Amps vs. Power


Thread Starter

Arvind Sharma

We are trying to establish a feedback loop by monitoring amperage for a dryer motor and controlling the feed rate to the dryer on that basis. We recently found out that monitoring dryer motor power and using power to proportionally control feed to the dryer would give us more linear control.
Few questions:
a) Is there a better way of accomplishing this feed back (instead of power)? Why?
b) In case power monitoring works out to be the best bet, could I get a recommendation on some known brand names that specialize in power monitoring equipment?

Thank you.
In regards to Item "b" below, measuring power, you might try Load Controls Inc. I've had good luck with them. You can find them at:
Bill Mostia
William(Bill) L. Mostia, Jr. PE
Independent I &E Consultant
WLM Engineering Co.
P.O. Box 1129
Kemah, TX 77565
[email protected]
These opinions are my own and are offered on the basis of Caveat Emptor.

O'Connor, Denis


Check out Veris' Hawkeye meter for your application. The thing that is interesting about the meter is that they have molded the kW transducer into the CT.

It speaks modbus and is available with pulse or ma outputs.

There are a lot of options for power meters these days.
My guess why you are more accurate with power instead of amperage is because you eliminate voltage and power factor differences.

Denis O'Connor
Manager of Operations
Alliant Energy Integrated Services - Cogenex
Boott Mills South, 100 Foot of John Street
Lowell, MA 01852

tel 978.441.0090 ext 240
fax 978.441.4626
E-mail: [email protected]
Responding to Arvind Sharmar's query on the subject:

An operating motor has a constant kVA characteristic. This means that if voltage drops then current will rise. Conversely, if voltage increases then current decreases.

A voltage variation will also affect magnetizing current and slip (speed) so that power out will vary somewhat. However, these effects are small compared to the variation experienced on the motor's input ampere reading.

If voltage variation exists, then the current will vary enough to make it a poor indicator of power, hence a poor control parameter. If you
want accuracy, then use the power measurement... not current.

Phil Corso, PE
(Boca Raton, FL)