DC motor question

[PDRUNEN]

Hi Group,

Please tell me what is going on here?

1: AC 120V to full wave bridge and I read 107Vdc on the meter.

2: Connect a DC motor across the bridge (with no freewheeling diode) and I get
13000 rpm as expected but the meter reads 136V.

3: As motor is loaded, the voltage decreases and get near the 107Vdc reading.

So, in #2, where is the extra 30 Volts coming from? Can I use it to determine
motor inductance or other?

Thanks,

Paul

 

[Wong Sy Ming]

One of your diodes is open?

 

[Spehro Pefhany]

Hi Group,

Please tell me what is going on here?

You have a DC voltmeter, it reads *average* voltage.

1: AC 120V to full wave bridge and I read 107Vdc on the meter.

Average voltage of full-wave rectified (unfiltered) 120VAC is about
108V, less a volt or so for the diodes.

(120 * 2* sqrt(2)/pi)

(The RMS voltage is still 120V, of course, ignoring diode losses, but
that isn't important here)

2: Connect a DC motor across the bridge (with no freewheeling diode) and I get
13000 rpm as expected but the meter reads 136V.

The back EMF from the motor is filling up the gaps in the waveform,
increasing the *average* voltage (the motor acts as a generator with
an voltage output of about 10mV/RPM, assuming it's a PM motor). If
there was no windage or other losses, you could expect to get as much
as 170V (sqrt(2) * 120).

3: As motor is loaded, the voltage decreases and get near the 107Vdc reading.

The motor slows down under load and the influence of the back EMF
decreases.

Best regards,
Spehro Pefhany