[Stoves] Question regarding motors

Crispin Pemberton-Pigott crispinpigott at gmail.com
Tue May 29 13:33:00 PDT 2012

Dear Frank


A variac is useful for a series wound motor like a skill saw or an electric
drill, and even then, the speed is very load sensitive.


I am not sure how much you are willing to spend, but a simple and reliable
solution is to get a NORD controller.



I have used their SK750 for years on fence making machines. It means 750
watts so you can get away with a smaller one like the SK500E. There is a 250
and a 380 Watt version. You can get about 1.5 times the rated torque out of


In case you want to stop or start slowly, you sent a ramping rate to
anything you like. For example you can have it speed up at a rate of 5
seconds per 100 Hz which at 60 Hz input would take 3 seconds to come up to


For you application, it is perfect in that you can enter the gear ratio into
one of the variable settings and the display will read out in RPM directly.
Setting 24, for example, you would put at 0.036 and Setting 23 at Cd (which
displays the output speed).  It automatically detects the number of poles in
the motor when it is first turned on. Pretty snazzy.


You can set it to change speed when you push the ‘UP’ or ‘DOWN’ buttons. The
convenience for you is that you can easily make it any speed you want, and
calibrate it as you like in case it is not quite exact on the speed reading.


You would be able to vary the speed from 250 down to quite slow and it holds
the speed with a varying load.


If you want you can use a potentiometer mounted somewhere convenient to
change the speed by turning a dial and still use the front panel screen as
the readout.


There are competing products all over the place. I chose this one because of
reliability and price.






From: stoves-bounces at lists.bioenergylists.org
[mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Frank Shields
Sent: Tuesday, May 29, 2012 3:27 PM
To: 'Discussion of biomass cooking stoves'
Subject: [Stoves] Question regarding motors





I want to turn a frame holding solution of biochar along with toxicants to
determine up-take potential. Also to turn wood and biochar  pellets to
determine hardness. It needs to turn at 30 and 50 RPM. 

Can I use a ¼ HP motor with a Variac ? I have a motor ordered that turns at
52 RPM (gearing included) and would like to fine turn it to 50 rpm then, if
it will not destroy the motor, adjust it down to 30 rpm with the same







Frank Shields


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