KL3073

DC Motors Starters and Breaking Methods

DC MOTOR STARTERS

In order for a dc motor to function properly it must have some special control and protection equipment

The purposes of this equipment are.To protect the motor against damagedue to short circuits in the equipmentfrom long-term overloadsfrom excessive starting currentsTo provide a convenient manner in which to control the operating speed of the Motor

DC Motor Problems on Starting

It must be protected from physical damage during the starting period.

At starting conditions, the motor is not turning, and so EA = 0 V.

The full-load current of this motor is:Since the internal resistance of a normal dc

motor is very low a very high current flows.This current is over many times the motor's

rated full-load current. This may damage the motor.

Solution to the problem of excess current

Solution to the problem of excess current

Shunt motor with an extra starting resistor.

In designing the starter it is important to properly pick the size and number of resistor segments.

Shuts the resistor bypass contacts at the proper time

shunt motor with a starting resistor

Selected Rstart so that the current flow equals say twice the rated current.

the increasing EA decreases the IA in the motor.

When the IA falls to rated current, a section of the starting resistor must be taken out to increase the starting current back up to 200 percent of rated current

the increasing EA decreases the IA in the motor.

Repeat until all segments are out

Solution to the problem of excess current

How many steps are required to accomplish the current limiting?

Rtot as the original resistance in the starting circuit

The total resistance left in the starting circuit after stages 1

Initial starting resistance must beresistance R, must be switched out

at 1st stage

How many steps are required to accomplish the current limiting?

After switching that part of the resistance out, the armature current must jump to

Equating previous 2 equation

By direct extension, the resistance left in the circuit after the nth stage is switched out is

How many steps are required to accomplish the current limiting?

At the boundary where RA = Rtot,n

Equating previous 2 equation

Solving for n yields

Example

Example 6-7. Figure 6-24 shows a 100-hp 250-V 350-A shunt de motor with an armature resistance of 0.05 ohms. It is desired to design a starter circuit for this motor which will limit the maximum starting current to twice its rated value and which will switch out sections of resistance as the armature current falls to its rated value.

(a) How many stages of starting resistance will be required to limit the current to the range specified?

(b) What must the value of each segment of the resistor be? At what voltage should each stage of the starting resistance be cut out?

DC Motor Starting Circuits

Devices commonly used in motor-control circuits

DC Motor Starting Circuits

One common motor-starting circuit

DC Motor Starting Circuits

One common motor-starting circuit

THE WARD-LEONARD SPEED CONTROLLER

The speed of a separately excited, shunt, or compounded dc motor can be varied in one of three ways:

by changing the field resistance, changing the armature voltage, or changing the armature resistance.

THE WARD-LEONARD SPEED CONTROLLER

figure below shows an ac motor serving as a prime mover for a dc generator, which in turn is used to supply a dc voltage to a dc motor by changing the field resistance.

This system is called Ward-Leonard system.

THE WARD-LEONARD SPEED CONTROLLER

Controlling the field current of the dc generator armature voltage can be controlled

This allows the motor's speed to be smoothly varied between a very small value and the base speed.

Higher speed can be achieved by reducing the motor's field current

THE WARD-LEONARD SPEED CONTROLLER

if the field current of the generator is reversed, polarity of generated voltage also reversed.

This reverse the motor's direction of rotation.

If the torque or the speed alone of the motor reverses while the other quantity does not, then the machine serves as a generator.

The operating range of a Ward-Leonard motor-control system

SOLID-STATE SPEED CONTROLLERS

The average voltage applied to the armature of the motor can be controlled by fraction of the time the supply voltage is applied to the armature.

fast on and off of the supply can be done by modern solid state devices such as SCR.

A simple dc armature voltage controller circuit using SCR is shown below

A two-quadrant solid-state dc motor controller

SOLID-STATE SPEED CONTROLLERS

A more advanced circuit capable of supplying an armature voltage with either polarity is shown below. This armature voltage control circuit can permit a current flow out of the positive terminals of the generator, so a motor with this type of controller can regenerate

A 4-quadrant solid-state dc motor controller

DC MOTOR BREAKING METHODS

There are three kinds of electric breaking, namely:

Rheostatic or dynamic breakingPlugging andRegenerative breaking

Electric breaking for shunt motors

The armature of the shunt motor is disconnected from the supply and it is connected across a variable resistor R.

The field winding is kept undisrupted and this breaking is controlled by varying the series resistor R.

This method used generator action.

Rheostatic or dynamic breaking

Electric breaking for shunt motors

the armature terminals are reversed to rotate the motor in the reverse direction

VT and the back Eb start acting in the same direction.

Plugging or Reverse Breaking

Electric breaking for shunt motors

In regenerative breaking, Eb is greater than Vt.

The direction of IA and the armature torque Tb are reversed

Regenerative Breaking

Electric breaking for series motors

In this method the motor is disconnected from supply. The field connection is reversed and the motor is connected through a variable resistance R.

Rheostatic or dynamic breaking

Electric breaking for series motors

it is similar to that of shunt motor.

Plugging or Reverse Current Breaking