Load test on a three-phase squirrel-cage induction motor
Brake load test on a three-phase induction motor: connections, tabular column, slip, torque, power factor and efficiency calculations, performance curves, and viva questions with answers.
Aim
To conduct a load test on a three-phase squirrel-cage induction motor using a brake drum, and to draw its performance characteristics — efficiency, slip, torque and power factor against output power.
Apparatus required
| Apparatus | Specification | Qty |
|---|---|---|
| Three-phase squirrel-cage induction motor | 3 hp, 415 V, 50 Hz, 4-pole, with brake drum | 1 |
| Three-phase autotransformer or DOL starter | 415 V, 10 A | 1 |
| Wattmeter (UPF) | 600 V, 10 A | 2 |
| Voltmeter (MI) | 0–600 V | 1 |
| Ammeter (MI) | 0–10 A | 1 |
| Spring balances | 0–20 kg | 2 |
| Tachometer | Digital or contact type | 1 |
Theory
A three-phase supply to the stator produces a magnetic field rotating at the synchronous speed N_s = 120f/P. The rotor, cut by this moving field, has an emf induced in it; the rotor bars are shorted, so a current flows, and the interaction of that current with the field produces torque.
The rotor can never reach synchronous speed. If it did, there would be no relative motion, no induced emf, no rotor current and therefore no torque. The fractional lag is the slip, s = (N_s − N)/N_s, and it is what makes the machine work — hence 'induction' or 'asynchronous' motor.
Input power is measured by two wattmeters. Below 0.5 power factor one of them reads backwards, and its reading must be entered as negative and subtracted. A lightly loaded induction motor sits exactly there, so the reversal is expected on the first reading or two, not a fault.
Output is measured mechanically at the brake drum. The two spring balances measure the tight-side and slack-side pull on the belt; their difference times the effective radius gives the torque, and 2πNT/60 converts torque and speed into shaft power. Efficiency is that output over the electrical input.
As load increases, slip rises roughly in proportion to torque, power factor improves from a very poor no-load value towards 0.8–0.9, and efficiency peaks somewhat below full load before the rotor copper loss pulls it back down.
Circuit connections
Check every point below against your board before switching on. There is no diagram here on purpose — a wrong diagram is worse than none, and this is the list a demonstrator actually walks through with you.
- Three-phase supply through the starter to the stator terminals, with the two wattmeter current coils in two of the three lines.
- Wattmeter pressure coils connected from their own line to the third line — the line without a current coil.
- Ammeter in one line, voltmeter across two lines.
- Belt over the brake drum with a spring balance at each end, both slack at the start.
- The motor must be started with no load on the brake.
Procedure
- 1Check the connections, make sure the brake is fully released, and start the motor on no load.
- 2Record the no-load set of readings: W₁, W₂, line current, speed, and both spring balance readings. Note the sign if a wattmeter pointer reverses.
- 3Tighten the brake in small steps. At each step let the speed settle, then record all six readings together.
- 4Continue up to rated current — read the motor's nameplate and do not go past it.
- 5Release the brake fully, let the drum cool, and switch off.
- 6Compute slip, torque, output, power factor and efficiency for each row, and plot them against output power.
Work out your readings
Type in the numbers off the meters. This fills the tabular column, works the calculation through step by step, plots the characteristic — and tells you when a reading cannot physically be right, which is the part a manual can't do. Everything stays on this device, and it works with the network off.
Nameplate and machine data
Drum radius + half the rope thickness
Load readings
W₂ may legitimately be negative at light load — enter it with the minus sign.
| # | W₁(W) | W₂(W) | I_L(A) | Speed N(rpm) | Spring S₁(kg) | Spring S₂(kg) |
|---|---|---|---|---|---|---|
| 1 | ||||||
| 2 | ||||||
| 3 | ||||||
| 4 | ||||||
| 5 |
Fill in the machine data and at least one complete row of readings to see the results, the worked calculation and the curve.
Precautions
- Start with the brake fully released. Starting an induction motor against a locked brake draws locked-rotor current — five to seven times rated — for as long as it takes to trip.
- Do not exceed the rated line current on the nameplate.
- Cool the brake drum with water if the setup provides for it, and never touch it during or just after the run.
- If a wattmeter reads backwards, reverse its pressure-coil connections and record the value as negative. Do not reverse the current coil.
- The effective radius is the drum radius plus half the rope or belt thickness, and it must be in metres.
Sources of error
Every record asks for these, and every record gets the same three lines copied from the one before. These are the errors this particular experiment actually has.
- Belt friction and drum heating: the friction coefficient changes as the drum warms, so the same brake setting is not the same torque ten minutes later.
- Spring balances are read while vibrating; the reading is an eye average and easily a few percent out.
- Using the drum diameter instead of the radius, or forgetting the half-thickness of the rope, puts a direct error into every torque value.
- Supply voltage sag on the bench as other groups start their machines — torque varies as the square of the applied voltage.
Viva questions with answers
Why can an induction motor never run at synchronous speed?
At synchronous speed there is no relative motion between the rotating field and the rotor, so no emf is induced, no rotor current flows and no torque is produced. The motor would immediately slow down. Some slip is essential for torque to exist at all.
One wattmeter reads negative at no load. Is the connection wrong?
No. In the two-wattmeter method one meter reads negative whenever the power factor is below 0.5, and an induction motor on no load runs at about 0.1–0.3 power factor. Reverse the pressure coil to get a readable deflection, record the value as negative, and the total is still W₁ + W₂.
What is slip and what is its typical value?
s = (N_s − N)/N_s, the fractional shortfall of rotor speed below synchronous. It is near 1 at standstill and typically 2–6 % at full load on a cage motor.
Why is the starting current of an induction motor so high?
At standstill the slip is 1, so the rotor emf and frequency are at their maximum and the rotor circuit impedance is at its lowest. The machine behaves like a short-circuited transformer, drawing five to seven times rated current.
Why is the no-load power factor so poor?
On no load the motor draws almost purely magnetising current to set up the air-gap flux, and very little active current. A large magnetising component with a small in-phase component is precisely a low power factor.
How does torque vary with applied voltage?
As the square of it. A 10 % drop in supply voltage costs about 19 % of the torque, which is why an under-voltage supply makes a loaded motor stall.
Why is a squirrel-cage rotor skewed?
To avoid magnetic locking or cogging between rotor bars and stator slots, to reduce the noise and vibration caused by slot harmonics, and to give a smoother torque as the rotor turns.
What is the relationship between rotor copper loss and slip?
Rotor copper loss = s × rotor input. So the loss is directly proportional to slip, which is why a high-slip operating point is an inefficient one, and why running an induction motor at half speed by increasing slip wastes half the power in the rotor.