Differences in AC and DC control, particular attention to coils?

[GMusgrove]

Hi,

I have just discovered this website and I am very impressed with the forum set up.
I am 20 years old and currently serving a 4 year Electrical apprenticship at Centrica Storage, a Gas Terminal in the UK.

The site Engineer asks me technical questions now and then to keep me on my toes and recently asked me to find "differences in AC and DC control, paying particular attention to coils?"

I appreciate this question is below the technical grade of other forum questions but any feedback would be welcome.

Regards,
GMusgrove

 

[itsmoked]

With regard to coils:

*GENERALLY*

DC coils consume far more power.
DC coils run hotter.
DC coil control wires can run further.

AC coils are per volume more powerful.
AC coils may be more noisy.
AC coils are variable impedance.

 

[jraef]

Also;
If you are controlling AC, you have AC already available and can use a simple transformer if it is the wrong voltage for the coils. To use DC in that application, you must also have a recitifier / power supply to convert the AC to DC. The same holds true the other way. To use an AC coil on a system where only DC is available, you must have an inverter, whereas to use DC in a DC system, a dropping resistor can change the voltage if necessary.

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[Barry1961]

With large DC coils on brakes it is common to reduce the voltage to the coil after it actuates. The voltage is dropped across a resistor so your system still needs the same power but the coil is cooler. A 200VDC coil is reduced to around 30VDC in many brakes.

You also have to have something to bleed the spike when you turn off a DC coil.

Barry1961

 

[coconutalley]

Gents,

I just want to add to the already good answers:

The differences between AC motors and DC motors are
basically: AC is electrically commutated [brushless] and DC are mechanically commutated [with the brushes].
An AC motor needs AC voltage to operate and DC needs DC
voltage. There are many flavors of each types so to answer the "paying close attention to coils" needs to specify types of motors. Are we talking permanent magnet DC motors vs. squirrel cage induction motors? To confused matters more, brushless DC motors have been thrown into the mix. Brushless dc motors are typically permanent magnet, e.g. rotor has magnets and stators have coils. The drive for brushless dc motor and AC motor can be almost identical, for the case of sinusoidally driven brushless DC motor except AC has to worry about slip. A six step current Drive for brushless is still a pseudo AC drive. Brushless DC motors are really a special case of AC motors.

 

[GMusgrove]

Thanks for all the feedback gents its been very helpful.

 

[briand2]

AC coils can vibrate / hum, sometimes to the annoyance of occupants, DC coils are quiet.