Two Basic Methods Used For Braking a Motor (DC Injection and Dynamic)

Braking a Motor

Two common basic methods used for braking a motor are DC injection braking and dynamic braking. We will look at both in detail, starting with electric braking.

1. DC Injection Braking
2. Dynamic Braking

1. DC Injection Braking

DC injection braking is a method of braking in which direct current (DC) is applied to the stationary windings of an AC motor after the AC voltage is removed. This is an efficient and effective method of braking most AC motors.
DC injection braking provides a quick and smooth braking action on all types of loads, including high- speed and high-inertia loads.

Recall that opposite magnetic poles attract and like magnetic poles repel. This principle, when applied to both AC and DC motors, is the reason why the motor shaft rotates.

Figure 1 – DC injection braking of a motor

In an AC induction motor, when the AC voltage is removed, the motor will coast to a standstill over a period of time, since there is no induced field to keep it rotating. Since the coasting time may be unacceptable, particularly in an emergency situation, electric braking can be used to provide a more immediate stop.

By applying a DC voltage to the stationary windings once the AC is removed, a magnetic field is created in the stator that will not change polarity.

In turn, this constant magnetic field in the stator creates a magnetic field in the rotor. Since the magnetic field of the stator is not changing in polarity, it will attempt to stop the rotor when the magnetic fields are aligned (N to S and S to N).

DC injection motor braking demonstration (VIDEO)

The only thing that can keep the rotor from stopping with the first alignment is the rotational inertia of the load connected to the motor shaft. However, since the braking action of the stator is present at all times, the motor is braked quickly and smoothly to a standstill.

Since there are no parts that come in physical contact during braking, maintenance is kept to a minimum.

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What Would Be The Best Conductor Material for Electrical Cables

HV copper cable (photo credit: businessinsider.com)

Al or Cu conductor…


The conductivity of copper is 65% higher than that of aluminium which means that the conductor size of similarly rated cables is proportionately smaller. Correspondingly less expense is then incurred in providing for insulation, shielding and armouring the cables themselves. Transport of the less-bulky cables is easier and so is installation. In limited spaces in cable ducts, the smaller volume and better ductility of copper cables can have an even larger benefit.

Copper cables are easily jointed because copper does not form on its surface a tough, non-conducting oxide. The oxide film that does form is thin, strongly adherent and electrically conductive, causing few problems.

Cleaning and protection of copper is easy and if joints are made as recommended they will not deteriorate to any great extent with age, which saves on maintenance costs.

For the same nominal current rating, the cable with the aluminium conductor is significantly larger in diameter, carries a proportionally greater volume of insulation and is not so easily installed because of being less flexible. Aluminium is notoriously difficult to joint reliably. Table 1 compares aluminium and copper conductors for equivalent current rating.


Table 1 – Comparison between Copper and Aluminium Conductors in XLPE Insulated Steel- Wire Armoured Cables.
 

Characteristic	Copper 300 m2	Aluminium 500 m2
Overall diameter (mm)	66.5	83.9
Minimum bending radius (mm)	550	700
Max DC resistance/km at 20o C (ohm)	0.0601	0.0617
Approx. voltage drop/A/m (mV)	0.190	0.188
Continuous current rating, drawn in to duct (amp)	496	501

(Cable: to BS 5467 (& IEC 502) 4-core, stranded conductors, XLPE insulation, PVC bedding, steel wire armour, PVC oversheath, rated at 0.6/1.0 kV)



These notes have largely been derived from reference to BS 7450 which is identical to IEC 1059. Both of these give full details of the variables to be considered and the ways in which optimum cable size determinations can be made.

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