Motor current reducer help

Thread Starter

Greg G

Joined Jan 25, 2009
8


Hello, this is my first post. I port cylinder heads for a living, and use a grinder like the one pictured above. It is a 14,500 RPM 11 amp grinder. The grinder turns too many rpm's to control the cutter well, so I "toggle" the switch manually to keep the spindle speed down to a manageable level.

I was interested in buying or possibly building a control for it so I can get the rpm's down to ~5000 rpm and stop toggling the switch. From what I've been reading the last few hours, I *think* this grinder is equipped with a *universal* motor. It does have brushes.

Would it be possible for me to build a 10 amp DC driver with a PWM controller? If it's a universal motor I can switch over to DC power, right?

Any help is appreciated.

Thanks,

Greg
 
Last edited:

mik3

Joined Feb 4, 2008
4,846
Yes you can drive it with DC power if it uses a universal motor. Note that the voltage has to be lower than the AC voltage.
 

Thread Starter

Greg G

Joined Jan 25, 2009
8
That's what I will get then. Thank you.

Just out of curiosity, what would a do-it-yourself solution consist of? A bridge rectifier for DC power, then a PWM circuit to control power to the grinder?

Thanks.
 

SgtWookie

Joined Jul 17, 2007
22,201
Just out of curiosity, what would a do-it-yourself solution consist of? A bridge rectifier for DC power, then a PWM circuit to control power to the grinder?
Something like that. But, as mik3 mentioned, the DC voltage level would have to be reduced.

If you rectified 120VAC to DC, you'd wind up with around 169VDC on the output filter capacitor under no-load conditions. So, you'd need a heavy-duty step-down transformer to reduce the 120VAC to around 85v before rectification. A transformer that large would be pretty expensive by itself. But then, you'd need yet another transformer to drive the low-voltage electronic circuit; perhaps 555 timer(s) and power MOSFETs or IGBT's.

You might as well just use a variable transformer (VARIAC) to start off with. It'll be very efficient, and you won't have to build anything.
 

mik3

Joined Feb 4, 2008
4,846
Something like that. But, as mik3 mentioned, the DC voltage level would have to be reduced.

If you rectified 120VAC to DC, you'd wind up with around 169VDC on the output filter capacitor under no-load conditions. So, you'd need a heavy-duty step-down transformer to reduce the 120VAC to around 85v before rectification.
He can rectify it directly, filter it and use PWM to produced the desired average voltage.
 

KMoffett

Joined Dec 19, 2007
2,766
An off-the-shelf solution you might consider is a Variac:
http://www.mpja.com/prodinfo.asp?number=15163+TR
Just plug it in and you're ready to go.

I've bought a number of items from that vendor; never been dissapointed.
A word of warning about that 15A variable autotransformer. One of our Physics prof's bought several of those to run heaters in his lab. When he tried to use them, they usually (not always) tripped the 20A breakers in the local panel for those branches. We tried them as the only device on the 20A branch...still tripping the breaker when we turned it on. We tried them with no load and they did the same thing. Disassembled one and could find no problem. I talked to a customer service person at MPJA, and he said "that was normal". ??? I thought that maybe it was a cheap Chinese design, so I called an engineer at "VARIAC" (the real deal) and he said the same thing. I guess that the extremely low resistance and low inductance of the primary compared to a "normal" transformer results in huge initial currents.

So there ya go, for what it's worth. :(

Ken
 

jpanhalt

Joined Jan 18, 2008
7,695

Thread Starter

Greg G

Joined Jan 25, 2009
8
Now I'm skeptical of buying a Variac. The circuit it would be on is 20 amps.

Why would a Variac draw so much current?
 

mik3

Joined Feb 4, 2008
4,846
I think you have misunderstood the maximum current rating of the Variac. The load is the one which determines the current through it and not the Variac. The Variac applies a voltage to the load and the load draws the appropriate current.
 

Thread Starter

Greg G

Joined Jan 25, 2009
8
I was referring to KMoffett's post:

"they usually (not always) tripped the 20A breakers in the local panel for those branches. We tried them as the only device on the 20A branch...still tripping the breaker when we turned it on. We tried them with no load and they did the same thing. "



Why would a Variac trip a 20 amp breaker just by turning it on with no load (which is how I read the post)?

Thanks.
 

KMoffett

Joined Dec 19, 2007
2,766
Mil3 and Greg G,

I think you should call the VARIAC people and talk to them about your concept. The 15A variable autotransformers we had, have a primary/secondary winding of about 75 turns of what looked like #10 copper wire on a toroidal core. I'm just telling you of our experience and what their response was.

Ken
 

jpanhalt

Joined Jan 18, 2008
7,695
I came across this information on the Avel Lindberg site to explain the high inrush currents with Variacs:

Due to the excellent magnetic circuit that toroidal cores create, and also due to the remanence that results from the more square hysteresis loop that these cores possess, high inrush currents can be encountered when switching on large toroidal transformers. These are higher than in laminated stack transformers and can last for a few half-cycles of the mains voltage. This is caused by the core saturating for a split second and is quite normal. However, this means that larger toroidals (1.5 KVA and higher) should not be switched on without some precautions.
John
 

Bernard

Joined Aug 7, 2008
5,227
Openion: An incadescent bulb will burn just as bright on either 120V ac or rectified 120V AC, assuming no filtering. Strange, I never had a problem with my 10A 'Variac" at turn on with 15A breaker. How about an adapter box, AC pigtail, socket, 10Ω, 10W resistor, 24V relay,10μF cap. 1250Ω @10W. !A diode. Diode,1250Ω,Relay coil in series across 120; cap parallel to coil; 10Ω between input & AC output. relay NO contacts in parallel with 10Ω. Best to add an AC switch to box. With auto-transformer plugged in to box ,sw on , initial surge limited by 10Ω, for 2-3 cycles, then shorted out to provide normal operation.
 

jpanhalt

Joined Jan 18, 2008
7,695
Strange, I never had a problem with my 10A 'Variac" at turn on with 15A breaker.
Neither have I. That is why I found the explanation from Avel so interesting. When you saturate a core, the current can go pretty high fast.

John
 

Bernard

Joined Aug 7, 2008
5,227
An other thought : Leave out all the junk except 10 ohm & 2 switches in box.Plug in auto transformer, turn on AC, half second pause flip on other sw. which is in shunt with resistor. Resister would get rather hot if left on.
 

KMoffett

Joined Dec 19, 2007
2,766
Openion: An incadescent bulb will burn just as bright on either 120V ac or rectified 120V AC, assuming no filtering. Strange, I never had a problem with my 10A 'Variac" at turn on with 15A breaker. How about an adapter box, AC pigtail, socket, 10Ω, 10W resistor, 24V relay,10μF cap. 1250Ω @10W. !A diode. Diode,1250Ω,Relay coil in series across 120; cap parallel to coil; 10Ω between input & AC output. relay NO contacts in parallel with 10Ω. Best to add an AC switch to box. With auto-transformer plugged in to box ,sw on , initial surge limited by 10Ω, for 2-3 cycles, then shorted out to provide normal operation.
OK, I did essentially that, but with light bulb and SSR....no change. And like others, I'd never had problems with the garden variety 10A VARIACs.

Thanks for the reference info guys.

Ken
 
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