Define Base Field (Flux) Amps

Answer 20

The field current where the motor delivers rated horsepower at its rated motor voltage at base speed.

Question 21

Define Weak Field (Flux) Amps

Answer 21

The field current where the motor delivers rated horsepower at its rated motor voltage at top speed.

Question 22

Define Constant Horsepower Range

Answer 22

A method used to achieve higher motor speed at maximum volts by weakening the motor’s field (flux).

Question 23

Define Crossover

Answer 23

The point at which maximum motor volts are reached and the field (flux) starts to weaken in order to achieve higher motor speed.

Question 24

Explain the effect of frequency on an AC motor.

Answer 24

Volts/hertz (V/hz) is used to describe AC motor control by frequency. The drive’s output frequency is related to the desired top speed. At no load, the motor is locked into this speed. At speeds below top speed, the voltage and frequency are lowered proportionately. As the load requirements of the motor increases, it will slow to its slip frequency speed at full rated load. To provide better open loop speed regulation, slip compensation is used. The speed reference is increased as the motor load
increases, based on the percentage of slip. Dual
speed motor rated DC motors are nameplated as
follows:

RPM:
1750 (base speed) / 2300 (top speed)

Field Amps:
5.0 (base field amps) / 4.0 (top speed amps)

Top speed amps are also called weak field amps.

For example:
The motor operates at base field (flux) amps when the motor delivers rated horsepower at its rated motor voltage at base speed. Once the top motor voltage is reached, the motor can run at its top speed by maintaining rated output volts and then lowering field amps.

Question 25

What happens when a motor reaches maximum voltage?

Answer 25

Once a motor reaches maximum voltage, the output power does not increase if its field (flux) is weakened to achieve higher speeds. The out- put torque is directly reduced as a function of the reduction of flux. This is referred to as oper- ation in the weak field range or in the constant horsepower range. The point where maximum voltage is reached and the field (flux) starts to weaken is called the crossover.