Oral Questions - Motor Testing Flashcards
When testing a 415V, 3-phase motor for insulation resistance to earth, what is the required test voltage?
500V
When testing insulation resistance between phase windings in a 3-phase delta connected motor, is it necessary to remove the links fitted at the motor terminal block?
Yes
Describe how you would determine if a motor was overloaded.
Determine full load speed and current from nameplate and measure actual speed and current under load conditions.
While testing a capacitor from a split phase motor, the analogue meter on ohms scale shows an initial low reading, which then climbs to a high reading. Is the capacitor faulty?
No – probably OK.
What would be the result if the centrifugal switch of a split-phase motor stayed open-circuited?
The motor would not start and would probably burn out.
What is the effect of the loss of a phase on a three- phase motor, which is running at full load?
The motor will run roughly and draw increased current. Should trip the overload relay.
A 4.0kW, 3 phase motor has the following winding resistances: • U1 - U2 4.0Ω • V1 - V2 4.6Ω • W1 - W2 4.1Ω Are these readings acceptable?
No, there is more than 10% difference, which indicates shorted turns.
Would an electric motor with an IP56 rating be suitable to use in a petrol bowser?
No, only a motor designed for hazardous use with an appropriate flameproof rating can be used.
A motor is fed via an electronic variable speed drive controller. Can the VSD unit be relied on for isolation purposes should the motor need to be disconnected?
No, leakage currents through the VSD can be lethal – a physical break or isolator must be used.
A motor blows the fuses immediately power is applied. What are the likely causes?
Low insulation resistance, faulty wiring or connections.
