[thingstodo]

This was a test to determine if the 5 HP 460VAC 1745 rpm motor that I am using for testing will really generate 220% starting torque if it is driven with about 300% rated full load current. It was being driven by a 5 HP 240 VAC three phase VFD.

The notes that I took, the readings that I recorded, and the theory do not match. I see that I need to repeat this test. But here is the information that I have.

The experiment is done with starting torque, since I can measure that with a fairly simple setup. The motor is rated for 1745 rpm, or 55 rpm of slip. That's not quite 1 Hz (60 rpm) of slip. The graph of the torque on a Design B motor appears to show maximum torque around 2 times the slip frequency. My testing should have varied the VFD frequency from 0.9 Hz (approximately slip frequency) to 2.7 Hz (approx 3X slip) to determine where maximum torque is. Somewhere along the line I got mixed up and did not get that testing done ...

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The following is a bit of a history lesson for three phase motors. Please tune out until the end of this section if you don't like reading this sort of thing. Why the history lesson? I'm trying to explain what I'm trying to test. I want to determine if I can get 220% of the rated torque from my test electric motor - a 1745 rpm, 5 HP, three phase 460V design B motor. And if I can, how much current it takes for me to achieve that torque. My estimate is 300% of the full load current.

There are 4 NEMA 'standard designs' for three phase squirrel cage induction electric motors, creatively labelled A, B, C and D. A few extra have been added for specialty applications and use the letters up to M

Industrial and commercial motors that are available surplus are mostly Design B. There are minimum standards for how much torque a Design B motor has, but not all manufacturers stick with the minimums. For the Design A,B,C,D if the equipment designer used the 'minimum' values, another vendor's motor will work in the application.

The sizes that could be used in a vehicle are normally 'T' frame motors. The 'T' frame motors have standardized dimensions (mounting holes, shaft sizes, distance from the mounting feet to the shaft, etc) that allow you to replace a T frame from one vendor with a T frame from another vendor and everything fits.

The specifications for a Design B motor state that it will generate at least 220% of it's rated torque at a certain frequency. That frequency is related to the speed that the motor is turning, and the motor's rated slip.

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The motor is mounted on a 2 x 8, which is bolted to a few 2 x 4s. This motor does not drive a belt, but a strap instead (it is rated for much more tension than any of the belts that I have). The strap is wrapped around the drive pulley a few times and secured to the pulley. It then runs to a wooden pulley with a couple of different diameters, and runs around half the pulley circumference where it is fitted into a slot so that it does not slip. The pulley is mounted to a rotating shaft with a bearing on each end. The wooden pulley has a 2x2 bolted to it that pushes down on a 2 x 6, which spreads that force across a bathroom scale, which is levelled. That scale is how the torque of the motor is measured, with a multiplier for the wooden pulley and the motor pulley. It gives me decent accuracy and a range of measurements. The motor is rated at 15 foot-lbs, so I need to use the pulleys to amplify this weight value to something that can be easily measured on the scale. I hope to measure 220%, which is 33 foot-lbs. My scale can measure 240 lbs, so the multiplier should be just over 7, but it ended up being just under 4.

The motor and pulley bearings are bolted to the 2 x 4s. This whole mess is suspended between a work bench and a metal rack. The scale is on the floor below.

When the motor is turned on, the rack is being pushed down or compressed, the work bench is being lifted. The work bench had to be over 200 pounds to make this work.

There is a sketch below that tries to show what is going on. I'm not that good at explaining - the people that I explained this test to have not understood what I'm doing. When I reviewed what I did and compared that to my explanation, I must confess that I have trouble figuring out what I did as well. It appears that I will be doing this test again, perhaps in the spring.


Here is the sketch, for what it's worth.