Grinder tests

[BobL]

This morning I was checking how much power a grinder was drawing and then I got carried away and measure the power draw and a few other things about my grinders.

Here is a compilation of the results


RPM
Columns 3 shows the RPM as listed on the motor's nameplate while column 4 shows the RPMs as measured by a tacho. No great surprises here compared to what I have seen elsewhere.

Power ratings
Column 5 shows the power ratings as listed on the motor's nameplate.
Only the Abbot and Ashby l(A&A) listed both the power input (i) and power output (o) while the Bergin only listed the power input.
When neither output or input power is indicated I assume it is power input.

Output power ratings take into account how efficient (electrically and mechanically) a motor is and is the true indication of the ability of the motor to perform real work.
To measure this requires a dynamometer
The ability of an electric motor to do work is directly related to the torque which is also related to how easily it loses RPM under load.
As the load is applied an electric motor will also draw more current and hence generate more power up to a point where the applied load is greater than the ability of the motor to perform more work and it stalls.
Ideally a grinder should lose no speed when used and in some cases this is more or less how it is used i.e. very light strokes but if you need to shape a serious piece of some metal then some load has to be applied

Column 6 shows the power input (in Watts as measure by a mains current meter) when the grinder is free running - i.e. no applied load
As expected this is well below the listed power rating - column 5

Column 7 shows the power (in Watts as measure by a mains current meter) drawn by the grinder when an activity is undertaken that drops the free running speed by 50 rpm.
I do this by watching the tacho and current meter and slowly applying a load to the grinder. As soon as the RPMs drops by 50RPM I note the current draw and determine the power from this.
Note how for the A&A the 504W is getting closer to the rated power output.

Column 8 is the difference between column 7s and 6, and effectively represents the additional power the grinder draws to reduce the speed by 50 RPM.
Note

Assuming the motor efficiency does not change significantly between the "no load" and "load" conditions I interpret this as follows
A low value for column 8 means that the grinder only has to have a light load applied to it to drop the RPM
A high value means the grinder has to be pushed harder to drop the speed.

Note how the A&A draws the highest power to lose 50 Rpm while the Cromco and the Bergin lose RPMS under a much lighter load.
The differences are so marked between the grinders that this should be evident when they are used and that is indeed the case. The Bergin and the Cromco have always lost speed relatively easily compared to the other two grinders.

I'd be interested in any other interpretations.

[Chas]

I think your assumption that the nameplate power rating refers to input power is incorrect. Nameplates for induction motors normally show only the output power, but in the unusual case of one of your grinders the input power is identified and shown as well. Input power can be calculated by amps X volts. In earlier days your grinders would have been rated 3/4 HP (550W) and 1 HP (750W). Input power ratings are commonly used in marketing material to give the impression of "MORE POWER". This usually indicates "less efficient".

Chas.

[Cliff Rogers]

Strewth Bob, you need a hobby, ever thought about taking up woodwork?

[BobL]

I think your assumption that the nameplate power rating refers to input power is incorrect. Nameplates for induction motors normally show only the output power, but in the unusual case of one of your grinders the input power is identified and shown as well.

Two of the grinders show "input power" on the nameplate, The A&A shows input and output power and The Bergin shows Input power only

Input power can be calculated by amps X volts.

Thats what I used to determine P1 and P2

In earlier days your grinders would have been rated 3/4 HP (550W) and 1 HP (750W). Input power ratings are commonly used in marketing material to give the impression of "MORE POWER". This usually indicates "less efficient".
Chas.

Yep, I have measure the input power of a number of motors on dust extractors and the quoted powers are nearly all input powers (V x I).
I still reckon the CROMCO and the NONAME grinder power ratings on their nameplates are input powers.

The next test I will conduct is the power near stall test. If the CROMCO and NONAME stall before they deliver 550 and 750 W respectively then the quoted powers cannot be their output power because the output must be less than the input.

[BobL]

Strewth Bob, you need a hobby, ever thought about taking up woodwork?

I am doing debs and drabs of WW but it's nothing to be posting about.
I do need to stop phaphing about and do some real work.
Currently working on a brake for my bandsaw - I came "this close' to injuring myself with the coasting sucker the other day.

[Pat]

Bob, electronic or manual braking?

[BobL]

Bob, electronic or manual braking?

Manual, I've made a disc and hub that fits onto the drive pulley and am using a couple of used disc pads mounted in a home made calliper which I just finishedlast night. Like most of my projects it's sort of evolving as I go. I might be able to perform a preliminary test today. I'll post some pics on the BS forum.